Have you always been interested in making custom rubber molds for casting concrete but haven’t given it a try?
The Concrete Mold Making Starter Kit may be a great place to start. This kit is designed for individuals that are trying a small-scale mold making project for the first time. With the exception of some items (listed below), this kit includes all of the basic materials that you’ll need to make your first mold (a list of included items can be found at the bottom of this post).
Create custom rubber molds for a variety of DIY projects:
Flexible Texture Mats
Not sure if this starter kit is the best option for you? Call our Technical Support (610.559.8629 or firstname.lastname@example.org).
This tutorial is just one example of how this kit can be used:
As 2015 comes to a close, we would like to share some events and projects from the past year. Unfortunately, we can’t list all of the interesting events we’ve attended and people we’ve worked with, but these are some highlights:
We’ll see you at World of Concrete 2016 at Booth #351 in the North Hall!
– NEW Concrete Mini-Catalog –
In February of this year, we were excited to introduce a mini-catalog that serves as a “starter” selection guide for our concrete customers. It features product recommendations for applications involving concrete casting, texturing and stamping:
A big thanks to everyone that stopped by our booth at The Precast Show in Orlando, FL. We’ll see you next year in Nashville (Booth #879)!
– Customer Project: The Whimsical Gardens –
John Cannon of The Whimsical Gardens brought his clay sculpture to our facility to make a mold of the figure and then reproduce it in plastic on a rotational casting machine. See the process of making the tin-cured silicone block mold here.
– New Tutorial: Lifecasting with PlatSil® Gel-25 –
One of our distributors, BITY Mold Supply, visited us in late 2014 to help with this PlatSil® Gel-25 lifecasting project. The video tutorial was posted this year:
In May, we attended CONstruct FX (hosted by The Compleat Sculptor) as a vendor. CONstruct is a brand new expo specifically focused on sculpting, molding and casting for special effects, prosthetics, prop making and makeup. We had a great time meeting attendees and industry professionals and we hope to attend in 2016!
– National Sculpture Society –
Thank you to everyone that stopped by our table at the National Sculpture Society Sculpture Celebration in Philadelphia, PA in June!
– Kiln-Casting with Glass –
Anna Boothe, who has worked with glass since 1980, gives us a behind-the-scenes look at her glass kiln-casting process. Steps include: making the original wax model, fabricating a flexible Poly 74-24 rubber mold, casting wax copies, pouring the refractory mold, melting the wax, kiln-casting and cold-working the glass.
– Concrete Convergence –
In September, we attended the first annual International Concrete Convergence to demonstrate some mold making techniques.
Thank you to everyone that stopped by our table at the Jerseyfest Model Kit & Statue Fair. As explained on jerseyfestfair.com, this event is a “[…] celebration of movies, television, and comic books, with the focus on monsters, science fiction and fantasy.” They even offer a variety of classes and demos.
– Concrete Decor 2015 –
The Concrete Decor Show was held in Indianapolis, IN this year. Thank you to everyone that stopped by our booth! We’ll see you next year in San Diego, CA.
Interested in learning how to make some of the decorative items you saw in our booth? The tutorials are listed below.
Decorative Concrete Panel Tutorial
This tutorial shows the process of making a decorative glass fiber reinforced concrete (GFRC) panel using various Polytek® mold rubbers and casting materials.
Wood-Textured Concrete Stools Tutorial
This tutorial details the process of making a polyurethane rubber mold of a log that will be used to cast concrete stools.
– October 2015 Mold Making & Casting Workshop –
Twice a year, we hold a Mold Making & Casting Seminar/Workshop. Take a look at some of the projects attendees brought to our October workshop.
Thank you to all of our customers and distributors for a great year!
Polytek® Mold Making & Casting Seminar/Workshops are held twice a year at our Easton, PA facility. Attendees are invited to bring their own mold making projects to work on over the course of the two-day class.
On October 15 & 16, we held our fall Mold Making & Casting Seminar/Workshop. Attendees of this two-day class are invited to bring their own project (as long as the project can be realistically completed in two days).
