Tek-Tip: Tools & Techniques for Cutting Rubber Molds

March 19th, 2015

Some rubber molds require one or multiple cuts in order to remove the original model and subsequent castings.

Poured Blanket Mold with Mold Shell
 

Cutting_RubberMold
 

Here are some tips regarding the tools and cutting techniques that we use here at Polytek:
 
 
Tools that we use: 

A tool like your kitchen paring knife will not work well for cutting rubber molds. At Polytek, we most often use the following:

  • Mold Key Knife
  • Scalpel
  • Razor Blade (one-sided for safety)
  • X-Acto Knife

Cutting Rubber Molds - Tools
[in order from left to right: mold key knife, scalpel, razor, X-Acto knife]

 
 
 General notes about these tools:

  • Mold Key Knives automatically create a tongue-and-groove pattern.
  • Scalpels, razor blades and X-Acto knives can fit into tighter spaces/corners than mold key knives. Often, a mold key knife is used to create the first cut (this cut should not reach the model) and then a scalpel/razor blade/X-Acto knife is used to make the final cut.
  • Depending on the rubber being cut, blades can degrade rather quickly (i.e., after cutting 2-3 molds), so it’s a good idea to keep multiple on-hand.

 
 
General notes on cutting rubber molds: 

  • It is very difficult to cut mold rubbers that are harder than Shore A45 (read more about Shore Hardness here).
  • Silicones are generally easier to cut than polyurethanes.
  • To avoid damaging the model, the first cut should not go completely through to the model. As mentioned in the previous section, the final cut should be made with a scalpel or X-acto knife as these tools are more precise.
  • Avoid making a straight cut in the mold; make a zig-zag/tongue-and-groove pattern for better alignment when casting.

 

ZigZag_CutinMold
 

Planning ahead:

  • If you are planning on cutting a mold, it is a good idea to create a thicker section of rubber in the area that you intend to cut (~0.75″). An example is shown on the far right below. You can also see this technique in a video tutorial on our YouTube Channel.

 

Rubber Flange for Cutting-01
 

For brush-on blanket molds (like the mold pictured above), we make these thick pieces of rubber by casting the rubber in strips (we have a silicone mold that we made specifically for this purpose) and then applying them to the mold:

 

Casting Rubber Strips-01
 

  • When selecting a location for the cut, choose an area where seam lines will not be as noticeable if they appear on the casting (e.g., avoid the face).
  • Repetitive opening and closing of the mold can cause tears over time. Embedding Tietex® Fabric in the area where the cut ends can improve strength. The red arrow below indicates an area where TieTex would be beneficial.

 

TieTex next to cut-01

 
TieTex Fabric for Cut Molds
[^TieTex Fabric]
 

Are you working on a mold making project and have a question about technique?

Call us at 800.858.5990.
Email us at sales@polytek.com.
Fill out this simple online form.

New Product Announcement: FormRub Series Liquid Rubbers

March 3rd, 2015

FormRub 50 Liquid Rubber - 80-lb Kit

FormRub Series Liquid Rubbers

Use FormRub Series liquid polyurethane rubbers to make high-performance molds that stand up to the rigors of high-production casting and forming of concrete.

The product options within this series offer simple, 1A:1B mix ratios (measured by weight or volume) and low-viscosity mixes that flow easily across original models.

Popular uses include molds for casting concrete veneer stones, architectural elements, edge and trim strips, decorative panels, tiles, countertops, tabletops, sinks, pavers, slabs, patio stones, fences, sound barriers and more.

 

 

FormRub Series Features

  • Shore A35 to A65 hardness mold rubbers
  • Low viscosity
  • 1A:1B mix ratios
  • High-strength, abrasion-resistant, long-lasting molds
  • Reproduces fine details
  • Pourable or brushable (brushable with the addition of PolyFiber II)
  • Can be accelerated for rapid cure
  • Economical & versatile
  • Ideal for the most demanding mold making applications
  • All rubbers can be demolded after ~16 hours

 

Individual Options

FormRub 35Shore A35 Hardness – ideal for models with deep relief or irregular undercuts that require a flexible, yet durable mold option.

FormRub 50Shore A50 Hardness – this low-viscosity option is very suitable for pouring large molds or molds with challenging flow conditions.

FormRub 60 - Shore A60 Hardness – provides a longer working time compared to other Shore A60 polyurethane mold rubber options that we offer.

FormRub 65Shore A65 Hardness – ideal for casting larger parts where dimensional stability is required.

 

Accessories

A range of additives can be used to adjust the characteristics of FormRub Series liquid rubbers, including accelerators, thickeners and softeners. View accessories on the FormRub Series page on www.polytek.com. 

 

Do you have questions about this new mold rubber line? Reach out to our Technical Support Team:

Call us at 800.858.5990.
Email us at sales@polytek.com.
Fill out this simple online form.

Tek-Tip: Painting with PlatSil® Gel-10 & Silicone Pigments

February 26th, 2015

Painting directly onto silicone can be challenging as there are not many paint materials that adhere well to silicone or stay in place when it stretches.

In this tutorial, we show you a painting technique that BITY Mold of Supply of Richardson, TX demonstrated when visiting our headquarters recently.
 
 
Supplies for Project

 

One benefit of this technique is that if you’re already making prosthetics or props out of PlatSil Gel, there is no need to purchase additional products.

In this tutorial , we paint a PlatSil Gel-10 face cast:

PlatSil-Gel-10-FaceCast

 

Measure & Mix PlatSil Gel-10

 
PlatSil Gel-10 has a mix ratio of 1A:1B by weight or volume. Carefully measure and mix a small amount.
 

PlatSil Gel-10_Painting
 

PlatSil Gel-10 Part A

[^Part A]

PlatSil Gel-10 Part B

[^Part B]

Mix PlatSil Gel-10

 

Carefully Mix in Naphtha

 
Add Naphtha to the mixed PlatSil Gel-10. We add the solvent at a 1:1 ratio, but you can add more or less depending on how thin you want the mixture to be.

Naphtha is a flammable solvent – care should be taken when handling. Make sure to use solvent-resistant mixing containers and tools.
 

Add Naptha

 
Very carefully mix the components – it is important to mix slowly as Naphtha is prone to splashing. Mix until the silicone breaks up and dissolves.

NOTE: The addition of Naphtha will increase working time. Standard working time for PlatSil Gel-10 is 30 minutes. The addition of this solvent could potentially double that time.
 

Mix Carefully

 

Combine with Silicone Color Pigments

 
Select the Silicone Color Pigments (the 8-color tray pictured below is available from BITY Mold Supply) desired for the project. Colors available from Polytek include: black, blue, green, red, yellow, fleshtone and white.
 

Silicone Pigments - Polytek

 
Place the desired pigment (or blend of pigments) into a mixing container and then add a small amount of the PlatSil Gel/Naphtha mixture to the pigment and mix.
 
