Choosing the Right Plastic

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When 3D-printing a firearm, picking out the right plastic is important. Not all plastics are created equal, and some may fail in a way that could harm the user if used in the wrong application.

This guide will go over some considerations and commonly-used types of plastics that tend to meet the needs of firearm design.

Considerations

The following are key reasons one may choose any given plastic over another:

Impact Resistance

The ability for a plastic to withstand an impact without breaking or yielding is called its impact resistance, and it's one of the most important characteristics for most parts of a firearm. Several functions rely on high impact resistance, most of them things that happen while the firearm cycles.

Heat Resistance

Each plastic has what's called a "glass transition temperature" (Tg) which is the temperature at which it starts to lose its crystalline structure and begins to flow and warp. If, say, your receiver hits Tg while firing, your gun may start to meltdown and malfunction. Some plastics have glass transition temperatures low enough that they cannot withstand the open sun in certain regions' summers.

In general: the higher the printing temperature, the higher the continuous round count.

Rigidity

Plastics are floppy, generally. The more plastic in one place, the more it can support itself, but some plastics are less floppy than others.

If a plastic is particularly flexible but has enough desirable qualities in other areas -- as is the case in most Nylons -- then carbon fiber/glass fiber infill may be added. This increases rigidity and oftentimes crystallinity, but at the cost of layer adhesion. How exactly these infills behave is dependent on the plastic at hand.

Hygroscopy

The tendency for a plastic to absorb water from its environment is referred to as its hygroscopy. It is a slow, gradual process that is never driven to completion but instead hits an equilibrium with the ambient humidity in the environment. Plastics that have absorbed water often exhibit lower rigidity and higher impact resistance.

Filaments that are particularly hygroscopic should be dried before printing and may even need to be printed from a specialized dry box so as to not reabsorb moisture over the duration of a print. When melting a plastic that has absorbed water, the water will boil to steam at the hotend and introduce voids in the print, ruining it.

Creep

Given constant pressure (such as that imposed by a spring), some plastics will permanently warp out of place over long periods of time. This phenomenon is known as creep. Creep can be mitigated for via fillings and through the use of clever part design, such as the use of reinforcement plates on an AR-15-compatible fire control group.

Failure Mode

The way in which a plastic fails is an important consideration when risk is high, such as in the case of a printed upper-receiver. Plastics tend to fail in two ways:

• Breaking, in which the plastic shatters violently; or
• Yielding, in which the plastic stretches and taffies out

Yielding is often more desirable as it tends to constitute less of an operational risk to the wielder.

Common Plastics

Below is a table of often-used plastics

Good Plastics

Plastic	Pros	Cons	Notes
PLA+	Does not warp very often Cheap and accessible Does not emit noxious fumes Does not require CF/GF infill Yields instead of breaking	Extremely low temperature resistance Can sometimes be confused with PLA-CF and Silks, which are not suitable	The go-to basic plastic and what most DIY firearm novices cut their teeth on.
PA-6	Very high temperature resistance Pleasing surface texture that hides layer artifacts	High print temperature required Most blends use CF/GF infill, which requires hardened steel nozzles Must be dried before and during printing	A tried-and-true durable filament and oftentimes the plastic of choice for experts.
PA-12	High temperature resistance Pleasing surface texture Cheaper than PA-6 with most of the same benefits Less hygroscopic than PA-6	Lower temperature resistance than most other Nylons	PA-6's little brother shares a lot of its benefits with less headache

Bad Plastics

Plastic	Reasons to Avoid	Notes and Exceptions
PETG	Failure mode is breakage Low impact resistance	In cases where risk is low, such as on furniture, weaker plastics may be acceptable for their UV resistance, heat resistance, or price point.
PLA	Failure mode is breakage Low impact resistance Low temperature resistance
Any recycled filament	Recycled filaments, even of those listed in the "Good Plastics" table, are significantly weaker than virgin plastics.