TPV Injection Molding Surface Defects: 10 Common Defects, 

Root Causes and Fixes

Every TPV molder knows the sequence: the trial shot is clean, production starts, and by the third shift the reject bin is filling with silver-streaked seals, flashed trim, sink marks behind ribs and yellowed parts. Surface defects in TPV injection molding are rarely one operator error — they are the visible output of three variables: material state, process balance and mold fit. Two can be fixed on the shop floor; the third can only be fixed in the granule you buy.

This guide comes from DOWREN NEW MATERIAL (CHANGZHOU) CO., LTD., a manufacturer of TPV granules supplying processors in more than 40 countries across Southeast Asia, the Middle East, South America and Africa. We make the pellets; our customers mold the seals and profiles. Below: each defect, its root cause, the correction that works, and the granule property that stops it returning.

Why Surface Defects Cost More Than Appearance

• Functional — voids cut sealing force and compression-set life; an unfused weld line cracks first in a flexing part.
• Assembly — flash on a sealing lip changes insertion force; drag marks hurt bonding and over-molding adhesion.
• Compliance — yellowing and black specks are the first thing a buyer's QA team photographs.
• Cost — sorted batches, re-inspection labor, rework, or a claim that eats the order's margin.

Surface defects are the cheapest quality signal you have: they appear before mechanical failure does.

The 10 Surface Defects: Causes and Fixes

TPV is a crosslinked rubber phase (typically EPDM) dispersed in a thermoplastic phase (typically PP) through dynamic vulcanization. That structure explains most defect behaviour: gas traps more easily than in single-phase resin, the PP phase carries a crystalline shrinkage signature, and melt strength sits between a rubber compound and a commodity plastic. Values below are typical starting points — confirm them on your machine, tool and grade.

1. Bubbles and Silver Streaks

Symptom: blisters, or white/silvery streaks with micro-voids beneath the skin.

Causes: moisture vaporising in the barrel, air carried in with the feed, blocked or shallow vents.

Fixes: dry 80–90 °C for 2–4 h (6 h above 60% humidity) to below 0.1% moisture · back pressure ~2 → 4–5 MPa · vents cleaned, 0.03–0.05 mm deep × 5–10 mm wide · gate speed ~50% → 20–30% · metering zone −5 to −10 °C.

Material side: sealed barrier packaging and low residual volatiles. Streaks appearing only in the second half of a container mean the drying schedule is fixing moisture a sealed bag would have kept out.

2. Sink Marks and Shrinkage

Symptom: shallow depressions at rib roots, bosses and wall-thickness transitions.

Causes: insufficient holding pressure or time; mold temperature too high; wall ratios above about 3:1.

Fixes: holding pressure at 60–70% of injection pressure · holding time ~10 → 15–20 s for walls over 5 mm · mold temperature ~60 → 40–50 °C · wall ratio below 2:1, rib thickness ½–⅔ of nominal wall.

Material side: shrinkage is predictable only if the material is — a drifting MFI shifts the shrinkage window and the molder chases settings instead of holding dimensions.

3. Flash and Overflow

Symptom: thin fins at the parting line, ejector-pin bores and insert gaps.

Causes: injection pressure above the clamp force the tool actually delivers; worn parting surfaces; nozzle temperature thinning the melt.

Fixes: thin wall ~150 → 120–130 MPa · thick wall ~100 → 80–90 MPa · clamp force at least projected area × injection pressure × 1.2 · parting-surface gap ≤ 0.02 mm · nozzle ~200 → 180–190 °C.

Material side: a grade that flows more easily than the tool was designed for flashes at an acceptable pressure. If flash appears after a supplier change at unchanged settings, the variable is MFI.

4. Scratches and Drag Marks

Symptom: fine linear marks in the demolding direction, or dull roughened bands.

Causes: cavity polish below spec; insufficient draft on a material that grips steel; uneven release agent; uneven ejection.

