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

September 15th, 2014

Kent Ullberg is a highly acclaimed wildlife sculptor whose work spans the globe and can be found in numerous museums, municipalities, corporate offices, and private collections. Ullberg is well-known for his monumental works, which include a 23-foot-tall elephant (installed at the St. Louis Zoo) and an 18-foot mako shark (installed at NOVA Southeastern University), among many others [www.kentullberg.net].

According to the Reporter-Herald, 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].
 

The Molding Process

The bronze casting process was completed by Art Castings in Loveland, CO. Spray Solutions, also located in Loveland, stepped in to create the rubber mold of the sculpture (one of the early steps in the casting process).

Spray Solutions utilized Polygel® Spray 35, a self-thickening, two-part (1A:1B mix ratio) polyurethane rubber, to make the mold. Because the sculpture was to be cast in 100 separate pieces, aluminum and plastic shims were strategically placed in order to construct the mold in 100 separate sections.

Polygel Spray 35 was applied to the clay sculpture with meter-mix spray equipment.

Some of the behind-the-scenes work can be seen below [photos courtesy of Sculpture Depot]:

Mastodon Sculpture - Shims
 

Kent Ullberg Mastodon Sculpture Mold
 

Monumental Mastodon Sculpture
 

Polygel Spray On Rubber - Mastodon Sculpture
 

Polygel Spray Polyurethane Rubber
 

Ullberg’s mastodon will arrive at the Denver Museum of Nature and Science at the end of September and the dedication date is late in October.

Visit Kent Ullberg’s website for biographical information and more examples of his work.

 

www.polytek.com  |  610.559.8620  |  sales@polytek.com  |  online contact form

Adding Fillers to Polyurethane Casting Resin

September 11th, 2014

There are a few common reasons that fillers are added to polyurethane casting resins (e.g., EasyFlo Series Liquid Plastics, Poly 15-Series Liquid Plastics):

  • to reduce cost
  • to help dissipate exotherm (exotherm can cause shrinkage in castings)
  • to achieve a different look or weight

 

NOTE: Other additives, like thickeners (Cabosil & PolyFiber II are dry fillers), softeners, retarders, and color dyes are often added to liquid plastics, but these types of additives will not be covered in this blog entry. Only dry fillers, used for the purposes above, will be discussed.

Dry Fillers-01

 

Things to Consider Before Adding Fillers

Moisture Sensitivity

Liquid polyurethane plastics are moisture sensitive, so it is important to only add dry fillers to these resins. Even some fillers that appear dry (e.g., wood powder, nut shell flours) may contain moisture and cause foaming of the plastic, so ensure that fillers are completely dry before using (baking may be necessary).

Moisture Sensitive Resin

[an example of foaming that can occur when water is introduced to EasyFlo 60 casting resin]

 

Change in Viscosity

Large amounts of filler can affect the flow characteristics of resin, so make sure to experiment before attempting a large project. Adding too much filler can increase viscosity which may result in air entrapment and bubbles in the finished casting.

 

Sinking & Floating

Heavy fillers tend to sink in resin and light fillers tend to float; therefore, it’s very important to mix the combination thoroughly before casting. Also, a fast-setting resin is generally a better option as it will begin to cure before fillers have the chance to settle or float to one side. A popular option is EasyFlo 60 Liquid Plastic, which has a 2 to 2.5-minute working time and a 15-30-minute demold time.

 

Weight & Appearance of Fillers

Fillers are often added to change the look or feel of a casting (see the “Adding Fillers to Change Look or Weight of Castings” section); however, it’s important to consider these physical changes when selecting a filler for other purposes, like cost reduction and to dissipate exotherm. For instance, heavy fillers absorb more exothermic heat, but they result in heavy castings, which may not be appropriate for the application at hand. Also consider that the color of the filler may affect the color of the finished casting.

 

Neutral-Density Option

For a neutral-density filler that will not affect the density of the casting, consider PolyFil ND. This filler can be used to reduce the cost of castings as well as dissipate exotherm.

p-3542-product_detailed_image_30128_312.jpg

 

Adding Fillers to Reduce Cost

Adding an equal volume of low-cost filler to a Polytek liquid plastic can potentially cut the cost of the casting in half.

Popular fillers for reducing cost are dry sand and limestone. As a reminder, these fillers will affect the viscosity of the casting resins.

Typical costs per cubic inch of these fillers are shown below, along with a few other fillers:

Filler $/lb Density (g/cc) $/in³ Typical Use
Dry Sand 0.05 2.7 0.01 Reduce Cost
Ground Limestone 0.15 2.7 0.01 Reduce Cost
Extendospheres CG 0.50 0.7 0.01 Make Lightweight Castings
Q-Cel 6014 or 3M K1 6.00 0.1 0.02 Make Lightweight Castings
Bronze Powder 15.00 8.8 4.79 Make Cold Cast Bronze Parts

 

Adding Fillers to Dissipate Exotherm

Exotherm is the heat of the reaction produced as the resin cures. In large castings, it can sometimes cause thermal shrinkage or distortion.

Large Plastic Casting-01

[shrinkage can occur in large castings, such as this EasyFlo 60 casting of a long, wood panel]

 

In our experience, Alumina Trihydrate performs well as a filler to disperse exothermic reaction and reduce shrinkage. Calcium carbonate and PolyFil ND are other options, but do not perform as well as Alumina Trihydrate.

 

Adding Fillers to Change the Look or Weight of Castings

Fillers are often added to resin to achieve a certain look or feel. For instance, bronze powder and other metal powders are used to create castings with a metal-like finish. In this example, castings are removed from the mold and then burnished with steel wool to expose the metal particles [see a cold cast bronze tutorial here]. Not only does the appearance change, but the weight increases.

The photo below features three EasyFlo 60 plastic castings with varying fillers. EasyFlo 60 naturally cures to a white color [the figurine on the right does not have any fillers]:

[left: Bronze Powder & Brown PolyColor Dye  |  center: Sand  |  right: No Fillers]

Polyurethane-Plastic-with-Fillers
 

Other dry fillers that are often used to create a distinct look include marble dust and glitter, which are more often featured in clear casting resins, like the Poly-Optic® 14-Series resins.

