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| Mono bug clamp - Technical Moulded Systems Ltd | |
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Designing the manufacturing process Now he needed some help - so he contracted a design-engineering company to develop the design in detail. Among the requirements of his brief to them was that the peg should look quite distinctly different from the established Italian competition and that the spring form (G)should be optimised (G) for maximum performance. The original spring had two straight arms but now a design was derived from cart springs as developed for cars, with two back-to-back. These springs taper through their curvature to be stiffer in the middle - for maximum power - and thinner at the ends - for maximum flexibility. Together these make what is called a 'progressive rate' spring effect. That is a spring that would be easy on your fingers to open, but would clamp hard when closed to keep the clothes securely on the washing line. To be able to revolve the spring into the peg a 'live hinge' was created. A live hinge is a very thin section of the material (15 thousandths of an inch) to allow it to fold. In any plastic suitable for the peg this hinge would be practically indestructible. The engineering designers produced five or six quite different designs - on their CAD system (G), one of which was selected by Ivor for development into manufacture.
The first prototype At about this time Ivor was talking to an old colleague who had recently installed an injection moulding machine in Warwick University's manufacturing department. He had seen their new 'rapid prototyping' unit which could produce three-dimensional models straight from a computer (CAD) file. So Ivor got the Warwick people to produce his refined design by rapid prototyping in nylon, and Eureka! he had a working peg - though in this material the spring wasn't quite as it would be. Otherwise, this was highly successful and only very small changes followed. So a CAD drawing of the design was finalized.
Second prototype This drawing (actually a full three-dimensional 'virtual model') went to the toolmakers for an injection moulding 'tool' or 'die' to be made which would make two pegs at once (a 'two-impression mould'). The toolmakers were responsible for ensuring that the mould would allow the molten plastic to flow efficiently throughout the mould cavity to produce high quality pegs, reliably. This was a complex design!
Getting the right material Having proved the design in general and refined the form of the peg, the right material needed to be found. The requirements were that:
To do this the first production moulding machine had to be purchased - and this development phase took a full year. The material chosen was a polypropylene. This was selected against the nearest competitor plastic as it was stiffer, less rubbery and had a stronger memory effect for the spring action. Various colours were tried including some very bright fluorescent ones.
Initial market testing The first products in the chosen material, called production prototypes (G)- were tested in use and given to others to test. On contacting the director of the biggest direct marketing (door-to-door sales) company that sold clothes pegs, Ivor was lucky. The director liked the design, was attracted by the fluorescent colours and the unique metal-free design. He was reassured by the cost of production and placed a large order very soon after. This gave Ivor's company, Technical Moulded Systems, five months to gear up for a high volume of production. The prototype tool, with just two cavities being moulded at once, was simply not going to produce quickly enough. Under-capacity (not being able to make enough products quickly enough) can be as much of a problem as over-capacity (having machines etc lying idle because not enough orders have been received to keep them busy). A big customer expects a reliable supply of products to keep up with demand from consumers, or their reputation will suffer, so this stage of production development was as important as any.
Production tooling and capital costs A new tool - or moulding die - had to be made and this was designed to produce not two but 32 clothes pegs from each injection of polypropylene. This tool, two pieces of steel with the necessary moulding and plastic feed cavities machined into it, cost over £30,000, on top of the £70,000 price of the injection moulding machine that it was to be run on. These costs are called capital costs (G). A moulding tool is a very demanding item to produce as it has to be machined with exceptional accuracy and to the highest possible quality of finish. Any blemishes would be reproduced in every moulded product - millions of times over. Extra to the basic moulding machine is the feed arrangement. This involves a store for new polypropylene granules piped through to a device which measures out the right mix of granules, colour pigment and re-ground spare material to ensure a consistent colour and quality of product. Molten polypropylene is injected into the mould along the feed channels to the peg-shaped cavities. These are machined half into each side of the mould (the mould is in two halves to allow it to open and release the new mouldings). You can see these in the photo of a mould half.
Plastic that cools in the feed channels is spare, not part of the pegs, and known as a sprue (G). A special feature of this mould is that at the end of each sprue are very small holes known as 'pin gates' which allow the plastic into the peg cavities. This means that as the mould opens the sprues break cleanly away so it is not necessary to trim them off by hand. Hand trimming is very common in the injection-moulding industry but it adds a lot to the costs. This is typical of the detailed thought that allows the company to produce these pegs cost-effectively (G).
Cycle time No - nothing to do with bikes! This term is used to refer to the time it takes the machinery to produce one set of 32 pegs and be ready to start on the next. The unit costs (G)are reduced the faster the product is produced, so shortening the cycle time is very important. This relies on the time taken after the batch of pegs comes out of the mould. With this peg, there are two important aspects that control this length of time:
Labour costs and work flow A major advantage of the one-piece, injection-moulded design was that the peg could be produced by an entirely automated process with 32 pegs coming off the machine every few seconds. However, this brings other problems - the rest of the process has to keep up with the rapid rate of production of the moulding machine. Even in this simple, one-piece product, although no other parts need to be assembled, the spring needs to be snapped into place after moulding. At first this was done by hand with the help of a lever-operated jig. This took only a moment or two but to do this 32 times took far longer than the machine took to mould the parts. Once production was fully underway it took ten people to clip springs into place fast enough to keep up with the moulding machine. Counting batches of 20 pegs, placing them in plastic bags and stapling on a label required another 6 people to keep up. For a company with only two other employees 16 was quite an increase in staff! And staff are a very expensive overhead (G). This situation can be described like this:
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The
injection moulding machine
