Injection Molding Defects: A Troubleshooting Guide

When a molded part shows a defect, the usual reaction at the machine is to raise the pressure or the temperature until it goes away. That often replaces one defect with another and leaves a process that runs at the edge of its window. A better approach is to identify the defect correctly, know its possible causes, and try the corrections in a fixed order from cheapest to most expensive. This guide covers eight common defects.

Four groups of causes

Every defect comes from one or more of four areas. Check them in this order, because the cost of a change rises from top to bottom.

The molding window

Good parts are produced inside an area bounded by short shots on the low-pressure side, flash on the high-pressure side, cold-flow defects on the low-temperature side and degradation on the high-temperature side. A robust process sits in the middle of that area. If the window is very small, the cause is in the mold or the part design, and no setting will make the process stable.

The eight defects

1. Short shot

The cavity is not completely filled.

2. Flash

A thin film of plastic at the parting line, around ejector pins or slides.

3. Sink marks and voids

Depressions on the surface opposite ribs and bosses, or hollow spaces inside thick sections.

4. Warpage

The part bends or twists after ejection.

5. Weld lines

A visible line or weak point where two flow fronts meet.

6. Burn marks

Brown or black marks, usually at the end of flow or in blind ribs.

7. Splay (silver streaks)

Silvery streaks spreading from the gate along the flow direction.

Target moisture before molding, as a guide: PA below 0.2 %, ABS below 0.1 %, PC below 0.02 %, PET and PBT below 0.02–0.04 %.

8. Jetting

A snake-like line on the surface starting at the gate.

A method for the machine side

1. Describe the defect exactly: where on the part, in which cavity, on every shot or intermittently, since when.
2. Ask what changed: material lot, dryer, mold service, machine, operator, ambient temperature.
3. Run a short-shot series: reduce the dose in steps with holding pressure off. The filling pattern shows where the flow fronts meet and where the air is trapped.
4. Change one setting at a time and wait until the process is stable, at least 5–10 cycles, more after temperature changes.
5. Do a gate-freeze study: weigh parts at increasing holding times. Set the holding time just beyond the point where the weight stops rising.
6. Record every change and its result. If the defect disappears, return the setting to prove the cause, then fix it properly.
7. Escalate to the material, then the mold, then the design if the window stays too narrow.

What to fix in the mold, once

Some defects return on every run because their cause is built into the mold. If the same correction is needed at each start-up, raise it as a mold action instead of a process setting:

• vents that are too few or too shallow at the end of fill and at weld lines;
• gates too small to pack the part, or placed in a thin section;
• unequal cooling between the two mold halves;
• unbalanced runners in multi-cavity molds;
• worn shut-off faces that need extra clamping force to stay closed.

A permanent mold correction usually costs less than one month of scrap and adjustment time.

Quick reference