Tutorial: Brush-On Rubber Mold of Decorative Architectural Element

November 18th, 2014

The following tutorial features the making of a one-piece, brush-on Polygel® 35 polyurethane rubber mold and a two-piece Poly 1512X Liquid Plastic mold shell.

The purpose of the mold is to later cast concrete copies of this decorative architectural piece.

 Plaster Model for Mold Making

Mold Making with a Plaster Model


Step 1: Prepare the Model

This model is composed of painted plaster. Because the plaster is already sealed with paint, additional sealing agent is not necessary. A release agent is required to prevent the polyurethane rubber from sticking to the model, so Pol-Ease® 2300 Release Agent is sprayed onto the model and surrounding baseboard and then brushed out with a dry brush.

This particular release agent is being used because a polyurethane mold rubber is going to be applied to the model. If a silicone mold rubber was selected as the mold material, a different release agent would be recommended (i.e., Pol-Ease® 2500 Release Agent). For more information on Polytek release agents and sealers, view our Sealer & Release Agent Selection Guide.

Pol-Ease 2300 Release Agent

Step 2: Brush On Liquid Mold Rubber

Polygel 35 Brush-On Rubber is selected as the mold material. Polygel 35 is a self-thickening, two-part polyurethane rubber that cures to a Shore A35 hardness. Upon mixing Part A & Part B (1A:1B mix ratio), the mix immediately thickens to a brushable consistency. It is a great option for those that would rather not add a separate thickener (i.e., all Polytek pourable liquid mold rubbers can be thickened with the addition of a separate thickener, like PolyFiber II).

Because there are no major undercuts on this model and Polygel 35 is a relatively soft rubber, a one-piece mold can be made. One-piece blanket molds that can be lifted or peeled off of models without a cut in the mold are often referred to as “glove” or “sock” molds.

Generally, blanket molds should be between 1/4″ and 3/8″ thick, which requires multiple layers.

Carefully apply the first layer of Polygel 35, making sure that all of the details of the model are filled with rubber. Also create a flange of rubber around the model on the baseboard (we often use melamine-laminated particle board as baseboards, like the example below).


Brush-On Polygel Rubber

Before applying the second layer of rubber, allow the first layer to gel enough that application of the second layer will not disturb or move it. This takes about an hour, depending on temperature and humidity.

To ensure uniform coverage of each layer, a small amount of PolyColor Dye can be mixed into the Polygel so that thorough coverage can be checked visually. In this example, Red PolyColor dye is added for application of alternating layers:

Polygel 35 Brush On Mold Rubber

Polygel 35 - Second Coat Finished

Allow the second layer to gel before applying the third layer.

Polygel 35 Mold Making Rubber

Allow the third layer to gel before applying the fourth, and final, layer of rubber.

Polygel 35 - Final Coat

Allow the rubber to cure for 8-12 hours at room temperature before constructing the mold shell.


Step 3: Construct Mold Shell

Poly 1512X Liquid Plastic is selected to construct the two-part mold shell. Poly 1512X has a 1A:1B mix ratio, a short working time of ~5 minutes and a demold time of ~30 minutes.

In this example, the parting line will run vertically down the middle of the “face” – it’s important to be aware of undercuts that could potentially lock the mold shell onto the model and mold.

Shims can be created from a variety of materials (e.g., metal flashing, plastic) – cardboard is used for this example. Make an outline of the shape of of the mold with a marker and then cut the shape out.

Draw Cutout for Shim

Place the shim over the designated parting line.

Place Shim over Model and Mold

To help prevent the plastic shell material from leaking onto the other side of the mold, caulk the edges with plasticine clay. Although it is not shown in this example, it is a good idea to place some clay “keys” on the cardboard shim in areas that will contact the shell material. This is done to register mold shell halves for improved alignment later.

Apply Clay to Edges

The mold rubber, cardboard shim and surrounding baseboard need to be coated in a proper release agent so that the polyurethane plastic shell does not bond. In this example, Paste Wax is applied with a brush and then Pol-Ease 2300 Release Agent is sprayed and brushed out with a brush.

Poly 1512X Liquid Plastic is a pourable plastic with a low viscosity of 400 cP (i.e., a viscosity similar to SAE 30 Motor Oil), so PolyFiber II thickener is added to the mixed resin to bring the plastic to a brushable consistency for application. Add PolyFiber II to the liquid plastic until a paste-like consistency is reached.

