Internal stress is invisible on a fresh part. It shows up weeks later as a warped housing, a cracked boss, or a lid that no longer fits. Two mechanisms create it: uneven cooling and frozen molecular orientation. Most warped parts suffer from both at once.
Buyers often blame the material. In practice the resin is rarely at fault. Stress comes from how the part was designed, where the gate sat, and how the mold was run. Each of these is fixable once you know which mechanism dominates.
Thermal Stress From Uneven Cooling
Plastic is a poor heat conductor. In a thick section, the surface touching the cold mold wall freezes fast. The core stays hot much longer. When the core finally cools and shrinks, it pulls the already-rigid skin inward. That tug-of-war locks stress into the part.
Rapid temperature swings make it worse. A thick boss molded against a thin wall sees the two regions freeze at very different rates. The thin wall is rigid while the boss core is still shrinking. Something has to give. Usually the part bends toward the hot side as it cools, or it cracks at the junction months later.
Annealing can relieve some of this stress. Heating the part after molding lets the molecules relax. It adds a process step and it can move dimensions. Better to prevent the stress than to bake it out afterward.
Frozen Orientation From Melt Flow
The second source is flow itself. Polymer chains start as random coils in the melt. Shear in the gate and runners stretches them into elongated shapes. Typical shear rates in injection molding run 1,000 to 5,000 per second. That is enough to distort the coils markedly.
Once past the gate, the stretched chains try to relax back to random coils. Cooling usually beats them to it. Mobility drops as temperature falls, and the orientation freezes in place along the flow direction. The frozen layer is thin, but its effect is real.
Frozen orientation weakens the part across the flow. Cracks propagate more easily in the flow direction because the stretched chains offer less resistance sideways. Stiffness rises along the flow for the same reason. The part becomes anisotropic: stronger one way, weaker the other.
Heat exposes frozen orientation fast. Warm a stressed part and the molecules finally relax. The part warps, sometimes grossly. A housing that was flat at 23°C can bow several millimeters at 80°C. If your part sees heat in service, frozen orientation is not a cosmetic issue.
Glass Fibers Multiply the Problem
Short glass fibers, typically 0.3 to 0.5 mm long, point along the flow just like the polymer chains. They lock the anisotropy in place. An unfilled resin shows modest directional differences. A 30% glass-filled grade can shrink two to three times more across the flow than along it.
Mineral fillers such as talc add some anisotropy too. Their plate-like particles orient in the flow, but the effect is smaller than with fibers. When a glass-filled part warps and its unfilled twin does not, fiber orientation is the first suspect.
How to Tell Which Stress Dominates
Cut the part and watch. Thermal stress usually bends thick sections toward the last area to cool. Orientation stress warps flat, thin parts along the flow direction. A part that is flat when molded but warps after a hot day in a car points to frozen orientation.
Another test is the solvent check. Highly stressed regions craze first when wiped with an aggressive solvent. If crazing clusters around the gate, orientation dominates. If it spreads across thick-to-thin junctions, thermal stress dominates. Our cracking and delamination guide covers the solvent test in detail.
Design Fixes That Cut Stress at the Source
Uniform wall thickness is the single best defense. Equal walls cool at equal rates. The thermal stress mechanism nearly disappears when the thickest and thinnest sections differ by less than 25%.
Generous flow channels come next. Wide runners and gates keep shear rates down. Low shear means less chain stretching and less frozen orientation. Thin gates look tidy but they work-harden the melt. Size the gate for the part, not for the degating convenience.
Gate placement matters as much as gate size. A gate that fills the part symmetrically gives symmetric orientation. A gate at one end of a long flat part gives maximum directional difference. For flat parts, center gating or multi-gate balanced filling cuts warpage more than any process tweak.
Process Fixes That Relax the Part
Raise the mold temperature. Thermoplastic molds typically run 10 to 120°C depending on the resin. Running at the high end of the range slows cooling, gives molecules time to relax, and cuts frozen orientation. The trade is longer cycle time, since cooling capacity sets the pace of the whole cycle.
Extend packing time past the gate seal point. Packing feeds extra melt into the cavity while the gate is still open. It compensates for shrinkage and evens out density. Packing pressure that is too low leaves the far end of the part starved. Packing time that is too short does the same. Both are direct causes of warp.
Cool the part evenly. Water circuits should follow the melt path, entering near the gate where the melt arrives hottest. A mold half that runs 15°C hotter than the other will bow every part toward the hot side. Check both halves with an infrared gun during the trial, not just the controller display.
Do not eject hot. A part pulled before it can support its own weight distorts under ejector pins and gravity. Longer cooling costs seconds per cycle. Scrapped warped parts cost far more. If cycle pressure is high, improve cooling channels instead of cutting cooling time.
Prevention Checklist
- Wall thickness uniform within 25% across the part.
- Gates sized generously; shear kept low.
- Gate position gives symmetric fill; weld lines out of stress zones per our weld line guide.
- Mold temperature at the high end of the resin range where warpage is a risk.
- Packing time extends past gate seal; packing pressure verified on the far end of fill.
- Cooling circuits balanced; both mold halves within a few degrees.
- Part fully rigid before ejection; cooling time set by part temperature, not by habit.
- Glass-filled grades flagged for orientation review; fill pattern simulated before steel is cut. Cross-check sink risk with our sink marks guide, since the same thick sections cause both defects.
