In this guide 6 sections
Uneven cure usually comes from one of four places: the compound lot, scorch before the mould closes, platen temperature drift, or poor heat inside the mould itself. Each leaves a different trace. If cure time, platen readings, lot number and mould number sit on one record, the pattern shows quickly.
Start with the compound, not the press
Most supervisors blame the press first because it is the thing they can see, yet the mix is often the real culprit. Two lots of one grade can cure at different rates when the mixing room ran a different dwell, a new filler lot or a longer shelf age.
Ask the mixing room for the rheometer curve of each lot, since the curve gives scorch time and the time to reach full torque. A lot that cures faster than the last one will look overcured on a press set for the old lot.
Compare the curve with what the moulders actually saw. If parts from one lot come out soft while another lot is fine on identical settings, the compound is your answer.
- Rheometer curve per lot, with scorch and full cure times
- Mixing date and shelf age at the time of moulding
- Filler, curative and plasticiser lot numbers
- Which lot fed which press and mould, shift by shift
Check for scorch before the cavity fills
Scorch is early cure that starts before the rubber has flowed, and it shows as flow lines, poor knit at the parting line or a skin that never bonds. The cause is heat history, not press time.
Preforms that sat warm on a tray by the press, or stock held after a long break, scorch more readily. A hot day in the shop makes it worse, and so does a slow loader who lets the charged mould sit open.
Record the preform age at loading. If bad parts cluster after tea breaks or at the end of a shift, you are looking at scorch, not at a cure setting.
Look for platen temperature drift
The controller on the panel says one number. The platen surface may say another, because heater cartridges weaken, thermocouples sit in the wrong hole and steam or oil lines scale up. A press can display its set point while the plate under the mould runs cooler.
Read the platen with a contact pyrometer on a fixed schedule, at the same points each time, and write down both readings. A growing gap between panel and surface is a maintenance job.
Drift is sneaky. It also shows up as a pattern across presses. If one press makes the soft parts for every compound, stop looking at the rubber.
- Panel reading and surface reading, top and bottom plate
- Cure time and pressure actually used
- Bump or breathe cycles counted per shot
- Date of the last thermocouple and heater check
Separate mould heating from press heating
A mould with thick steel, a blocked vent or a deep core takes longer to reach cure temperature than a thin plate. Parts from cavities nearest the platen edge may cure well while the centre cavities lag.
When one cavity keeps producing the weak part, suspect the mould. Look at cavity wear, heat transfer through dowels and spacers, and residue built up on the steel after many shots. A mould that has been cleaned or repaired also behaves differently for a while.
Tag every defect to a cavity number, not just to a mould. That small habit separates a mould problem from a press problem within a week.
Put the four records on one sheet
You cannot tell these causes apart from memory. The press operator knows the platen, the mixer knows the lot, the toolroom knows the mould, and nobody holds all three. That is the gap.
Join them by order, so each moulding order carries the compound lot, the mould and cavity numbers, the press, the measured platen temperatures and the defect notes. After a month, sort by each field in turn and see which one the soft parts follow.
Start this week by adding lot number and cavity number to the existing shift sheet. Review it on Friday.
Uneven cure diagnosis sheet
One page ruled into four columns, one for each suspect: compound lot, scorch, platen drift and mould heating. Rows list the evidence to collect, such as the rheometer curve, preform age, panel against surface temperature and cavity number of each defect. A final row holds the verdict and the corrective action owner.
Open a blank worksheet to printQuestions owners ask
How do I tell scorch from undercure?
Scorch shows as flow lines and poor knit, because the rubber set before it filled the cavity. Undercure shows as soft, tacky or sponge-like parts with good surface flow. Preform age and time between loading and closing help confirm which one you have.
Does a rheometer curve really matter for small shops?
Yes, if you can get one. Even a single curve per lot from the mixer or supplier lets you tie cure behaviour to the lot instead of guessing. Without curves, record the lot number and the mixing day.
Why do parts from the same mould differ between cavities?
Heat reaches each cavity differently, depending on position, steel thickness, vents and wear. Recording the cavity number of each rejected part shows whether the fault follows one position.
How often should platen temperature be measured?
Often enough to catch drift before it makes scrap. A fixed check at the start of each shift on every press is a sensible starting point, with the readings written against the press and the date.