Mix Silicone Pigments & Solvent

 
Mix Gel-10 & Pigments
 
 
Gel-10 Mixture & Silicone Pigments
 
 
Blue Silicone Paint
 
 
Gel-10 Silicone Paints

 

Begin Painting

 
The overall look is at the hands of the artist; this is just a basic demonstration using two colors.

Start painting with one color (for this demonstration, a combination of red and brown is used first to bring out detail).

Red & Brown Silicone Paint

 
Painting with Gel-10

 
First Color

 

Run a Hair Dryer Between Application of Colors

 
When finished with one color, make sure to run a hair dryer over the surface of the model for a few minutes before proceeding to the next color. If you add a second color before running the hair dryer, the colors will bleed together.
 

Use Hair Dryer Between Colors

 
A second color (blue) is then added.
 
Add Blue Silicone Paint
 
 
Blue PlatSil Gel-10 Mix

 
When application of the blue silicone paint is complete, the hair dryer is run again.
 
Painting Gel-10 face

 
Lastly, to produce the look of freckles, a brush is cut and used to “splatter” a bit of brown/red paint.
 
FreckleTechnique

 
A towel can be used to pull back color when necessary.
 
Blot if Necessary

 
The hair dryer is run once more.
 
PlatSil Gel-10 - Polytek

 
Repeat this process until the desired effect is reached. When the final touches are added, allow the silicone to fully cure before handling.
 
PaintingPlatSilGel10

 

Baby powder can be dusted on to the surface to reduce shine.
 
 
 
Do you have questions about this process? Reach out to our Technical Support Team:

Call us at 800.858.5990.
Email us at sales@polytek.com.
Fill out this simple online form.

New Concrete Catalog: Liquid Mold Rubbers for Concrete Casting, Texturing & Stamping

February 11th, 2015

OuPolytek Liquid Mold Rubbers for Concrete Casting, Texturing & Stampingr newest “mini-catalog” features product recommendations for applications involving concrete casting, texturing and stamping:

 

  • Cast Stone Veneer
  • Form Liners
  • Decorative Surfaces & Decor
  • Hardscape
  • Architectural
  • Casting Plastics & Hardcoats
  • Accessories

 

View the catalog here. 

 

Do you need assistance in selecting an appropriate mold material for your next concrete casting project? Ask our technical support team:

Call us at 800.858.5990.
Email us at sales@polytek.com.
Fill out this simple online form.

Mold Making Tutorial: Old-Growth Cypress Concrete TableTop

January 29th, 2015

SureCreteWe recently visited SureCrete Design Products, a decorative and specialty concrete manufacturer, at their headquarters in Dade City, FL. While there, we did some rubber mold making demos to showcase how flexible molds can be used to create a variety of unique, decorative concrete products. Typical applications include, but are not limited to:

  • full countertops & tabletops
  • individual countertop or tabletops elements such as sink inserts, decorative edging/trim strips and drain boards
  • decorative panels and tiles
  • outdoor and indoor furniture
  • planters
  • textured pool decks, walkways, patios, driveways, stairs, countertops with the use of rubber stamping and texture mats

 
For one demo, SureCrete provided us with a slab of old-growth cypress (pictured below). It was approximately 48″ x 20″ x 3″ and a “mold box” was already built around it using melamine-laminated particle board.
 

Old-GrowthCypress

 
UndercutsonCypressSlab
 
 
KnotsinCypress

 
Here are the basic steps that we took to make a mold of this model during the 90-minute demo:
 

Seal the Model

Porous models must be sealed prior to the mold making process. There are many sealing options available (view our Sealer & Release Agent Selection Guide here), but we chose Poly PVA Solution for this model. PVA is a water-soluble solution that can be washed off after the mold making process and is available in clear and green options.
 
We applied it by brush and then allowed it to dry.
 

PVASealer

 
PVASolution

 

Create Block-Outs

Because of the protrusions on one side of the slab, there are large empty spaces within the mold box.
 

UndercutsonCypressSlab

 
To cut down on the amount of rubber needed, we cut foam to block out those spaces.
 
ReducingWalls

 
CuttingFoam

 
We coated the foam in EasyFlo 60 liquid plastic (thickened with PolyFiber II) to give it a smoother, non-porous surface.
 
PlasticCoatedFoam

 
FoamCutoutsinPlace

 

Apply Release Agent

We applied Pol-Ease® 2300 Release Agent to the cypress, baseboard, mold box walls and foam, and then brushed it out with a dry brush to promote even coverage.
 

MoldMakingReleaseAgent

 
PolEase2300

 

Calculate Amount of Rubber Needed

We used Poly 74-45 Liquid Rubber to make this mold. It is a mid-range hardness, polyurethane mold rubber with a mix ratio of 1A:1B.
 
To following calculation was used to determine how much rubber was needed for the mold:

  • calculate the volume of the mold box (23″ x 50.25″ x 3.5″ = ~4,045 in³) and the approximate volume of the cypress (20″ x 48″ x 3″ = ~2,800 in³)
  • subtract the volume of the cypress from the volume of the mold box (4,045 in³ – 2,800 in³ = 1,245 in³)
  • take that result and divide by the specific volume of Poly 74-45 liquid rubber (1,245 in³ ÷ 27.5 in³/lb = ~45 lb of Poly 74-45 Liquid Rubber)

 

calculate rubber needed-01

 

Measure, Mix & Pour Liquid Rubber

We measured Part A & Part B separately by volume, then combined and mixed them thoroughly in a clean mixing container. We used a Poly Paddle mixing tool to mix by hand.
 

PartBPoly74-45

 
PartAPoly74-45

 
Mix74-45

 
MixRubberThoroughly

 
We poured the rubber into the mold box and allowed it to rise over to the model. It is best to pour rubber into rubber and avoid pouring it directly onto the model.
 
Poly 74-45 Liquid Rubber

 
PourLiquidRubberOverModel

 
PolytekLiquidRubber

 
AllowRubbertoFlow

 
AllowRubbertoCure

 
We allowed the rubber to cure overnight (~16 hours) at room temperature before demolding.
 

Demold

To demold, we began by removing the mold box walls and foam.
 

DeconstructBox

 
PryMoldBox

 
RemoveMoldBoxWalls

 
SidesofMold

 
We carefully loosened all edges of the mold with putty knives and crowbars.
 
LoosenEdgeofMold

 
LoosenEdgesandCorners

 
We peeled back the mold from the model, paying attention to areas where the rubber may have caught (i.e., in knots in the wood).
 
DemoldRubberMold

 
KnotsonWood

 
UrethaneRubberMold

 
We carefully freed the rubber from the knots using a utility knife.
 
Cut

 
FinishedMold

 
KnotDetail

 

Cast Concrete

SureCrete’s Xtreme Series precast mix was poured into the mold (a particle board block-out was used).
 

