Polyester Casting Resin

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Trade name: "Clear Cast", "Easy Cast", "Silmar 41", et. al.
Material: Polyester

There are a wide variety of resins consisting of polyester dissolved in styrene, with additives to promote curing at different rates for different ambient temperatures (called promoters), and potential other ingredients as well. Some even include acrylic compounds to increase clarity and add UV resistance. If you are going to make your own blank, be sure to get a resin intended for casting, not for fiberglass layup. Polyester resins generate heat while curing, and that heat accelerates the reaction. Fiberglass resins contain more promoters so that they will cure in thin layers. Thick castings retain more heat and using the wrong resin could potentially cause a fire.

The heat produced during casting affects the result in two important ways. First, it causes the resin to expand, and then shrink when cooling. The more heat, the more it expands, and the more it subsequently shrinks. This makes it easier to remove from the mold, but also means that <acronym title="Google Page Ranking">PR</acronym> may pull away from materials it is cast with. That is why <acronym title="Google Page Ranking">PR</acronym> is not usually the resin of choice for "worthless wood" casting. The second way heat affects <acronym title="Google Page Ranking">PR</acronym> is that it speeds up the hardening process. If the reaction proceeds too quickly, then the crystalline structure doesn't generate as many cross-links between the polyester macromolecules and you wind up with a cast that is brittle.

Polyester Resin will gel and harden by itself over time, but the casting process is typically initiated by adding the hardener methyl-ethyl-ketone-peroxide (MEKP). The speed of the reaction and the heat produced is dependent on the promoters mixed in with the resin, the ambient temperature, and the amount of MEKP used. Insufficient MEKP will produce a weak cast (the peroxide is needed to form the cross-links). Too much MEKP will yield a brittle cast (again, from insufficient cross-links).

MEKP is a strong oxidizer and should be treated as a hazardous chemical. It can cause burns if it gets on your skin. The fumes are a strong irritant. MEKP can start fires if mixed with paper or wood shavings. Handle it with care.

Styrene (the liquid that carries the polyester) has a strong odor that most people find unpleasant. That's what gives fiberglass it's distinctive smell. Cast polyester continues to cure and off-gas for some time after initial hardening, so a <acronym title="Google Page Ranking">PR</acronym> pen blank might smell bad to you.

If the top surface of a polyester cast is tacky, that's OK. It's actually an important property when doing multi-pour casts, or multi-layer fiberglass layups. The uncured surface allows the next layer to form a chemical bond. There are ways to eliminate the tackiness, or you can just cut/turn it off.

Whew! That's more than I intended to write, but I hope it will help explain the different characteristics of different brands of <acronym title="Google Page Ranking">PR</acronym> blanks.

Make at home: Yes

Polyester Casting Resin is very popular for home-cast blanks. There are a wide variety of dyes, pigments, and other colorants (including mica powder) for <acronym title="Google Page Ranking">PR</acronym>, and you can embed object in it easily. <acronym title="Google Page Ranking">PR</acronym> is also commonly used for tube-on casting, but you may have mixed success if the shrinkage causes the resin to pull away from the tube.

Bubbles in <acronym title="Google Page Ranking">PR</acronym> casting can be reduced with vibration (ultrasonic or not), vacuum degassing, or pressure casting. My personal preference is pressure but others have great success with different techniques.

Drilling: Varies.

Most <acronym title="Google Page Ranking">PR</acronym> blanks drill easily, but some can be brittle. How brittle a blank is depends mainly on the casting conditions (see above).

Turning: Varies widely.

Some turn easily. Some are so brittle that they require great technique to turn without chipping or shattering.

Finishing: <acronym title="Google Page Ranking">PR</acronym> generally tends to sand and polish to a high gloss fairly easily.

Plastic polish or buffing yields a glass-like shine.

Threading: Varies, but generally difficult.

<acronym title="Google Page Ranking">PR</acronym> can be tapped and threaded, but the result is highly dependent on the brittleness of the cast.

Embedding: Yes.

Small objects embed well. <acronym title="Google Page Ranking">PR</acronym> can pull away from worthless wood and some tube-on castings (due to shrinkage when cooling) as noted above.

Laser engrave: Yes.

Color fill is usually necessary to achieve good contrast.

Durability: Varies widely.

Polyester can be a very durable material. They make bowling balls out of it. How durable a particular pen blank is depends on how it was cast.


Many commercially sold blanks are made from Polyester Resin, and it's one of the easiest and least expensive materials to use to make your own. The colorants and objects you can incorporate are seemingly endless so there's lots of room for your own creativity. Be sure to cast outdoors, or with good ventilation, as <acronym title="Google Page Ranking">PR</acronym> has a strong odor. I plan to write separately about some of the properties of certain brands of manufactured polyester pen blanks.