Rubber Compound Mixing Procedure, Internal and Open Mill

Purpose. This procedure sets out how a rubber compound batch is weighed, mixed on the internal mixer, finished on the open mill, tested and released at the Wingfield works.

1.Before mixing

The mill operator must read the job card and the compound recipe, confirm every ingredient lot in the Raw Materials and Compounds Stock system, check that extraction over the mixer and mill is running and that the mill trip bar has been tested and recorded. Weigh each ingredient on the calibrated scale to the tolerance on the recipe and record the actual weights and lot numbers on the batch card. Curing agents are weighed separately and kept apart from the masterbatch ingredients.

• Recipe version and job card match
• Ingredient lots recorded
• Extraction running, mill trip tested
• Scale calibration current
• Curatives weighed and held separately

2.Masterbatch mixing

Charge the polymer, carbon black, fillers, oils and process aids to the internal mixer in the sequence and at the times on the recipe, with the ram down and the rotor speed set. Monitor the batch temperature and drop the batch at the dump temperature on the recipe. Do not exceed the maximum temperature, which will scorch the compound. Record the mixing time, energy and drop temperature.

3.Open mill finishing

Drop the batch onto the open mill with the nip set to the recipe gap and band the rubber on the front roll. Add the curatives at the mill temperature on the recipe and blend by cutting and folding from the sides with the mill knife, never reaching across or behind the nip. Sheet off to the thickness on the recipe onto the cooling rack or batch-off with anti-tack.

4.Testing and release

Take a sample from each batch for the rheometer. Compare the cure curve, scorch time and torque with the acceptance limits on the recipe, and run hardness and specific gravity where the recipe requires. A batch outside limits must be tagged, entered in the Quality Nonconformance Register and held for the technical team. Record the test results on the batch card and release the batch in the stock system with its batch number.

5.Housekeeping and chemicals

Clean the mixer and mill between compounds to prevent cross-contamination and record the clean-down. Return curatives and chemicals to their stores, with peroxides to the cool cabinet, and update the register quantities. Sweep carbon black spills with the vacuum, not the broom.

6.Records

File the batch card with the job card. Batch numbers must be traceable to every belt, sheet or moulded product made from the compound.