Cracking, delamination, and distortion look like three unrelated defects. They share one root family: stress the molding cannot carry. Cracking needs stress plus an aggressive environment. Delamination needs stress plus a weak layer. Distortion needs stress plus premature handling. Remove the stress and all three fade.
Buyers sometimes treat these as material failures and switch resins. That rarely helps. The same resin molds cleanly when the stress is gone. Fix the stress first, then judge the material.
Cracking and Crazing Need Two Ingredients
A stress-free part resists chemicals well. A stressed part can crack in surprisingly mild environments. Low ozone levels in ordinary air attack stressed plastics. Plain petroleum jelly crazes stressed surfaces. Neither seems aggressive until the stress is factored in.
This combination has a name: environmental stress cracking. The stress opens microscopic paths. The chemical wicks in and breaks the polymer chains. The crack grows slowly, then all at once. Parts often pass initial inspection and fail in the customer’s hands.
The only reliable cure is a low-stress molding. Raise mold temperature to let orientation relax. Pack the part fully so density is even. Cool it uniformly so no region fights its neighbor. A part with minimal frozen stress shrugs off the same chemicals that destroy its stressed twin. Our internal stress guide details both stress mechanisms.
Delamination Means Layers That Never Bonded
Delamination looks like onion skin peeling off the part surface. The classic case is a washing-up bowl: liquid detergent poured into the center, then hot water added. The concentrated detergent, the sudden heat, and the frozen stress in the bowl base combine. The surface peels in sheets.
Three shop-floor causes cover most cases. The first is contamination. Mixing polythene with polystyrene, or any two incompatible resins, gives layers that refuse to bond. Purging between material changes must be thorough. A few grams of the wrong resin in the barrel is enough to delaminate a whole run.
The second is excessive mold release agent. Silicone-based releases migrate into the plastic and weaken layer bonding. They should be used sparingly or not at all. Polythene molding should never see silicone release. A well-designed mold with correct draft and polish needs no release agent. If parts stick, fix the mold or the process instead of spraying more release.
The third is cold melt meeting cold mold. Low barrel temperature or a cold mold freezes the flow front before layers can knit. The skin forms over a melt that never fused to it. Raising melt temperature and mold temperature together usually resolves it.
Distortion Is Not Warping
Distortion means the part never had the right shape. Warping means it had the right shape and lost it. The distinction points at different causes.
The most common cause of distortion is early ejection. A part pulled while still soft cannot hold its own shape. Ejector pins stretch the soft plastic. A thick section sags under its own weight. The part cools into whatever bent shape it landed in.
Handling damage right after ejection does the same. Parts tossed into a bin while warm take a set from the parts piled on top. A warm part stacked under weight is a distorted part by morning.
Jig cooling can hold shape, but it treats the symptom. The real fix is a cooling time long enough for the part to support handling. Cooling capacity, not the plasticizing unit, sets the cycle pace on most molds. Spend the seconds. The alternative is sorting bent parts.
How to Diagnose Each One
Start with where the failure sits. Cracks that start at the gate or at a weld line point to frozen orientation. Cracks at thick-to-thin junctions point to thermal stress. Either way, the stress map tells you where to act.
For delamination, try the peel test. If a layer lifts with finger pressure, bonding failed during molding. Check the purge log and the release agent usage first. Both are cheaper to fix than re-cutting steel.
For distortion, check flatness straight out of the mold versus after 24 hours. A part that is true at ejection and bent the next day warped. A part that is bent at ejection was distorted. The first needs stress work. The second needs cooling time or gentler handling.
Fixes That Work Across All Three
Cut frozen stress with hotter molds and fuller packing. Thermoplastic molds run 10 to 120°C. Moving toward the top of the resin’s range relaxes orientation. Packing past the gate seal point evens out density and kills the weak zones where cracks start.
Keep materials pure. Dedicate barrels and dryers where possible. Purge fully on every changeover. Ban silicone release agents from the molding floor unless a specific part cannot run without them.
Respect cooling time. Cavity pressures run 300 to 1,200 bar during filling, but pressure means nothing if the part leaves the mold half-molten. Set cooling by measured part temperature, not by the cycle time the last job used.
Design out the stress risers. Sharp internal corners concentrate stress the same way a notch concentrates it in metal. A 0.5 mm fillet at a boss root costs nothing in the mold and buys real crack resistance. Keep weld lines away from load paths; their strength can drop to 20% of the base resin. See our weld line strength data before placing gates.
Prevention Checklist
- Mold temperature high enough to relax orientation for the resin in use.
- Packing time past gate seal; density even from gate to far end.
- No material contamination; full purge on every changeover.
- Mold release agents minimized; silicone banned unless specified.
- Cooling time set by part rigidity, not by habit; handling fixtures for warm parts.
- Fillets on all internal corners; weld lines out of stress zones.
- Chemical exposure in service reviewed against the resin’s stress-crack resistance.
- Suspect parts checked with the solvent wipe test before shipment; full diagnostic flow in our defect root cause guide.
