Delamination is a part surface that separates into thin layers you can lift with a fingernail or a knife, much like mica. The leading cause is contamination with a second resin that does not mix with the first. Start with what else has been through the barrel, the hopper, the dryer and the grinder. Excess mold release, moisture and high shear at the gate come after that.
Also described as: peeling, flaking, layers separating, skin lifting, mica-like surface.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| The skin peels in thin sheets over most of the part. It started after a material change. | Resin from the previous job is still in the barrel, hopper, loader or dryer, and it does not mix with the resin you are running. | What ran before; the purge; the hopper, loader and hoses |
| Peeling comes and goes. It follows the regrind. | The regrind contains a foreign resin, from a shared grinder or a mixed scrap bin. | A run on virgin resin only |
| Peeling started with a new color or a new concentrate. | The carrier resin in the color concentrate does not mix with the base resin, or the let-down ratio is too high. | The carrier resin named by the color supplier; the let-down ratio; a short run in natural |
| Peeling only near the gate. The rest of the part is sound. | High shear at the gate is separating the skin from the core: fast fill, a small gate, or a cold melt or mold. | Injection speed; melt and mold temperature; gate size |
| Layers along with silver streaks or small blisters. | Moisture. Steam reaches the melt front and is trapped between the skin and the core. | Pellet moisture at the feed throat; dryer temperature and dew point |
| The surface flakes or feels greasy. Paint or labels do not stick. Mold release is being sprayed. | Mold release is building up on the steel and being folded into the surface of the part. | Stop spraying, clean the cavity and run 20 shots |
| Peeling started with a new lot, on virgin resin with no color. | The lot is cross-contaminated, or it is not the same material as the last one. | COA of the good lot against the bad lot; a retained sample; a close look at the pellets |
Checks in order, cheapest first
- Find out where it peels. Pick at the surface with a knife on several parts. Peeling only at the gate points to shear. Peeling everywhere points to contamination.
- Check the moisture. Test pellets taken at the feed throat, not at the dryer outlet. Confirm the dryer is at temperature and the hopper holds enough material for the full drying time.
- Raise melt and mold temperature and slow the fill. A hotter melt and mold let the layers knit together. Slower injection lowers the shear at the gate. Stay inside the supplier’s range.
- Stop the mold release spray. Clean the cavity and run without it. If the part now sticks, treat that as its own problem. See parts sticking in the mold.
- Purge, clean the feed path and run virgin natural resin. Empty and wipe the hopper, loader, hoses and dryer, purge the barrel fully, then run with no regrind and no color. If the peeling stops, add regrind and color back one at a time. See purging and color changes.
- Compare lots and check the color carrier. Compare the COAs. Look at the pellets for mixed shapes, sizes or shades. Unfilled polyethylene and polypropylene pellets float in water, and most other molding resins sink (filled grades can sink too), which makes a quick check for one common contaminant. Ask the color supplier which carrier resin the concentrate uses.
- Look at the tool. A small gate or a sharp corner just inside the gate raises shear. Opening the gate or adding a radius can remove peeling that is limited to the gate area.
- Change the resin last. Delamination is usually contamination, not a fault in the resin itself. If a clean system on virgin resin still peels, the grade and the lot are the next places to look.
The numbers
Moisture is one of the causes you can rule out with a measurement. Typical drying conditions and moisture targets for the resins that need drying and are often involved:
| Resin | Drying temperature | Drying time | Target moisture |
|---|---|---|---|
| ABS | 176–203°F / 80–95°C | 2–4 h | 0.1% general; 0.02–0.05% for plating |
| Polycarbonate | 248–250°F / 120–121°C | 2–4 h | ≤0.02% |
| PC/ABS | 212–230°F / 100–110°C | 2–4 h; 4–6 h for regrind or filled grades | 0.02–0.04% |
| Nylon 6, unfilled | 176°F / 80°C | 2–6 h | 0.03–0.12% |
| Nylon 66, unfilled | 175–180°F / 80–82°C | 2–4 h from a fresh bag | 0.03–0.2% |
| PBT | 250°F / 121°C typical | 2–4 h | 0.02% |
| PET, injection grade | 250–275°F / 120–135°C | 2–4 h virgin; 4–6 h regrind | <0.02% (200 ppm) |
| Acrylic (PMMA) | 158–208°F / 70–98°C | 2–4 h | <0.02% strict; 0.05–0.1% non-critical |
Polyethylene and polypropylene are not hygroscopic and normally need no drying.