When added at the proper ratio, Extendospheres CG, Q-Cel 6014 or 3M K1 can make castings float or carve more easily.
 
 
Dry fillers can be a useful addition to resin for many reasons, but not all fillers will perform well. If you have questions about adding fillers to Polytek polyurethane casting resins, please get in touch with us:

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!

Foaming Additive for Poly 15-6 Polyurethane Liquid Plastic

September 3rd, 2014

Poly 15 Part F Foamer can be added to Poly 15-6 Liquid Plastic (a two-part polyurethane plastic in the Polytek Poly 15-Series) to create a rigid, open-cell foam with densities as low as 6 lb/ft³.

 

Liquid Plastic with Foam Additive

 [Poly 15-6 Liquid Plastic and 1-ounce Poly 15 Part F Foamer]

 

Without foamer, Poly 15-6 Liquid Plastic cures to a rigid, Shore D72, tan/off-white plastic. It has a 1A:1B mix ratio, 5-minute pour time, and 1 to 3-hour demold time.

 

How to Use Poly 15 Part F Foamer

Stir Part F Foamer into Part B of Poly 15-6 before adding Part A. Experiment to determine the best amount of Part F Foamer for the application at hand, but never use more than 3% (of the total weight of the mix) of this additive or the final physical properties may be affected.

Poly 15 Part F Foamer

 

Uses for Poly 15-6 Liquid Plastic & Part F Foamer

One way to create durable, lightweight castings is to make a thin, hollow casting with a two-part casting resin (e.g., EasyFlo 60 Liquid Plastic) and then backfill the hollow piece with a two-part rigid or flexible foam (e.g., PolyFoam R-5 Casting Foam).

This casting technique is used widely for decor, props, displays, and more.

As noted, this process typically requires two different products: a two-part casting resin and a two-part casting foam. By using Poly 15-6 and Part F Foamer, the benefit is that you only have to stock one two-part casting resin and the one-part foaming additive (available in 1 oz, 1 pt, 1 gal, and 5 gal sizes) to make this type of casting.

 

Product Demonstration

The following demonstration shows the process of making a decorative architectural element with a rigid, plastic exterior and rigid foam interior using Poly 15-6 Liquid Plastic & Part F Foamer.

 

Step One: Make a Hollow Poly 15-6 Casting with Rotational Casting Machine

Spray Pol-Ease® 2300 Release Agent into the mold cavity and brush out with a dry brush. This step is only necessary when working with polyurethane rubber molds; a silicone mold would not require release agent.

Pol-Ease 2300 Release Agent
 

Apply Release Agent to Mold
 
 
Carefully weigh appropriate amounts of Poly 15-6 Part A & Part B on a scale.
 
Weigh Plastic on Scale

 
Combine Part A & Part B and mix thoroughly.
 
Combine Parts A & B

 
Stir Casting Resin

 
Pour the Poly 15-6 plastic mixture into the mold cavity.
 
Pour Plastic into Mold Cavity

 
Only a small amount of resin is necessary for rotocasting or slush casting (read more about slush casting here).
 
Resin for Rotocasting

 
Before attaching the lid to the mold shell, spray and brush out Pol-Ease 2300 Release Agent onto the areas of the lid that will contact the plastic.
 
Lid and Plug for Rotocasting

 
Insert a plug to prevent plastic from leaking out of the mold while on the rotational casting machine.
 
Lid for Rotocasting

 
Securely attach the mold to the rotational casting machine and rotate for about one hour.
 
Rotocast Hollow Part

 
Rotocasting Polyurethane Mold

 
Remove the mold from the rotational casting machine and set aside the plug.
 
Remove Plug

 
Step Two: Backfill the Hollow Poly 15-6 Casting with Foam

Carefully weigh an appropriate amount of Poly 15-6 Part B and then add the desired amount of Poly 15 Part F Foamer (we added 3%). Mix Part B and the foamer additive thoroughly and then add the appropriate amount of Poly 15-6 Part A. Mix all ingredients together; the foam will begin to rise quickly, so make sure that the mold is nearby.
 

Add Foamer to Plastic

 
Pour the mixture into the mold cavity. The demold time of Poly 15-6 without foamer is 1 to 3 hours; however, the demold time of Poly 15-6 with foamer is about 10 minutes.
 
Pour Foam into Mold Cavity

 
Foaming Additive

 
Resin and Foam Part

 
Excess foam that comes out of the pour hole can be easily trimmed off.
 
Remove Mold from Mold Shell

 
Remove Casting from Mold Shell
 
 
The finished casting:
 
Poly 15-6 Architectural Element
 

For more information on casting material options for your next project:

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!

Customer Project: Decorated Venus Figurine – EasyFlo Liquid Plastic

August 28th, 2014

At the March 2014 Mold Making & Casting Workshop, attendee Ro-Z Esposito (more info. on this artist at the end of the blog) brought the plaster Venus figurine pictured below. She wanted to make a rubber mold of the figure and then cast plastic copies.

Plaster Venus Model
 

Ro-Z would later decorate the plastic casting by gluing intricate beading to the surface (pictures of the finished piece are at the end of this blog entry).

After a discussion with Polytek staff members at the workshop, a strategy was decided for the making of the mold:

  • Mold Making Method: Two-Part Poured Block Mold
  • Mold Product: PlatSil® 73-40 Silicone Rubber
  • Casting Material: EasyFlo Series Liquid Plastics

 

The following steps were taken to complete the mold and make the plastic castings:

 

Step 1: Test the Model Material for Cure Inhibition

Platinum silicone mold rubbers, like PlatSil 73-40, can suffer from cure inhibition when in contact with certain materials (e.g., sulfur, tin compounds), so a small amount of a fast-setting platinum silicone rubber was applied to the model surface to ensure that it would cure properly against the plaster. You can read more about cure inhibition and “test cures” here.

Testing Platinum Silicone for Inhibition-01
 

Step 2: Embed Half of the Model in Clay

As mentioned previously, the two-part poured block mold method was selected for this model. To begin, a parting line (i.e., the line where sections of the mold divide) was determined and then the process of embedding half of the model in plasticine clay was started. The selected parting line essentially split the front and the back of the model in half.

