Use of Moulds with HR

HR systems are vulnerable to damage. The additional requirements relating to the design of other functional systems and automated operation raise the level of responsibility for proper management of the moulds. This relates to  

HR moulds are for use in mass production, i.e., continuous operation, and the high cost of any downtime must be considered. Only highly-skilled staff may be involved in work with these kinds of moulds. Supervisory staff must be familiar with the mould documentation to be able to identify the HR system used and adapt process instructions to it. This also means that when a ready-made mould is purchased, the contract must insist on supply of full technical documentation and process instructions for use. 

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Improved utilisation of injection moulding machines. Use of moulds with HR makes it possible to ... 

For a rough assessment a simplified calculation of the threshold number of cycles, designated as glc, i.e., production volume/number of cavities, above which it pays to invest in a mould with an HR system is used. The formula does not include the reduction in personnel (cost G) or the rise in mould cost (maintenance cost L and interest M), and the result is approximate. 

Note. Full thermal balance and correct functioning of the HR system may only be achieved through automatic operation of mould and injection moulding machine, thermostatic control of the mould and the use of plastic material with no mechanical impurities. 

Two special injection moulding methods have been developed to get around this problem, both based on HR systems with multiple gating sequential moulding and cascade moulding. Both methods require the use of shut-off nozzles with independent control. 

In view of the advantages of HR systems with internal heating (described in Chapter 4.2.2), they are also used in stack moulds. Of particular importance here is the potential to reduce the mould height. Figure 9.14 shows a fragment of a stack mould with a long sprue bushing, internally-heated manifold and two-way torpedo. A characteristic feature of this system is that the manifold is the mould plate. 

Figure 9.14 Example of use of an HR system with internal heating in a stack mould 1 - sprue bushing 2, 3 - cartridge heaters 4 - manifold 5 - two-way torpedo Reproduced with permission from D-M-E Belgium)...

A clear improvement of the mechanical characteristics with regard to tensile and impact characteristics of multilayer web reiirforced PLA composites was achieved by using modified process parameters (compression moulding technique CP-2). In comparison to the processing technique CP-1 a pre-drying step was inserted (2 hr at 105°C) and the compression molding time in the hot press was reduced to 5 min. The cooling of the composites occurred in a cold press for 5 min at 25°C. 

Unsaturated polyester (Butanox M-60) mixed with 1.5% of Methyl Ethyl Ketone Peroxide (MEKP) as catalyst was selected as a resin for the current work. Treatedbetelnut fiber reinforced polyester (T-BFRP) composite was fabricated using hand lay-up technique. In composite preparation, a metal mould (100 x 100 x 12 mm) was fabricated. The inner walls of the mould were coated with a thin layer of wax as release agent. The first layer of the composite was built by pouring a thin layer of polyester. A prepared mat was placed carefully on the polyester layer. Steel roller was used to arrange the mat and eliminate trapped bubbles. This process was repeated until the composite block was built containing 13 layers of fiber mats and 14 layers of polyester. The prepared blocks were pressed at approximate pressure of 50 kPa in order to compress the fiber mats and to force out the air bubbles. The blocks were cured for 24 hr and then machined into specimens in the size of 10 x 10 x 20 mm. 

A type (a) mould of three-plate design with heated sprue bushing is converted using an HR manifold and nozzles with side gating. The cycle time has been reduced by 35%, and the production cost has fallen by 43%. 

Various types of heater are used, depending on the requirements made of individual HR system elements and their design (see Chapter 4). They are manufactured and made available by specialist firms or by the manufacturers of standard mould bases with special recommendations for their installation and use. 

Manifold design. The choice of manifold is not as problematical as the choice of nozzle, since the range of manifolds is much more modest, and in most cases the manifold is made to order, and hence adapted by the HR system manufacturer to use with a specific plastic, product and mould. Manifolds to some extent have their shapes and some dimensions standardised, which should be borne in mind when planning the cavity layout. The minimum cavity spacing is additionally governed by the nozzle diameter. 

In a mould for radio dials (Figure 8.5), three HR nozzles have been used [3]. The cavity was fitted with a pressure meter and thermocouple, enahling the mould temperature and holding pressure to he monitored directly, thus minimising shrinkage of the plastic and making it reproducible. Too much shrinkage could cause delamination of the decorative layer. 

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