A variety of mold making techniques were used at the workshop, including poured block molds, two-piece block molds, poured blanket molds and brush-on blanket molds. Take a look at some of the projects below:
Check the Polytek website for upcoming workshop dates or get in touch with us:
Preparation of the original model is a very important step in the mold making process. Improper preparation can lead to the rubber bonding to the model or improper cure of the rubber [for general information on how to prepare models when making polyurethane rubber molds, read this blog entry.]
For porous models that are brought in from the outdoors, it’s important to allow them to fully dry as moisture can affect the cure of polyurethane rubbers.
Porous models, such as this log (below), must be thoroughly sealed to prevent rubber from penetrating the pores and becoming stuck. Many sealers can be purchased through local home improvement stores while others are available directly from Polytek.
In some cases, a couple of layers of primer spray paint will be enough to seal the model; however, this log is extremely textured and porous, so we decide to paint on a layer of paint by hand. Brushing on the paint allowed us to “push it” into the texture better and seal the surface more thoroughly.
Once the model is thoroughly painted, a release agent must be applied. Again, since this log is extremely prous and the bark texture is very deep, we decided to use PolyCoat. PolyCoat is a silicone-based, semi-permanent sealer andrelease agent.
The log is turned onto its side to help prevent runoff and allow the PolyCoat to fully penetrate the irregular surface. In total, four layers of PolyCoat are applied.
PolyCoat should be allowed to cure in between application of each layer and the final layer should be allowed to cure before application of any liquid rubber.
When the PolyCoat has cured, a large crack in the bottom of the log is filled with warmed plasticine clay to help prevent rubber from seeping into it (the side of the log pictured below will be the bottom of the finished concrete stool.) A clay “snake” (i.e., clay that has been rolled out into a long, thin strip) is also placed around the outside edge of the bottom of the log to act as a gasket when the log is mounted onto the baseboard.
Step 2: Construct a Containment Area (a.k.a. Mold “Box”)
The log is about 12″ in diameter, so we manufacture a plywood baseboard with a 14″-diameter recess. A 14″-diameter cardboard concrete tube (e.g., Quik-Tube® or SonoTube®) will fit snugly into the recess and will act as the containment area. The recess is only about 1/2″ deep, but it will help to keep the tube stable when the mold rubber is poured.
A 14″-diameter tube is selected so that the mold walls will be about 1″ thick.
The baseboard is placed onto the bottom of the stool and secured with screws. It is important to make sure that the work surface and log are level throughout this process.
Vent holes are made in the baseboard so that air in the model may escape. Without vent holes, this air may become trapped in the rubber.
The edges of the log are sealed with plasticine clay to help prevent rubber from seeping beneath the log. The mold rubber will pick up the clay texture, so the clay is flattened/smoothed out as best as possible.
Clay is also placed around the edge of the recess to help prevent rubber from leaking outside of the containment tube.
A cut is made down one side of the tube; this will make the demolding process easier, but will also weaken the tube.
Before placing the tube around the log, we tape the inside and outside of the cut.
Vaseline is then applied to the entire inside of the tube with a dry brush. The exterior side of the bottom of the tube is also coated just in case rubber leaks outside of the tube.
Tape is wrapped around the tube and then the tube is placed snugly into the recess. Mold straps are used to secure the tube and baseboard together.
Step 3: Measure, Mix & Pour FormRub 35 Liquid Rubber
To determine how much rubber is needed to make this mold, we do the following calculation:
Determine the volume (in³) of the containment area.
Estimate the volume of the log and subtract it from the volume of the containment area.
Take that result and divide by the specific volume of the mold rubber (the specific volume of FormRub 35 is 27.2 in³/lb).
This final result is the amount of rubber, in pounds, needed to complete the mold.
This log mold requires approximately 65 lb of FormRub 35 Liquid Rubber.
FormRub Series rubbers can be mixed by weight or volume, but we highly recommend weighing on a digital scale.
Because these mixing pails only hold 40 lb of material, we mix three separate batches of rubber.
Part B is added to the mixing pail first because it is lower in viscosity and less likely to cling to the sides.
Part A is then weighed into the bucket.
The components are thoroughly mixed. It’s important to scrape the sides and bottom of the bucket multiple times during the mixing process.
