Vacuum Forming Plastic Choices

Styrene, High-Impact poly styrene (HIPS)

Generally the best version of thermoforming plastic for modelers. This plastic is very "friendly" to and its use is well known by the modeling community. Because the plastic is fairly low-density, it is easy to work with small hand tools. There is a world of paints that will work very well on it. The use of paint is recommended because this type of plastic is sensitive to UV light (ultra-violet light, a part of sunlight or prolonged fluorescent light exposure) and can break down, becoming brittle. In addition, the use of a primer is also recommended because some paints can adversely affect styrene over time.

Styrene comes in many thicknesses, as thin as one thousandth of an inch (.001"). Common thicknesses are .020" (said, "twenty thousandths of an inch"), .030" and .040" these are most useful in that they are easily cut, drilled or glued. There are standard dimensions that include two sheet sizes, a common 48"x96" (4 x 8 feet). And 40"x72" (3½ x 6 feet). Both are useful for various reasons. The larger can be useful for the largest projects, or maximum cuts-to-dollar-spent efficiencies. The smaller can be a space-saver or, alternately, it can be cut in half for even more storage flexibility. It should be noted that when ordering the 4x 8 foot, full-sheet size, it will come a little bit larger than that, by about a 1/2". This is so that the sheet can be trimmed exactly to 48"x96" or intermediate cuts taken from that full-sheet can be "perfect" dimensions. For example, if you wish two pieces that are 24"x96" you can have a center cut taken by a type of cutter that removes a blade's thickness ("kerf"). This would affect only the hardiest of efficiency seekers, but if you have built a large cutting table to accept a 48" dimension, it will not fit!

There is also a manufacturing tolerance to consider. A thickness of .030" can deviate slightly from that ideal number. It could be thicker or thinner, but not by much. These are standard manufacturing tolerances. If dimensions are critical, use a micrometer to measure your plastic.

The only limit to working with styrene is heat. For heat-forming the concern is not to let it catch fire. The resulting smoke can be a lung irritant. It takes direct contact with a flame for it to catch, so under normal conditions that is a remote possibility. Some at-home vac-formers use the top-burners of their stove to soften the plastic. Experience here is useful, but the risks must be known in advance. Other heat concerns are powered cutting and drilling. Almost any moderate speed of either will cause the plastic to melt. A slow band saw can work well. A very slow drill will work; this usually requires a cordless variable speed hand-drill or a lot of space to use a full-size hand drill that can be "dialed down" to a very slow speed. A hand-powered drill will work well, but usually requires both hands and again, requires a lot of space with which to drill in. Almost all Dremel and Dremel-type drills will require an additional speed controller to take them down to styrene-working speeds. Some modelers have made their own slow-speed drills.

Gluing styrene is almost always best done with a solvent-type glue. This melts the two pieces of plastic to make them one and is very effective. Beware! There are several types of solvent glues, some of which are "hotter" or more aggressive on styrene than others. Experiment first or a thin piece of styrene can be dissolved before your eyes. If it took a while to make that shape, it can be a heart-breaker. There are other "friendlier" methods of attaching styrene that involve solvent mixed with a carrier that forms a thicker material. Again, experimentation can be your friend.

Styrene can absorb a certain amount of moisture. This can be troublesome on some forming projects. The result can be steam-produced irregularities in an otherwise smooth surface. The solution is to warm the plastic before it is used in a vac-forming session. A temperature far below the boiling point of water for a short time will help. Also store the plastic in a dry place.

Cellulose Acetate Butyrate (CAB)

This is a very precisely formed plastic that is very clear. Sometimes erroneously referred to as "acetate" it is not—that is a different form of plastic that is very flammable. With the addition of butyl, a type of rubber, this plastic is more flexible that other versions. This makes its heat-forming abilities superior to other clear types.

This material is prized for its ability to be formed as replacement canopies for aircraft. Of course, it can be formed into any other type of windscreen. CAB has excellent "stretch" properties and can be counted on to retain a lot of strength even when pushed 200-250%.

The need for tinted clear elements of many model subjects has resulted in a line of plastic tinting paints. The range is usually for tinted windscreens or turn signals so gray to red or blue is the norm. For any other colors, one has to count on the property of plastics to absorb some colors from dyes. Soaking an aircraft replacement canopy in a chrome yellow dye overnight can yield an acceptable "gold plated" surface. Experiment to suit.

CAB comes in many thicknesses but generally is quite hard to find thinner than .020". On the other hand, forming over some shapes can result in a thinning of the material that may work for you. Experience and experimentation can help if you need something much thinner. CAB comes in an odd "standard" sheet size: 21"x51".

CAB responds to several solvent type glues, Weldene and ProWeld plus others.

Polyethylene Terephthalate (commonly referred to as PET-G)

This material shares many properties of styrene but is clear. Pet-G is also precisely formed and is generally not available thinner than .020". "Vivak™" is a brand name. Because PET-G can be thermo-formed and is FDA approved for food-contact, it is primarily used for custom chocolate candy making molds or similar, low-temperature candies. Because it can be used as a "food contact" material, it is protected on either side by sheets of vinyl that must be removed before heat forming or working.

PET-G is sometimes used as a substitute for polycarbonate (commonly known by its brand name, "Lexan") because of its impact-resistance and clarity after forming. But PET-G, like polycarbonate, can be over-heated during the softening phase of warming and become brittle. Care must be taken not to allow PET-G to become too hot before forming. For the hope vac-former this can only be achieved through experience and experimenting.

PET-G is available in full 48"x96" sheets with the same "over-size" warning as styrene. With the addition of the protective film, PET-G is quite a handful to maneuver in tight spaces and is much heavier than similar, nominal thicknesses of material. It is highly recommended that cuts to the material be hade by the supplier.

PET-G responds well to solvent-type glues.