Begin to Build Clay Base
 

Meanwhile, the platinum silicone test sample was removed and we found that it cured properly.

Cured Silicone
 

The “pour hole” (i.e., where the casting material will later be poured) was built in at the “feet” of the model.

Embed Model in Clay-01
 

The surface of the exposed clay was smoothed and a mold box, tall enough to accommodate an appropriate mold wall thickness, was constructed. Melamine-laminated particle board and C-Clamps were used to make and secure the mold box.

Two-Part Block Mold - First Half Embedded
 

Marbles were partially embedded in the clay to create “keys” that would align the two halves of the mold when complete. The interior corners of the mold box were then sealed with clay to help prevent rubber from leaking out of the mold box.

Marbles Keys in Clay
 

Step 3: Apply Release Agent

A few sealer/release agent options could have been used in this case (e.g., Vaseline, Pol-Ease® 2500 Release Agent), but Pol-Ease 2350® Sealer & Release Agent was ultimately selected. Pol-Ease 2350 is white petrolatum dissolved in mineral spirits and can be used when working with liquid silicone or polyurethane mold rubber. It was sprayed on to the model, clay, and mold box walls with a Sure Shot Atomizer and then brushed out with a dry brush.

Apply Release Agent to Plaster
 

When working with Pol-Ease 2350, it is important to make sure that it has sufficiently evaporated before applying any rubber (evaporation takes ~1 hour and varies depending on temperature, porosity of the model, and thickness of the layer) – if the Pol-Ease 2350 has not evaporated sufficiently, it can interfere with the cure of the rubber.

 

Step 4: Measure, Mix & Pour the Mold Rubber

The mold rubber selected for this project was PlatSil® 73-40 Silicone Rubber. It is a 1A:10B mix (by weight), platinum-cured silicone rubber with a 45-minute pour time and a 16-hour demold time.

One of the reasons that a platinum silicone rubber was recommended as the mold material for this project is because release agent is not required when casting polyurethane plastic. Glue (used later in the process to apply items directly to the surface of the casting) does not adhere well to plastic coated in release agent and many release agents are difficult to wash off of plastic, so we eliminated the need for release agent entirely.

Parts A & B of PlatSil 73-40 were individually weighed using a digital scale and then combined and mixed thoroughly.

Mix PlatSil 73-40
 

The silicone rubber was poured into one corner of the mold box and allowed to flow across the model until it reached the top of the mold box walls.

Pouring PlatSil 73-40 Silicone Rubber
 

Pouring Platinum Silicone Rubber
 

Allow Rubber to Cure
 

The rubber was allowed to cure, undisturbed, for 16 hours. Heat (e.g., heat lamps) or PlatSil® 71/73 Part X Accelerator can be used to speed the cure.

 

Step 5: Make the Second Half of the Mold

To begin the process of making the second half of the mold, the mold box was temporarily set aside and all of the clay and marbles were removed.

Remove from Mold Box and Flip Over
 

Then the first half of the mold was turned over and the model was kept in the same position.

Remove Marbles
 

The mold box was repositioned around the first half of the mold and model and the interior corners of the mold box were sealed with clay. Silicone will bond to silicone if there is no release agent applied, so Pol-Ease® 2500 Release Agent was sprayed on to the model and exposed silicone and then brushed out with a dry brush to ensure even coverage.

Put into Mold Box
 

Pol-Ease 2500 Release Agent
 

Again, the rubber was carefully measured, mixed, and poured to the create the second half of the mold.

Pour Rubber to Make Second Half of Mold
 

Pour Second Half of Mold

 

Step 6: Demold

Upon cure of the second half of the mold (~16 hours), the mold box was removed and two halves were parted.

Finished Platinum Silicone Mold-01

 

Step 7: Casting EasyFlo Series Liquid Plastic

EasyFlo Series Liquid Plastics are fast-setting, two-part polyurethane plastics. All options within the series have simple 1A:1B mix ratios by volume.

Release agent is not necessary when casting EasyFlo Series liquid plastics in platinum silicone molds, so once the two halves of the mold are properly aligned (we used large rubber bands to further stabilize the mold), casting material can easily be poured into the pour hole, which is indicated by arrows in the pictures above and below.

The first casting that Ro-Z made was a solid pour of EasyFlo 60 Liquid Plastic, but she opted to slush cast the second time around to produce a lightweight, hollow part.

Casting in Two-Part Mold-01
 

For slush casting, a small amount of EasyFlo 120 was poured into the pour hole and the mold was manually rotated in all directions for about 5 minutes [you can learn more about slush casting in this blog entry].

EasyFlo 120 Casting Resin
 

Slush Cast EasyFlo Resin-01
 

Slush Cast EasyFlo 120 Part
 

EasyFlo 120 Liquid Plastic
 

In the picture below, the original model is displayed on the left, while the slush-cast, hollow EasyFlo 120 part is on the right.

EasyFlo Casting Resin
 

Ro-Z took this casting home and completed her design with intricate beading:

You can view Ro-Z Esposito’s work at www.ro-zesposito.com and can contact her at mypaintbrush@gmail.com.

Ro-Z has provided the following photos of her finished piece: 

Venus1
 

Venus2
 

Venus3
 

Do you have a mold making or casting project you would like to discuss with a Polytek Technical Support staff member?

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!

Blending Liquid Rubber & Plastic: Poly-Optic® Series

August 20th, 2014

Poly-Optic 14-Series Casting Resins are two-part, room temperature curing, polyurethane systems that result in water-clear castings. They can be colored with PolyColor Dyes to achieve transparent, colored castings.

Polytek Poly-Optic Clear Casting Resin
 

There are six individual product options within this series, most of which result in a rigid, Shore D80-D85 plastic. One option; however, is a firm rubber that cures to a Shore A70 hardness. As points of reference, Shore D80-D85 is comparable to the hardness of a hard hat, while Shore A70 is similar to that of a car tire [more examples of Shore hardness are below].