Fixes: polish to Ra ≤ 0.8 µm · draft 1° → 2–3° (5° on complex geometry) · neutral silicone release agent, sparingly · about one ejector pin per 10 cm².

Material side: persistent drag marks are often mold deposit, not mold finish — pellet fines, dust and excess lubricant migrate to hot steel and build a film over a few thousand shots.

5. Yellowing and Black Spots

Symptom: a yellow cast across the part, or discrete black specks through the surface.

Causes: barrel excursions above the material's ceiling (typically 220 °C); long residence time or unplanned stoppages; an antioxidant system insufficient for the thermal history; dust in the feed.

Fixes: metering zone 190–210 °C, avoiding sustained operation above 220 °C · on stops over 10 minutes drop the barrel to 150–160 °C · purge with low-melting PE or fresh TPV for 5–8 shots · sieve the feed and cover the hopper.

Material side: two pellets can match on hardness and tensile figures yet differ in heat-stability package — one survives a stopped machine, the other produces yellow parts for the next hour.

6. Flow Marks and Weld Lines

Symptom: flow marks are streaks aligned with the melt front; weld lines are witness marks where two fronts met without fusing.

Causes: injection or barrel temperature too low, a gate too small for the part; for weld lines, fronts cooling below fusion temperature and air trapped at the confluence.

Fixes: raise injection speed ~30% → 50–60% (short of folding air into the melt) · compression zone +5–10 °C · gate width 2–3× wall thickness · at the confluence speed 60–70% and mold temperature ~40 → 50–60 °C, with vents added.

Material side: a weld line can be hidden but not strengthened afterwards. Where a load path crosses one — bellows, flexing boots — specify for flow, not hardness alone.

The Prevention Framework: Material State, Process Balance, Mold Fit

Process and mold are decided on your floor. Material state can only be fixed at the source — and it sets the ceiling for the other two.

What to Specify in the TPV Granule You Buy

Most defect investigations end the same way: the purchase specification named a hardness and a colour and left the rest to chance. A production-ready specification covers:

1. Moisture at bag opening with sealed barrier packaging — controls bubbles and silver streaks.
2. MFI with a stated tolerance band — controls flash risk and the shrinkage window.
3. Hardness and compression set target — sets sealing force and recovery under load.
4. Heat-stability package sized to your thermal history — prevents yellowing during stoppages.
5. Fines content and pellet size distribution — reduces mold deposit, drag marks and black specks.
6. Base polymer system (EPDM/PP, PP/IIR, AEM-based, silicone-modified) — sets the temperature, oil and weathering envelope.
7. Flame-retardant requirement, if any — halogen-free grades where fire performance and smoke limits apply.
8. Documentation: COA per batch, TDS, RoHS and REACH statements — shortens incoming inspection.

If your specification covers items 3 and 7 only, defects 1, 2 and 5 will keep returning regardless of how much time your team spends on the machine.

DOWREN TPV Grades: Engineered to Reduce Defects

DOWREN produces TPV granules across a range of hardness, flow and fire-performance targets, so the grade can be matched to the tool rather than the tool being tuned around the grade.

Hardness, MFI, flame rating and compression-set values are grade-specific: confirm them against the current technical datasheet before ordering.

Why Processors Choose DOWREN

• Own production lines with grade separation between injection and extrusion types, so a formulation change on one line does not contaminate the next. 
• Quality control from incoming material to packaging — in-process checks on pellet geometry and fines, moisture-controlled packaging, a batch-level COA per shipment. 
• Lot-to-lot consistency — hardness and MFI held within a stated band, so a validated parameter set stays valid next quarter.
• Technical support on the defect, not just the price — we review tool layout, wall thickness and flow path before recommending a grade. 

Request a Quote, Sample or Technical Review

Send us the part drawing or application conditions and we will return a recommended grade, the granule specification to write into your purchase order, and a processing window to start from.

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