Concrete in Mold

 
Later in the day, we demolded.
 
TurnOverMold

 
DemoldConcrete

 
SureCreteConcrete

 
ConcreteCasting

 
ConcreteKnotDetail

 
The concrete was then stained with SureCrete’s EcoStain. 
 
MoldMakingProject

 
FinishedConcreteTabletop

 
Do you need assistance in selecting an appropriate mold material for your next project? Ask our technical support team:

Call us at 800.858.5990.
Email us at sales@polytek.com.
Fill out this simple online form.

Or leave a comment right here on the blog!

Mold Making: Poured Blanket Mold vs. Poured Block Mold

January 19th, 2015

In this blog entry, we will demonstrate two different mold making methods using the same plastic lion head model [pictured below] and the same silicone mold rubber. The methods are the poured blanket mold technique and the poured block mold technique.

Plastic Model for Mold Making

 
Based on the size and shape of this particular model, either mold making method could be used with success. The following information is designed to give you a better idea of the advantages and disadvantages of each technique as you decide which method to use for your mold making project. Below, we note the basic comparisons of each technique and then detail each method with pictures and instruction.
 

Basic Comparisons:

  • The poured block mold is considered the simplest type of mold, while poured blanket molds can take some time to master.

Mold Complexity-01

  • Poured blanket molds generally require less mold rubber than poured block molds.
    • In this specific tutorial, 4.3 lb of silicone mold rubber is needed for the poured blanket mold and 12.5 lb of silicone mold rubber is needed for the poured block mold (i.e., less money is spent on mold rubber). NOTE: Keep in mind that poured blanket molds do require construction of a mold shell (next bullet point), which requires more materials.
  • Poured blanket molds require construction of a “mold shell” or “mother mold”, while poured block molds require a containment area often referred to as a “mold box”. The following materials are often used to make mold boxes: melamine-laminated particle board, plywood, PVC pipe, plastic pails, metal flashing and many other options. The construction of mold shells take practice, while the construction of a box can be rather simple.
  • Removing castings from poured blanket molds can sometimes be easier because these molds are generally more thin and flexible compared to block molds. This depends heavily on the shape of the model, however.

 

Demonstration of Each Method:

The first step in both methods is to construct a containment area around the model. For the poured blanket mold method, a mold shell is constructed from liquid plastic. For the poured block mold method, a mold box is constructed from melamine-laminated particle board.

Rubber Blanket Molds vs Rubber Block Molds-01

 
Both methods are detailed below:
 

Poured Blanket Mold: Construction of Mold Shell

First, secure the model to the baseboard. We insert a wood screw through the bottom of the baseboard and into the plastic model. Another option is to adhere the model to the baseboard with epoxy. The baseboard in this tutorial is a piece of melamine-laminated particle board.
 

SecureModeltoBaseboard_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Place plastic wrap over the model.

 

CoverModelwithPlastic_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

 

CoverModelwithPlasticWrap_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Form a uniform layer of clay over the model and plastic wrap. Make sure to use a sulfur-free, oil-based clay (we use plasticine clay) as sulfur can inhibit the cure of silicone mold rubbers.

The layer of clay should be approximately 1/2″ thick and should fill in any undercuts on the model. The clay represents the space that the mold rubber will later fill, so uniform thickness is important (i.e., you do not want thin spots or holes in your finished mold).

 

Clay over MOdel

Poured Blanket Mold: Construction of Mold Shell

 

Create a clay flange that extends ~1 to 1.5 inches beyond the model. Smooth out the clay as best as possible.

 

CoverModelwithClay_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Build-up an area that will serve as the pour hole, which is where mold rubber will later be poured.

 

Build-UpPouredHole_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

 

SmoothOutPourHole_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Again, smooth out the clay as best as possible.

 

UniformLayerofClay_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Spray Pol-Ease® 2300 Release Agent onto the clay and surrounding baseboard. This will help remove the plastic mold shell from the clay and particle board later in the process.

 

Pol-Ease 2300 Release Agent_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

 

Apply Release Agent to Clay_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Select a mold shell plastic such as Poly 1512X Liquid Plastic. This polyurethane plastic has a working time of 5 minutes and a demold time of ~30 minutes.

 

Mold Shell Plastic_BlanketMold

Poured Blanket Mold: Construction of Mold Shell

 

Poly 1512X has a 1A:1B mix ratio and can be measured by volume or weight.

Measure Part A:

 

Measure Mold Shell Plastic_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Measure Part B:

 

Measure Poly 1512X Mold Shell Plastic_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

 

Plastic Measured by Volume_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Thoroughly mix together Part A & Part B and then add PolyFiber II thickener to bring the mixture to a brushable consistency.

 

PolyFiberII Thickener_BlanketMold

Poured Blanket Mold: Construction of Mold Shell

 

 

Mix Mold Shell Plastic with Thickener_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Add PolyFiber II thickener until a thick, non-sag consistency is reached.

 

PlasticBrushableConsistency_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Apply the thickened Poly 1512X to the layer of clay, leaving an opening at the top of the pour hole. Note: get the Poly 1512X plastic out of the mixing pail as quickly as possible; it will cure faster in larger masses (i.e., sitting in the pail).

 

Apply Mold Shell Plastic to Clay_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Spread the plastic evenly over the plasticine clay using disposable dry brushes or other tools like stainless steel spatulas.

 

Apply Plastic with Brush_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

 

Mold Shell Creation_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

 

Distribute Mold Shell Plastic Evenly_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Create a plastic flange that extends 1-2 inches beyond the clay on the baseboard.
 

Mold Shell_Poly1512X_BlanketMold

Poured Blanket Mold: Construction of Mold Shell

 

Before the plastic completely sets, add legs that will hold and level the mold and mold shell for casting later.

 

LevelWoodenLegs_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

 

AddWoodenLegs_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

When the Poly 1512X plastic has set (after ~30 minutes – this time will vary depending on temperature and humidity), create a reference point with a permanent marker indicating the location of the mold shell in relation to the baseboard for proper re-positioning later in the process.

 

Mark Location of Mold Shell_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Drill holes evenly throughout the plastic flange – the mold shell will later be re-positioned and screwed into place in these locations.

 

Secure Mold Shell to Baseboard_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Loosen the edges of the mold shell and carefully remove it from the baseboard. CAUTION: Edges of the plastic mold shell can be very sharp.

 

PryMoldShell

Poured Blanket Mold: Construction of Mold Shell

 

 

Remove Mold Shell from Model_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Remove the plastic wrap and clay from the mold shell.

 

Clean Mold Shell_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

 

Clean Interior of Mold Shell_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

If necessary, trim the top of the pour hole.

 

Clean Pour Hole_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

 

Finished Plastic Mold Shell_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Apply a release agent, like Petroleum Jelly, to the interior of the mold shell.

