The sandwich moulding process

The original ICI sandwich moulding process had certain disadvantages which were overcome by redesigning the mode and sequence of injecting the skin and foam-core melts. All further comments refer to the modified process which is now used almost exclusively. 

The principle of the two-component process is as follows. Compact, i.e. skin, material is admitted to the mould first, closely followed by foamable material, thus skin and core materials are being injected simultaneously for most of the injection part of the cycle. At the end of the shot a little compact melt is injected so that, (a) when the sprue is cut off the cellular core is not exposed, and (b) the nozzle contains compact melt in preparation for the next cycle. The process is easily visualized using the analogy of a rubber balloon being blown up inside the mould cavity. The balloon corresponds to the skin material and the air within the balloon to the foam core. 

The heart of the process lies in the special nozzle block which is, in effect, a composite shut-off nozzle consisting of two nozzles, one inside the other, and actuated by separate hydraulic cylinders. 

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On the basis of these results, the sandwich moulding process was investigated by use of the controlled formation of chemical bonds for the improvement of the bond strength in the interface of the two components - the skin and the core component. The advantage of this process is a longer melt contact time during the mould filling period. 

COMPARISON OF THE SANDWICH MOULDING AND STRUCTURAL FOAM PROCESSES... 

This chapter describes the development and manufacture of the Thorn 1 IC 40 Model television-screen frame, sandwich moulded in high-impact polystyrene (HIPS) by Elco Plastics Ltd, Hemel Hempstead. The sandwich moulding (SM) process is described in some detail in PST 2 and comparisons are drawn with structural foams (SF) which share many common features with SM. 

There are a number of variations of the injection moulding process, one that is particularly applicable to recyclates is coinjection moulding. Co-injection moulding produces a sandwich like structure that can be used to encapsulate recyclate materials, covering them in a layer of virgin skin material. 

A variation of thermoforming which does not involve gas pressure or vacuum is matched die forming. The concept is very simple and is illustrated in Fig. 4.53. The plastic sheet is heated as described previously and is then sandwiched between two halves of a mould. Very precise detail can be reproduced using this thermoforming method but the moulds need to be more robust than for the more conventional process involving gas pressure or vacuum. 

The sandwich process extends the scope of injection moulding and (as an instance of this) makes it possible to provide fibre-filled mouldings with surface finish of very high gloss. It is applied to produce reflectors for headlights in motor vehicles—where the substrate is heavily filled and resistant to distortion by heat yet the surface is mirror-like. 

Component parts with PTFE polyamide materials as the skin component and reinforced polyamides as the core materials made by sandwich moulding will lower the material and processing costs and reduce the amount of PTFE polyamide... 

A good example of such a continuous moulding process that incorporates Foam Film technology is a system developed for the production of industrial vehicle carpets. These products are usually made out of a sandwich of two or more layers of different materials. Individually, they provide different features aesthetics and function (a textile carpet or a synthetic mat), sound deadening (a layer of polyurethane foam) and protective (a cheap layer to protect the foam from moisture and degradation). 

In the next group of studies the structural foam (SF) and sandwich moulding (SM) techniques are featured. PST 2 deals with the principles of these processes and the design possibilities they offer, and there is also a comparison with the conventional process, known as compact injection moulding, or CIM. 

The term structural foam refers to materials which consist of a cellular core surrounded by a solid skin. The core and skin are made from the same material and the composition is characterised by a dramatic increase in flexural stiffness compared with similar products - weight for weight -produced from solid polymer. The term sandwich moulding also refers to solid skin/cellular core materials but in this case two or more polymer melts are injected from separate chambers into the mould cavity the SF process involves injecting a single material into a mould cavity from one screw. In contrast to SF the sandwich construction very often comprises different polymer types for skin and core, as we shall see later. 

The Mono-Sandwich technique for co-injection moulding was described in Section 10.8.2.3 to which the reader should refer for machinery details. This technique can also be used for over-moulding by using the core back technique, again described earlier. In this technique termed the monosandwich 5 process, an additional valve is required in the runner system that ean release different valves as necessary . Once the melt is layered, the first component is injeeted. The valve is switched within the mould to expand the cavity and then the rest of the shot is injected to ereate a multi-shot component. 

As an example of another form, co-extruded blow mouldings have been introduced to the market as containers for liquid detergents. The process used is known as sandwich blow moulding and provides a means by which recycled high density polyethylene can be incorporated again into a useful form by being employed between two layers of virgin material. 

The co-injection moulding of PVC-U with other thermoplastics (glass fibre reinforced PVC, polypropylene, ABS and polycarbonate), was investigated using the mono-sandwich process and the properties determined. Polypropylene was the only polymer not to exhibit good adhesion. The mechanical properties of the other samples were intermediate between those of the constituent polymers (104). 

The fine control over pore size and pore size distribution in CMPs [11, 19] suggests that these materials might also be promising for gas separation applications, although it would first be necessary to solve the non-trivial problem of synthesizing or processing these materials as defect-free thin films or as a moulded, sandwiched layer. 

This process which was used by Plastics Recycling Ltd. in the UK in the 1980s can accept mixtures of any plastics and a small proportion of non-plastic materials e.g. paper and metals). It does not involve a screw extruder and instead waste granulated polymers are laid on a black plastic film and covered by an identical film to form a sandwich. The mixed plastics are sintered by passing through an oven and finally compression moulded into a board. The product is very tough but has... 

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