Alongside the undoubted advantages of HR technology, there are also limitations to its use.
Information on the limitations of HR systems may be of particular assistance when making an objective assessment of whether to install one.
Technical benefits from using HR are:
• Simplification of the design of certain types of mould. Use of CR moulds with an extra parting line (moulds known as three-plate moulds or with a so-called intermediate plate) (see Figure 2.5a) have major restrictions. The functioning of such moulds is difficult to automate because of the sprue removal aspect, and because of the tendency for the sprue to get stuck between the mould plates. The heavy moving plate of the mould can be a cause of rapid wear to the plate guide system. The same could be said of its drives and mechanical travel limiters. Besides this, the opening and closing time for a mould of this type is always longer than for moulds with a single parting line. The proportion of the sprue in the overall injection mass in three-plate moulds is also greater. • Eliminating the fall in melt temperature which occurs in a cold runner, has enabled the use of a longer flow path in the cavity. This is of significance in the case of partially- crystalline plastics, which feature a narrow temperature window that prevents compensation of the temperature drop in the CR by raising the injection temperature. • The plastic flowing into the cavity is at a controlled temperature (assuming precision
• The lower pressure loss in the HR means a higher cavity fill pressure is available. • In moulds for large products, there is greater freedom to select the optimum location
for the injection points, which leads to a more uniform filling, with a smaller loss of temperature and pressure in the mould cavity. Engineering mouldings show less differential shrinkage and lower internal stresses.
• A reduction in the injection pressure required to fill the cavity, which enables the required injection moulding machine locking force to be reduced.
• The potential to regulate the holding time by controlling the time for which the gate is live or by mechanically closing it. The reproducibility of self-cooling conditions for a plastic under pressure, and thus also of moulding shrinkage, are thus improved. Holding pressure loss is also less than for CR, since the gating runner has a larger operating cross- section which does not diminish through the formation of an insulating layer.
• HR make it possible to design moulds of, for example, the modular type for thin- walled packings, bottle preforms, and so forth, and also moulds for sequential moulding, moulds for in-mould decoration and lamination, and so on.
• The potential for further development of stack moulds and for design of moulds with gating via a long core.
Limitations of the use of HR:
• An HR system must be selected for a particular application and plastic. A change of colour of the plastic may be difficult and time-consuming if this has not been allowed for in the design (this is also true of systems with internal heating). A change in the type of plastic may be difficult or even impossible if, for example, nozzles of a different type have to be used for the new plastic.
• There is an increased risk of damage to thermally sensitive plastics, which, following plasticisation in the injection cylinder, must also feature resistance to over heating in the HR system. The ‘dead space’ that occurs in some systems causes stagnation and the risk of breakdown of the plastic. The pros and cons should always be weighed up when an HR system is being considered for use with polyvinyl chloride (PVC), high temperature plastics and for plastics with additives that reduce flammability. The thermal loading on the plastic may be considerable, especially during breaks in operation. • The system is vulnerable to mechanical contamination of the raw material, which
may cause gate blockage. This is countered by locating an additional filter in the injection nozzle or sprue bushing.
• A certain amount of experience is required to avoid gate stringing or drooling of some plastics from the nozzle.
• The distribution and number of small cavities is limited by the nozzle diameter. • The use of an HR system has a substantial bearing on increased mould height, which
may not exceed an admissible value for the given injection moulding machine. • One condition of proper functioning of an HR system is the automatic operation
of mould and injection machine, as far as possible without between-shifts or maintenance stoppages.
• Mould operations may run into problems if trained staff are not available. Start-up of the HR system, change of plastic, stopping and, if necessary, cleaning of hot runners cannot be done without some knowledge and observance of certain principles. For the same reason, repairs and maintenance should be carried out by trained staff only. Damage to moulds caused by unskilled servicing generally leads to serious financial losses.
• A considerable expansion of the scope of maintenance is required.