 

Apply Petroleum Jelly to Mold Shell_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Apply release agent to the model and surrounding baseboard before placing the mold shell back over the model. We have selected a silicone rubber as the mold material, so Pol-Ease® 2500 Release Agent is used.

 

Pol-Ease 2500 Release Agent_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Spray the release agent onto the model and then brush it out with a dry brush to encourage even and thorough coverage.

 

Brush Release Agent onto Plastic Model

Poured Blanket Mold: Construction of Mold Shell

 

Re-position the mold shell in its original position over the model. Align the marker and holes.

 

Reposition Mold Shell over Model_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Secure the mold shell to the baseboard with screws.

 

SecuretoBaseboard_Blanket

Poured Blanket Mold: Construction of Mold Shell

 

Seal around the screws and the edges of the mold shell with plasticine clay to help prevent rubber from leaking out.

 

SealEdgesofMoldShell-01

Poured Blanket Mold: Construction of Mold Shell

 

Construction of the mold shell is now complete.
 

Poured Block Mold: Construction of Mold Box

Start by securing the plastic model to the baseboard.

 

Bottomboard

Poured Block Mold: Construction of Mold Box

 

Seal the edges of the model with oil-based, sulfur-free clay (e.g., plasticine clay). Hot glue and caulk also work.

 

Caulk Edges with Clay_Block

Poured Block Mold: Construction of Mold Box

 

 

Smooth Edges_Block

Poured Block Mold: Construction of Mold Box

 

Smooth out and flatten the clay as best as possible.

 

Prevent Model from Floating_Block

Poured Block Mold: Construction of Mold Box

 

Apply Pol-Ease® 2500 Release Agent to the model and surrounding baseboard and then brush out with a dry brush. This step can be completed after the mold box is constructed; however, it is a bit easier to accomplish prior to construction.

 

Apply 2500 Release Agent_Block

Poured Block Mold: Construction of Mold Box

 

 

Spray 2500 Release Agent_Block

Poured Block Mold: Construction of Mold Box

 

 

Brush Out Release Agent_Block

Poured Block Mold: Construction of Mold Box

 

Construct walls around the model. We recommend leaving at least 1/2-inch of space between the model and mold box walls (sometimes more space is required, depending on the size/shape of the model and the casting material that will later be used).

We are using melamine-laminated particle board for the mold box. These adjustable mold boxes are available for purchase on www.polytek.com.

 

Construct Mold Box_Block

Poured Block Mold: Construction of Mold Box

 

 

Particle Board Mold Box_Block

Poured Block Mold: Construction of Mold Box

 

Secure mold box walls together. We use C-Clamps.

 

Secure with C-Clamps_Block

Poured Block Mold: Construction of Mold Box

 

Secure the mold box walls to the bottom board with screws; this will help to prevent rubber from seeping between them and lifting the walls.

 

Secure Baseboard to Walls_Block

Poured Block Mold: Construction of Mold Box

 

Seal the interior edges and corners of the mold box with clay (or hot glue or caulk) to help prevent leaking.

 

Seal Interior Edges_Block

Poured Block Mold: Construction of Mold Box

 

 

Smooth Clay in Mold Box_Block

Poured Block Mold: Construction of Mold Box

 

Here are the finished containment areas pictured side-by-side.

 

Finished Containment Areas_Blanket Mold_Block Mold

Mold Shell  /  Mold Box

 

Poured Blanket Mold: Pouring the Mold Rubber & Demolding

For both molds, we use PlatSil® 73-20 Silicone Rubber. It is a soft, platinum-cured silicone rubber with a 5-minute working time and a 1-hour demold time.

 

Polytek PlatSil® 73-20 Platinum Silicone Rubber

Poured Blanket Mold: Pouring the Mold Rubber

 

PlatSil 73-20 has a 1A:1B mix ratio. Carefully weigh out Part A & Part B and then combine and thoroughly mix. Scrape the sides and bottom of the mixing container several times.

 

Two-Part Platinum Silicone Mold Rubber

Poured Blanket Mold: Pouring the Mold Rubber

 

 

Mix Silicone Rubber

Poured Blanket Mold: Pouring the Mold Rubber

 

 

Poly Paddle Mixing Tool

Poured Blanket Mold: Pouring the Mold Rubber

 

4.3 lb of PlatSil 73-20 rubber is necessary to complete this mold (compared to 12.5 lb of rubber needed to complete the poured block mold).

Carefully pour the mixed rubber into the pour hole until it reaches the top.

 

Fill Mold Shell with Silicone Rubber_Blanket

Poured Blanket Mold: Pouring the Mold Rubber

 

 

Fill to Top of Mold Shell_Blanket

Poured Blanket Mold: Pouring the Mold Rubber

 

Allow the rubber to cure for about an hour at room temperature and then carefully remove the mold shell.

 

Remove Plastic Mold Shell from Mold._BlanketJPG

Poured Blanket Mold: Demold

 

 

Silicone Poured Blanket Mold_Blanket

Poured Blanket Mold: Demold

 

Loosen the edges of the silicone mold and then remove it from the model.

 

Peel Mold from Model_Blanket

Poured Blanket Mold: Demold

 

 

Loosen Edge of Mold_Blanket

Poured Blanket Mold: Demold

 

 

Finished Poured Blanket Mold_Blanket

Poured Blanket Mold: Demold

 

When ready for casting, place the mold back into the mold shell and align properly.

 

Place Silicone Mold in Mold Shell_Blanket

Poured Blanket Mold: Finished

 

 

Place Mold in Shell

Poured Blanket Mold: Finished

 

 

Poured Blanket Mold_Polytek_Blanket

Poured Blanket Mold: Finished

 

Poured Block Mold: Pouring the Mold Rubber & Demolding

Mixing procedure for the mold rubber is the same for both options.

12.5 lb of PlatSil 73-20 rubber is necessary to complete this mold (compared to 4.3 lb of rubber needed to complete the poured block mold).

Pour the mold rubber into one corner of the mold box and let it rise. Pour rubber into rubber and avoid pouring directly on the model.

 

Fill Mold Box with Rubber_Block

Poured Block Mold: Pouring the Mold Rubber

 

Fill the mold box to at least 0.5″ above the model.

 

Fill to Top of Mold Box_Block

Poured Block Mold: Pouring the Mold Rubber

 

Spraying release agent across the surface of the rubber immediately after pouring can help to release air bubbles.

 

73-20 block mold

Poured Block Mold: Pouring the Mold Rubber

 

Allow the rubber to cure for approximately 1 hour and then demold. Remove the screws that secure the baseboard to the mold box walls.

 

Remove Screws from Mold Box_Block

Poured Block Mold: Demold

 

Remove the C-clamps and mold box walls.

 

Remove C-Clamps_Blanket

Poured Block Mold: Demold

 

 

Remove Mold Box Walls_Block

Poured Block Mold: Demold

 

Loosen the edges of the mold and then remove it from the model. A prying tool may be helpful.

