Annealing Delrin and Acetal: Stress Relief for Precision Machined Parts

Acetal is typically annealed by one of two methods. Air annealing heats the part in an oven near 320F (just below melt) and holds it, then cools it slowly, the standard for general stress relief. Oil or inert-liquid annealing immerses the part in a heated medium, which gives more uniform heat transfer and avoids oxidation and surface degradation on thick or critical parts, though it adds cleanup. Either way the controlling parameters are temperature, soak time (roughly 30 minutes per quarter-inch of thickness is a common starting point), and a slow cool-down to avoid re-introducing thermal stress. The ramp matters as much as the soak. Cooling acetal too fast creates a temperature gradient between surface and core that locks in fresh stress, defeating the purpose, so parts are cooled slowly, often left in the oven to cool with the heat off. Over-annealing or overheating risks surface degradation and, near the melt point, loss of dimensional control, so the temperature window is fairly tight. The buyer guidance is to specify annealing for any precision acetal part and to expect a controlled slow cycle. As with the metals in this catalog, the order of operations is rough machine, anneal to relieve stress, finish machine after stabilization, especially for tight-tolerance gears and sealing components.

Delrin 150 is a general-purpose acetal homopolymer grade, homopolymer acetal has the highest stiffness, strength, and crystallinity in the family, making it the choice for precision mechanical parts, but it has a characteristic of potential centerline porosity in larger extruded shapes and is slightly more sensitive to high-temperature degradation. Acetal copolymer (such as Delrin's competitor grades) trades a little stiffness for better long-term thermal stability, better resistance to hot water and chemicals, and freedom from centerline porosity, which is why copolymer is often chosen for parts exposed to hot water, hot air, or aggressive media. Thermally, both anneal the same way for stress relief, but copolymer's broader processing window and better thermal stability make it a bit more forgiving during annealing and in elevated-temperature service. Homopolymer's higher crystallinity gives slightly better mechanical properties and wear, but the centerline-porosity tendency means thick homopolymer parts should be inspected and may benefit from careful stock selection. For buyers, the choice between homopolymer (maximum stiffness and wear, precision mechanical parts) and copolymer (better chemical and hot-water resistance, no centerline porosity) is a service-environment decision. The annealing approach is essentially the same, stress relief with a slow controlled cycle, and neither is heat treated for hardness.