Keep the pour time of the product in mind when mixing. FormRub 35 has a pour time of 15 minutes, but this time varies from product to product.
A Poly Paddle, used below, is a good tool for hand mixing.
The rubber is poured into one side of the mold. We continue to pour the rubber into the same spot and allow it to rise.
When working with multiple batches, pour the next batch as soon after pouring the previous batch as possible.
Generally, mold rubber should be poured to at least 1/2″ above the top of the model. For this mold, we pour the rubber about 1″ over the top of the log.
FormRub 35 can be demolded after 16 hours at room temperature.
Step 4: Demold & Cut the Mold
The tape that covers the cut in the tube is cut and then the tube is removed from the mold.
The screws are removed from the baseboard and the edges of the mold are loosened with a flat tool (e.g., putty knife). The mold is then turned over (this may require two people) so that the bottom of the log is exposed.
A cut will be made down one side of the mold to remove the log and subsequent castings. A sharp scalpel or blade is a good tool for cutting soft, polyurethane rubbers.
It can be helpful to have a second person pry open the cut; this gives a better view of where the model is. The location of the cut of this mold is not incredibly important because a seam line in the final casting is well-disguised in the deep textures of original model. Seam lines can be more noticeable depending on the shape and texture and original model, so cutting the mold along an existing line in the model is important in those instances.
Cutting in a zigzag pattern (compared to a straight cut) is helpful for realignment of the mold when casting.
If rubber has seeped into any deep knots in the log, it can usually be cut away easily with a scalpel.
While one person holds the mold apart, a second person removes the log.
Step 5: Cast Concrete
To cast concrete, the tube is placed back around the mold and secured with a mold strap; this will help to prevent deformation of the mold from the weight of the concrete. It is situated so that the cut in the tube is on the opposite side of the location of the cut in the rubber mold. This will provide more stability during the casting process.
The mold is coated with Pol-Ease® 2650 Release Agent. Pol-Ease 2650 is a silicone-free, oil-based release agent that does not require any drying time before casting concrete. There are a number of other concrete release agents that are suitable for use with polyurethane mold rubbers.
The concrete is mixed and poured into the mold.
To reduce the weight of the finished stool, a small amount of concrete is poured into the mold and then a 6″-diameter foam plug is inserted.
A weighted pail is placed on top of the foam insert so it doesn’t float and then the remainder of the concrete is poured into the mold.
Once cured, the concrete is removed from the mold.
Excess foam is trimmed from the insert.
If desired, the concrete can be stained.
When properly taken care of, this mold can be used to cast hundreds of concrete stools. Video Tutorial
Supplies and tools used for this project:
Original Log Model
FormRub 35 Liquid Rubber
PolyCoat Sealer & Release Agent
Pol-Ease® 2650 Release Agent
Cardboard Concrete Tube
Mixing Containers & Mixing Tools
Scalpel or Sharp Blade
Would you like to speak with Polytek Technical Support about your concrete casting project?
Our latest video tutorial details the process of making a FormRub 35 rubber mold of a log in order to cast concrete stools. The original log is sealed and released and then placed in a cardboard concrete tube. The mold rubber is poured around and on top of the log to make the mold. FormRub 35 is very low in viscosity which allows it to easily flow into the deep textures of the log.
1. Prepare Original Model (i.e., the log)
2. Construct Containment Area Around the Log
3. Measure, Mix & Pour the Mold Rubber
5. Cast Concrete
Watch the Video Tutorial:
Do you have additional questions about mold rubbers for concrete casting projects? Get in touch with our Technical Support Team:
Without the proper equipment, it can be difficult to achieve bubble-free castings with clear polyurethane casting resins such as Polytek Poly-Optic® 14-Series resins. These clear resins are two-part polyurethane systems formulated specifically for applications where optical clarity is needed.
[below: an example of Poly-Optic casting (left) next to the original model (right)]
NOTE: Some of these methods require special equipment; always seek the advice of a professional when sourcing and using this type of equipment.
In each method, the first step is to measure and mix the resin. Poly-Optic 1411 has a mix ratio of 1A:1B by volume (100A:90B by weight).
