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 and can contact her at

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




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

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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.



 [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.


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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]



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




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?
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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.



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:


 [^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.



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


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?

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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?

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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).

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.
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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
Fill out this simple online form.

Or leave a comment right here on the blog!

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

July 9th, 2014

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.

According to IMA:

Cambodian artist Sopheap Pich’s “A Room” fills the Efroymson Family Entrance Pavilion with nearly 1,200 real and artificial bamboo strips extending 40 feet from the atrium’s ceiling to floor. The 26-foot space inside the bamboo is a place for contemplation, allowing visitors to experience natural light bouncing off of and piercing between the bamboo strips to evoke the sense of light within Cambodia’s bamboo forests. In using bamboo, Pich creates a contrast between the aluminum, glass, and cement structure of the Pavilion, the warmth of the bamboo and the various bright and subtle colors of the different bamboo castings.

This exhibit runs from February 27 – August 24, 2014.

Sopheap Pich - A Room

 [All photos are courtesy of Indianapolis Fabrications.]

iFab used bamboo supplied by Sopheap to create silicone rubber molds and then cast 2,800 colored polyurethane plastic reproductions using EasyFlo Clear casting plastic and PolyColor Dyes. EasyFlo Clear is a fast-setting polyurethane plastic that cures to a translucent light amber color – it can be colored with PolyColor Dyes (available in black, brown, blue, green, yellow, red, and white) to achieve a variety of colors. iFab also used Polytek tin silicone mold rubber and release agents.


Sopheap Pich - A Room - Bamboo

Sopheap Pich - A Room - Silicone Molds

Sopheap Pich - A Room - EasyFlo Clear

Brian McCutcheon of iFab made a 3-week trip to Cambodia to assist Pich with the project, give instruction on mold making, and set-up a small foundry for Pich to cast scrap aluminum. iFab also helped the Indianapolis Museum of Art assemble the installation.


Sopheap Pich - A Room - Clear Plastic Bamboo

Indianapolis Fabrication - A Room - EasyFlo Clear

Indianapolis Fabrication - Sopheap Pich - A Room

The following video, detailing Sopheap Pich’s work on this installation, can be found on IMA’s YouTube Channel.


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
Fill out this simple online form.

Or leave a comment right here on the blog!

New Product Announcement: PlatSil® Gel-25 with New Accessories

July 1st, 2014

We are happy to announce a new addition to the PlatSil® Gel product line-up. PlatSil Gels are primarily used to create prosthetic appliances and for life casting. PlatSil Gel-OO and PlatSil Gel-10 have been the industry standard for special effects in television, theatre, and film for years, but now there’s a new player.


PlatSil® Gel-25: Industry Tested, Versatile, Low Viscosity

PlatSil Gel-25 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.

New PlatSil Gel-25 Silicone_Polytek-01

^By Imaginarium FX.

PlatSil Gel-25 was introduced and featured at IMATs London by Neill Gorton (Neill Gorton Prosthetics Studio) & Mouldlife. Photo courtesy of Mouldlife.

If you juggle several projects at once and they each require platinum silicones with varying hardness, PlatSil Gel-25 is a great option. With the use of one silicone product, you can create a theatrical prosthetic with a Shore A10 skin, make a Shore A40 mold for casting prosthetics, and make a super soft gel as an appliance filler.

Gel25 16 lb kit

^PlatSil® Gel-25 is available in 2 lb, 16 lb & 80 lb kits


View PlatSil Gel-25 on the Polytek website.

Scroll down to view more photos of PlatSil Gel-25 demos from IMATS London.


New Accessory: PlatSil® Deadener LV

Like Smith’s Theatrical Prosthetic Deadener, PlatSil Deadener LV can be used to soften PlatSil Gels without oil leaching. The major difference between Smith’s Deadener and PlatSil Deadener LV is that Deadener LV has a lower viscosity (1520 cP compared to 5800 cP). Either Deadener can be used with any PlatSil Gel for maximum versatility.

DeadenerLV 1.8

You will also notice that the demold time decreases with the additional of Deadener LV. The table below details the effects of PlatSil Deadener LV on PlatSil Gel -25:


Effects of PlatSil® Deadener LV on PlatSil® Gel-25

Mix Ratio 1A:1B:0D 1A:1B:0.25D 1A:1B:0.5D 1A:1B:1D 1A:1B:2D
Pour Time (min) 5 7 8 10 10
Demold Time (hr) 1 0.5 0.5 0.5 0.5
Shore Hardness A25 A15 A4 (OO45) OO20 (OOO60) OOO30
Tack No No No Yes Very


New Accessory: PlatSil® Part H Hardener

PlatSil Part H Hardener can be used to increase the Shore hardness for any of the PlatSil Gels. For PlatSil Gel-25, a mix of 1A:1B:1H has a 7-minute working time and cures to a Shore A~40 rubber in under two hours.

