Same machine. Same settings. Same material, on paper. Yet today’s parts don’t quite match last week’s. This is one of the most frustrating problems in injection moulding, because nothing on the machine has changed.
Here are five common causes of inconsistent moulded parts, and how to track them down.
Why “Nothing Changed” Is Rarely True
The machine is usually the last thing to blame. It’s simply reacting to small changes elsewhere in the process. These changes are easy to miss, since nobody touched a setting.
Consistency depends on several things staying stable at once: material condition, mould temperature, cooling time, and even factory conditions. A shift in any one shows up in the part.
5 Causes of Inconsistent Moulded Parts
- Material Moisture Variation
The most common cause. Inconsistent drying means the material going into the machine changes slightly, batch to batch. A dryer running to a timer, not a confirmed dew point, is a frequent source of this drift. - Regrind Ratio Changes
An uncontrolled regrind ratio shifts part properties between batches. Reprocessed material often flows differently to fresh material. - Mould Temperature Drift
A mould running a few degrees off target changes shrinkage, warping, and surface finish. Often caused by a temperature control unit no longer holding its set point, due to scale build-up or an undetected fault. - Inconsistent Cooling Time
Small cycle time variation changes how long a part cools before ejection. Parts eject at slightly different temperatures, showing up later as dimensional variation. - Ambient Environmental Changes
Factory temperature and humidity shift with the seasons. A process dialled in during cool, dry weather can behave differently once the factory warms up.
How to Track It Down
Check what actually changed, not what you assume didn’t:
- Check moisture readings against spec, not just the drying schedule
- Check mould temperature against its set point, not just power status
- Review cycle time logs for small drift
- Compare a “good” batch against a “bad” one using data, not memory
Why It’s Worth Chasing Down
Inconsistent parts cost more than scrap alone. They erode confidence in a trusted process, and often lead to over-adjusting machine settings, which creates new problems on top of the original one.
See the Plastics Technology guide to resin drying if moisture looks like a factor. Also worth reading: our piece on material contamination, a related issue with different root causes.
Get in Touch
If your process has drifted and you can’t pin down why, we’re happy to help. Sometimes it’s a quick check; sometimes it’s worth a site visit.
Geiger Handling UK Ltd
Phone: 01782 630555
Email: sales@geigerhandling.co.uk