In this guide 6 sections
Rubber moulding rejects trace back to four places: the mould, the cavity, the compound lot and the press settings. Flash points to clamp or tool wear, short shots to low charge or trapped air, blisters to gas or under-cure. Record a reject code per defect to see the pattern.
Flash points to the tool or the charge
Thin rubber leaking past the parting line is flash. Start with the mould. A worn land, a damaged cavity edge or dirt on the plate lets rubber escape.
Then check the charge. Weigh the preforms. Too much preform weight pushes extra rubber out. Low clamp force or an uneven platen can do the same.
If flash appears in one cavity only, suspect that cavity. If it appears on every cavity, look at the preform weight and the press.
- Worn or dirty parting line and lands
- Preform weight above the set value
- Platen parallelism or clamp force
- Cavity damage found on one cavity only
Short shots and incomplete fill
A part with a missing corner or an unfilled edge is a short shot. The usual causes are too little compound, trapped air or a compound that has begun to cure before it flows.
Look at the lot first. Old or scorchy material stiffens and will not fill a fine edge. Then check preform size, vent condition and the closing speed of the press.
A short shot that repeats in the same cavity usually means a blocked vent. Clean it. Then watch the next run.
- Preform weight below the set value
- Compound lot age and scorch test
- Blocked or missing vents
- Closing speed and bump cycles
Blisters and trapped gas
Blisters are pockets of gas or air inside or under the surface. They appear when moisture in the compound turns to steam, or when the cure is too short and the part is opened while still soft.
Compare the cure time on the work order with what the operator ran. A press that was opened early looks fine in the sheet and fails in the oven. Check the post-cure schedule as well.
Blisters clustered in one lot suggest moisture or contamination in the mix. Check the lot sheet.
- Cure time and temperature as run
- Moisture in the compound or filler
- Breathing cycles at mould closure
- Post-cure oven loading and time
Backrinding, tears and marks
Backrinding is a ragged edge at the parting line, often where rubber was cured to different states. Tears happen at demoulding, from poor release or a part that was removed too soon.
Check mould release, demoulding technique and the age of the cavity surface. A coating that has worn off causes sticking, and operators then pull harder on every part.
Record the cavity number whenever a mark is found. Surface marks are nearly always local to one cavity. Name it.
Reject codes that make scrap counts mean something
A scrap figure without a reason tells you little. Give every reject a code, and tie it to the mould, cavity, lot, press and shift. Keep the list short enough that an operator can pick one without hunting.
After a month, sort by code and then by mould. The same few combinations will repeat. Fix those first. The rest can wait.
Print the code list and tape it at each press this week.
Rejects-by-cause checklist
A single chart with one row per defect: flash, short shot, blister, backrinding, tear, porosity and contamination. Columns show likely tool cause, likely compound cause, likely press cause and the first check to make. A strip along the bottom lists reject code, mould, cavity, lot and press for the sample in hand.
Open a blank worksheet to printQuestions owners ask
What causes flash in rubber moulding?
Most often a worn parting line, an overweight preform or low clamp force. If one cavity flashes and others do not, look at the tool. If all do, look at the charge and press.
How do I tell a tool fault from a compound fault?
Compare defects across cavities and lots. A fault in one cavity across several lots belongs to the tool. A fault across all cavities in one lot belongs to the compound.
Why do blisters show up only after post-cure?
Trapped gas or an under-cured section can look sound at the press. Heat in the oven expands it. Check the press cure time before blaming the oven.
How many reject reasons should an operator choose from?
Keep the list short, around eight to twelve, and use plain names. Operators who must scroll a long list will pick the first one they see.