Moulding defects are diagnosable, and the useful question about any of them is whether the cause is in the process, the tool or the part design. Process problems can be adjusted out this afternoon. Tool problems need the tool off the machine. Design problems cannot be fixed at all without changing the part, which is why naming the cause correctly matters so much.
Defects the design owns
Sink marks over thick sections, ribs and bosses. Warp from uneven wall thickness or from a filled resin shrinking differently along and across the flow. Weld lines wherever flow fronts meet after passing a hole or core. Voids in thick sections. All of these follow from geometry, and a moulder who is asked to tune them out is being asked to do something impossible.
Defects the tool owns
Flash, where material escapes at the parting line because of damage, wear, poor shutoff or insufficient clamp. Burn marks where trapped air cannot escape because a vent is missing or blocked. Drag marks and stuck parts from insufficient draft or a damaged cavity surface. Ejector pin witness where ejection is too aggressive or the part is not rigid enough yet.
Defects the process owns
Short shots from insufficient material, pressure or temperature. Splay and silver streaking from moisture in undried resin. Burns from injecting too fast. Dimensional drift from packing pressure or cooling time. These are the ones a good moulder resolves at the machine, and the ones a process record should show being resolved.
Diagnosing it in the right order
Ask when it started. A defect present from the first shot is usually design or tool; a defect that appeared after months is usually tool wear or a process or material change. Ask whether it is on every cavity or one, since one cavity points at the tool. And ask what changed, because a resin lot, a regrind ratio or a machine change is behind a surprising share of them.
Questions people ask about common injection molding defects
What causes sink marks?
A section too thick for its surroundings, most often a heavy wall, a rib thicker than about two thirds of the wall, or a solid boss. It is a design cause, not a process one.
What is flash?
Material escaping at the parting line, caused by tool wear or damage, poor shutoff, insufficient clamp force or over packing. It is a tool or process problem.
Why does my part have silver streaks?
Almost always moisture in the resin. The material was not dried enough before moulding, and no machine setting will remove it.
How do I tell a tool problem from a process problem?
Ask whether it affects one cavity or all of them, and whether it was there from the first shot. One cavity and sudden onset both point at the tool.