 

Shore Hardness Scale_Polytek
 

Blending Poly-Optic Products to Achieve Varying Hardness

If you require a clear polyurethane plastic that has a hardness between Shore A70 and Shore D80, Poly-Optic 1410 (a hard, Shore A80 plastic) and Poly-Optic 14-70 (a firm, A70 rubber) can be blended at different ratios to achieve varying hardness.

 

PlasticRubberMix

 [Samples of blended Poly-Optic cast in a slate tile mold:

25% 1410/75% 14-70, 75% 1410/25% 14-70, 50% of each]

 

In the table below, you will find what ratios (by weight) these two Poly-Optic products should be mixed at to achieve varying Shore hardness.

Ratios for Blending Poly-Optic Plastic & Rubber
NOTE: Poly-Optic 1410 is a non-yellowing formula (note: it is not recommended for long-term exterior use); however, Poly-Optic 14-70 is not a non-yellowing formula, so the addition of any amount of Poly-Optic 14-70 added to Poly-Optic 1410 will cause the entire casting to yellow more quickly when exposed to UV light than a casting without any Poly-Optic 14-70.

 

Do you have questions about blending rubber and plastics?

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!

Plasticine Clay: Multiple Uses in Mold Making

August 13th, 2014

Poly Plasticine is a non-hardening, sulfur-free modeling clay. It is non-toxic, odorless, and has a smooth, firm consistency.

Plasticene Clay - Polytek Development
 

It’s great for sculpting, which might be what most people think of when they hear modeling clay; however, we use this clay for varied purposes in our mold shop. You may be able to incorporate some of these uses into your everyday mold making routine.

We mentioned earlier that the clay that we use is sulfur-free. This is important because sulfur can cause cure inhibition in platinum-cured silicone rubber (you can read more about cure inhibition here). It is best to use an oil-based, sulfur-free clay, especially when working with platinum silicones.
 

Clay as Caulking

For the purposes of sealing, clay can often be easier to work with than hot glue or silicone caulking (and clay is recyclable!), especially when it’s warmed for easier application. We always keep of batch of clay in our oven, set at 140°F, so it’s ready to go at any time.

The photo below shows the edges of this stepping stone sealed with clay. This is a common practice that helps prevent rubber from leaking beneath the stone once the mold box is constructed around the stone and rubber is poured over it to make the mold (shown in the second picture below).

Seal Mold Box Edges with Clay

 [see this full project tutorial here]

DSC05270

 

In a similar scenario, pictured below, clay is used to seal the interior edges of the mold box to help prevent rubber from leaking outside of the mold box [see this project tutorial here].

Seal Mold Box with Clay
 

Below, clay is used to seal the exterior edges of a mold box that’s being used to make a two-part baluster mold [see the full tutorial here].

Seal Mold Box for Baluster
 

Plasticine clay can also be used to the seal the edges of mold shells. The mold shell below is constructed from Poly 1512X polyurethane plastic; the clay seals the edges as well as the areas around the hardware that is securing the mold shell to the base board.

Clay - Sealing Edges of Mold Shell-01
 

The plastic tubing pictured below is being used to create a through-hole in the mold. Clay is applied around the tube to help prevent leaks while the mold is being made.

Using Clay as Caulk
 

In the picture below, clay is being used to partition off an area of the mold that needs to be repaired with PolyBond adhesive [see the full tutorial here].

Position Clay in Corner-01

Apply PolyBond to Corner - Plug with Clay

 

Clay as Mold Boxes & Containment Areas

Not only is plasticine clay great at sealing mold boxes, it can can actually serve as a “mold box”, or containment area, itself. An example is pictured below.

Building a Clay Mold Box-01
 

In the picture below, we use clay to create a small containment area on the model to test for cure inhibition against the model material.

Clay for Test Cure

 

Two-Part Block Molds

Clay is an integral part in the making of two-part block molds, which are constructed by pouring one piece of the mold, allowing it to cure, and then pouring the second piece.

Before pouring rubber to make the first half of the mold, one portion of the model is embedded in clay, up to the desired parting line (example below).

Two-Piece Block Mold - Using Clay
 

Polytek Silicone Rubber

Once the rubber has cured, the clay is removed and the second half of the mold is made - you can see the full tutorial for this project here.
 

Poured Blanket Molds

Clay also plays a very important role when making poured blanket molds. In this process, the model is covered in a layer of clay (plastic wrap is often placed over the model before doing this) and then a mold shell is constructed around it (pictures below).

Following construction and cure of the mold shell, the shell and clay are removed and then the shell is re-positioned back over the model. Mold rubber is then poured into the mold shell, covering the model and filling the space that the clay once occupied. Read more about the basics of blanket molds here or view this poured blanket mold tutorial.

Clay for Blanket Mold

Plasticine Clay for Blanket Mold
 

DSC04030 

 

Keying

Keys are tongue-and-groove or button-like impressions that align molds or mold shell sections. The brush-on silicone mold pictured below is ready for mold shell construction. Before brushing on the first half of the mold shell, a cardboard shim is constructed to establish a parting line for the two halves of the mold shell and then two clay keys are positioned on the shim. The first half of the mold shell is brushed over the shim, clay keys, and mold. When the second half of the mold shell is constructed, the keys will provide an area of impression where the two halves will align nicely.

 

Clay Keys for Mold Shell-01

 [view this full tutorial on YouTube - Part I & Part II]

Clay Keys on Mold Shell-01 

 

Shims to Make Parting Lines for Mold Shells

While cardboard was used to make a shim in the example above, clay is used in the example below. The finished mold shell is also pictured.

Plasticine Clay Parting Lines-01
 

Two-Part Polytek Plastic Mold Shell

 

Modifying Models

Clay can be used to modify models before the mold making process. For instance, the person making a mold of this medallion wanted the final concrete castings to be thicker, so, prior to making the mold, he increased the height of the piece by adding a uniform layer of clay beneath the medallion.

Increase Height of Model with Clay-01
 

In this example, the holes in the model below were filled with clay prior to mold making because the holes were not wanted in the final castings.

Plugging Holes with Clay

What else do you use clay for around your shop?

Would you like to discuss your next mold making or casting project with a Polytek Technical Support staff member?
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!