Typical values from published drying guides. Targets differ between producers and grades, so confirm against the data sheet for your grade. The full list is in the resin drying chart.
Why parts delaminate
- Most plastics do not mix with each other. A small amount of a foreign resin stays as separate droplets in the melt. As the melt flows along the cold wall, those droplets are stretched into thin sheets, and there is little or no bond across them. The classic case is polyethylene or polypropylene in ABS, polycarbonate, nylon or polystyrene.
- It does not take much. A few pellets left in a loader hose, a shared grinder, or a color concentrate on the wrong carrier can be enough to make a show surface peel.
- Shear makes it worse. The skin of a molded part is highly oriented. A cold melt pushed fast through a small gate can split the skin from the core, most often in blends and impact-modified grades, which already contain two phases.
- Moisture and mold release act as a barrier between layers. Steam or a film of release on the steel keeps one layer of melt from bonding to the next.
- The part is weaker than it looks. A delaminated part can pass a visual check and still fail in impact or under load, because the layers do not share the stress.
Safety. Acetal and PVC must never meet in a hot barrel. Each speeds the breakdown of the other and the mix releases irritating, corrosive gas. Purge fully with polyethylene or polystyrene between them.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A color concentrate on a carrier matched to the base resin, or a pre-colored compound | Removes the foreign carrier resin that causes peeling in colored parts. | A separate concentrate for each resin family, so more items to stock. A pre-colored compound usually costs more and comes with larger minimums. |
| Less regrind, or regrind kept separate by resin and job | Lowers the chance that a foreign resin gets into the feed. | More virgin resin, more scrap to store or sell, and more labor at the grinder. |
| A compatibilized grade for mixed or recycled streams | A compatibilizer helps two resins that do not mix bond to each other. | A higher price, properties that differ from the virgin grade, and a new grade for your customer to approve. It works for a known pair of resins, not for random contamination. |
| A grade with an internal mold release | Lets you stop spraying release on the tool. | It can interfere with painting, printing, plating and adhesive bonding. A new grade for your customer to approve. |
| Recycled content from a better-sorted source | Recycled resin with a foreign polymer in it is a common source of delamination. | Usually a higher price and fewer sources. Ask what testing comes with each lot. |
Any resin change moves your dimensions. The tool was cut for the shrinkage of the resin you run now. A resin that shrinks less makes a bigger part from the same steel, so check critical dimensions before you approve a swap.
Often confused with
- Splay. Silver streaks on the surface that follow the flow. The surface does not lift when you pick at it. See splay and silver streaks.
- Blisters. A raised bump with gas under it, in one spot. Delamination is flat layers over an area. See voids and bubbles.
- Gate blush and flow marks. A dull patch or ripples near the gate that cannot be peeled. See flow lines and flow marks.
- Coating failure. Paint or plating that peels and leaves sound plastic underneath is an adhesion problem with the coating. Mold release on the surface is a frequent cause of both.
Quick answers
Why is the surface of my part peeling?
A peeling surface is most often contamination with a second resin that does not mix with the first. Start with what else has been through the barrel, hopper, dryer and grinder. Excess mold release, moisture and high shear at the gate come after that. Peeling only at the gate points to shear, and peeling everywhere points to contamination.
What causes delamination in injection molding?
The leading cause is a small amount of foreign resin, which stays as separate droplets in the melt and is stretched into thin sheets with little or no bond across them. Sources include the previous job, regrind from a shared grinder, and a color concentrate on the wrong carrier. Moisture, mold release and gate shear also cause it.
Why does my part flake apart in layers?
Layers form when something keeps one layer of melt from bonding to the next. Usually that is a foreign resin, and sometimes steam from moisture or a film of mold release on the steel. Purge, clean the feed path and run virgin natural resin alone. If the peeling stops, add regrind and color back one at a time.
Is delamination caused by the resin or the process?
Delamination is usually contamination, not a fault in the resin itself. If a clean system on virgin resin still peels, the grade and the lot are the next places to look. Compare the COAs and look at the pellets for mixed shapes, sizes or shades. Polyethylene and polypropylene pellets float in water, and most other molding resins sink.
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