injection molding services Defects FAQ: Causes, Prevention, and How to Fix Them
No matter how carefully a mold is designed, plastic parts can still come out of the machine with defects. Warpage, sink marks, flash, short shots, weld lines, and burn marks are the six most common problems buyers encounter when their new supplier ships the first samples. The encouraging part is that almost every defect is traceable — to part design, material behavior, mold construction, or process settings — and each one has a known fix. Drawing on the questions customers at RCH Plastic ask most often when sourcing injection molds from China, this FAQ explains what causes these defects and what a reliable mold maker should do to prevent them before parts ever reach your assembly line.
1. Why Do Injection Molded Parts Warp, and How Can It Be Prevented?
Warpage is the deformation of a part after ejection — corners lifting, walls bowing, or flat surfaces twisting out of plane. It happens because plastic shrinks as it cools, and when different regions of the same part shrink at different rates or at different times, internal stress builds up and the part bends to relieve it. The usual triggers are uneven wall thickness, non-uniform cooling across the mold, poorly placed gates, and materials with naturally high shrinkage such as semi-crystalline resins like POM and PA66.
Prevention starts long before the mold is cut. Designing with uniform wall thickness is the single most effective measure, because it lets the whole part shrink at the same rate. Next, a proper mold flow analysis balances the filling pattern so the material packs evenly, and a well-laid-out cooling channel system removes heat uniformly from every region of the cavity. On the machine side, correct packing pressure and a controlled cooling time allow the stress to relax before ejection. If warpage still appears, adjusting the packing profile or adding cooling to the problem area usually brings the part back into spec.
2. What Causes Sink Marks, and How Are They Fixed?
Sink marks are small depressions on a part surface, usually appearing directly opposite thick sections such as ribs, bosses, or gussets. The mechanism is simple: the thick section holds heat longer and shrinks more than the surrounding thin wall, so the surface material gets pulled inward as it solidifies. Sink marks are purely cosmetic on many parts, but on visible A-surfaces they are a common reason buyers reject first articles.
The best fixes are made at the drawing board. Keep rib thickness at about 60 percent of the wall thickness and core out bosses wherever possible, so no region is significantly thicker than its neighbors. When thick sections cannot be avoided, place them opposite non-cosmetic surfaces, where a slight depression does not matter. During DFM review, an experienced mold maker will flag these features before tooling begins. On the process side, raising packing pressure and adding local cooling under the affected area reduces the shrinkage differential that creates the mark.
3. Why Does Flash Occur on Injection Molded Parts?
Flash is the thin excess plastic that escapes along the parting line, around ejector pins, or past sliding cores — the feather-like edge you often have to trim off molded parts. It occurs when cavity pressure forces molten plastic into gaps that should be closed. Common causes are insufficient clamping force for the projected area of the part, a mold that is not closing fully because of debris or a damaged parting surface, venting grooves cut too deep, or cavities filled at too high a pressure.
Fixing flash means finding which gap is opening. A mold maker first verifies the press tonnage is adequate and the parting surfaces are clean and undamaged. Vent depth is then checked — for most thermoplastics vents should be only 0.02 to 0.05 mm deep, deep enough to let air out but too shallow for melt to escape. If flash appears at a specific location, balancing the fill and slightly reducing injection pressure at the end of packing usually closes the gap. Flash that reappears in the same spot can also signal tool wear, which is why regular mold maintenance is part of the answer.
4. What Causes Short Shots, and How Are They Avoided?
A short shot is an incomplete part — the cavity did not fill completely, leaving thin sections, ribs, or the far end of the part missing or rounded off. The plastic simply did not reach every corner of the mold. Typical causes are insufficient injection pressure or speed, melt temperature set too low, a material with poor flow for the part geometry, trapped air blocking the flow front, or a gate and runner system too small for the required fill rate.
Avoidance starts with good communication between fill rate and part geometry. For parts with long, thin flow paths, the mold should be designed with adequate gate and runner sizing and sufficient venting so air can escape ahead of the melt instead of blocking it. On the machine, raising melt and mold temperatures and increasing injection pressure and speed gives the material the energy to complete the fill. If a short shot keeps appearing at the same spot, it is usually a venting or gate issue rather than a machine setting issue, and a mold flow analysis will pinpoint it quickly.
