Help Me Select a Casting Material
Great, you need assistance choosing a Polytek casting material. Let’s get started. The selection process is not always straight forward, but we are going to try to simplify it as much as possible. Polytek’s casting materials include liquid plastics, rubbers and foams. If you are looking to cast concrete, plaster, wax or other materials not available from Polytek, there are many sources. If you need assistance locating a product, just contact us.
The casting material you are about to choose is based largely upon the following criteria:
- Appearance – What color? Should it look like glass, wood, stone or bronze?
- Weight – Lightweight, heavy (like bronze or stone), hollow?
- Function – Impact-resistant, heat-resistant, fire-retardant, certain feel, etc.
- Exterior Durability – Is this important or is it for interior use only?
- Cost & Time – How fast do you need to create your parts and what are the economic considerations?
Let’s discuss the appearance required of your casting(s) first. Each Polytek casting material (plastics, rubbers or foams) has an intrinsic color specific to that product. Some Polytek products are translucent or clear.
- If a single color is all that is required, our PolyColor dyes (for polyurethanes) or SiliColor pigments (for silicones) can be used to create almost any color you desire. If the object is to be multi-colored painting can be done on a casting that has been well cleaned with a detergent (like Cascade) or a citrus cleaner/degreaser product to remove any residual release agent. Some paints do not adhere well to plastics. We recommend applying an auto body primer first (allowed to dry overnight) then painting.
- If the object is to be clear and hard, Poly-Optic plastics are excellent. If the object needs to be flexible and clear and nearly colorless, GlassRub, PlatSil 71-40 or Poly-Optic 14-70 should be considered. These products can be tinted with PolyColor dyes and SiliColor pigments as needed. There are “clear” products which have an amber color (varies slightly) such as Poly 74-30 Clear (a rubber) and EasyFlo Clear (a plastic).
- If a casting needs to look like stone, fillers such as marble dust, calcium carbonate, Fillite and others can be used. If these fillers are used with clear products such as Poly-Optic plastics, the casting can be made to have a translucency near the surface. Solid colors or swirls can be introduced to enhance the effect. In addition, paints can be sprayed on lightly and removed with a paint thinner wipe to highlight texture or provide an aged patina effect.
- To make an object look like metal such as bronze, there are several techniques to consider. Some metal fillers such as our bronze powder can be dusted onto the surface of the silicone rubber mold then poured solid with a plastic such as EasyFlo 60 tinted brown with PolyColor (this insures no white color shows through the bronze if the casting gets scratched). Bronze or other metals can also be mixed into a plastic such as EasyFlo 60 or EasyFlo Clear then brushed or slushed into a mold to coat the entire surface. Backfilling with plastic without the bronze keeps cost very low. This can be done by pouring (to make a solid casting) or by slushing/laying-up (to make a hollow casting). These casting can be burnished with 0000 steel wool and highlighted with the patina/paint effect described above.
- To make a casting look like wood, it must be made in a mold that has suitable texture from the master. Polyurethane rubbers and plastics can then be poured into the mold (often tinted with PolyColor brown) to capture this wood grain texture. The casting can be “stained” to look like real wood by applying either stain or paint followed by a paint thinner wipe. This accents the deeper grain texture. Different colors can be used to vary effects. PolyFil ND (neutral density- not heavy, not light) or a filler like calcium carbonate (heavy) can be mixed into the plastic before casting so paints and stains adhere better to make for a more realistic part.
Let’s discuss the weight of your casting. There are several ways to vary the weight of a casting. Most Polytek plastics and rubbers have a density of ~1.0 g/cc. These products can be filled to vary the weight of a solid casting to make it lighter or heavier. Here are some details and options.
- For heavier castings that feel like stone, use Poly 15-3 or Poly 15-3X as supplied. These polyurethane plastics are 1:1 mixes and exhibit low shrinkage when cured. These plastics are great as a backfill when making cold cast bronzes so the casting feels heavy like the real bronze metal.
