Melt Temperature Check - Best Practice Method
Graham Webster
In carrying out this procedure, you must be aware that there are burning risks.

Ensure that you only carry out the procedure using the necessary safety equipment.
The safety equipment required is as follows:

  • Safety spectacles or face shield
  • Heat resistant gauntlets.

These are in addition to the usual good practice of wearing overalls and protective shoes/boots. Ensure that other personnel, including operatives, are clear of the machine during the procedure. Pay special attention to the rear side of the machine and any personnel working on adjacent machines. Make them aware of your intentions to carry out the process.

Equipment required for the process is a follows:

  • Pyrometer fitted with 'needle' type probe suitable for immersion in the melt. Do not use a surface contact type of thermocouple probe.

The best practice process is as follows and for accuracy, should be carried out after at least moulding 10 cycles

Wear all safety equipment.

Set the machine to 'HAND' operation.

Withdraw the injection unit as far as possible so that the nozzle is clear of the back of the fixed platen.

Adjust the injection speed and injection pressure to low values for safety.

Place a piece of cardboard on the bed of the machine under the nozzle.

Operate 'SCREW ROTATE' to create a charge of melt at the front of the barrel.

Ensure that the nozzle guard is closed and operate the 'INJECT' or 'SCREW FORWARD' control - the machine will purge a shot of melt onto the cardboard.

Plunge the needle probe into the centre of the purged melt and move the probe around within the melt for some seconds, whilst watching the read-out on the instrument. When the temperature indicated has stabilised at a constant figure, this is the true melt temperature. Record the temperature on a note pad.

Remove the needle probe as soon as the temperature has been recorded to avoid the melt freezing onto the probe.

Remove the purging with the cardboard and dispose of it into a metal bin. Clean the probe immediately, removing all traces of melt from it so that it is ready for the next usage.

Finally compare the temperature measured in this way with the indications from the machine controllers - differences of more than 10°C are not uncommon. Variations greater than this may indicate a fault on the machine instruments or the thermocouples, or may be the result of long residence times. If the latter, adjust set values until desired melt temperature is achieved.

Plastic.IT 2005

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