5. What Are Weld Lines, and Can They Be Eliminated?
Weld lines — also called knit lines — are the visible seams that form where two flow fronts meet and fuse together. They occur around holes, inserts, or wherever the melt has to split and rejoin. The two fronts do fuse, but if the melt has cooled too much by the time they meet, the bond is weak and the line remains visible, and on structural parts it can be a genuine weak point.
Weld lines are rarely eliminated completely, but they can be minimized to the point of being nearly invisible. Repositioning the gate so the flow fronts meet at a less visible or lower-stress location is the first option, and raising melt temperature and injection speed helps the fronts weld more completely. In some cases an overflow well is added near the weld line so the cooler material at the meeting point is pushed out of the visible surface. Materials with better flow also reduce the temperature drop at the junction. During mold design, this is exactly the kind of trade-off a mold maker should discuss with you before steel is cut.
6. What Causes Burn Marks on Injection Molded Parts?
Burn marks appear as dark, brown, or black streaks, usually near the end of fill or at the far corners of the cavity. There are two distinct causes. The first is trapped air: as the melt advances, it compresses the air in the cavity, and at very high compression the air superheats and scorches the plastic at the flow front. The second is material degradation: melt temperature set too high, or resin left in the barrel too long, breaks down the polymer and produces dark streaks.
For trapped-air burns, the solution is better venting — letting the air escape before it is compressed — combined with a slightly lower injection speed in the final stage of fill so the flow front does not over-compress the remaining air. For degradation burns, reduce the melt temperature and shorten the residence time by matching barrel capacity to shot size. Both fixes are straightforward once the root cause is identified, which is why burn marks are usually resolved in a single round of mold trials.
7. How Can I Tell Whether a Defect Is a Design Problem or a Process Problem?
This is the question that separates a mature supplier from a reactive one. As a rough rule, defects that appear at the same location on every shot — a sink mark over the same boss, a short shot at the same rib, flash at the same corner — usually point to the tool or the part design. Defects that move around or vary in severity between shots are usually process related: material moisture, inconsistent melt temperature, or a cycle that drifts through the day.
There is also a timing clue. If a defect appears on the very first trial and never improves, it is most often designed into the part — a feature that is simply difficult to mold, such as an extremely thin flow path or a thick section under a cosmetic surface. If the same part runs clean after a process adjustment, the root cause was in the process, not the geometry. A competent mold maker should be able to walk you through this logic during the trial, showing the trial records that prove how each defect was diagnosed and eliminated — which is the standard we follow on every mold design project at RCH Plastic.
Defects Are Not Random — They Are Information
Every defect tells a story about the design, the material, the tool, or the process. A supplier that treats defects as normal and keeps sending revised samples is not solving your problem — it is managing it. The reliable way to avoid defects is to prevent them at the source: a thorough DFM review before tooling, mold flow analysis to validate fill and cooling, and a structured mold trial process that documents every adjustment.
At RCH Plastic, we apply this discipline to every tool we build — from simple two-plate molds to multi-cavity production tools — and we share the trial documentation with you so you can see exactly how defects were identified and eliminated. If you are sourcing a new mold or troubleshooting an existing one, send us your part drawings and we will run a free DFM review. Contact us to get started.
Injection Molding Defects — Key Takeaways
Warpage is a shrinkage imbalance — fix it with uniform wall thickness, balanced cooling, and correct packing. Sink marks sit above thick sections — core out the mass or pack harder. Flash is melt escaping through gaps — check clamp force, parting lines, and vent depth. Short shots mean the cavity did not fill — raise pressure, temperature, and venting. Weld lines are fused flow fronts — reposition gates and raise melt temperature to make them disappear. Burn marks come from trapped air or degraded material — improve venting and check melt temperature. Most importantly, choose a mold partner that catches these issues in the DFM and mold flow analysis stage, not after thousands of parts have been molded.