Polyurethane plastics cure faster in larger masses, so it’s important to work relatively quickly and get the plastic out of the mixing container and onto the mold. Dry brushesstainless steel spatulas, and Poly Paddles are good tools to use to move the plastic into position. Completion of half of a mold shell may require more than one batch of plastic, depending on the size of mold shell and how much plastic is being mixed at a time. If the shell requires a second layer, allow the first layer to gel before applying the second layer.

Make sure to create a mold shell flange that extends beyond the mold.

Construct First Half of Mold Shell

Wooden legs are added before the plastic completely sets. They are used to level the mold for casting later.

Wooden Sticks

Allow the first half of the mold shell to cure (the demold time is ~30 minutes; however, thin sections may take longer) and then remove the cardboard shim and prepare to make the second half of the mold shell.

Allow Plastic to Cure

Apply Paste Wax followed by Pol-Ease 2300 Release Agent to the Polygel 35 rubber, mold shell flange and surrounding baseboard. Also apply these to the opposite side of the mold shell flange in case any plastic accidentally spills over during application.

Apply Paste Wax

Construct the second half of the mold shell.

Construct Second Half of Mold Shell

Add the wooden legs at the same height as the opposite side.

Add Wooden Legs

Once the shell has completely cured, drill holes and insert hardware (e.g., bolts, washers, nuts) through the mold shell flange (not pictured here) to secure the two sides together for casting later.

Finished Plastic Mold Shell

Carefully pry the mold shell off of the baseboard using a putty knife or crowbar – it’s best to loosen all edges before removing the entire thing. The edges of the plastic mold shell will be very sharp, so sanding is recommended.


Your mold is now ready for casting!

As mentioned previously, this mold was designed for casting concrete; however, numerous casting materials can be poured into polyurethane molds. This includes plaster, waxes and resins and foams (with use of the proper release agents). For assistance in choosing an appropriate mold material, visit our website.

Pictured below: A painted EasyFlo 60 Liquid Plastic casting made in the Polygel 35 mold.


Plastic Architectural Element

Video Tutorial

The following video tutorial shows the poured blanket mold technique with a different model and mold material:

Do you need assistance in selecting a mold material, mold making method, or casting material? Contact us:

Fill out our simple online contact form.

2015 Polytek® Mold Making & Casting Seminar & Workshop Dates

November 11th, 2014

We have just announced the 2015 dates for our Mold Making & Casting Seminar & Workshop:


Join us for this two-day seminar and workshop on:

March 26 & 27, 2015


October 15 & 16, 2015






Polytek Workshop 80


For details on what takes place during this two-day seminar and workshop, visit the seminar information page on our website.


To read about some of our most recent seminars/workshops, visit the following blog entries:  October 2013  |  March 2014  | October 2014


For additional information, pricing, or to sign up, please contact us at 800.858.5990 or sales@polytek.com.


We look forward to seeing you there!

New Catalog: Mold Making & Casting Materials for Sculptors & Foundries

November 3rd, 2014

Mold Making Materials for Sculptor and FoundriesGiven the amount of mold making and casting products available for sculptors/artists and foundries, it can be difficult to filter down the options when researching for a specific project. In our Mold Making & Casting Materials for Sculptors & Foundries catalog, we narrow down our large product line to a select group of materials that have proven successful for mold makers year after year.

This group of products includes polyurethane and silicone mold rubbers, casting plastics, release agents, and accessories.

If you have questions about your project after reviewing the catalog, please get in touch with us. Our technical support staff is available Monday through Friday, 8:30am – 4:30pm [EST] to answer your questions:

Fill out our simple online contact form.


How to Make Silicone Rubber Molds for Casting Wax Candles

October 29th, 2014

The following video tutorial features the making of three PlatSil® 73-25 Silicone Rubber molds to cast wax in to make candles. Two of the models used to make the molds are branches from a fallen tree. The third model is a plastic pine cone.

Candle Models

We cut the branches down to an appropriate size for candles:


Models for Candle Making

PlatSil® 73-25 Silicone Rubber is used to make the molds. It is a two-part platinum silicone rubber with a 1A:1B mix ratio, Shore A25 hardness, 20-minute pour time, and 4 to 5-hour demold time.


