Injection Molding Defects: Causes, Fixes and How to Prevent Them

Every molder eventually faces the same moment: the press is running, the parts are coming out — and something is wrong with them. A short shot half-filled, a sink mark over a rib, warpage that no one can explain, or a weld line right where the surface must be perfect. Injection molding defects are rarely random. Almost all of them trace back to one of three sources: the process settings, the mold itself, or the part design. The trick is diagnosing which one — quickly, and in the right order.

This guide walks through the most common defects, what actually causes them, how to react on the press, and — most importantly — how to prevent them before steel is ever cut.

Diagnose in the Right Order: Process, Mold, Design

Before touching anything, sort the problem into one of three buckets. Getting the order right saves days of wasted trial shots:

• Process — temperatures, pressure, speed, hold time, cooling time. Cheapest to change, so try first.
• Mold — gate size and location, venting, cooling layout, draft, ejection. Fixable but costs toolroom time.
• Design — wall thickness, rib ratios, undercuts, material choice. Almost always decided before the mold exists, and the most expensive to fix late.

If a defect appears on only one cavity of a multi-cavity tool, it is a mold or flow-balance issue, not a process setting. If it appears identically on every cavity, suspect the process or the design first.

The Most Common Defects and Their Root Causes

Defect	What you see	Likely root causes	First fix
Short shot	Part incompletely filled	Low injection pressure/speed, cold melt, thin gate, poor venting, wall too thin	Raise injection pressure and speed; check gate size
Sink marks	Depressions opposite thick sections/ribs	Over-thick ribs or bosses, insufficient hold pressure/time, fast cooling of skin	Increase hold pressure and time; thin the rib
Warpage	Part bends or twists after ejection	Uneven cooling, non-uniform wall, wrong gate location, excessive packing	Balance cooling; review gate and wall
Weld lines	Visible lines where flow fronts meet	Multiple gates, holes/inserts splitting flow, low melt temperature	Raise melt temp; relocate gate to move the line
Flash	Thin excess material at parting line	Excessive injection pressure, low clamp force, worn/insufficient parting-line fit	Increase clamp tonnage; inspect parting line
Silver streaks	Silvery streaky marks along flow	Moisture in resin, entrapped gas, too-cold melt, excessive shear	Dry the resin; check venting and temperature
Burn marks	Dark/black discolouration at flow end	Poor venting trapping gas, too-high injection speed, contaminated resin	Add vents at the last-fill area; reduce speed
Drag/ejection marks	Scratches or whitening on walls	Insufficient draft, undercut, rough cavity, poor polish	Add draft; polish the wall; check ejector balance

The table is a starting point, not a verdict. A sink mark can be cured on the press today and still return next month because the real cause was a rib that was too thick in the design. Treating the symptom without finding the source is how projects end up in endless trial loops.

Process Fixes vs. Steel Fixes

Every defect has two possible homes: the machine or the metal. Sorting them correctly protects your schedule.

• Try the press first when the defect is a filling, temperature or pressure issue — short shots, some sink marks, silver streaks and weld-line visibility often respond to process changes within a single shift.
• Expect toolroom work when the defect is structural — flash at the parting line, drag from missing draft, or gas that has nowhere to vent. Adding a vent, polishing a wall or adjusting a shut-off is real work with a real lead time.
• Escalate to design when the same defect keeps returning across process windows. A wall that is too thick, a rib that feeds a sink mark, or a gate that lands in the worst cosmetic zone are design decisions — and so is the material and shrink behaviour behind them.

Prevention: Review the Design Before the Mold Exists

The cheapest defect to fix is the one that never happens. Nearly all the defects above can be predicted and designed out during a manufacturability review — wall thickness, draft, rib ratios, gate location, cooling and shrink are exactly what that review examines. If the design is never audited, the defects simply wait until the trial to appear, when every fix is at its most expensive.

Our DFM analysis guide covers that pre-tooling review in detail: which geometry features to check, the numeric targets to apply, and what a real DFM report should contain.

The material and tool steel are the other half of prevention. A glass-filled or corrosive resin changes both the shrink behaviour and the steel that resists it — a decision that must be settled before the cavity is machined. See our injection mold steel selection guide for how material grade drives the steel choice.

Troubleshooting Checklist

• ☐ Confirm the defect appears on all cavities or only some — this alone splits mold from process
• ☐ Check that the resin is the specified grade and is properly dried
• ☐ Verify melt temperature, injection pressure/speed and hold profile against the process sheet
• ☐ Inspect gate size and location; confirm filling balance across cavities
• ☐ Confirm venting reaches the last-fill areas, not just the perimeter
• ☐ Review cooling layout against thick sections where warpage and sinks originate
• ☐ Measure wall thickness and rib ratios against the DFM targets
• ☐ Classify the fix: process, steel or design — and fix the source, not the symptom
• ☐ Record the settings and outcome in the process sheet so the fix is repeatable

Fix the Cause, Not the Symptom

Defects are feedback. Each one points back to a decision — a process setting, a vent that was never added, a rib that was too thick. Reading that feedback correctly is the difference between a mold that runs cleanly from trial one and one that bleeds money through endless adjustments.

Precimold designs and builds injection molds for silicone and plastic parts, and reviews each part design before tooling so the common defects are removed in CAD rather than discovered on the press. Send your 3D model and annual volume, and we will flag every manufacturability and filling risk while it is still a design change. We reply within 12 hours.