- To make unfilled polyurethane plastics heavier, they can be filled with dry fillers such as sand, calcium carbonate, marble dust, etc. If the filler is not dry, the plastic may foam.
- To fill a plastic to make the cast part less expensive without affecting it’s density, use PolyFil ND (the ND stands for neutral density – same as the plastic).
- For lighter castings that feel like wood use Poly Lite Cast as supplied or you can fill other plastics with micro balloons, glass or ceramic spheres.
- The inside volume of a casting can sometimes be “blocked out” so that casting material is prevented from flowing into that space. The “block out” can be removed once the casting material cures or it can be left in place. Block outs can be made from wood (solid or assembled), foam, pipes, etc. If a block out is porous, it must be sealed or positioned not to create air pockets.
- For very light castings, you can choose and PolyFoam product. These foams come in varied densities and can be hard as well as flexible.
- Rotational casting (with a machine) or slush casting (by hand) can produce very lightweight castings. If the thin layer of plastic is not durable enough, the hollow part can be backfilled with a PolyFoam system to enhance its structural integrity.
- Lightweight castings can also be made by brushing, laying-up, or by spraying liquid plastic or rubber into an open back mold.
Let’s discuss the functional requirements for your casting.
- Obviously the hardness, elongation and other physical properties that are important can be accommodated by our wide array of rubbers and plastics.
- Impact resistance – If this is important, a strong plastic or firm rubber might be a good choice. Obviously, rubbers are by nature impact resistant. Some Polytek plastics are stronger than others allowing them to absorb impact, of say, being dropped. Poly PT Flex and Poly Plasti-Flex rubbers fit this bill as do Poly 1511, Poly 1512X, Poly-Optic 1411 (if clear is needed), and EasyFlo 120 plastics.
- If heat-resistance is important, the question is how high? Poly-Optic 1412 has a heat deflection temperature (HDT) of 227 F (109 C). In general, plastics will soften at higher temperature. This phenomenon can be reduced with the correct choice of material as well as its geometry.
- If fire retardant parts are required, there are a number of Polytek products which can be used. EasyFlo 100 FR is a pourable plastic. EasyFlo Spray FR is a sprayable plastic. EasyFlo Spray Foam FR is a sprayable foam. Other Polytek rubber and plastics can be made fire retardant for special applications. Contact us for more information.
Let’s discuss exterior durability. This means parts will see all the elements from mother nature, the most damaging of which is UV radiation from the sun. Just as UV light can cause sunburn on humans, so too can it yellow, chalk and break down the surface of rubbers and plastics. Obviously, some rubbers (like tires on a car) are resistant to this damage, but these rubbers cannot be easily poured like Polytek systems. Here are some options.
- Poly-Optic plastics are, by their design, more resistant to UV degradation than other polyurethane plastics. If the best possible UV protection is to be afforded, Poly UV Additive should be added to Poly-Optic plastics for years of exterior durability.
- Poly UV Additive can be added (~0.5 to 1.0%) to other Polytek polyurethane rubbers and plastics to improve exterior durability. Poly 75-80 is very good in outdoor environments and Poly 75-65 to a slightly lesser degree.
- Any Polytek plastic or foam can be painted to insure excellent UV resistance.
Let’s consider the cost and time constraints as related to casting. Cost and time are one and the same unless you are a hobbyist who is not “on the clock” to get the job done. In either case, the cost of a project is driven largely by material cost. The more weight of material used and the more it costs, the more the final casting will ultimately cost. Here are some things to consider:
- Fillers can be added to plastics as described above to reduce cost.
- Block outs can be used as described above to reduce cost.
- Foams can be used to reduce cost because they expand nicely to fill irregular spaces.
- Shorter pot-life and therefore faster demold products can allow more parts to be made in any given period of time.
- Castings that are made by pouring take less time to make than those made by lay-up or spraying.
- Slush casting takes time since someone needs to rotate the mold while the casting material gels.
- Rotational casting takes less time since a machine does the rotating but there is the cost of the machine.
Confused or need help? Just call us at 800-858-5990 or email us at email@example.com.