In all three method examples below, a small polyethylene container will serve as the mold.
Open Pour Method
The open pour method does not involve any special equipment. After mixing Parts A & B together, simply pour the resin into a properly prepared mold and allow it to cure.
Vacuum degassing requires a vacuum chamber, which can be expensive. To degas, the mixed resin is placed inside a container and then placed inside the vacuum chamber. The container holding the resin should have enough headspace to accommodate three times the original volume of the resin.
Vacuum is pulled to 29 inches of mercury.
The air is sucked out of the resin which causes the bubbles to rise up and then collapse. This process typically takes a couple of minutes. NOTE: Resins with short working times should not be vacuum degassed.
When the resin is removed from the chamber, it appears bubble-free.
The resin is then poured into the mold. Although the resin has been degassed, bubbles may be generated in this step.
To pressure-cast a part, the resin should be mixed and poured into the desired mold prior to placing it in the pressure pot. Pressure casting essentially crushes the air bubbles in the resin.
NOTE: Molds must be made under the same pressure or deformation may occur.
We recommend setting the pressure pot at 60 psi; the closer to 100 psi, the better.
The mold should remain in the pressure pot for the length of the demold time.
The open pour method results in hundreds of tiny air bubbles which are quite obvious in the final casting.
The vacuum-degassed casting is an improvement, but the act of pouring the degassed resin into the mold produced some air bubbles.
The resin cast under pressure resulted in the casting with the least air bubbles.
The recommended casting method for bubble-free, clear parts is pressure casting.
Do you have additional questions about Poly-Optic casting resins? Get in touch with our Technical Support Team:
Anna Boothe, former faculty member in the Glass Program at Tyler School of Art (Temple University) and current freelance artist, provided us with a behind-the-scenes look at her lead crystal kiln-casting process. You can find Anna’s bio at the end of this blog entry or view her work at www.annaboothe.com.
Photo Credits: Anna Boothe
Step 1: Create the Original Model
From wax, Anna hand-carves a form that she wishes to transpose into glass. All details and surface textures will be replicated.
Original models do not necessarily need to be carved from wax. They can be made from any number of materials. The composition of the original may affect which mold material (i.e., silicone or polyurethane) is used in the next step.
Step 2: Fabricate a Flexible Rubber Mold
To create multiples of the original wax model, Anna fabricates a polyurethane rubber mold. Silicone mold rubber is also an option.
The original wax is removed and archived; future waxes of this model are generated by pouring molten wax into the mold.
Step 4: Prepare to Make Refractory Mold
In preparation to pour a refractory (heat-resistant) plaster-based mold, Anna places the wax model on a clay base and then secures a tar paper coddle (cylinder into which the liquid refractory will be poured) around the model and base.
Step 5: Make the Refractory Mold
A refractory mixture predominately comprised of silica, plaster and fiberglass is mixed with water and poured into the coddle.
Step 6: Remove Coddle & Melt Wax
When the plaster has cured, Anna removes the coddle. In the photo below, the mold has been turned over to expose the sprue (i.e., the opening into which glass will be poured once the wax has been removed).
The wax-filled mold is placed upside down inside an industrial pressure pot, atop a grate that is suspended above a water bath. Pressurized steam created by heating the pot over a propane stove causes the wax to melt out of the mold and into the water bath.
Step 7: Kiln-Casting the Glass
Anna fills the empty mold with leaded glass frit (crushed glass) and places it in a special kiln. The glass is melted at 1500°F degrees and cooled slowly. Anna explains that for molds this size, the whole firing process takes approximately 3 to 4 days.
Step 8: Break Away Mold
Once cooled, Anna carefully breaks away the mold from the cast glass object. This type of mold is called a “waste” mold because it can only be used one time.
Step 9: The Glass is “Cold-Worked”
After the glass has been washed of mold debris, it is “cold-worked” (i.e. cut, ground and polished using a variety of machinery and hand tools that rely on diamond and other abrasives).
Step 10: The Finished Sculpture
Anna’s finished sculpture incorporates multiple “hand flower” forms, all of which have been cast individually and assembled.
