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Injection Molding Defects Troubleshooting Guide

When a part comes out wrong, a structured list of likely causes saves time at the press. This reference table groups common injection molding defects by process area and lists the causes and first checks that usually resolve them.

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Defect	Main area	What you see	Common causes	First checks
Short shot	Fill	Cavity not completely filled, missing corners or ribs	Low shot size, low injection pressure or speed, cold melt or mold, blocked vent or gate, worn check ring	Shot size and cushion, melt and mold temperature, vents, check ring
Flash	Mold	Thin excess plastic at the parting line or around pins	Clamp force too low, worn parting line, excess pack pressure, high melt temperature, vents too deep	Clamp tonnage, parting line condition, pack and hold pressure
Sink marks	Pack and cool	Depressions over thick sections, ribs or bosses	Low pack pressure or time, gate freezes early, thick walls, hot mold, rib too thick	Hold pressure and time, gate size, rib to wall ratio
Voids	Pack and cool	Internal bubbles in thick sections	Insufficient packing, early gate freeze, volatile moisture in material, melt too hot	Pack time, gate seal time, resin drying
Weld lines	Fill	Visible line or weak seam where flow fronts meet	Cold flow fronts, low injection speed, poor venting at the meeting point, gate location	Melt temperature, injection speed, vents at the line
Burn marks	Fill	Dark or brown marks at the end of fill	Trapped air compressed by fast injection, blocked vents, overheated melt, long residence time	Vent cleanliness, injection speed, melt temperature
Splay or silver streaks	Material	Silver streaks along the flow direction	Moisture in hygroscopic resin, volatiles, high melt temperature, excess decompression	Dryer temperature and dew point, drying time, decompression
Warpage	Pack and cool	Part bends or twists after ejection	Uneven cooling, differential shrinkage, high ejection temperature, high packing stress, orientation	Cooling balance, mold temperature split, cooling time
Jetting	Fill	Snake-like flow pattern from the gate	Melt jets into open cavity, fast initial injection speed, gate facing open space	Initial injection speed, gate type and location
Flow lines	Fill	Wavy lines or rings near the gate	Cold melt or mold, slow fill, thin sections after thick ones	Melt and mold temperature, fill speed profile
Delamination	Material	Surface layers peel off like skin	Contamination with another resin, moisture, excessive shear, wrong regrind	Material contamination, drying, regrind source
Black specks	Material	Dark particles in the part	Degraded resin in the barrel or hot runner, contamination, long residence time	Purge, residence time, hot runner temperatures, material handling
Ejector marks	Mold	Pin impressions or whitening on the part	Part too hot or too soft, uneven ejection, high ejector speed, insufficient draft	Cooling time, ejector speed and position, draft
Drag marks	Mold	Scratches along the ejection direction	Insufficient draft, rough or undercut cavity, heavy packing	Draft angle, surface polish, pack pressure
Brittleness	Material	Parts crack under light load	Degraded polymer, excess regrind, moisture hydrolysis, residual stress, wrong grade	Drying, regrind ratio, melt temperature, residence time
Gate blush	Fill	Dull ring or halo around the gate	High shear at the gate, fast injection, small gate	Injection speed at gate, gate size, melt temperature
Nozzle drool	Machine	Resin leaks from the nozzle between shots	Nozzle too hot, excess decompression, wrong back pressure, open nozzle type	Nozzle temperature, decompression, nozzle style
Inconsistent shot weight	Machine	Part weight varies shot to shot	Worn check ring or screw, inconsistent cushion, feed throat problems, variable melt	Cushion variation, check ring, feed throat temperature

General guidance only. Always change one parameter at a time, wait for the process to stabilise, and record the change. Confirm material limits with the resin supplier.

Track defects by mold and resin lot

Optional: we can review your defect coding and inspection records and show how to analyze them by mold, press and lot.

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How to use it

Search or filter by the defect name or by what you see on the part. The main area tells you where to start: fill problems relate to melt, speed and venting, pack and cool problems to hold pressure and cooling, material problems to drying, contamination and degradation, mold problems to venting, wear and ejection, and machine problems to screw, check ring and nozzle. Export the table to Excel to add your own plant specific notes.

A disciplined approach

Defects often have several causes, so a table narrows the field but does not decide it. Start with material, since moisture and contamination mimic process faults. Then confirm the process is stable using cushion and shot weight. Change one variable at a time and wait for several cycles before judging. Document the setting that fixed the defect on the process sheet, because the next shift will meet the same problem.

Check the material and drying records first
Check cushion and shot weight repeatability
Adjust one parameter, wait, then re-evaluate
Log the cause and fix for the next time

Defects that mislead

Splay is often blamed on the mold but is frequently moisture. Flash looks like a clamp problem but may be excess pack or a worn parting line. Sink and voids are the same mechanism in different places: shrinkage with no melt available to feed it. Warpage can be a cooling issue, a packing issue or a design issue, and should be studied across several parts.

Common mistakes

Changing several settings at once, adjusting the process to hide a worn mold, ignoring regrind ratio, and fixing the symptom without recording it. Another frequent error is raising melt temperature to cure everything, which can trade one defect for degradation and longer cooling.

Frequently Asked Questions

What causes most short shots?

Usually insufficient melt reaching the cavity: low shot size, low injection pressure or speed, cold melt or mold, or blocked vents and gates. Check shot size and cushion first, then temperatures and vents.

How do I tell flash from a clamp problem?

Look at where it appears and whether it grows with pack pressure. Flash at a worn parting line or along pins points to the mold. Flash that disappears when pack or injection pressure drops points to process.

Why do I get splay with dried material?

Moisture can be re-absorbed between the dryer and the barrel, or the dryer may not reach its dew point. Also check excess decompression, high melt temperature and residence time, which create volatiles.

Is this table complete?

No. It covers frequent defects with typical causes. Your resin, mold and machine will have specific behaviors, so add your own notes after export.

Netray can connect defect codes captured at inspection to molds, machines and resin lots in your ERP, so repeat problems show up in a report instead of a memory.

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