Rubber Blanket Molds: The Basics

August 7th, 2014

The rubber blanket mold technique is a great method to master as a mold maker. For those unfamiliar with it, it can be a bit intimidating, so let’s go over the basics:

 

 

What is a “Blanket” Mold?

As indicated by the name, a blanket mold resembles a thin blanket of rubber placed around the model. Rubber blanket molds are generally 1/4″ to 1/2″ thick and are made either by 1) brushing or spraying flexible mold rubber onto a model and then creating a mold shell around it (the brush-on method generally requires application of multiple layers of rubber) or by 2) pouring the rubber into a previously constructed mold shell that is situated around the model.

In all cases, construction of a rigid mold shell, also known as a “mother mold”, is required in order to keep these thin molds in the proper shape and position for casting.

Scroll to the bottom to see a video tutorial of each method. Visit this blog entry for a tutorial on method 2.

Polytek Blanket Mold-01
 

Methods of Construction

Apart from varying application methods (i.e., brush, spray, pour), blanket molds can be constructed in different ways:

 

  • Constructed in one piece and then removed in one-piece. This blanket mold is often referred to as a “glove” or “sock” mold as they can be turned back on themselves and peeled off the model like a glove or sock. This method is typically done on low relief or pyramid-shaped models and sometimes requires lubrication with silicone release or soapy water for easier demolding.

Rubber Glove Mold
 

  • Constructed in one piece and then later cut up one side for removal of the model and then removal of castings. Plan ahead for the cut location by building a thicker rubber flange along the cut line. This thickened area can be cut with a mold key knife or scalpel when the rubber has cured; the cut line should be irregular (as pictured below) to provide keys or locating features to adjoin the rubber mold halves properly when casting. Molds can be cut in more than one place if necessary.

 Poured Blanket Mold with Mold Shell
 

  • Constructed in two or more pieces. The rubber mold is created in sections and each section is allowed to cure before moving onto the next one.

 

IMPORTANT NOTE ABOUT MOLD SHELLS: Although some rubber blanket molds may be made in only one piece, mold shells are almost always made in multiple pieces (and connected to each with hardware of some kind). Because mold shells are most often made of rigid material, they can easily lock onto even the smallest undercuts on the model and mold (e.g. if you covered a statue of a man with a one-piece mold shell, there would be no way to remove it). Carefully assess the undercuts on your model before beginning your mold making project in order to determine how many mold shell pieces you will need to construct. As an example, the bust below required a two-piece mold shell; the parting line for the two pieces essentially split the front and back of the bust in half.

Brush-On Mold and Mold Shell - Polytek

[left: clay sculpture, center: sculpture covered in a brush-on mold rubber, right: two-piece plastic mold shell]

 

Complexity of Blanket Molds

In terms of mold complexity, blanket molds are generally more complicated than block molds. Block molds are made by placing the model in a containment area (e.g., mold box) and then pouring mold rubber over the model to fill the contained area (click here for a one-piece block mold tutorial). Generally, blanket molds require less mold rubber, but often require more time and expertise.

Mold Complexity-01

 

Why Make a Blanket Mold Instead of a Block Mold?

Location, size, and shape of the model often determine the mold making method.

 

Location

For instance, a block mold would not be feasible for this vertical column located 10 feet above the ground, so a brush-on blanket mold method is used:

DSC03486

 [^view this tutorial on the blog or on YouTube]

Also consider, for example, a large gargoyle perched on the roof of a tall building. It would not be practical to build a large mold box around the gargoyle and pour the rubber; a brush-on or sprayed blanket mold would be the most suitable option.

 

Size & Shape

Blanket molds are also preferred for large or irregularly shaped models. For instance, the poured block mold technique would not be a suitable method for a life-size sculpture of a man, especially if his limbs are extended in varying directions; a brush-on or spray-on blanket mold would be the most appropriate choice.

 

Cost

Some choose the blanket mold technique because it requires less rubber. This does cut down on the cost of rubber; however, the materials needed for the mold shell also need to be factored in.

 

What are the Best Mold Rubbers to Use for Blanket Molds?

In the past, latex rubber was the go-to option for blanket molds; however, more and more mold makers are moving to polyurethane and silicone mold rubbers because these molds do not take nearly as long to complete. Because each layer of latex rubber must dry before the next layer can be applied, creating adequate mold thickness can take weeks. Polyurethane and silicone blanket molds can be completed in one day.

 

Product Options for Poured Blanket Molds

The majority of Polytek mold rubbers are considered “pourable”, which means that their viscosity is low enough that they can easily be poured over a model. There are numerous rubbers that can be used for this method, including polyurethane and silicone options. The casting material that you will pour into the finished mold and the Shore hardness of the rubber will play major roles in the final selection (click here for more information on mold rubber hardness or click here for some help in selecting a mold material).

Poured Blanket Mold PlatSil 73-20

[PlatSil® 73-20 silicone rubber being poured into a mold shell and over a model to make a blanket mold]

 

Product Options for Brush-On Blanket Molds

The pourable rubbers mentioned above will run off of vertical surfaces, so in their natural state they are not suitable for brush-on application. By adding a thickener, like PolyFiber II (for polyurethane rubbers), Polytek pourable rubbers can be thickened to a consistency that can be applied by brush. Cabosil or liquid thickeners should be used for silicone mold rubbers. Accelerator can be added to rubbers with long pour and cure times in order to reduce the amount of time it will take to complete the mold. Again, final selection of a rubber is heavily based on the casting material you will be using as well as the mold rubber hardness best suited for the application.

Poly 74-20 Brush-On Mold

[Poly 74-20 Liquid Rubber with PolyFiber II being brushed onto a clay sculpture of a bear head]

Polytek also offers self-thickening rubbers, including Polygel® 35 (a polyurethane rubber) and TinSil® Brush/Spray 25 (a tin-cured silicone rubber), that immediately thicken to a brushable consistency when Part A & Part B are properly mixed together; there is no need to add a thickening agent.