PlatSilHardener 2 lb

All other existing PlatSil Gel accessories can also be added to Gel-25 to achieve varying effects:

Pigments Use Silicone Color Pigments to create custom colors in silicone rubber. Available in fleshtone, red, blue, green, yellow, black and white.
Thickener Add PlatThix liquid thickener to PlatSil Gels to thicken the mix to a light-bodied, non-sag gel.
Retarder Add PlatSil® 71/73 Part R Retarder to slow the cure of PlatSil products.
Accelerator Add PlatSil® 71/73 Part X Accelerator to accelerate the cure of PlatSil products.
Thinning Agent/Softener Add Silicone Fluid 50 cSt to the mixed rubber to thin the mix. More than 10% fluid addition may exude from the cured rubber. To soften without oil leaching, use Smith’s Theatrical Prosthetic Deadener.


More photos, courtesy of Mouldlife, from IMATS London:

Neill Gorton _ PlatSil Gel 25

 ^By Neill Gorton [photo below from Neill Gorton Prosthetics Studio Facebook page]

PlatSil Gel-25_MouldlifePhoto_1

 ^By Gary Christensen

PlatSil Gel-25_MouldlifePhoto_2

PlatSil Gel-25_MouldlifePhoto_7

PlatSil Gel-25_MouldlifePhoto_3

 ^By Anthony Davies

PlatSil Gel-25_MouldlifePhoto_4

PlatSil Gel-25_MouldlifePhoto_5

PlatSil Gel-25_MouldlifePhoto_6

 ^By Leigh Cranston

PlatSil Gel-25_MouldlifePhoto_8

PlatSil Gel-25_MouldlifePhoto_9

Danny Marie Elias

PlatSil Gel-25_MouldlifePhoto_10

For questions about this new addition to the PlatSil Gel Series:

Call us at 800.858.5990.

Email us at

Fill out this simple online contact form.

Gary Christensen


Tek-Tip: How to Store Rubber Molds to Extend Library Life

June 24th, 2014

The Library Life of a rubber mold is the length of time a rubber mold will remain useful after being placed in storage. When properly taken care of, molds can have a library life of up to 10 years or more.


Rubbers with Long Library Life

If long library life of a rubber mold is required, consider polyurethane rubber or platinum-cured (addition-cure) silicone rubbers. Avoid tin-cured (condensation-cure) silicone rubbers as they only last two to four years depending on conditions. Also avoid Poly-Fast 72-40, a polyurethane rubber that degrades quickly in storage.

Polyurethane and silicone mold rubbers-01

 [a small sample of Polytek polyurethane and platinum silicone rubbers]


Store in a Cool, Dry Area

Store molds in a cool, dry area and out of sunlight. These mold rubbers are not UV-resistant and will chalk and yellow when exposed to sunlight.


Store Molds in Original Shape

Store molds in their original, undistorted shape. If rubber is left in a distorted position (e.g., a corner is curled up, the mold is placed over top of another object, an object is laying on top of the mold) for any length of time, it may never recover to its original shape.

Store block molds flat and do not stack them on top of or underneath other molds or objects:

Store Rubber Molds Flat

Store brush-on or poured blanket molds in their mold shells (also known as mother molds):

How to Store Rubber Molds

Some complicated blanket molds should be stored with the original model in place as the mold may collapse in on itself over time and in certain circumstances. In these cases, and depending on the material, the model may need to be sealed to prevent migration of oils between the model and mold. For instance, when you have a blanket mold that is stored in a mold shell made of porous material (e.g., bare plaster), the mold shell surface that is in contact with the mold should be sealed (e.g. with shellac). Distortion can occur from migration of oil between the two.



Two-piece mold shells should be secured together with the proper hardware for storage:



Don’t Mix Rubbers in Storage

Do not allow molds made of one type of rubber to remain in contact with molds made of other rubbers when in storage. This even applies to rubbers within the same series; for instance, don’t allow one polyurethane rubber (e.g, Poly 74-20) to rest against another polyurethane rubber (e.g., Poly 75-80) in storage. This may cause migration of oils or plasticizers which can cause swelling, shrinking, or distortion.


If you have questions about the best way to store your rubber molds or if you have a question about an upcoming mold making or casting project:

Call us at 800.858.5990.
Email us at
Fill out this simple online form.

Or leave a comment right here on the blog!