Common Injection Moulding Defects and How to Prevent Them
Most injection moulding defects can be traced back to one of four sources: the part design, the tool, the material, or the process settings on the machine. Understanding where a fault comes from makes it far easier to fix, and far cheaper to prevent in the first place.
At Plas-Tech Injection Moulding, we design and build our own tools and run production on machines from 60 to 320 tonnes, so we see the full picture from CAD model to finished component. Below are the defects we are most often asked about, along with how an experienced plastic injection moulding company keeps them off your parts.
Sink Marks and Voids
Sink marks are small depressions on the surface of a part, usually found opposite thick sections or bosses. They form when the inner material keeps shrinking after the outer skin has cooled and set, pulling the surface inwards. Voids are the same problem hidden inside the part as air pockets. Both are best tackled at the design stage by keeping wall thickness consistent and coring out heavy sections. On the machine, adjusting hold pressure and cooling time helps the material pack properly before the part is ejected.
Short Shots
A short shot happens when molten plastic does not fill the cavity completely, leaving a part with missing sections or incomplete edges. Common causes include walls that are too thin for the material to flow through, gates in the wrong position, trapped air, or melt temperatures that are too low. Good gate placement and venting designed into the tooling prevent most short shots, with injection speed and pressure fine-tuned during sampling.
Flash
Flash is a thin layer of excess plastic that escapes along the parting line of the tool or around ejector pins. It usually points to worn or poorly fitting tool faces, or to injection pressure that is higher than the clamp force can hold. Flash adds trimming time and can stop parts from fitting together. Accurately machined and well maintained tools keep it under control, along with choosing a machine with the right tonnage for the part.
Warping
Warping is when a part twists or bows out of shape as it cools, often because different areas shrink at different rates. Uneven wall thickness and poor cooling channel layout are the usual culprits. Balanced cooling in the tool and the right choice of plastic make a big difference, as does allowing enough cooling time before ejection.
Weld Lines and Flow Marks
Weld lines appear where two flow fronts of plastic meet and do not fully bond, leaving a visible line and a potential weak point. Flow marks are wavy patterns or streaks near the gate caused by the material cooling unevenly as it fills the cavity. Moving the gate or raising the melt and mould temperature can reduce them. On cosmetic parts, we plan gate positions early so any unavoidable weld lines sit in areas that are out of sight or under less stress.
Preventing Defects Before Production Starts
The most reliable way to avoid injection moulding defects is to design them out before any steel is cut. Reviewing wall thickness, draft angles, gate positions, and cooling in the CAD stage costs very little compared with modifying a finished tool. Because our tool room and moulding shop sit under one roof, we can spot an issue during sampling and make adjustments quickly. Every project is produced to our ISO 9001 quality standards, with first-off inspection before a full production run begins.
Talk to Us About Your Injection Moulding Project
If you are seeing recurring defects with an existing supplier, or want to get a new part right from the start, our team can review your design and recommend practical changes. Find out more about our injection moulding services or request a quick quote to get started.