 

Remove Block Mold from Model_Block

Poured Block Mold: Demold

 

 

Lift Mold from Model_Block

Poured Block Mold: Demold

 

The mold is now ready for casting.
 

Resin Casting with EasyFlo 60

We prepare enough EasyFlo 60 Liquid Plastic to pour into both molds. EasyFlo 60 is a two-part, low-viscosity polyurethane plastic in the EasyFlo Series. This fast-setting plastic has a 2 to 2.5-minute working time and a 15 to 30-minute demold time.

Measure EasyFlo 60 by volume (1A:1B) or by weight (100A:90B) and mix thoroughly.

 

Fast-Setting Casting Resin_EasyFlo60

EasyFlo 60 Liquid Plastic

 

 

Measure Out EasyFlo Liquid Plastic

EasyFlo 60 Liquid Plastic

 

Carefully pour the mixed resin into the molds.

 

Poured Resin into Mold Cavity_BlanketMold

Poured Blanket Mold: Resin Casting

 

 

Pour EasyFlo Resin into Silicone Block Mold

Poured Block Mold: Resin Casting

 

Allow 15 to 30 minutes before demolding. This resin is a translucent yellow/amber color in liquid form, but cures to an opaque white color.

Allow EasyFlo to Cure
Demold the poured blanket mold by first removing the plastic mold shell.

 

Demold Poured Blanket Mold

Poured Blanket Mold: Demold

 

Peel back the blanket mold to remove the casting.

 

Peel Back Silicone Mold from Model_Blanket

Poured Blanket Mold: Demold

 

 

Demolding Plastic Model_BlanketMold

Poured Blanket Mold: Demold

 

 

Finished Casting_Blanket Mold

Poured Blanket Mold: Demold

 

To demold the poured block mold, loosen the edges and pull out the casting. This demold requires a bit more effort as the model has some undercuts and the mold rubber is thicker and less flexible compared to the blanket mold.

 

Demold Block Mold

Poured Block Mold: Demold

 

 

Remove Model from Mold_Block

Poured Block Mold: Demold

 

 

Demold Plastic Lion Head_Block

Poured Block Mold: Demold

 

We put some finishing touches on the plastic castings with black and green wax.

Apply Wax to EasyFlo Casting
 

EasyFlo 60 Plastic Head with Wax
 

Black and Green Wax Applied to Plastic Model
 

 

Poured Block Mold vs Poured Blanket Mold

left: casting from the poured block mold | right: casting from the poured blanket mold


 

Video Tutorial

This is one of the projects that we worked on with BITY Mold Supply (a distributor of Polytek products) when he paid us a visit back in December. Here is this video tutorial of the project:

 

 

Do you need assistance in selecting an appropriate mold making method and material for your next project? Ask our technical support team:

Call us at 800.858.5990.
Email us at sales@polytek.com.
Fill out this simple online form.

Or leave a comment right here on the blog!

A Year in Review: 2014 Mold Making & Casting Projects & Events

December 31st, 2014

As 2014 comes to a close, we would like to share some events, customer projects and new product announcements from the past year:

 

– World of Concrete 2014 –

We had a great time meeting attendees and fellow exhibitors at the World of Concrete in Las Vegas.

Click here to see how we the made the concrete Polytek® signs mounted to our booth.

World of Concrete 2014 - Polytek
 

Concrete Signs

 

We also gave away Poly 74-20 rubber coaster molds (pictured below) at this event. You can learn how to make these here.

Coaster Molds - Polytek
 

We’ll see you at World of Concrete 2015!

 

– The Precast Show 2014 –

The Precast Show, held in Houston, TX this year, was another great concrete-related tradeshow – we had a great time meeting attendees and fellow exhibitors.

Precast Show
 

We’ll see you at the 2015 Precast Show in Orlando!

 

– Customer Project: The Wildlife Casting Project –

Sarah Madigan is a wildlife rehabilitator, educator, and creator of the Wildlife Casting Project. Rooted in devotion to wildlife conservation and awareness, Sarah describes the goal of the Wildlife Casting Project:

My deepest hope for my own work is that as people encounter the animal casts, their wonder and imagination for the wild world will increase. I believe……continue reading.

Sarah graciously sent us a casting she made of a jaguar’s paw (Panthera onca – a black panther) [pictured below].

Wildlife Casting Project
 

– Celebrating St. Patty’s Day with Resin Casting –

In celebration of St. Patty’s Day, we made beer mug replicas. View the video tutorial here.

Resin Casting St Pattys
 

– March Mold Making & Casting Seminar & Workshop –

Two times a year, we hold a Mold Making & Casting Seminar & Workshop. Attendees are invited to bring their own projects with them (some examples are pictured below). Click here to read more about this workshop.

Mold Making & Casting Workshop
 

– Architectural Restoration in Philadelphia –

A couple of Polytek staff members headed to a commercial building in downtown Philly to help make a brush-on mold of this damaged column. You can read about the process on our blog or view the video tutorial on our YouTube Channel.

 

Restoration of Decorative Column
 

– “Communities In Schools of the Lehigh Valley” Chemistry Demo with Polytek Products –

Polytek was happy to be a part of Communities In Schools of the Lehigh Valley back in June. This organization is dedicated to helping children stay in school, overcome obstacles, and thrive in life. David Salisbury, a chemist and the President of Polytek, spent part of the afternoon with students at Easton Middle School. After introducing the basics of chemistry via whiteboard, David got started on showing the students how chemistry applies to products that we use or see every day. Read more about the visit here.

Communities in School
 

– New Product Announcement: PlatSil® Gel-25 –

In July, we announced a brand new product: PlatSil® Gel-25. It is a new platinum silicone gel system similar to Gel-10 & Gel-OO, but varies in its versatility and physical properties. Gel-25 offers lower mixed viscosity (6,000 cP compared to Gel-OO and Gel-10’s 15,000 cP viscosity) and greater versatility in Shore hardness. Using new PlatSil Deadener LV and PlatSil Part H Hardener, PlatSil Gel-25 can be made as soft as Shore OOO30 and as hard as Shore A40. More information here.

PlatSil Gel-25 Special Effects Silicone
 

– Customer Project: Indianapolis Fabrications Assists Sopheap Pich – “A Room” Installation –

Commissioned to assist Sopheap Pich with the “A Room” installation at the Indianapolis Museum of Art (IMA), Polytek customer Indianapolis Fabrications (iFab) was asked to produce artificial bamboo elements to be combined with natural bamboo for this meditative space. They also assisted in finishing cast aluminum elements (cast at the Herron School of Art and Design Sculpture Departments) for the installation. Read more…

Sopheap Pich - A Room
 

– Customer Project: Cast Stone Magnolia Buds for Tattnall Square Park –

West Side Stone Works of Macon, Georgia took on a mold making and casting project to create cast stone magnolia buds that would serve as finials on the columns situated at the entrance of Tattnall Square Park in historic Macon. Read more…

Cast Stone Magnolia Buds Finished
 

– Loveland Sculpture Invitational –

We were happy to be part of the last Loveland Sculpture Invitational in August. We were there with our friends from Sculpture Depot.