Polygel 35 Brush-On Rubber

[Polygel® 35 Brush-On Rubber being applied to a clay sculpture]

 

Product Options for Spray-On Blanket Molds

Polygel® Spray 35, Polygel® Spray 50, and TinSil® Brush/Spray 25 are designed to be used with meter-mix spray equipment, like Plas-Pak Spray Guns, for making spray-on blanket molds.

Polygel 50 Spray Mold

[A Polygel® Spray 50 mold being removed from the model]

 

Polytek Plastics for Mold Shells

Mold shells can be made from a variety of materials, including plaster, plaster and hemp, plaster and burlap, polyester resin and fiberglass, polyurethane foam, firm polyurethane rubber, and polyurethane plastic.

One of Polytek’s most popular options for the creation of mold shells is Poly 1512X liquid polyurethane plastic. When combined with PolyFiber II thickener, it can be easily applied by brush. It has a fast working time of ~5 minutes and can be demolded after approximately 30 minutes.

 

1512X Plastic Mold Shell
 
OLYMPUS DIGITAL CAMERA

 

Video Tutorials

The video below demonstrates the making of a one-piece brush-on blanket mold with a plastic mold shell. Polygel® 35 polyurethane rubber is used for the mold and Poly 1512x Liquid Plastic with PolyFiber II is used for the mold shell. There are no cuts necessary in the mold because the model is pyramid shaped and the rubber mold can simply be peeled off of the model:

 

The video below demonstrates the making of a one-piece brush-on blanket mold with one cut and a plastic mold shell (this is Part I of a two-part tutorial). TinSil® 80-30 is used as the silicone mold rubber and Poly 1512X is used to make the mold shell:

 

The following video tutorial demonstrates the poured blanket mold method. Poly 1512X liquid plastic is used to construct the mold shell and TinSil® 70-25 silicone rubber is used to make the rubber mold:

 

Do you need assistance selecting the most appropriate mold rubber and mold making method?

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!

How to Make a Rubber Mold to Reproduce Concrete Stepping Stones

July 31st, 2014

DSC05123The following tutorial details how to reproduce concrete stepping stones (or pavers) from an existing stepping stone. You might use this method if, for instance, you would like to reproduce a unique stepping stone that is no longer available for purchase or if you’d like to produce concrete stepping stones from a model you created (e.g., clay, wood, 3D-printed or other machined models).

This tutorial only shows the making of one mold, which would be used for relatively low-volume reproduction. For high production applications, consider making a “gang” mold, discussed at the end of the tutorial.

Mixing Rubber
Supplies & Suggested Tools for this Project:

  • A model (in this case, a stepping stone)
  • Poly 75-70 Liquid Polyurethane Rubber
  • Pol-Ease® 2300 Release Agent
  • Pol-Ease® 2650 Release Agent
  • Poly PVA Solution
  • Plasticine Clay
  • Dry Brushes
  • Mold Box
  • Drill
  • Screws
  • Mixing Container
  • Poly Paddle or other mixing tool
  • Digital Scale
  • Putty Knife
  • Crowbar or similar prying tool, if necessary
  • Concrete Mix

 

A video tutorial of this project is available at the end of this blog entry. 

 

The Project:

The walkway pictured below has a few crumbling stepping stones that need to be replaced:

Concrete Stepping Stone Walkway_Polytek
 

Crumbling Concrete Pavers

Step 1: Select a Model to Make the Mold of

Select the most intact stepping stone from the walkway. Wash off or dislodge debris from the stone as best as possible; rubber molds will pick up every detail of the stone (e.g., clumps of dirt).

Paver Stone Model - Mold Making

Step 2: Apply a Sealing Agent

If you plan to put the stepping stone back into its original position in the walkway, you might consider using a removable sealing agent, like Poly PVA Solution (below). It can be washed off with water after the mold making process. If you have a very porous model, like the one in this tutorial, you may have to apply more than one coat of Poly PVA Solution. We applied two coats of this sealer, waiting for it to dry (~ 1 hour) in between coats and before moving on to the next steps.

You don’t need a removable sealer? Consider paste wax, lacquer, paint, Pol-Ease® 2350 Sealer & Release Agent (you don’t need additional release agent if you use this), or PolyCoat Sealer & Release Agent (you don’t need additional release agent if you use this) as additional options.

PVA Clear Sealer for Concrete

Step 3: Construct Mold Box & Seal Edges

Find a suitable bottom board for your mold box and create vent holes; venting is required with porous models to allow trapped air to escape and prevent air from migrating into the rubber causing bubbles.

We often use melamine-laminated particle board (pictured below) for both our bottom boards and mold box walls (the mold box used in this tutorial is available for purchase from Polytek).

Bottom Board for Model
 

Drill Holes in Bottom Board
 

Concrete Stepping Stone on Bottom Board
 

Seal the edges of the stepping stone to help prevent rubber from leaking beneath the stone. We used warmed plasticine clay in this instance; hot glue or caulk are other options.

 

Seal Edges with Clay
 

Construct the mold box walls. We left 0.75″ of space between the stepping stone and mold box walls to create 0.75″ thick mold walls.

We secured our mold box walls together with C-Clamps and also secured the mold box walls to the bottom board with screws.

 

Construct Mold Box
 

Secure Mold Box Walls to Board
 

Seal the outside edges of the mold box (where the walls meet the bottom board) to help prevent rubber from leaking outside of the box. Again, we used warmed plasticine clay.

 

Seal Outside Edges with Clay
 

Also seal the interior corners.

 

Seal Interior Corners

Step 4: Apply & Brush Out Release Agent

Apply Pol-Ease® 2300 Release Agent to the stepping stone, mold box walls and mold box bottom board and then brush it out with a dry brush to encourage even coverage.

Pol-Ease 2300 Release Agent
 

Apply Pol-Ease 2300 to Paver
 

Brush Out Release Agent with Dry Brush

Step 5: Determine How Much Rubber is Needed

Here are our calculations for this specific project:

1. Calculate Volume of Mold Box = 19.5″ x 19.5″ x 2.5″ = ~950 in³ [NOTE: We calculate the mold box volume based on a height of 0.5" above the stepping stone so the thickness of the bottom of the finished mold will be 0.5".]

