Delamination in Injection Molded Parts: Buyer Review Before Approval

Delamination points to material contamination or preparation failure more than simple process drift. When a part sheds thin layers, flakes near a gate, or peels under impact, the surface skin failed to bond with the material underneath. That compromises strength. Treat it as a structural issue, not a waive-able cosmetic mark. Verify root cause before you approve the lot.

Identifying Delamination

Delamination looks like a flaky, layered surface you can lift with a fingernail or peel when the part flexes. It often resembles fish scales, especially near the gate or sharp transitions. Sometimes layers only separate at an impact point or fracture line instead of showing a solid cross-section.

Delamination can stay hidden until the part sees mechanical stress. A bench sample that passes visual inspection can delaminate in assembly or the field. Unlike splay (silver streaks), which stays on the surface, delamination is physical separation inside the wall thickness.

Why Delamination Blocks Approval

An unbonded wall cannot carry load. Even if the part looks acceptable, it will likely fail drop, snap-fit, or fatigue testing when layers shear. Treat delamination as structural failure. Require the supplier to name the mechanism (cross-contamination, excess moisture, incompatible regrind, or release agent) and prove corrected parts are fully bonded through the cross-section.

Root Causes to Verify

Incompatible material contamination: Mixing resins that do not chemically bond, such as polypropylene (PP) into polycarbonate (PC), creates layers that will not fuse. This is the most common cause. It often traces to incomplete purging, a shared hopper or dryer, or contaminated regrind.

Excess moisture: Hygroscopic resins must be dried properly. Wet material in the barrel flashes to steam at the melt front and creates a separated skin. Related to moisture-driven splay, but the result is physical layer separation, not only surface streaking.

Release agent or oil migrations: Excess mold release or leaking hydraulic oil on the cavity creates a boundary that stops the advancing melt from bonding to the cooling skin.

Foreign particles or film: A flake of degraded polymer, carbon, or unmelted material at the gate can seed a delamination layer. If dark inclusions ride along, see black specks.

High shear or local overheating: Aggressive gate geometry, or injecting too fast at too high a temperature, can degrade the polymer skin at the gate and destroy local bond.

Technical Questions for the Supplier

• Where exactly does the delamination occur, and is it isolated to specific cavities or shots?
• What precise grade is running? Was any regrind, masterbatch, or purge compound introduced into the barrel?
• Was the resin dried to the manufacturer’s specifications? Can you provide the dryer log with dew point and residence time?
• Was the machine, hopper, or dryer used for a different material family immediately before this run?
• What mold release is used, how heavily is it applied, and was the tool cleaned thoroughly before these samples?

Required Verification Evidence

Do not accept a generic “process adjusted” close. Demand records:

• High-resolution, marked-up photos of where layers separate. If available, cross-section or fracture-surface photos.
• Material lot record with exact regrind percentage and masterbatch let-down ratio.
• Drying log (temperature, time, dew point) for moisture-sensitive resins.
• Purge and changeover log for shared material-handling equipment.
• A corrective action plan that names the mechanism (contamination vs. shear) and how the fix was validated.

When to Hold Tool or Production Approval

Halt approval if:

• Delamination appears on a load-bearing or structural feature.
• The supplier cannot rule out cross-contamination from shared dryers, hoppers, or regrind streams.
• The supplier tries to call delamination at a break or impact point cosmetic.
• The proposed fix is only a process tweak, and the supplier cannot explain why layers failed to bond.

Securing Material Integrity in the RFQ

State material purity requirements in the RFQ. Define whether regrind is allowed and set a hard maximum percentage. Ask for changeover and purge validation between resin families. For structural parts, specify a mechanical acceptance test (drop, flex, or impact) instead of visual inspection alone. Delamination often hides under the surface.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, inspect tooling, or certify suppliers. Delamination causes are material- and process-specific. Verify root causes through the supplier’s material records and require structural evidence that the correction worked.