Control unit mould 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. 

Machine runs for the MDI flexible moulded foam were conducted on a Krauss-Maffei, cylinder displacement series, high pressure machine. Fresh premixes, consisting of the appropriate polyols, water, crosslinker, surfactants and catalysts for each formulation were charged to the machine. Modified MDI was used throughout the entire study. All chemical temperatures were held at 25 °C 2 °C via the machine s internal temperature control units. Foam pours were made into an isothermally controlled heated aluminium mould maintained at 60 °C 2 °C. The mould was a typical physical property tool designed with... 

Cold runner systems have a similar temperature control requirement to the plasticising and injection unit, since the cold runner acts as an extension to the nozzle, controlling the rubber temperature to preclude any build up of scorched material while it awaits injection into the mould. 

For higher hard control requirements and for all cases involving soft temperature control, fluid temperature control units (forming units, heating units and cooling units) are used, which pump up a temperature control medium (water and oil) through the temperature control system of the mould. The cycle temperature of the temperature control medium is adjustable, and is adjusted for a pre-set value. 

Mould temperature regulation can be used to reduce the effects of these interference factors considerably. A temperature sensor in the mould acts as an actual value pick-up, and signals the actual mould temperature to the temperature control imit. The flow temperature is now no longer kept at a constant value, but is adjusted by means of a regulator until the actual temperature in the mould corresponds to the theoretical temperature for the mould set on the temperature control unit. 

Matching Temperature Control Units and Mould Maintenance... 

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. 

If the dimensions of the temperature control unit were too large, that would not do any damage, but it would not be economic. If the dimensions are too small, this will increase the temperature variations in the mould, in every case. The temperature control units must therefore have available a sufficiently high pmnp delivery (1/min), with a sufficiently high transmission pressme level. It must be possible to overcome the flow resistances of the temperature control circuit with a suitable delivery. 

Unfortunately, the transmission pressme on a munber of temperature control imits on the market today is insufficient. A safety valve is frequently built into such units, which opens a parallel circuit within the unit, once a limiting pressure has been exceeded. Unfortunately this is not always noticeable to the user. Only a fraction of the flow delivery then flows through the mould, and the temperature control suffers accordingly. Naturally, the flow resistances of the temperature eontrol circuits should be kept as low as possible. To this end, there should be sufficiently thiek hoses, as short as possible, between the temperature control unit and the mould. The temperature eontrol channels in the mould must be of suitable dimensions. 

Four heater cartridges of240 Volts (V), 380 Watts (W) are used to heat the blow mould eavity block. A temperature control unit with an on / off function is used to control mould temperatures during each trial. 

Fig. 12. Schematic diagram of the experimental set-up for plastisol moulding into thin flat moulds 1 — loading of raw materials 2 — damper 3 — pump unit 4 — plunger 5 — discharge tube 6 — inlet channel (round runner) 7 — metallic bottom half mould 8 — top half mould (of lucite for low-temperature moulding and of steel for moulding into hot mould) 9 — pressure gauge 10 — stub tube for control 11 — drain cock 12 — conical flat tube instead of round point runner bolts for controlling the width of forming cavity within the limits of 0.15-1.5 mm...

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