PlatSil 73-25 Silicone Rubber

The full tutorial is provided below, followed by a list of materials and tools used for the project:



  1. Select a Model
  2. Construct Mold Boxes & Prepare Models
  3. Measure, Mix & Pour Silicone Mold Rubber
  4. Demold
  5. Melt & Pour Wax
  6. Remove Candle from Mold





Materials & Tools Used

– Original Models
PlatSil® 73-25 Silicone Rubber
PolyPoxy® Quick Stick Epoxy Adhesive
Pol-Ease® 2350 Sealer & Release Agent
Pol-Ease® 2500 Release Agent
– OPTIONAL: EasyFlo 60 Liquid Plastic
– Wax, Wax Colors, Wick, Clamp for Wick
Dry Brushes
– Mixing Containers & Mixing Tools
– Mold Containers (e.g., PVC pipe, particle board, pail)
Plasticine Clay
Tongue Depressors
– Drill
– Screws
– Digital Scale
– Putty Knife/Crowbar
– Scalpel


Additional Tutorial

Click here for another candle-making tutorial based on a decorative glass object.


For more information on mold making supplies for candle making:

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

Or leave a comment right here on the blog!

Mold Making & Casting Seminar & Workshop: October 2014

October 23rd, 2014

Last week, we held our fall Mold Making & Casting Workshop (see a recap of our March 2014 workshop here). Workshops are two days long and include the following:


  • Make your own molds and castings
  • One-on-one guidance from Polytek staff members
  • Work with a wide variety of mold rubbers and resins
  • Learn a variety of mold making and casting techniques
  • Meet others with similar interests
  • Buy material at low trial unit prices during the seminar


Attendees are invited to bring their own project to the workshop or choose from our large supply of in-house models.

For more details, visit the Polytek website.


Here are some photos from the workshop:

Mold Making and Casting Workshop


Demonstrations from Polytek staff members:

Polytek Workshop 1

Polytek Workshop 2

Polytek Workshop 3

Polytek Workshop 4

Polytek Workshop 11

Polytek Workshop 6

Mold making and casting photos:

Polytek Workshop 5

Polytek Workshop 53

Polytek Workshop 10

Polytek Workshop 9

Polytek Workshop 31

Polytek Workshop 7

Polytek Workshop 17

Polytek Workshop 16

Polytek Workshop 15

Polytek Workshop 14

Polytek Workshop 19

Polytek Workshop 12

Polytek Workshop 8

Polytek Workshop 13

Polytek Workshop 28

Polytek Workshop 29

Polytek Workshop 94

Polytek Workshop 54

Polytek Workshop 92

Polytek Workshop 91

Polytek Workshop 85

Polytek Workshop 32

Polytek Workshop 79

Polytek Workshop 84

Polytek Workshop 90

Polytek Workshop 62

Polytek Workshop 63

Polytek Workshop 83

Polytek Workshop 30

Polytek Workshop 43

Polytek Workshop 81

Polytek Workshop 82

Polytek Workshop 74

Polytek Workshop 78

Polytek Workshop 80

Polytek Workshop 77

Polytek Workshop 76

Polytek Workshop 75

Polytek Workshop 86

Polytek Workshop 72

Polytek Workshop 48

Polytek Workshop 64

Polytek Workshop 70

Polytek Workshop 87

Polytek Workshop 89

Polytek Workshop 69

Polytek Workshop 66

Polytek Workshop 65 (2)

Polytek Workshop 61

Polytek Workshop 20

Polytek Workshop 21

Polytek Workshop 22

Polytek Workshop 58

Polytek Workshop 45

Polytek Workshop 96

Polytek Workshop 57
Polytek Workshop 55
Polytek Workshop 50

Polytek Workshop 51

Polytek Workshop 49

Polytek Workshop 47

Polytek Workshop 44

Polytek Workshop 42

Polytek Workshop 41

Polytek Workshop 40

Polytek Workshop 34

Polytek Workshop 33

Polytek workshop 25

Polytek Workshop 35

Polytek Workshop 91

Polytek Workshop 93

Polytek Workshop 97

Polytek Workshop 99

Polytek Workshop 100


Check the Polytek website for upcoming workshop dates or 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!

Tutorial: How to Make a Silicone Rubber Candle Mold

October 15th, 2014

The following tutorial details the process of making a silicone rubber mold for casting wax to make candles.


Supplies Needed for Project:

Rubber Molds for Candle Making


Step 1: Select a Model

Select or create a model to replicate. The final wax candle will be a copy of this original model. Many candle makers sculpt their original models with clay, wood or wax or utilize computer-based options like 3-D printers and CNC routers to produce their designs.

The model that we selected for this particular project is made of glass:


Glass Object for Making Mold


This glass model is hollow; however, we would like the final candle to be solid, so we fill the model with wood and clay:


Inserting Wood

Wood Inserts

Fill Model with Clay


The mold rubber will pick up every detail of the model, so the clay is carefully smoothed out:


Even out Clay

Smooth Clay in Model


Step 2: Construct a Mold Box & Secure the Model

To make a one-piece poured block mold, construct a mold box or select a suitable container to place your model in. Make sure that there is enough space between the model and the mold box walls to create mold walls with an appropriate thickness.

