When the screw within the barrel of the injection moulding machine rotates, it draws material from the hopper via the feed throat and propels it along inside the barrel.Once inside, and specifically as the material passes through the compression zone on the screw, it melts or plasticises. This is due to the combined heating effect of the heater bands around the barrel and the shear heating caused by the rotation of the screw. The actual temperature of the molten material is referred to as the melt temperature.
Most thermoplastics can be processed over quite a wide melt temperature range, but the material supplier’s data sheet should always be consulted in order to identify the recommended temperature range for the material. If such data sheets are not available, the temperature ranges indicated in the below table, can be taken as a reasonable starting guide for the more commonly encountered materials. Note that these temperatures values are
for the actual melt temperature, which may differ considerably from the set temperatures on the machine barrel temperature controllers.
Table of melt temperatures.
Material Melt temperature range (C)
ABS 190–290
Acetal 180–220
Acrylic 200–250
HDPE 210–270
HIPS 180–280
LDPE 180–240
PA6 230–290
PA66 270–300
PA11 220–250
PBTP 240–275
PC 280–320
PP 200–280
PS 170–280
PVC (Flexible) 170–190
PVC (Rigid) 160–210
SAN 200–260
Measurement of Melt Temperature.
In order to measure the actual melt temperatures, about 150 cm3 melt is purged out of the cylinder into a suitable container and measured with a pyrometer. [Note: This is a potentially dangerous operation, and must be carried out with extreme care.] A needle, or
melt probe attached to a suitable pyrometer is then plunged into the melt and slowly moved around to search out the hottest spot. The maximum temperature indicated is taken to be the melt temperature.
Degradation of Materials During Plasticising.
With any material it is inadvisable to exceed the maximum melt temperature as specified by the material manufacturer. If this happens the polymer may undergo degradation, leading to changes in flow characteristics, reduction in mechanical strength, and discoloration. Overheating of heat-sensitive materials such as PVC or the acetals can lead to very rapid degradation and the liberation of substantial quantities of noxious fumes.
It should also be remembered that molten polymers, even at temperatures within the recommended processing range, can also decompose quite considerably if they are held at such temperatures for an excessively long time in the cylinder. The length of time a polymer can be held at a particular temperature without the risk of
undue degradation is known as its maximum residence time at that temperature, and this time should never be deliberately or accidentally exceeded. For this reason, it is always good practice to purge the cylinder empty if a machine has to be stopped for more than a few minutes.
Walsh's Plastic consulting. 