Introduction to polymers
Introduction to polymers

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Introduction to polymers

6.4.2 Manufacturing the boat

Filling the mould is a serious problem in injection moulding: the lower the MFI, the more difficult it is to push molten polymer down narrow tubes into a metal mould. The engineers at Rolinx, the trade moulders who initiated the thermoplastic project, were working at the limits of their machinery in moulding such large objects in one operation. In the event, they were forced to blend the low MFI copolymer grade with a higher MFI grade material (GY702M) in order to achieve their objective (Figure 64). Following injection of the hull and deck, the two parts are fused together near the waterline by incorporating a tape made from metal wire and PP fibre at the joint. When a large electrical current is passed in the wire, it heats up and the two parts fuse together, a process aided by the application of an external load (Figure 65). Apart from several fixed parts such as protective plates for the rudder zone, the accessories (mast, rudder, daggerboard, etc.) can then be added by hand assembly, a particularly easy task since the boat is designed for ease of disassembly for the consumer or user. Indeed, these items are usually packed in kit form for ease of transport to the point of sale.

Figure 64
Figure 64 Molecular mass distribution of propylene copolymer determined by high temperature GPC. The curve shows two peaks since it is a blend of low and high MFI materials
Figure 65
Figure 65 A section through the hull-deck weld showing how the flange on the hull section fits into a recess in the deck section, and the joint fusion welded using sacrificial metal tape

The sailing, if not commercial, success (total sales about 2000) of the original GRP Topper provided the Rolinx engineers with design guidelines which were explored in depth with the original designer, Ian Proctor. In one sense, the early GRP boat replaced a conventional prototype, but with the benefit of market feedback. Switching to thermoplastic produced many design changes, some such as a higher rake angle for injection moulding (Figure 66). In addition, most accessories were replaced by thermoplastic equivalents which have proved successful in the new version. At the same time, the redesigned hull incorporated several features which enhanced its ease of assembly, performance and integrity: for example, the sacrificial daggerboard box, which provides failsafe protection for the hull against grounding forces (Figure 67), and the mast locking gate which enables rapid insertion of the sail and mast on launching (Figure 68). The rudder and daggerboard are parts which must be able to withstand severe impact blows without failure, and both are made from glass-reinforced polypropylene. To reduce the weight as well as improve manufacturing efficiency, both are made by foam moulding, where a foaming agent is added to the solid polymer before injection, so that it expands on heating the tool to create a foamed interior.

Figure 66
Figure 66 The GRP (left-hand side) and (right-hand side) Topper boats compared: (a) deck sections, (b) hull sections. The redesigned boat incorporated a higher rake angle on the mast access slot and a daggerboard box with failsafe plate
Figure 67
Figure 67 Sacrificial dagger board plate attached to base of hull – grounding forces will only damage the plate and cracks cannot propagate into the hull itself

By 1984, sales had exceeded 23 000 and the boat was still selling over 2000 per annum, so the high capital investment in the tools and machinery had been well justified. The story of the Topper is one of relatively small businesses and companies spotting the potential for efficient mass production of a high added-value consumer product. Design in polymeric materials demands a bipartisan approach – design for function and design for manufacture (but see Box 12).

Figure 68
Figure 68 Mast locking gate for the Topper dinghy redesigned in glass-filled polypropylene: (a) split apart to show component parts. Upper and lower plates and rotating cam activate by drawcord; (b) shown in situ in closed position without mast. The composite moulding replaced a single marine ply board drilled with a single hole for the mast

Box 12 The Toucan boat

Following the successful development of the Topper boat, attempts were made to design a larger boat using injection moulding. It was thought that a large rowing boat could be designed in the same way, having a significant competitive edge over wooden equivalents owing to the rot-resistant properties of polypropylene. The resultant prototype, the Toucan, is shown below in Figure 69, compared with the Topper at left.

Although the lower hull was moulded in one piece, the structure was by its very nature, an open-sided shell. The deck was positioned below the top of the hull, and was itself composed of several subsidiary moulded parts. So the project involved more than the two tools needed for the Topper, in itself increasing the development costs. The stiffening element of the monocoque structure of the Topper was much less significant with the Toucan, making the boat more flexible (especially when loaded in torsion). Finally, the design did not exploit the lightweight properties of the plastic to its greatest advantage. The Topper is designed to hydroplane over the surface of the water, giving the user exciting sailing at speed when under wind. The same couldn't be said about the Toucan, a boat with a deep hull designed more to plough through the water rather than skim along the top. The project was eventually abandoned following poor performance in trials, showing the limits of plastic materials when simply used to copy existing conventional designs without allowing for the problem of high flexibility. Existing wooden dinghies exploit the properties of wood, a natural composite, to its greatest advantage. Thus the side timbers of the hull are planks following the grain, so are intrinsically stiff and resistant to imposed bending and torsion loads. This gives a stiff structure capable of resisting sailing stresses, and so giving safe and predictable sailing.

Figure 69
Figure 69

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