Casting Delrin and Acetal: The Term Is Wrong, Here's the Real Process

Acetal is a semi-crystalline engineering thermoplastic prized for stiffness, dimensional stability, low friction, and excellent machinability, the default material for gears, bearings, bushings, and precision mechanical parts. But its processing window is narrow and unforgiving. POM melts around 165 to 175 C and must be processed not far above that, and if it is held too hot or too long it depolymerizes, unzipping back toward formaldehyde monomer, which both degrades the part and releases toxic, flammable formaldehyde gas. Injection molders manage this with tight barrel-temperature control, purpose-built screws, and minimal residence time. That thermal sensitivity is one reason a gravity-cast process makes no sense for acetal: you cannot hold it molten in an open pot the way you would a casting metal or a thermoset resin without it degrading and off-gassing. Acetal is also a thermoplastic, not a curing thermoset, so unlike epoxy or polyurethane casting resins it does not exist as a liquid that polymerizes in a mold; it is supplied as solid pellets or stock that you melt or machine. So 'casting Delrin' is a category error, and the productive move is to translate the request into the real processes. Acetal parts are injection molded for volume or machined from extruded rod, plate, and tube for prototypes, low volume, and precision. The 'cast plastic' intuition again comes from thermoset casting resins, which acetal is not. Everything below is about molding and machining, the two routes that actually produce acetal parts.

Injection molding is the production route for acetal and the reason POM is everywhere in consumer and automotive products. Pellets melt in a temperature-controlled barrel and inject into a steel mold, producing net-shape parts, gears, clips, fasteners, fittings, conveyor components, at high volume and low unit cost with excellent repeatability. Acetal molds well and holds tight tolerances, but it shrinks significantly and somewhat anisotropically on cooling (mold shrinkage around 1.8 to 2.5 percent for homopolymer), which the tool design must compensate for, and thick sections can develop voids and sink marks because of that shrinkage. Tooling runs $10,000 to $80,000 depending on complexity. Machining from extruded or cast-shape stock is the route for prototypes, low-to-moderate volumes, large parts, and very tight tolerances. Acetal is one of the most machinable plastics, often cited as the benchmark for plastic machinability, it cuts cleanly with sharp tools, produces well-broken chips, and holds dimensions well, so it is a favorite for precision turned and milled mechanical parts. Stock comes as rod, plate, and tube; for the best dimensional stability, stress-relieved (annealed) stock is used because acetal can move as molded-in or extrusion stresses relax. Note on terminology: extruded acetal rod, especially larger diameters, is sometimes loosely called 'cast' or made by a continuous-casting-like extrusion process, which may be where some 'cast Delrin' searches originate. That stock is then machined; the part itself is not cast. The decision is the standard plastics one: high volume and net shape favors injection molding; prototypes, low volume, large parts, or precision tolerances favor machining from stock. Many mechanical acetal parts, especially low-volume gears and bushings, are machined precisely because the tolerances and small quantities do not justify a mold.

The real decision in acetal is not the (nonexistent) casting method but homopolymer versus copolymer, because they differ in ways that matter for real parts. Acetal homopolymer (Delrin is the well-known homopolymer) has slightly higher strength, stiffness, hardness, and fatigue resistance, and better creep performance, making it the choice for highly loaded mechanical parts, precision gears, and components where maximum mechanical performance is needed. Its one well-known weakness is a tendency toward centerline porosity in larger extruded rod (a low-density core), which matters if you machine a part that exposes the core, and reduced resistance to hot water and strong alkalis compared with copolymer. Acetal copolymer (such as the POM-C grades) trades a little strength for better chemical resistance, especially to hot water, hydrolysis, and alkalis, more uniform extruded stock without the centerline porosity issue, better thermal stability in processing, and lower tendency to depolymerize. It is the choice for parts exposed to hot water, steam, or aggressive chemicals, food and water contact applications, and large machined parts where the homopolymer's centerline porosity would be a problem. Delrin 150 is a specific homopolymer grade, a general-purpose, medium-viscosity acetal homopolymer used broadly for machined and molded mechanical parts. The buyer's practical guidance: choose homopolymer (Delrin) for maximum strength, stiffness, and fatigue life in dry mechanical service, and choose copolymer for chemical and hot-water resistance, dimensional consistency in large machined parts, and food or water contact. Both machine and mold well; the choice is about the service environment and whether you are machining large cross-sections. As with all acetal, neither is cast, they are molded or machined, and the grade choice plus proper stress relief is what determines whether your part holds tolerance and survives its environment.