Last updated: 2026-09-05 · 5 min read
Delamination in Injection Molding — Why Do Thin Layers Separate Inside the Part, What Resin and Process Conditions Cause It and How Do You Fix Each One?
Delamination is the separation of thin layers inside the molded part — the material splits along planes parallel to the surface, creating internal voids or surface blisters. The cause: the melt does not bond to itself during filling. Common triggers include contaminated resin (mixed lot, regrind with foreign material), excessive moisture in hygroscopic resins, and too-low melt temperature. Fix by drying the resin, checking for contamination, and raising melt temperature 10–20°C.
Key Facts
- Delamination appears as thin, flat separations parallel to the part surface — like layers of plywood peeling apart. On the surface, it shows as blisters or raised areas that sound hollow when tapped. Inside, the delamination surfaces are smooth (not rough like a fracture) because the layers never bonded. The defect is most common in PC, ABS and nylon — resins that are sensitive to moisture or contamination.
- Moisture is the number one cause for hygroscopic resins (PC, nylon, PET, PBT). Water absorbed by the resin turns to steam at molding temperature — PC at 280°C generates steam pressure of 50–100 bar inside the melt. The steam creates micro-voids between layers, preventing bonding. For PC: moisture must be below 0.02%. For nylon: below 0.1%. If the dryer is not achieving these levels, the part will delaminate regardless of other settings.
- Contamination from regrind or mixed resin lots is the second most common cause. If regrind from a different resin (e.g., ABS mixed into PC) enters the melt, the two resins do not bond — they form separate layers that split. Even 2–5% contamination from a different resin family causes delamination. Check the regrind stream: same resin, same color masterbatch, same supplier? If regrind from multiple sources is blended, contamination risk is high.
How Do You Distinguish Delamination from Other Internal Defects?
| Defect | Appearance | Location | Cause | Fix |
|---|---|---|---|---|
| Delamination | Flat layers, hollow sound | Parallel to surface | Moisture, contamination | Dry resin, check regrind |
| Void | Round bubble, single cavity | Center of thick section | Packing too low | Increase packing pressure |
| Burn mark | Black/brown discoloration | At flow end, traps | Trapped gas | Add vents |
| Splay | Silver streaks on surface | Random, along flow | Moisture or shear | Dry resin, reduce speed |
Delamination is distinguished from voids by shape and location. Voids are round bubbles in the center of thick sections — caused by insufficient packing pressure. Delamination is flat, layer-like separation parallel to the surface — caused by moisture or contamination. If you cut the part open and the separation surface is smooth and flat, it is delamination. If the void is a round hole with no layer structure, it is a void.
Desiccant dryer — a dryer that uses a desiccant material (silica gel or molecular sieve) to create a dew point below -30°C, pulling moisture out of hygroscopic resin pellets. Unlike hopper dryers (which only warm the air), a desiccant dryer achieves the 0.02% moisture target required for PC, PET and PBT. The hygroscopic resin drying guide covers dew point targets and drying times for each resin family.
What Three Fixes Eliminate Delamination?
- Dry the resin to the specified moisture level — this is the first step for hygroscopic resins. For PC: dry at 120°C for 4 hours to reach 0.02%. For nylon: 80°C for 4 hours to 0.1%. For PET/PBT: 130°C for 3 hours to 0.02%. Use a desiccant dryer with dew point below -30°C. Check moisture with an analyzer before molding — if above spec, extend drying by 1–2 hours. The short shots guide shows how moisture also causes flow issues beyond delamination.
- Eliminate regrind contamination — segregate regrind by resin type and color. If the regrind stream contains mixed resins (e.g., ABS and PC in the same bin), contamination causes delamination. Segregate regrind by resin family: PC regrind only into PC parts, ABS regrind only into ABS. If regrind ratio is above 20%, reduce to 10–15% — high ratios increase contamination risk from degraded material. For critical parts (optical, structural), use zero regrind until delamination is resolved.
- Raise the melt temperature — this improves layer bonding by keeping the melt fluid longer. If the melt temperature is too low, the layers of melt do not fuse during filling — they stack without bonding. Raising melt temperature by 10–20°C increases the melt’s ability to self-bond. For PC: raise from 280°C to 300°C. For ABS: raise from 220°C to 240°C. Do not exceed the resin manufacturer’s maximum — above that, the resin degrades and creates gas that causes the same delamination you are trying to fix.
Regrind contamination — the presence of foreign resin in the regrind stream. Even 2–5% of a different resin family (e.g., ABS in PC regrind) causes delamination because the two resins do not bond — they form separate layers that split. Prevention: segregate regrind by resin family and color. The material cost per part guide covers how regrind ratio affects cost and quality.
Delamination: layers separate parallel to surface. Cause: moisture (PC <0.02%, nylon <0.1%), contamination (mixed regrind), or low melt temp. Fix: dry resin, segregate regrind, raise melt temp +10–20°C. Distinguished from voids: flat layers vs. round bubbles.
- Moisture below spec before molding — PC <0.02%, nylon <0.1%, PET/PBT <0.02%. Measure, don’t assume.
- Regrind segregated by resin family — mixed regrind is the #2 cause of delamination.
- Melt temperature raised 10–20°C — improves layer bonding; do not exceed resin max.
- Mold temperature uniformity checked — cold spots (30°C delta) prevent layer fusion.
- Delamination vs. void confirmed — flat layers = delamination; round bubble = void.
Frequently Asked Questions
Why does delamination appear days after molding, not immediately?
Residual moisture continues to migrate after molding. If the part has 0.05% moisture (above the 0.02% spec for PC), excess moisture is trapped inside. Over 24–72 hours, moisture migrates to layer interfaces and creates micro-voids that grow into visible delamination. The part looks fine at T1 but delaminates on the shelf. Fix: ensure resin is dried to spec — not “close” but actually below threshold. For PC: 0.02% maximum.
Can delamination be caused by the mold, not the resin or process?
Yes — if the mold has a cold spot (a cooling circuit that is too efficient or blocked), melt freezes against the cold surface before bonding with incoming melt. The cold surface creates a skin that does not fuse with subsequent layers. Check mold temperature uniformity — if one area is 30°C cooler than surrounding areas, the cold spot is the cause. Fix: balance the cooling circuit or raise overall mold temperature by 10°C.
Is delamination the same as layer lines from the molding process?
No. Layer lines (flow lines or knit lines) are visible on the surface but do not represent a structural separation — the material is continuous beneath the surface. Delamination is an actual separation — the layers can be pried apart with a knife or fingernail. If you can insert a thin blade into the defect and separate the layers, it is delamination. If the surface mark is only cosmetic and the material is solid beneath, it is a flow line. Delamination is a structural defect; flow lines are cosmetic.
Key Takeaways
- Delamination: layers separate parallel to surface. Hollow sound.
- Moisture #1 cause: PC <0.02%, nylon <0.1%. Dry to spec.
- Contamination #2: mixed regrind. Segregate by resin family.
- Fix: dry resin, segregate regrind, raise melt temp +10–20°C.
Send the resin lot certificate, the drying log and photos of the delamination — the troubleshooting review identifies whether the cause is moisture, contamination or mold temperature, and specifies the fix needed to eliminate the layer separation.
Sources: Plastics Technology — Delamination Diagnosis and Prevention · Plastics Technology Online — Moisture Effects in Engineering Resins · Society of Plastics Engineers (SPE) — Delamination and Layer Separation Technical Papers · BASF — Processing Guidelines for Engineering Resins · DuPont — Resin Drying and Moisture Control Handbook
— Arthur, Tooling Engineer at aCatMould