Backfilling PlatSil® Gel-OO Skin with Flexible Foam

If you’re using silicone to create props, it may make more sense to create a silicone skin backfilled with foam instead of a solid silicone part. Silicone skin backfilled with foam is much lighter and easier to handle and it costs less to manufacture.

This article shows how to make a partial prop head with silicone and flexible foam. A big thanks to BITY Mold Supply for the demonstration.


PlatSil® Gel-OO & PolyFoam F-3

Prepare to cast PlatSil Gel-OO into a suitable mold.

The mold used in this tutorial (pictured below) is made of EasyFlo 120, a polyurethane plastic. No release agent is needed when casting platinum silicone in EasyFlo 120.


EasyFlo120 Rigid Mold

PlatSil Gel-OO is an option in the PlatSil Gel Series. PlatSil Gels are platinum-cured silicone rubbers used for a wide variety of special effects applications, including lifecasting, theatrical prosthetics and mold making. Gel-OO has a Shore OO30 hardness, 6-minute working time, 30-minute demold time and can be easily colored with silicone pigments.
Measure out PlatSil Gel-OO Part A & Part B by weight or volume. We measure by volume below:



PlatSil Gel-OO



PlatSil Gel-OO Part B



PlatSil Gel-OO Part A and B

To make the silicone more life-like, color with silicone pigments (e.g., fleshtone, red and blue) before mixing Part A and Part B together. If you mix Part A and Part B together and then try to pigment the silicone, you may run out of working time.
Combine Part A and Part B and thoroughly mix, scraping the sides and bottom of the mixing container several times.



Mix PlatSil Gel-OO

Brush a layer of PlatSil Gel-OO into the mold, making sure to work the silicone into the details.






PlatSil Gel-OO First Thin Layer






PlatSil Gel-OO Silicone Skin

While the first layer of silicone is gelling, prepare to mix up a second batch of PlatSil Gel-OO thickened with TinThix. TinThix is a liquid thickening agent that can be added to tin-cured and platinum-cured silicones (no more than 5%) to bring them to a brushable consistency. We did not add thickener to the first layer because we wanted it to be as thin as possible in order to capture detail. When the first layer has gelled enough that application of the second layer will not disturb it, brush in a layer of thickened Gel-OO.




Immediately after application of this layer, add a layer of cotton or polyester fiber. PolyFoam F-3, a polyurethane foam, will not chemically bond to silicone; therefore, a mechanical bond must be made.



Add Cotton or Polyester Fiber to Silicone



Bonding Silicone and Foam



Put Cotton in Silicone

When the second layer of PlatSil Gel-OO has cured, pull the loose, excess fibers out of the silicone.



Pull Excess Cotton from Silicone

Prepare to backfill the silicone with PolyFoam F-3.
PolyFoam F-3 is a flexible, polyurethane foam with a free-rise density of 3 lb/ft³. It has a mix ratio of 1A:2B by weight, a 25-second cream time and a 10-minute demold time.



PolyFoam F-3 Flexible Foam

Carefully weigh out PolyFoam F-3 Part A and Part B. Once these parts are added together, work quickly to mix and then pour the foam into the mold. If too much time elapses, the foam will rise in the mixing container and the mix may be lost.
We recommend packing PolyFoams to a minimum of 2 to 3 lb/ft³ above their free-rise density to achieve good surface detail. PolyFoam F-3 should be packed to 5-8 lb/ft³.



Mix Flexible PolyFoam

Pour the PolyFoam and brush around to thoroughly coat the silicone.


Brush in PolyFoam



Brush in PolyFoam F-3

Allow the foam to rise and cure. Some may choose to add a lid (with vent holes) to close the mold – doing so will provide better compaction and produce a better part in most cases. If using a lid, it should be securely clamped down to the mold before the foam begins to rise.



Allow Foam to Cure

After ~10 minutes, the piece can be demolded. Carefully loosen the edges of the silicone before removing the whole casting.



Demold Silicone and Foam




PlatSil Gels for PropMaking




Silicone Head


The polyurethane foam can be trimmed to better fit the silicone skin around the edges.
Another Option: PlatSil® Gel-OO & PlatSil® SiliFoam
PlatSil SiliFoam is a platinum-cured silicone foam with a free-rise density of 15 lb/ft³. This product has a 1A:1B mix ratio, ~45-second cream time and ~30-minute demold time.
Unlike polyurethane foam, silicone foam will bond to silicone rubber.



Silicone Foam

The same process for applying the PlatSil Gel-OO skin is used (i.e., one thin, detail layer and one thickened layer); however, the addition of cotton or polyester fiber is unnecessary. After the second layer of PlatSil Gel-OO has been applied, PlatSil SiliFoam can be mixed up and poured into the mold. Casting the foam before the silicone has fully cured is the most effective method.



Mix Silicone Foam

Silicone pigments, like Fleshtone, can easily be added to the foam before mixing Part A and Part B together.



Fleshtone Silicone Pigment

After mixing thoroughly, pour the silicone foam into the mold and brush around.



PlatSil Silicone Foam



Backfill Silicone with Silicone Foam



Silicone Skin Backfilled with Silicone Foam

Allow the silicone foam to cure before loosening the edges of the silicone and removing the casting.



Loosen Edges and Remove



SiliconeSkin with Silicone Foam



Silicone Head Backfilled with Silicone Foam

Ready to move on to the next step? Learn how to paint on silicone with PlatSil Gels and silicone pigments here.
Do you have questions about this casting process? Reach out to our technical support team:



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