Industrial Engineering & Services (IES)
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Factors Responsible for Batch to Batch Variation in Pvc Compound
Abstract:
In order to get consistency in product properties, it is necessary to provide consistent inputs and control the process. In mixing process, PVC and additives are inputs and free flowing PVC compound (Dry blend) is an output.
- In order to get consistency in product properties, it is necessary to provide consistent inputs and control the process. In mixing process, PVC and additives are inputs and free flowing PVC compound (Dry blend) is an output.
- In case of PVC products, compounding and mixing is responsible for 90% processing and quality problems.
- It is strongly advisable to have a separate “process owner” and skilled operators for compounding & mixing process, to get rid of further cascading effects right up to customer end.
In batch mixing process consistency of dry blend and batch to batch variation depends on variation in following factors –
- Batch quantity – Should not be more or less than the mixer capacity. Ensure that the volume is such that formation of vortex during high speed mixing is perfect.
- Weight of ingredients – The tolerance limit for the total additives batch should be +/- 2 g. Also the % of rework should be fixed and not used haphazardly.
- Speed and condition of the impeller (varies for U PVC & soft PVC depending on plasticizer level) – The angular velocity should follow the appropriate Froude Number. Froude Number (dimensionless ) = R.ω^2/g, where,
R = Blade radius, ω = Angular velocity (radians/sec), g = Acceleration due to gravity.
- Angle of deflector – Deflector breaks the moving vortex and deflects the material towards center of the vortex. Deflector should be set at 45 degrees and should be vertical (very important factor).
- Sequence of addition of ingredients – Should be standardized and consistent for all batches. It can be automated. All ingredients should not be added in the beginning.
- Temperature of addition and addition rate – Should be standardized and should be same for all batches. This will ensure that the compound is free flowing “Dry blend”.
- Rate of temperature rise – Should be @ 8-10 deg C/min. (depends on impeller blade condition).
- Drop temperature – Should be 120 deg C for UPVC. It will vary for plasticized PVC depending on type and quantity of plasticizers used.
- Cycle time – Should be almost same for all the batches (max 10 seconds variation).
- Torque at different stages – Should be same for all the batches with respect to time.
- Compound temperature in the cooler mixer – Should be taken out at 45 deg C.
- Variation in all these factors ultimately results in variation in the consistency of the compound in terms of free flowing property, bulk density and temperature.
- It is therefore necessary to monitor above 11 factors and take appropriate actions.
- Compounding and mixing process owner should take a round in extrusion, molding or calendaring shop floor (immediate customer) to see how his compound is performing.
- He should discuss with the operators for processing feedback to improve processability and QC people about what improvement he can provide in quality.
- This process owner can also be instrumental in producing a consistent compound at lowest cost without sacrificing quality.
- Can we call it value addition?
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