We leave approximately 1″ of space for mold walls and the mold bottom in this example.

Plywood is being used to construct the mold box. Porous materials like this must be sealed prior to pouring liquid mold rubber. The plywood will be sealed in Step 3.

Some other possible options for mold boxes or mold containers: Plexiglas®, plastic pails, PVC pipe, Sonotubes®, melamine-laminated particle board (you can purchase a 24″ H mold box directly from Polytek).


Cutting Plywood for Mold Box Walls


Using a hole saw, we create a hole in the bottom board of the mold box to hide this part of the model (we do not want this protrusion on the final candle):


Protrusion in Glass


Cutting Hole in Wood

Hole in Plywood


Securely adhere the model to the bottom board so it does not float when the mold rubber is poured around it.

Using PolyPoxy®  Quick Stick Adhesive, a two-part, fast-setting epoxy adhesive, we adhere the model to the bottom board:


Epoxy Adhesive for Mold Making


PolyPoxy® Quick Stick Adhesive has a 1:1 mix ratio and sets in 3-5 minutes.


Apply Epoxy Adhesive

Adhere Model in Bottom Board with Epoxy

Model on Baseboard (2)


While the epoxy is curing, construct the walls of the mold box:


Construct Mold Box

Construct Plywood Mold Box

Finished Mold Box


Step 3: Apply Sealer & Release Agents as Necessary & Finish Preparing the Model

Apply appropriate sealers and/or release agents to the mold box.

As mentioned previously, the plywood used for this mold box is not sealed. We coat it with Pol-Ease® 2350, a white petrolatum dissolved in mineral spirits, which serves as both a sealer and release agent. No additional release agent is necessary.

When using a non-porous mold box, consider applying Pol-Ease® 2500 Release Agent instead (Use this option when working with silicone rubber. Use Pol-Ease 2300 Release Agent when working with polyurethane rubbers).

Not sure which release agent to use for your particular project? Our Sealer & Release Agent Selection Guide may be helpful.


Pol-Ease 2350 Sealer and Release Agent

Apply Sealer to Mold Box


Apply a release agent to the model.

We also apply Pol-Ease 2350 Sealer & Release Agent to the model itself. Pol-Ease® 2500 Release Agent (an aerosol spray) is another option that could be used in this case (for use with silicone mold rubbers).


Polytek Sealer and Release Agent


Allow sufficient time for the solvent in Pol-Ease 2350 to evaporate (~1 hour, depending on temperature and humidity) before moving on to the next steps.

Seal any areas around the model where rubber may leak. There are small gaps at the base of this model, so we seal that area with plasticine clay (warmed up for easier application). Other options for sealing include caulk and hot glue.


Clay Snakes


Smooth and flatten the clay as best as possible:


Smooth Clay

Clay Caulking


Place the model in the mold box and secure the bottom board to the mold box walls:


Assemble Mold Box

Secure Mold Box


Place the mold box and model in the proper position for pouring the liquid rubber:


DSC06980 - Copy


Seal the edges of the mold box with plasticine clay to help prevent rubber from leaking outside of the box:


Clay Snakes - Caulk Box

Mold Box Sealed with Clay


Step 4: Measure, Mix & Pour Silicone Rubber

The mold rubber selected for this particular project is PlatSil® 73-15 Silicone Rubber.

Basic Specifications for this Rubber:

  • Type of Silicone: Platinum-Cured
  • Mix Ratio: 1A:1B
  • Shore Hardness: A15
  • Pour Time: 20 minutes
  • Demold Time: 4-5 hours
  • Color: White Translucent
  • Mixed Viscosity: 2,500 CP


NOTE ABOUT CURE INHIBITION: Contamination from amines, sulfur, tin compounds, polyester resins, some 3D-printed plastics, and some other materials can cause cure inhibition in platinum-cured silicone rubbers, so ensure that your model and other materials coming into contact with the silicone do not contain these inhibitors. It is best to perform a small test cure on your model before pouring a large mold.

This particular mold requires approximately 10 lb of PlatSil 73-15 rubber (5 lb of Part A & 5 lb of Part B).