2. Calculate Volume of Stepping Stone = 18″ x 18″ x 2″ = 648 in³

3. Subtract Volume of Stepping Stone from Volume of Mold Box = 950 in³ – 648 in³ = 302 in³

4. Take Result & Divide by Specific Volume of Poly 75-70 Liquid Rubber = 302 in³ ÷ 27 in³/lb = ~11.2 lb of Poly 75-70 Liquid Rubber

 

Step 6: Measure, Mix & Pour Mold Rubber

For this particular project, we used Poly 75-70 liquid rubber. Poly 75-70 is two-part polyurethane system that cures to a gray, Shore A70 rubber. Molds made from this rubber exhibit high tensile strength, tear strength, and elongation.

We recommend the use of polyurethane mold rubbers for most concrete casting applications (here’s why we usually recommend polyurethanes over silicone mold rubbers).

Poly 75-70 liquid rubber has a 1A:1B mix ratio, 40-minute pour time, and a 16-hour demold time.

Poly 75-70 Liquid Mold Rubber
Pour Rubber Over Stepping Stone
 

Poly 75-70 is a relatively firm rubber with a Shore Hardness of A70  (A70 is a bit harder than a standard car tire. More information on Shore Hardness here); we are able to use a rubber this hard because there are no undercuts on the sides of this stepping stone.

Mixing Rubber
 

No Undercuts on Stepping Stone-01
 

Intricate stepping stones or stones with undercuts on the sides would require a softer rubber for demolding purposes. For instance, a rubber like Poly 74-45 (Shore A45) might be used for the stone below so the sides of the mold can be flexed to allow for removal of concrete castings.

 

Undercuts on Stone - Mold Making
 

Carefully weigh or measure Part A & Part B into a clean mixing container (we often use polyethylene pails). Thoroughly mix the rubber, scraping the bottom and sides of the mixing container several times.

 

Mixing Rubber
 

Carefully pour the rubber into one corner of the mold box and allow it to flow across the model. Pour rubber into rubber; avoid pouring rubber directly onto the model. Do not scrape the sides of the bucket as you pour as unmixed material tends to cling the sides of the mixing container.

Pour Rubber Over Stepping Stone
 

Pouring Poly 75-70 Liquid Mold Rubber
 

Immediately after pouring, spray Pol-Ease 2300 Release Agent across the surface of the liquid rubber to help remove surface air bubbles.

Allow Poly 75-70 to cure, undisturbed, for ~16 hours before demolding. NOTE: Although you can demold this rubber after 16 hours, we do not recommend use of the mold for at least 48 hours.

 

Step 7: Demold

Remove the mold box walls.

Remove Mold Box
 

Cured Poly 75-70 Mold
 

Carefully remove the stepping stone from the mold. This may require the use of a prying tool.

Remove Paver From Mold
 

Rubber Mold of Concrete Paver
 

Trim the edges of the mold if necessary.

Polytek Rubber Mold of Concrete Stepping Stone

Step 8: Cast Concrete

Apply a suitable concrete release agent, like Pol-Ease® 2650 Release Agent, to the mold.

Pol-Ease 2650 Concrete Release
 

Apply Concrete Release to Mold
 

Choose a concrete mixture most suitable for reproduction of your stepping stone and properly mix it. We had some SureCrete Xtreme Concrete Mix laying around our shop, so we used it for the purposes of this demonstration.

Mix Concrete
 

Pour the concrete mixture into the mold and vibrate if necessary.

Pour Concrete into Rubber Mold
 

Remove the concrete casting from the mold and finish and seal the stepping stone as necessary. Use the mold to reproduce as many stepping stones as needed – polyurethane molds can last for many years and can cast hundreds to thousands of parts when well taken care of.

Concrete Paver Casting from Rubber Mold
 

Wash the Poly PVA Solution off the original stepping stone model with water and then place the original stepping stone and reproduced stepping stones into the walkway.

For high volume casting operations, you might consider making a “gang mold”, or multi-cavity mold (pictured below), instead of a single-cavity mold.

Multi-Cavity Mold
 

 

Video Tutorial

 

Do you have a mold making or concrete casting project you would like to discuss with a Polytek Technical Support staff member?

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

 

Poured Blanket Mold of Architectural Element for Concrete Casting

July 23rd, 2014

Two times a year, we hold a Mold Making & Casting Seminar & Workshop. Attendees are invited to bring their own models (there is a size limit because the workshop is only two days long).

At our March 2014 Workshop, this decorative architectural element was brought in:

Architectural Decor Model for Mold Making
 

Architectural Element - Concrete Casting
 

This particular attendee wanted to make a mold of this plaster element to later cast concrete.

 

After a discussion with Polytek staff members, a strategy was decided for the making of the mold:

  • Mold Making Method: Poured Blanket Mold – this technique is backwards from most mold making methods; the mold shell is made first and then the rubber mold is made.
  • Products: Poly 74-20 Polyurethane Liquid Rubber (for the mold) & Poly 1512X Liquid Plastic (for the mold shell).

 

The following steps were taken to complete the mold:

 

Step One: Cover the Model with a Uniform Layer of Clay

Apply a uniform layer of clay (e.g., plasticine clay) to the entire model (often, plastic wrap is placed over the model prior to clay placement). This clay represents the space that the mold rubber will eventually fill – it should be approximately 1/2″ thick and should fill any undercuts on the model so the mold shell does not lock onto the rubber mold.

Applying Clay for Blanket Mold
 

Applying Clay - Blanket Mold
 

Mold Making - Blanket Mold
 

After a uniform layer has been applied, add a clay extension that will later be used as the pour hole for the liquid rubber. Also build-up a thick parting line in the clay that will later serve as the area where the finished rubber mold is cut. When possible, situate this line in an inconspicuous area of the model – along the corner, for instance, so parting lines are less obvious on castings.

 

Pour Hole and Shims on Blanket Mold Making-01

 

Step Two: Construct Shims to Separate Mold Shell Sections

This particular model required two mold shell pieces. To define the parting lines, shims were constructed from flashing and duct tape:

Constructing Shims - Blanket Mold

 

Step Three: Apply Release Agent to Clay & Shims

Apply Pol-Ease® 2300 Release Agent to the clay and shims and then brush out with a dry brush to ensure even coverage. Release agent is applied to prepare for the construction of a plastic mold shell.