Loveland Sculpture Invitational
 

Loveland Sculpture Invitational
 

– Customer Project: Decorated Venus Figurine – EasyFlo Liquid Plastic –

At the March 2014 Mold Making & Casting Workshop, attendee Ro-Z Esposito brought a plaster Venus figurine. She wanted to make a rubber mold of the figure and then cast plastic copies. Ro-Z would later decorate the plastic casting by gluing intricate beading to the surface. Read more…

Decorated Venus Figurine
 

– Polygel Spray-On Mold of 19-Foot Tall Mastodon Sculpture –

Kent Ullberg’s most recent monumental sculpture, a 19-foot-tall, 24-foot-long mastodon, is a scientific reconstruction based on findings by scientists from the Denver Museum of Nature and Science, the Smithsonian Institution (Washington, DC), and a Michigan university [www.reporterherald.com]. Spray Solutions in Loveland, CO used Polygel® Spray 35 to make the enormous mold. Read more…

Kent Ullberg Mastodon Sculpture Mold
 

– Warping Concrete –

In September, we used rubber molds to “warp” GFRC concrete with the help of Trinic LLC & Majestic Concrete Design.

How to Bend or Warp Decorative Concrete in Rubber Molds:

Casting “Warped” Concrete Tables in Rubber Molds:

 

 

– Concrete Decor Show –

In October, we featured warped concrete at the Concrete Decor Show. We also showcased a wood grain textured concrete table top made by Trinic (view the video tutorial here).

Polytek Decorative Concrete Booth
 

We’ll see you at the 2015 Concrete Decor Show!

 

– October Mold Making & Casting Seminar & Workshop –

Our October Mold Making & Casting Workshop was full of interesting attendee projects. See more pictures here…

October Mold Making & Casting Seminar
 

View our 2015 Workshop Dates.

 

Thank you to all of our customers for a great 2014! 

Creating Permanent Masters for Mold Making

December 18th, 2014

This blog entry focuses on how to make hard rubber or plastic copies of original masters. There are a number of good reasons to create “permanent masters” of originals:

  • Some original masters (e.g., stone) can easily break or crumble.
  • Using original masters multiple times requires re-application of sealing agents, release agents and caulking.
  • If you produce molds in large volume, multiple identical masters allow for production of multiple molds simultaneously.

 

How to Make Copies of an Original Master

In this example, we’ll focus on a veneer stone master, which is composed of multiple side-by-side stones affixed to a baseboard. This master is designed to create a multi-cavity mold.

Veneer Stone Master

 
To create a duplicate master of these veneer stones, we choose to use a hard polyurethane rubber: Poly 75-80 Liquid Rubber.

On the Shore Hardness Scale, Poly 75-80 falls on the A scale at 80. It is a hard material, but will not break if it falls from a shelf, for instance.
 

Shore Hardness Scale_Polytek

 
Polyurethane plastics in the Poly 15-Series are also very popular for this use.
 

Step 1: Prepare the Existing Mold

The mold pictured below is made from Poly 74-20 liquid rubber. It was poured over the veneer stone master pictured above and was made several years ago (for a tutorial on how to make a veneer stone mold, visit our YouTube Channel).

veneerstonemold

Place the mold on a baseboard. We use melamine-laminated particle board for this tutorial.

Poly 74-20 Rubber Mold

To prepare the mold, first start by cleaning it off so there is no dust or debris that comes out with the master copy. Denatured alcohol is a good cleaning solvent, but care should be taken due to its flammability. Apply denatured alcohol to a rag or towel and wipe the mold.

Clean the Rubber Mold

Seal the edges of the mold so that casting rubber does not run beneath it. Typically, we seal edges with warmed plasticine clay; however, this particular mold has been exposed to quite a bit of release agent over the years and the clay has difficulty sticking directly the rubber.

We apply hot glue to the edges and smooth it out with a tongue depressor.

Seal Edges of Rubber Mold

Then we apply warmed plasticine clay over the glue and smooth it out.

Seal_Edges_with_Clay

Smooth_Clay

 

Step 2: Construct Mold Box

Construct a mold box to contain the casting material that will be poured into and over the existing mold.

Prior to constructing mold box walls, spray release agent onto the exposed baseboard and brush it out with a dry brush (it will be more difficult to reach once the walls are constructed). The casting material is a polyurethane rubber, so Pol-Ease® 2300 Release Agent is used.

Thoroughly_Apply_Release_Agent

Assemble mold box walls and secure them. We use melamine-laminated particle board for the walls and secure them together with C-Clamps.

Assemble_Mold_Box

Melamine-laminated_mold_box

Secure_with_C-Clamps

In this case, the finished duplicate master will have built-in walls, so the mold box is lined up 1/2″ beyond the exterior walls of the existing mold. The great benefit of having built-in walls is that there is no need to create a containment area around the permanent master every time a mold is made.

Mold_Box_for_Poured_Block_Mold

To prevent the mold box walls from lifting or “floating” when the rubber is poured, we secure them to the baseboard with screws.

Secure_Mold_Box_Walls_to_Bottom_Board

Seal the exterior edges of the mold box with clay so rubber does not seep out (hot glue or caulking also work).

Seal_Exterior_Edges_of_Mold_Box

Also seal the interior corners of the mold box.

Seal_Interior_Corners_of_Mold_Box

 

Step 3: Apply Release Agent to the Existing Mold

Very thoroughly coat the existing polyurethane mold with Pol-Ease 2300 Release Agent. Liquid polyurethane rubber will adhere readily to cured polyurethane rubber, so it is imperative to thoroughly cover the mold.

Pol-Ease_2300_Release_Agent_Polytek

Brush out the release agent with a dry brush to promote even coverage.

Brush_Out_Release_Agent

 

Step 4: Calculate Amount of Casting Material Needed

To calculate the amount of casting material needed to make the duplicate master, use the following calculation:

  1. Determine the volume of the mold box (L” x W” x H”). In this calculation, incorporate at least 1/2″ of space above the top of the existing mold.
  2. Subtract the volume of the existing mold from the volume of the mold box.
  3. Take the result of that calculation and divide by the specific volume of the casting material (the specific volume of Poly 75-80 is 26 in³/lb). This final result is the amount of rubber (lb) needed to complete the duplicate master.

Calcuate_Mold_Rubber_Volume

 

Step 5: Measure, Mix & Pour Liquid Rubber

Poly 75-80 Liquid Rubber has a mix ratio of 2A:1B, a pour time of 45 minutes, and a demold time of 16 hours.