PlatSil 73-15 Silicone Rubber for Candle Making - Polytek

Weigh out Part B on a digital scale – we recommend pouring Part B first because it is lower in viscosity than Part A and is less likely to cling to the sides of the mixing container:
Part A of PlatSil 73-15 Silicone Rubber

Tare the scale and weigh out Part A into the same mixing container.
Part B of PlatSil 73-15 Silicone Rubber

Thoroughly mix Part A and Part B together, scraping the sides and bottom of the mixing container several times:
Mix Silicone Rubber Thoroughly

Carefully pour the mixed rubber into the mold box. Pour into one corner and allow the rubber to rise. Do not pour rubber directly onto the model:
Pour Silicone Rubber into Mold Box

Soft Silicone Rubber

Allow Silicone Rubber to Cure

PlatSil 73-15 can be demolded in 4 to 5 hours at room temperature. If a different rubber is being used, make sure to check the demold time of that particular rubber. Demolding too soon can cause permanent warping of the mold.

Step 5: Demold & Cut Mold (if necessary)

Carefully remove the mold box bottom board and walls. A putty knife may be helpful in this process:

Remove Mold Box

Remove Mold Box Bottom Board

Carefully Remove Mold Box

Remove Wood and Clay

Depending on the shape of the model, the mold may have to be cut to remove the model and subsequent castings. We use a scalpel to cut an irregular pattern on one side of the mold (irregular patterns realign better for casting than straight cuts):
How to make a cut in Silicone Mold

Make Irregular Cut in Mold

Carefully remove the model from the mold:


Polytek Silicone Mold

Finished PlatSil 73-15 Silicone Candle Mold

Make a small hole in the bottom of the mold to later accommodate a wick:
Make Hole in Rubber Mold for Wick

This next step is completely optional. Before we move on to casting wax, we make a plastic copy of the original model just in case we need to make more molds at a later date (we broke the original glass model when demolding).
Wooden boards and duct tape are used to support the mold when casting (you could also use the original mold box for support):
Prepare Mold for Casting Plastic

EasyFlo 60, a fast-setting polyurethane plastic, is poured into the mold to make the copy:
Casting EasyFlo 60 Liquid Plastic

This plastic can be demolded in 15-30 minutes:
Plastic Copy of Candle

EasyFlo 60 Decorative Element


Step 6: Pour Wax into Mold

Feed a wick through the hole created earlier:


Place Wick in Silicone Mold

Candle Making with Silicone Mold

Temporarily tie a knot in the wick:
Knot in Wick

Wick in Silicone Mold

Secure the wick above the pour hole of the mold. Make sure it is centered:
Suspend Wick for Wax Casting

Secure Wick with Clip

Melt wax with the appropriate equipment and add dyes and fragrances if desired:
Wax in Parrafin Bath


Carefully pour the wax into the mold:
Casting Wax in Polytek Silicone Mold

Allow Wax to Cool in Silicone Mold

Allow the wax to completely cool:
Demold with Wax is Cooled


Step 7: Remove Candle from Mold

Carefully remove the candle from the mold:

Remove Candle from Mold


Trim the wick as necessary:
Trim Wick on Both Ends

Hundreds of candles can be made in this single mold. A finished candle is pictured below:
Decorative Candle from Silicone Mold

Close-Up of Candle Detail from Mold

Candle Making Tutorial


We utilized a soft, silicone rubber for this tutorial; however, polyurethane rubbers are also a viable option for candle making. Some waxes perform better in silicone molds vs. polyurethane molds. Please get in touch with us to discuss your specific candle-making 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!

Warping Decorative Concrete Elements in Rubber Molds

September 30th, 2014

Last week, we posted a video tutorial on making a rubber mold to cast “warped” side-by-side concrete tables. Our latest tutorial (below) focuses on casting “warped” decorative concrete elements in Poly 74-20 polyurethane molds. The original model was constructed through a series of steps involving wood, silicone rubber (PlatSil® 71-10), and polyurethane casting resin (EasyFlo 60).

The finished concrete elements can be used for a variety of purposes: tables, light housing, planters, garden decor or trellis, decorative panels (when cast flat), and much more.


For more information on liquid rubbers that can be used to make molds for warping concrete:

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!

Making Rubber Molds to Cast & “Warp” GFRC Tables

September 25th, 2014

Our latest video tutorial is now posted on our YouTube Channel and can also be viewed below. It features the making of a Poly 74-45 rubber mold designed to cast side-by-side “warped” concrete tables.

The original model is constructed from MDF and coated in primer paint. The mold is made from Poly 74-45, a two-part, mid-range hardness, polyurethane rubber. Shark-Tooth Inserts (patent pending: 62/045.228) are used to control the size and location of the bend in the mold.


Do you have questions about the liquid mold rubber used for this process?

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!

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.



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]


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.

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