Common Injection Molding Defects: Identification, Causes, and Solutions

Identify a molding defect by three facts before changing anything: where it appears on the part, whether it appears on every shot or only some, and whether it appears in every cavity or only one. Location points to the stage of the cycle and the area of the mold; consistency separates a mold or design cause from a process or material variation; cavity pattern separates a mold or runner cause from a machine or material cause. With those three facts recorded, each defect has a short list of causes to test one at a time.

The Three Questions

Record the defect with a photo, the shot and cavity number, the time, and the material lot before adjusting anything. The cycle stages these defects trace back to are described in the molding process from pellet to finished part. Then change one variable at a time and record the result; changing three settings at once may fix the part without showing which one mattered, and the problem returns.

Fill Defects

Short shot (incomplete part). The flow front stops before the cavity is full. Areas to look at first are the end of flow and thin sections. Test, in order: shot size and cushion, whether fill is pressure-limited, vents at the last-to-fill area, gate freeze, melt temperature and material lot viscosity. Full sequence: short shots troubleshooting.

Flash (thin film beyond the part edge). Melt escapes at the parting line, shutoffs, ejector pins, or slides. Test: parting line and shutoff condition, separating force against clamp setting, transfer point and pack pressure, melt viscosity. Full sequence: flash causes and elimination.

Jetting (snake-like pattern from the gate). Melt shoots across the cavity before spreading. Test: gate position against a wall or pin, gate size, initial injection speed.

Shrinkage and Cooling Defects

Sink marks (surface dents over thick features). The part’s interior shrinks after the surface has set and pulls it inward. Test: hold pressure and time against gate freeze, gate size and position, rib and boss proportions, local mold temperature. Full sequence: sink marks.

Voids (internal bubbles). The same shrinkage, where the skin is rigid enough not to sink, so a cavity forms inside; moisture and trapped gas can also form bubbles. Test: packing as for sink, material drying, and section the part to see the void’s position.

Warpage (part distorts from its intended shape). Uneven shrinkage from uneven cooling, wall thickness differences, or fiber orientation. Test: mold half temperatures, wall uniformity, gate position and orientation in filled grades, measurement after conditioning. Full sequence: warping prevention and correction.

Surface Defects

Dimensional and Ejection Defects

• Out-of-tolerance dimensions. Check whether the variation is shot-to-shot (process, cushion, material) or consistent (mold steel size, shrinkage assumption). Measure after the conditioning time in the specification.
• Ejector pin marks, cracks, or distortion at ejection. The part is too soft at ejection or sticks: cooling time, draft, mold temperature, ejector layout and speed.
• Sticking in the mold. Overpacking, insufficient draft, undercut-like texture, or vacuum on deep parts.

Root-Cause Areas

A practical order is to rule out quick, reversible checks first (material lot and drying, setup sheet against current settings, cushion, heater function), then process changes measured on the press, then mold maintenance, and only then design or tooling changes. Changes to the mold or part design cost the most and are justified by evidence from the earlier steps.

Keeping the Fix

When a cause is confirmed, record it with the evidence and the corrective action: the setting window, the maintenance item, or the design change. A defect log by mold and cavity turns the next occurrence into a lookup rather than a new investigation, and the inspection side of that record keeping is set out in quality control methods and standards.