SANDWICH MOULDING

Example 2.8 A polypropylene sandwich moulding is 12 mm thick and consists of a foamed core sandwiched between solid skin layers 2 mm thick. A beam 12 mm wide is cut from the moulding and is subjected to a point load, IV, at mid-span when it is simply supported over a length of 200 mm. Estimate the depth of a solid beam of the same width which would have the same stiffness when loaded in the same way. Calculate also the weight saving by using the foam moulding. The density of the solid polypropylene is 909 kg/m and the density of the foamed core is 6(X) kg/m. ... 

Example 3.15 A sandwich moulding is made up of solid skins with a foamed plastic core. The skins and core may be regarded as isotropic with the following the properties ... 

This is an injection moulding method which permits material costs to be reduced in large mouldings. In most mouldings it is the outer surface of an article which is important in terms of performance in service. If an article has to be thick in order that it will have adequate flexural stiffness then the material within the core of the article is wasted because its only function is to keep the outer surfaces apart. The philosophy of sandwich moulding is that two different materials (or two forms of the same material) should be used for the core and skin. That is, an expensive high performance material is used for the skin and a low-cost commodity or recycled plastic is used for the core. The way that this can be achieved is illustrated in Fig. 4.44. 

The surface finish of injection-moulded foamed articles are not ideal because of external defects, burst bubbles, etc. This can be avoided by sandwich moulding. This is accomplished by initially injecting a slug of polymer melt into the mould followed immediately by a polymer containing a blowing agent, which chases the initial polymer into all the interstices. 

The polymer compound and components moulded from it usually are coloured uniformly throughout (that is, the colours of the surface and the interior are the same). However, with a view to reducing costs of colouring and of compounding, methods have been developed for colouring only the surface, by sandwich moulding or by co-extrusion of product and surface layer. 

Interface Reactive Sandwich Moulding as a Special Two-Component Injection... 

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. 

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... 

Fig. 16 Reactive sandwich moulding - image of fractured specimen afitea- tensile test, PTFE polyamide bulk material on the fracture surface of the polyamide material...

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. 

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

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. 

Sandwich Moulded TV Screen Frame Elco Plastics Limited - Thorn EMI Limited... 

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. 

Sandwich moulding (SM), 78, 111, 113, 116 comparison with structural foam, 89 high quality finish, 117... 

---