Poly_75-80_Liquid_Rubber_Polytek

Measure out Part B on a digital scale (we pour Part B into the mixing container first because it is lower in viscosity and is less likely to cling to the sides of the mixing container).

Weigh_PartB_Poly75-80

Measure out Part A on the scale.

Weigh_PartA_Poly75-80

Thoroughly mix the rubber, scraping the sides and bottom of the mixing container several times. Mixing in a “figure 8″ pattern is also a good technique. We use a Poly Paddle to mix the rubber.

Thoroughly Mix Poly 75-80

When thoroughly mixed, pour the rubber into one corner of the mold box and allow it to rise.

Poured Rubber Over Mold

Pour rubber into rubber; avoid pouring directly onto the mold.

Pour Into One Corner

Pour Rubber into Rubber

The rubber should be at least 1/2″ over the top of the mold.

Permanent Master from Existing Mold

To help eliminate surface air bubbles, spray Pol-Ease 2300 Release Agent across the surface of the rubber immediately after pouring.

Pol-Ease 2300 Release Agent Aerosol Can

Allow the rubber to cure for 16 hours before demolding. Final cure properties are reached in about 7 days; however, molds can be used, with care, after curing for 24-48 hours.

16 Hour Demold Time

 

Step 6: Demold

Remove the screws from the baseboard and then remove the mold box walls; a putty knife can be helpful for wall removal.

RemoveMoldBoxWalls

RemoveMasterfromMold

Turn the mold over for easier removal if necessary.

Flip Mold Over for Easier Removal

FlipOverMold

Carefully loosen the edges of the mold from the new master before removing the entire thing.

Loosen Edges of Mold

Demold

Veneer_Stone_Permanent_Master

Mold Making Tutorial

The finished permanent master with built-in containment walls:

Firm Rubber Permanent Veneer Stone Model

Poly 75-80 Liquid Rubber to Make Duplicate Model

Poly 75-80 Liquid Rubber - Polytek

Trim the edges if necessary.

Trim Edges of Master

 

Step 7: Making a New Mold

To make a new mold of this master, first select the appropriate mold rubber. We have selected Poly 74-20 liquid rubber (the same rubber as the original mold) for this tutorial.

Poly 74-20 is soft polyurethane rubber (Shore A20) with a 1A:2B mix ratio; it is very popular for veneer stone applications.

Apply a suitable release agent (i.e., Pol-Ease 2300 Release Agent) to the master and brush it out with a dry brush.

Apply Release Agent to Rubber Model

Measure, mix and then pour the Poly 74-20 rubber onto/into the new Poly 75-80 rubber master.

Pour Poly 74-20 Liquid Mold Rubber

Poly 74-20 Mold Rubber

Poly 74-20 Veneer Stone Mold

Low Viscosity Poly 74-20 Mold Rubber

Poly 74-20 Urethane Mold Rubber - Polytek

Demold after 16 hours.

Demold New Rubber Mold

New Master & Rubber Mold

 

 

Do you have questions about this process? Get in touch with Polytek Technical Support:

www.polytek.com
800.858.5990
sales@polytek.com
Fill out our simple online contact form.

Tek-Tip: How Cold Weather Affects Polytek® Products

December 3rd, 2014

With temperatures dropping across the country, we would like to address how cold temperatures affect the following Polytek products:

  • Latex
  • Pol-Ease® 2601 Release Agent
  • Polyurethane Rubbers, Casting Plastics & Foams
  • Silicone Rubbers

 

 

Latex

Our latex-containing products: Poly Latex 60 & Poly Latex False Face Compound

Polytek Latex-01
 

Unlike the majority of our products, latex will freeze and become permanently unusable. In fact, we do not ship latex if temperatures are too low.

If you currently have latex in storage, make sure that temperatures do not drop below 40°F. Store at temperatures between 50ºF – 70ºF.

If you are making a latex mold in colder temperatures, it will slow the evaporation process.

For further technical information on using latex, view the Poly Latex 60 Technical Bulletin or Poly Latex False Face Compound Technical Bulletin.

 

 

Pol-Ease 2601 Release Agent

Pol-Ease 2601 Release Agent
 

Like latex, this water-based release agent (primarily used when casting plaster or concrete in rubber molds) will freeze in cold temperatures and become unusable.

 

 

 

Polyurethane Mold Rubbers, Casting Plastics & Foams

 

Polyurethane Rubbers and Plastics-01

 [product examples — left: EasyFlo 120 Liquid Polyurethane Plastic | right: Poly 74-24 Liquid Polyurethane Rubber]

 

Transporting liquid polyurethane products in cold temperatures will not affect them long-term, but these liquids should always be brought up to room temperature (60ºF – 90ºF) before use. Surface and air temperatures should be above 60°F during application and for the entire curing period. For best results, these products should also be stored at room temperature.

Mixing and using polyurethane products at low temperatures will increase viscosity and slow the pour and cure times. Polyurethanes are moisture sensitive, and extended cure times increase the likelihood of moisture contamination. Moisture contamination can cause improper cure of products and, in some cases, may prevent a product from curing entirely.

 

 

Silicone Rubbers

PlatSil_7320_SiliconeRubber

[product example — PlatSil® 73-20 Silicone Rubber pictured above]

 

Similar to polyurethanes, transportation in cold temperatures will not have any long-term impact on liquid silicone rubber. For best results, they should be stored at room temperature (60ºF – 90ºF).

Before mixing and using silicone, always bring the liquid back up to room temperature (60ºF – 90ºF). Working with liquid silicone rubbers at low temperatures will increase the viscosity and slow the pour and cure times (this delay is especially noticeable in platinum silicone rubbers).

 

Making a mold of a frozen model? Try an accelerated, tin-cured silicone rubber like TinSil® 70-30 or TinSil® 80-30 with TinSil FastCat Accelerator.

 

Do you have questions about your upcoming mold making or casting project? Get in touch with our technical support team:

www.polytek.com
800.858.5990
sales@polytek.com
Fill out our simple online contact form.

Tutorial: Silicone Blanket Mold with Foam Mold Shell

November 26th, 2014

This tutorial features the making of a poured blanket mold (for general information on blanket molds, visit this “blanket mold: basics” blog entry).

Unlike brush-on blanket molds, where the mold shell is created after the rubber mold is made, the poured blanket mold technique requires that the mold shell be created before the rubber mold is poured.

There are many mold materials and mold shell options available for this mold making method. For this tutorial, PlatSil® 73-40 Silicone Rubber is selected as the mold material and PolyFoam R-5, a rigid casting foam, is selected as the mold shell material.

The model, pictured below, is a skull and bones prop composed of sealed plaster.

Skull and Bones Blanket Mold 004
[the original sculpture is by Scott Bonelli / Big Head Studio]
 

Step One: Apply Uniform Layer of Clay to Model

Prior to creating a layer of clay on the model, we recommend outlining the perimeter of the model with a marker or pencil – the model will need to be placed back in this position later in the process and this will help to re-position it.

