Mould temperature control

The mould temperature controls the degree of residual sircu in the moulding and. in crystalline polymerv the crysialbniiy (see CTiaptcr 2i Both... 

At this point, a warning should be given that trial injections should not be carried out without monitored mould temperature control. Not only would this have the consequence that later, in production conditions, the dimensions obtained could in certain circmnstances be completely different, but also that the cycle times could be altered for the worse. 

Fluids are usually used for mould temperature control, because both heat extraction and heat supply can be controlled equally well. Electrical heating for soft temperature control is certainly very rapid, but still not fast enough on its own it would cause excessively high temperature variations. In all cases, electrical heating is used in conjimction with fluid temperature control, the latter being there to provide - if required - for sufficiently rapid heat extraction (so as to keep the temperature variations low). This combined temperature control system does indeed obtain the best results, but of course costs are significantly higher, and only rarely economically acceptable. So in what follows only pure fluid temperature control will be considered. 

Satisfactory results can only be obtained from mould temperature control if the performance of the temperature control unit is suitable for the quantities of heat to be exchanged in the mould. For example, if the temperature eontrol unit not only operates well, but also sufficiently rapidly, so that only slight temperature variations occur in the mould. 

Measuring the Mould Temperature - Checking the Uniformity of the Mould Temperature Control... 

The copolymers have been used in the manufacture of extruded pipe, moulded fittings and for other items of chemical plant. They are, however, rarely used in Europe for this purpose because of cost and the low maximum service temperature. Processing conditions are adjusted to give a high amount of crystallinity, for example by the use of moulds at about 90°C. Heated parts of injection cylinders and extruder barrels which come into contact with the molten polymer should be made of special materials which do not cause decomposition of the polymer. Iron, steel and copper must be avoided. The danger of thermal decomposition may be reduced by streamlining the interior of the cylinder or barrel to avoid dead-spots and by careful temperature control. Steam heating is frequently employed. 

Fig. 4.61 illustrates that the mould temperature is quite different from the set oven temperature (330°C) or indeed the actual oven temperature, throughout the moulding cycle. An even more important observation is that in order to control the rotational moulding process it is desirable to monitor the temperature of the air inside the mould. This is possible because there is normally a vent tube through the mould wall in order to ensure equal pressures inside and outside the mould. This vent tube provides an easy access for a thermocouple to measure the internal air temperature. 

Control of the temperature of rubber compound whilst being processed in extruders and injection moulding machines is vital if the product from these processes is to be uniform in quality. Failure to control temperatures in processing equipment can lead to scorching of the compound as it emerges from the extruder die or injection machine nozzle. Most modem items of processing equipment are fitted to a temperature control unit using either oil or water as the circulatory medium. 

To produce mouldings with high surface gloss the finishes of the cavities and cores of moulds must be excellent and free from faults. Since the temperature affects the rate of output and the finish on mouldings, precise control of this also is very important. 

The optimum horizontal tracking speed was found to be 40 mm/min acquiring 1100 data points, i.e. a horizontal spacing of 36 microns/data point. For measurement, a sample of liquid chocolate which has received a known temperature-time-shear profile is placed within a solidification rig that consists of a mould with known coefficient of expansion, contained in a temperature-controlled holder. 

Proper temperature control channels to maintain correct balance between heated plastic and cooling via metal mould. 

The two components are kept in temperature-controlled tanks, with pumped re-circulation when injection is not taking place. For the polyurethane system, one tank contains an isocyanate (usually MDI) and the other a mixture of polyol, chain extenders, catalyst and mould release (and possibly blowing agent or reinforcing additives). An amine catalyst accelerates the initiation of the polymerisation, while an organotin catalyst... 

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