Temperature control system mould

Moulds with a temperature-control system incorporated into the design. 

During the cooling of the component, the heat is drawn from it by the mould. The amount of heat involved depends on the component weight, the type of plastic (specific heat content), the ejection temperature, and the cycle frequency. The mould passes this heat to the environment (by heat conduction to the machine and by radiation or convection to the ambient air), if no special temperature control system is connected up. The higher the mould temperature, the greater the amount of heat released to the environment. So, when the machine starts up, the mould initially heats up to the temperature at which the amount of heat supplied by the solidifying and cooling plastic can be released to the environment. 

Again this clearly shows how important a good temperature control system for the mould is (it must be able to exehange the requisite amoimts of heat fast enough). The temperature control device must also be suitable for the specific system (so as to be able to supply and extract these quantities of heat). 

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. 

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. 

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... 

The mould temperature is controlled by t>assing temperature-equilibrated fluids through interconnecting drilled holes in the mould. It is the function of the thermostatted fluid to remove heat as quickly as possible from the mould and transfer it to the cooling system. The requirements are ... 

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. 

Earlier it was stated that press manufacturers have made substantial improvements to the temperature control of platens. This work has to be extended to ensure that the moulds become integrated into the system. Some thermal control systems are available which allow such control, but cost is cited against their use to eliminate cavity-to-cavity variation. It is not logical to expect any system to maintain close temperature control over an area of half a square metre from one pair of thermocouples Greater control will pay real dividends by allowing the technologist to optimise a cure time that is based on equal crosslink density rather than the cure time required for the coldest cavity. 

Exactly the same kind of factors determine the material choice for the new electronic control systems. The role of the plastics material is to provide encapsulation, or a housing, or a circuit board component, keeping the conductors separate and functioning over a wide temperature range, often in the presence of hostile chemicals. A sensor, for instance, may be located permanently within a rear axle, like the AB Elektronik speed sensor for BMW. The part of the encapsulation which is in permanent contact with the hot oil, at up to 160°C, is in PA 46, while the less vulnerable part is moulded in PBT (see Fig. 7.6). Other sensors, with temperature exposure confined to the —40 to -H30°C range, used for so called drive-by-wire systems con-... 

Use of mixed CR and HR systems has the advantage (leaving economic considerations aside) of simplifying the problem of temperature control in the HR nozzle gate area, and additionally enables the cold plug from the injection moulding machine nozzle to be halted. 

The whole HR system is divided into temperature control zones temperature measurement and control are carried out with the aid of automatic regulators separated from the mould. 

The thermal equilibrium of any HR system is governed by variable and fixed values of all components of the thermal balance of the mould/injection moulding machine/ambient conditions system. The fact that variable factors are always present means that automatic temperature control and regulation of the HR system must be used to keep deviations from the required temperature to a minimum. The following initial conditions must be fulfilled ... 

The control system is there to ensure repeatability during moulding operation. It monitors both the hydraulic system and the process parameters such as temperature, injection speed, screw retraction speed and injection and back pressure. The ability to control the process has a direct impact on final part quality, part to part consistency and economy. The nature of the control system may vary from a simple relay switch to a complex microprocessor system with closed-loop control. Some of the components of the machine control system will now be introduced. 

The conventional injection system, that is, the non-temperature controlled, mould-integrated sprue runner, provides the simplest way of achieving this. Injection systems such as those with thermoplastic processing are possible here. 

This is as much a matter of the level of mould temperatures as of their imiformity and repeatability. Economic quality improvement in injection moulding is not possible without good and imiform temperatures in the mould. Even with a more expensive injection process control or adjustment system, the negative influence of unsatisfactory mould temperatures can not usually be balanced out. If it is a question of narrowing the tolerances of the components, the first step is to check the mould temperature data, and, if necessary, these must be improved before, for example, doing any research into injection pressure control on the basis of the internal mould pressure. 

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