“Release” 2008 17” ht. x 17” top diameter x 8.5 bottom diameter Assembled kiln-cast lead crystal.
More on Anna Boothe:
With a BFA degree in sculpture and MFA in glass from Rhode Island School of Design and Tyler School of Art (Temple University), respectively, Anna Boothe has worked with glass since 1980. Her primarily kiln-cast and pate de verre work has been exhibited at the Tittot Glass Art and Bergstrom-Mahler Museums, Museum of American Glass, and Kentucky Museum of Art and Design, and is in the permanent collections of the Corning Museum of Glass and the Tacoma Museum of Art. Comprised of both sculptural and decorative objects, her works are shown in galleries throughout the US.
A member of Tyler’s glass faculty for 16 years, Anna also helped to develop and institute Salem Community College’s (NJ) AFA glass art degree program in 2004 where she also chaired the school’s International Flameworking Conference. She has taught and lectured extensively at other schools that include the Corning Museum of Glass, the Pittsburgh Glass Center, Urban Glass, Pilchuck, and venues in Belgium, Switzerland, Turkey, and Japan. In recent years, the artist has presented her work at the Rochester Institute of Technology, Virginia Commonwealth University, the University of Wisconsin at Madison, Illinois State University, Illinois-Wesleyan University, Sheridan College (Toronto) and the Everhart Museum (Scranton, PA). In 2012, Anna was awarded a Corning Museum of Glass Collaborative Artist Residency with NY area sculptor Nancy Cohen, the result of which was exhibited at the Accola Griefen Gallery in Chelsea, NYC in 2013 and reviewed in Glass Quarterly Magazine.
From 1998-2006, Anna served on the Glass Art Society Board of Directors and as the organization’s President (’04-’06). Until recently, she was the Director of Glass at Philadelphia’s National Liberty Museum where she curated glass exhibits and was in charge of organizing the museum’s annual Glass Now auction. Currently, Anna is a freelance artist and maintains a studio northwest of Philadelphia.
For more information on rubber mold materials for use in glass casting processes, get in touch with the Polytek technical support team:
The shelf life of all Polytek® liquid polyurethane products (rubber, plastic, foam) is six months from the date of shipment in unopened containers. After this time, the product may either cure improperly or may begin to solidify in the container.
Only purchase as much rubber, plastic, or foam needed for six months and use all of the product as soon as possible once opened (i.e., avoid keeping partially filled containers).
Exposure to Atmospheric Moisture & Water
A primary concern with liquid polyurethane components is exposure to atmospheric moisture and water, which can cause irreversible solidification, especially in Part A. Even if the product has not visibly hardened, it may have absorbed moisture which can cause foaming or an improper cure when the A and B components are mixed together.
Exposure can occur when the lid is left off of the product container, when the lid is placed back onto the container but not sealed well, when there is too much “headspace” in the container and when the product is exposed to water.
Here are some tips to help reduce the risk of moisture contamination:
Always tightly reseal lids immediately after using the product (it is a good idea to also clean the rim of container after the product has been poured and before resealing the lid; you can use a solvent like ethanol/denatured alcohol on a clean paper towel).
After dispensing the product, spray PolyPurge into the open container before replacing the lid. PolyPurge is a heavier-than-air dry gas blanket that displaces moist air. When the product is in drums, use Drierite® Cartridges.
If there is a large amount of headspace in the container after dispensing, consider transferring the liquid to a smaller container (i.e., a clean metal or plastic container, excluding PVC) with less headspace. As always, use proper personal protective equipment when handling liquid components.
Ensure that any fillers added to polyurethanes products are completely dry.
Always work at room temperature (~77°F). Working in cold temperatures can slow the cure of the product (or may prevent the cure altogether) which may increase the likelihood of exposure to ambient moisture.
For best results, store liquid products indoors at temperatures between 60°F and 95°F. Long-term exposure to temperatures below and above this range may cause the product to act improperly. Polytek polyurethane products should always be at room temperature before use.
Do you have additional questions about storing Polytek polyurethane products? Ask our technical support team:
Properly release the mold prior to pouring the casting resin. This PlatSil® 71-30 Silicone Mold does not require any preparation prior to casting because release agent is not necessary when casting polyurethane resin in silicone molds.
