Delrin and Acetal Injection Molding: Taming Shrinkage and Centerline Voids

Delrin is DuPont's brand name for acetal homopolymer, and the homopolymer-versus-copolymer distinction has real consequences for both molding and performance. Homopolymer acetal (Delrin) has higher crystallinity, giving it slightly greater strength, stiffness, and fatigue resistance, which makes it the choice for highly loaded gears, springs, and structural mechanical parts. Its downsides are a stronger tendency to form centerline voids in thick sections and somewhat lower resistance to hot water and certain chemicals. Acetal copolymer (such as Celcon or Hostaform POM-C) gives up a little stiffness in exchange for better long-term thermal stability, superior resistance to hot water, bases, and hydrolysis, and a lower tendency toward centerline porosity, which makes it more forgiving to mold in thick or varying wall sections. Delrin 150 specifically is a medium-viscosity general-purpose homopolymer grade common for injection molding mechanical parts. The practical rule: choose homopolymer when you need peak mechanical performance and fatigue life, and choose copolymer when chemical and thermal robustness or easier thick-section molding matters more.

Acetal shrinks a lot, typically 1.8-2.5% mold shrinkage, which is much higher than amorphous plastics like ABS (around 0.4-0.7%). This high shrinkage comes from acetal's high crystallinity: as the polymer crystallizes and solidifies it densifies significantly. The good news is that the shrinkage is consistent and predictable, so toolmakers design it directly into the cavity dimensions. The complications are that shrinkage is somewhat anisotropic (slightly different along the flow direction versus across it) and sensitive to wall thickness and packing pressure, so hitting tight tolerances requires the molder to use grade-specific shrinkage data and good process control. The high shrinkage also drives acetal's signature defect, centerline voiding in thick sections, because the material can pull a vacuum void at the core if packing cannot feed enough melt as it shrinks. With proper wall design and packing, molded acetal still holds tight tolerances around ±0.05 mm on small features, which is why it is a premier precision-part material despite the high shrinkage.

Centerline voids in acetal come from its high crystallization shrinkage combined with thick part sections. As acetal cools and solidifies from the mold walls inward, the still-molten core continues to shrink; if packing pressure cannot feed enough additional melt into that core before the outer skin freezes off, the core pulls a vacuum void. Thick sections are most vulnerable because they take longer to solidify and require more feeding. Prevention works on two fronts. In design, keep walls uniform and moderate in thickness, core out heavy sections to eliminate thick masses, and position generous gates so they can feed the thickest areas directly while they are still molten. In processing, apply adequate hold (packing) pressure and time, and an appropriate melt temperature so the material stays feedable. An experienced acetal molder will flag thick-section void risks during the design review, before the tool is cut, rather than discovering them in first articles, so involving the molder early in part design is the single best defense against acetal voids.

Machine Delrin or acetal copolymer instead of molding it for prototypes, low-to-mid volumes, large or thick parts prone to molding voids, and one-offs. Acetal machines exceptionally well, it cuts cleanly at high speeds, produces excellent surface finish, and is dimensionally stable, and rod, plate, and tube stock are readily available off the shelf, so you can have finished parts without waiting for or paying for a tool. The economic crossover is driven by volume and the cost of the injection mold (typically $10,000-$60,000): once you need thousands of identical precision parts, a multi-cavity acetal tool wins decisively, producing parts at a small fraction of machining cost with no secondary operations, which is why molded acetal gears and bearings dominate high-volume mechanical applications. Below that volume, or for geometry with thick sections that would void in molding, machining from Delrin or acetal copolymer stock is the honest and usually cheaper route. A common path is to machine acetal for prototypes and initial production, then move to a molding tool once volume is proven and the design is frozen.