Pol-Ease 2300 Release Agent - Mold Making
 

Brush Out Release Agent

 

Step Four: Construct Plastic Mold Shell

Poly 1512X, a fast-setting polyurethane plastic, was used in conjunction with PolyFiber II, a fibrous thickening agent, to make the mold shell in this instance:

Plastic Mold Shell Construction
 

Poly 1512X Plastic Mold Shell
 

When the first side is complete and cured, remove the shims, apply release agent to the clay and mold shell (only the parts of the mold shell that will touch the second half of the mold shell) on the opposing side, and construct the second half of the mold shell:

 

Plastic Mother Mold - Polytek

 

Step Five: Remove Clay from Model, Place Mold Shell Back Over Model & Add Hardware

Once the second half of the mold shell has cured, mark the location of the mold shell on the base board (so you can return it to that exact position when the liquid rubber is poured). Remove the mold shell from the model and then remove the clay from the model.

Blanket Mold - Mold Making
 

Once all of the clay has been removed, apply a proper release agent to the model (e.g., Pol-Ease 2300 Release Agent) and reposition the mold shell over the model making sure the positioning is the same as when it was initially created. NOTE: this plaster model was sealed by the attendee before arriving at the workshop – if it had not been sealed beforehand, we would have applied a sealing agent (e.g., mold soap, paste wax) prior to applying release agent.

Add hardware (e.g., nuts and bolts) to secure the two mold shell pieces together and add hardware securing the mold shell to the base board. Caulk possible leak points with plasticine clay:

 

Insert Hardware into Mold Shell-01

 

Step Six: Measure, Mix & Pour Mold Rubber into the Mold Shell

Poly 74-20 Liquid Polyurethane Rubber, the mold rubber used in this instance, has a 1A:2B mix ratio, a 30-minute pour time, and a ~16-hour demold time.

One way to estimate the amount of rubber needed is to weigh the clay that was removed from the model (the rubber will be filling the same space as the clay). Once weighed, calculate the volume of the clay by multiplying its weight by its specific volume (18.4 in³/lb), then divide the clay volume by the specific volume of the liquid rubber (27.5 in³/lb).

Example:
If 5 lb of clay was removed from the model:
5 lb x 18.4 in³= 92 in³ of clay
92 in³ ÷ 27.5 in³/lb = ~3.34 lb of Poly 74-20 liquid rubber

Carefully weigh and mix the rubber and then pour it into the pour hole located at the top of the mold shell.

To decrease cure time, we applied heat:

Heat Lamps on Rubber Mold

 

Step Seven: Demold

Poly 74-20 can be demolded after ~16 hours (less when heat is applied). Remove hardware and separate mold shell halves.

Poly 74-20 Blanket Mold with Mold Shell
 
 

Rubber Blanket Mold with Mold Shell
 

While the mold is still on the original model, make a cut through the thickened part of the mold created earlier with clay. We used both a mold key knife and scalpel for this process.

 

Polyurethane Rubber Mold - Polytek
 

The mold is now ready for casting!

 

Original Model with Rubber Mold and Mold Shell
 

Not sure if the poured blanket mold technique is best for your mold making 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!

Customer Project: Cast Stone Magnolia Buds for Tattnall Square Park

July 16th, 2014

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. Local artist Amy McCollough Hellis sculpted the magnolia buds from clay and West Side Stone Works took over from there. The original sculptures are approximately 3 feet tall and 20 inches in diameter.

Here are pictures of the original magnolia buds at varying angles:
 

[all photos are courtesy of West Side Stone Works]

Magnolia Bud_Clay Sculpture_3
 

Magnolia Bud_Clay Sculpture_4
 

Magnolia Bud_Clay Sculpture_5
 

Magnolia Bud_Clay Sculpture_6
 

Magnolia Bud_Clay Sculpture_1
 

Magnolia Bud_Clay Sculpture_2
 

Because of the significant undercuts on these models, West Side Stone Works utilized our most flexible polyurethane mold rubber, Poly 74-20.

 

Basic Specifications: Poly 74-20 is a Shore A20 polyurethane rubber with a 1A:2B mix ratio, 30-minute pour time and 16-hour demold time (see other popular uses for this rubber here).

 

Poly 74-20 is a pourable rubber with low viscosity. To make it suitable for brush-on application, they thickened the liquid rubber with PolyFiber II, a shredded fiber thickening agent. They also accelerated the cure and demold time by adding Poly 74/75 Part X Accelerator.

 

Prior to brushing on the liquid rubber, Pol-Ease® 2300 Release Agent was applied to the models so the cured rubber would release well from the clay when demolded:

 

Release Agent for Clay Sculpture
 

This is a picture of one of the cured Poly 74-20 brush-on molds prior to mold shell construction:

 

Poly 74-20 Brush-On Mold
 

Because brush-on blanket molds are thin, a mold shell is required for support. West Side Stone Works decided to use Poly 1511 Liquid Plastic with PolyFiber II thickening agent to make their multi-part mold shells. They used clay to make shims for this process.

 

Brush-On Plastic Mold Shell
 

Liquid Plastic for Mold Shell
 

Removing Mold Shell from Polyurethane Mold
 

Upon cure of the plastic, the rubber mold and mold shell were removed from the model to prepare for casting. The picture below is a view of the interior of one of the finished Poly 74-20 molds:

 

Mold for Clay Sculpture
 

After cleaning the molds and placing them back in their mold shells, a concrete mixture was poured into them. The final pieces are shown below prior to installation:

 

Cast Stone Sculpture
 
Artificial Lime Stone from Rubber Mold
 

Finally, the cast stone magnolia buds were installed atop these columns in Tattnall Square Park as part of a revitalization effort:

 

Cast Stone Magnolia Buds Finished
 

West Side Stone Works is located in Macon, GA and can be reached at 478-474-8585.

 

Do you have an upcoming mold making or concrete casting project you would like to discuss with a Polytek Technical Support staff member?

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!