Cover the model with plastic wrap; this will make removal of the clay easier in later steps:

Skull and Bones Blanket Mold 006
Apply a uniform layer of clay over the model at about 1/2″ thick. We are using plasticine clay, an oil-based and sulfur-free clay. It is important to use a sulfur-free clay as sulfur can inhibit platinum silicone rubbers.

Skull and Bones Blanket Mold 008
Carefully cover the entire model and also extend the clay beyond the model to create a flange (~1.5″ long).

Skull and Bones Blanket Mold 010
Smooth the clay as best as possible; a tongue depressor works well for this process. The mold rubber will eventually take the place of this clay, so it’s important to make sure that there are no thin spots or holes in the clay, as they will also appear in the rubber mold.

If you intend to cut the finished mold, you should consider creating a raised clay parting line – this will create a thick area in the finished rubber mold that will allow for a better cut.

This is also the point in which you should build clay parting lines if you intend to create a multi-piece mold shell (see this example here). In this case, only a one-piece shell is necessary and a cut will not be made in the mold, so no parting lines are created.

Skull and Bones Blanket Mold 014
 

Step Two: Construct Mold Box

This step is not always necessary. We often utilize thickened liquid plastic (e.g., Poly 1512X Liquid Plastic + PolyFiber II) to make mold shells (example here) – a mold box is not required when using thickened liquid plastic because it generally strong enough on its own and does not need to be “contained”. In this example, we are using a rigid casting foam (a less expensive option compared to plastic) to create the mold shell. Because this foam expands, it needs to be contained by a mold box. The mold box, which will become a permanent fixture in the mold shell, also provides strength during the casting process.

Several pieces of wood are glued and screwed together to form a shape similar to that of the model. It should be designed so that the edges of the clay flange surrounding the model meet the interior edges of the mold box.

>Skull and Bones Blanket Mold 016
 

Skull and Bones Blanket Mold 017
Place the model and mold box onto a baseboard (we use a piece of melamine-laminated particle board) and secure it from underneath with wood screws.

Ensure that the clay meets the interior edges of the mold box.

Skull and Bones Blanket Mold 019
 

Mark reference points for the position of the mold box compared to the baseboard so that it can be re-positioned in the same location later in the process. Marking the entire perimeter of the mold box with a marker or pencil is also a good idea.

Skull and Bones Blanket Mold 024
 

Step Three: Apply Sealers & Release Agents

In this case, we want the foam to stick to the wooden mold box so it becomes a permanent part of the mold shell, so we do not apply any release agent to the wood. Paste Wax is applied to the plasticine clay so that the foam does not stick to it.

Skull and Bones Blanket Mold 027
 

Step Four: Cast the Foam Mold Shell

PolyFoam R-5, a rigid polyurethane casting foam with a free-rise density of 5 lb/ft³, is selected as the mold shell material. To cast the foam shell, cover the top of the mold box walls with another piece of laminated particle board (we apply Paste Wax to bottom of the lid before securing it to the mold box walls). This piece of particle board should have a pour hole in it. Secure the lid to the mold box walls (e.g., wood screws, mold straps) and then prepare to mix the PolyFoam R-5.

PolyFoam R-5 has a 1A:1B mix ratio, a cream time of 30 seconds, a rise time of 3 minutes and a demold time of 30 minutes. It is best to mix foam with a Turbo Mixer or other high-speed mixer.

Mix for approximately 15 seconds and then pour the foam into the pour hole of the lid; PolyFoams being to rise quickly, so it is important to work fast.

Skull and Bones Blanket Mold 029
 

Skull and Bones Blanket Mold 030
 

Wait ~30 minutes before carefully removing the lid.

Skull and Bones Blanket Mold 033
Unscrew the the mold box, flip it over and remove the plastic wrap and clay from the model.

Skull and Bones Blanket Mold 035
 

Skull and Bones Blanket Mold 037
 

Step Five: Make Pour & Vent Holes in Mold Shell

Create a pour hole (large enough to pour the mold rubber into) and vent holes where necessary; we made one vent hole in each skull cavity. A hole saw or drywall saw work well.

Skull and Bones Blanket Mold 038
 

Step Six: Test, Measure, Mix & Pour the Silicone Mold Rubber

PlatSil 73-40 Silicone Mold Rubber is selected to make the mold. It is a platinum-cured silicone rubber with a 1A:10B mix ratio, a Shore A40 hardness (similar to the hardness of a pencil eraser), a 45-minute pour time and a 16-hour demold time.

This plaster model was, at one point, in contact with a material that can sometimes inhibit the cure of platinum silicone rubber, so we do a small test cure on the surface to ensure proper cure (read more about test cures here) before pouring the entire mold.

Skull and Bones Blanket Mold 039
 

Place the model in it’s original position on the baseboard using the perimeter marks you made earlier.

Spray Pol-Ease® 2500 Release Agent onto the model and brush out with a dry brush.

Pol-Ease2500_Release Agent
Place the mold box/foam mold shell over top of the model in the proper position (line up the “A” side of your mold box with the “A” on the baseboard and follow perimeter marks) and screw it into place. It is also a good idea to “seal” the exterior edges of the mold box (where it meets the baseboard) with clay to prevent rubber from potentially leaking.

At this point, there will be an empty cavity in the space that the clay once filled.

 

How to measure how much silicone is needed:

The volume of clay removed from the model equals the volume of liquid rubber needed. Calculate the volume of clay by multiplying its weight by its specific volume:

For instance, if 8 lb of clay was removed from the model: 8 lb x 18.4 in³/lb (the specific volume of the clay) = 147.2 in³ of clay.

Next, divide the clay volume by the specific volume of the liquid rubber to get the weight of rubber needed:

147.2 in³ ÷ 22.0 in³/lb (the specific volume of PlatSil 73-40 Silicone Rubber) = 6.69 lb of PlatSil 73-40 Silicone Rubber

 

Pour the silicone into the pour hole of the foam shell; small amounts of silicone will rise from the vent holes.

Skull and Bones Blanket Mold 040
Allow 16 hours before demolding. Unscrew the mold box walls/shell from the baseboard and lift it off of the model. Your silicone blanket mold with foam shell is complete!

Skull and Bones Blanket Mold 045
The rubber mold picks up every detail from the original model.

Skull and Bones Blanket Mold 046
 

Casting

A variety of materials can be cast in a PlatSil 73-40 silicone mold, many of which don’t require a release agent. This includes polyester, epoxy, wax, concrete, plaster, and polyurethane resins and foams.

In the example before, we brush Poly 1512X Liquid Plastic with PolyFiber II into the mold.

Skull and Bones Blanket Mold 049
 

Here is the plastic casting after being painted:

Do you have questions about your upcoming mold making or casting project?

www.polytek.com
800.858.5990
sales@polytek.com
Fill out our simple online contact form.