Plastic injection molding defects are usually reported as a single symptom — "the parts have sink marks" — when the underlying cause sits in one of four places: the part design, the mould, the process settings or the material. Separating those four is the whole skill. A buyer who can tell a design-limited defect from a process-limited one avoids paying for tool changes that fix nothing.
The Defect Map
| Defect | Typical root cause | Usual fix |
|---|---|---|
| Short shot | Insufficient pressure, cold melt, blocked flow path | Raise injection pressure or speed, check gate and venting |
| Flash | Excess clamp force mismatch, worn parting line, over-packing | Correct tonnage, re-fit parting surfaces |
| Sink marks | Thick sections cooling unevenly, low hold pressure | Thin the rib root, extend hold time |
| Warpage | Uneven cooling, differential shrinkage, gate location | Balance cooling circuits, revise gate |
| Silver streaks | Moisture, volatiles, contaminated regrind | Dry the resin, check regrind ratio |
| Weld lines | Melt fronts meeting cold | Raise melt temperature, move gate, add vent |
| Burn marks | Trapped air compressed in the cavity | Add or clear vents, slow injection at the end |
| Dimensional drift | Inconsistent cycle time or cooling | Stabilise cycle, check mould temperature control |
Reading the Evidence Before Touching the Settings
Three observations narrow the cause faster than any adjustment.
- Where does it appear? A defect at the same location on every part points to the mould or the part design. A defect that moves points to process or material.
- Did it start suddenly? A step change usually means a material lot, a regrind batch or a temperature controller fault. A gradual change points to wear.
- Does it appear on all cavities? If one cavity is affected and others are not, the problem is in that cavity's feed or cooling, not in the process window.
Design-Limited Versus Process-Limited
Some defects cannot be tuned away. A rib as thick as the wall it stiffens will sink; a boss with no draft will drag; a corner with a sharp internal radius will concentrate stress and crack. These are design decisions, and the fix lives in the drawing rather than in the machine.
Other defects are purely process: insufficient hold pressure causes sink even on a well-designed part, and a short cooling time causes warpage that no gate change will cure. Telling these apart before a tool modification is the difference between a one-week fix and a two-month rebuild.
Escalating Without Guessing
When a defect persists, the useful report contains four things: a photograph of the defect in place, the cavity number, the production batch, and the process record for that batch. Suppliers can act on that; they cannot act on "the quality is bad". Ask for a corrective action that names the cause, not just the adjustment — a report that says "increased hold pressure" without explaining why the pressure fell is a temporary fix. If the same defect returns after two corrective actions, the issue is usually in the tool or the design, and the conversation should move to the drawing rather than to the machine settings.
Prevention Before the First Shot
- Mould flow analysis on parts with thin walls, long flow paths or tight flatness
- Wall thickness held as uniform as the function allows
- Draft on every vertical face, including texture-adjusted draft
- Venting designed into the tool, not added after the first burn mark
- A written defect classification with photographs, agreed before production
The last item matters most commercially. Without an agreed standard, "minor flash" and "reject" are both arguable, and the argument costs more than the parts. Our note on the injection molding process covers the process side in sequence, while tooling economics are set out in injection molding and mold cost. Buyers who need a second opinion on a defect pattern should put it to verified moulding suppliers rather than to the tool shop alone, since the same symptom can originate on either side.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