This mold is of a stair runner architectural detail (typically found on the side of steps). The following is an example of a polyurethane resin cast in this silicone mold without any PolyColor Dyes or fillers.
Step 2: Prepare All Components
EasyFlo 60 Liquid Plastic has a very short working time (2 to 2.5 minutes), so it’s important to prepare all components ahead of time.
First, measure out Parts A & B of EasyFlo 60 by volume (1A:1B) or weight (100A:90B).
Next, prepare the PolyColor Dyes desired for marbling. Some may choose to only use PolyColor Black for natural-looking marble striations. Others may choose to use a variety of colors.
PolyColor Dyes are available in black, brown, blue, green, red, yellow and white.
Pour a small amount of each desired color into individual cups. In this example, we use blue, purple (a combination of PolyColor Red and PolyColor Blue), and yellow.
Ensure that you have a mixing tool for each color.
Step 3: Mix Primary Batch of Resin
Combine EasyFlo 60 Parts A and B and mix thoroughly.
By mixing in a thickener, you may be able to gain more control over the marble pattern. Thickened resin will not flow as easily, allowing more control of its placement and dispersion. In the following example, we do not add any thickener, but we will show some examples of marbled resin with thickener later in this blog entry.
Keep in mind that the working time of this product is 2 to 2.5 minutes once these components are mixed.
Step 4: Dispense Small Amount of Resin into PolyColor Containers
PolyColor Dyes are reactive; if they are not thoroughly mixed in with the A & B components of the primary casting material, they will not cure properly and may “bleed” out of the cured casting. Do not pour PolyColor Dyes directly into the primary batch.
Dispense small amounts of the primary EasyFlo 60 batch into each of the containers that contain the PolyColor Dye.
Then, individually mix each color.
Step 5: Pour PolyColor Mixes into Primary Batch & Pour Into Mold
Pour each PolyColor mix back into the primary EasyFlo 60 batch but do not mix.
Pour the mixture into the mold. Some may choose to pour into one spot and allow the resin to flow across the mold while others may pour the resin into different spots in the mold. The technique used to pour the resin will affect the final look of the casting.
We choose to pour the resin in one spot and allow it to flow across the mold.
When EasyFlo 60 begins to set, it turns from a transparent yellow/amber color to an opaque white color; therefore, the liquid mixture that you pour into the mold will change color quite significantly as it cures.
Step 6: Demold
After 15-30 minutes, the casting can be removed from the mold.
You’ll also notice that the front of the casting can look quite different compared to the back.
Using Thickeners for Marbling Resin
EasyFlo 60 and PolyColor Dyes are both very low in viscosity; some may choose to add thickeners to the resin in order to gain more control of the striations. We show some examples below of the results we got when using thickeners in varying scenarios.
The thickener used in the following examples is PolyFiber II. This product is traditionally used to thicken polyurethane rubbers and plastics for brush-on application.
In the example below, we thicken a batch of mixed EasyFlo 60 with PolyFiber II and then divide that mixture among the cups that contain PolyColor Dyes. Then we add those individual PolyColor Dye mixtures into a batch of EasyFlo 60 that does not contain thickener.
The colors appear to be more concentrated with this method.
In the example below, we use PolyFiber II to thicken both the PolyColor Dye mixtures and the primary batch of EasyFlo 60. The thickened PolyColor Dye mixtures are added to the thickened EasyFlo 60 primary batch and then poured into the mold.
In the example below, we pour mixed EasyFlo 60 resin into the mold and then drizzle the EasyFlo 60 + PolyColor Dye mixtures into the resin.
As you can see in the final casting, the color dyes did not make their way to the front of casting very well.
An important takeaway is that even though the same PolyColor Dyes and base resin are used for each casting, they turn out quite different based on the thickeners added to the resin and the pour technique as it’s poured into the mold.
Marbling Clear Resin
A very different look is achieved by using a clear resin instead of a white resin. In the finished casting, it’s difficult to see the striations without holding it up to light.
Do you have questions about coloring polyurethane resin? Get in touch with our Technical Support team: