Forensic polymer engineering

Transcription

We regard ourselves as being pioneers in our type of industry and pioneers in developing new manufacturing techniques and taking them to their ultimate. Flymo's success in developing air-cushion mowers has come from this attitude, evidence in their injection moulding techniques and in centralising their polymer processes on one site for automatic manufacture. Polymers are figuring more and more in new products and concepts and a knowledge of polymer engineering is absolutely essential. When competition is fierce and margins are slim, the use of polymers has given this particular company an added advantage both in terms of design for function and ease of processing to shape. In a moment we'll be looking at the application of both ideas to a specific product. We'll also be looking at the way polymer processes can be readily adapted for rapid, automatic production. As the series progresses we'll come to many factories like this one to look at both good practice and new ideas in action. The start of any design exercise is selecting the right material for the job. This is an example of one of the earliest air-cushion lawnmowers made. It was heavy, it was rather noisy in use because of the steel vibration, and it was relatively unattractive. It had problems with corrosion initiated by impacts caused by stones being thrown up against the side of the hood. It's pretty basic in its construction. The steel hood had a great number of limitations which included such things as the number of operations that had to be carried out once it was pressed to make it acceptable for assembly. For example, things like these lugs that locate the handle had to be attached on by welding, holes that had to be drilled and tapped and so on. The painting process was expensive and time consuming and we found that the demand was being stifled because of the manufacturing problems associated mainly with the deck. The next material Electrolux tried was GRP, here being used to produce motor car bodies. Being lighter than steel and less noisy, it gave the designers more freedom. But it still had limitations. It needed trimming and painting and a number of parts still had to be produced separately and then attached, so increasing assembly costs. But above all, production rate was too slow and couldn't meet demand. So what was the solution? Fortunately in the early '70s, Borg-Warner, Corporated in America came up with a grade of ABS thermoplastic which offered us a number of interesting properties for hood manufacture. So what are these properties? Well, the hood must be light enough to ride the air cushion without using a high-powered motor. It must also be stiff enough to support the motor internally. And tough enough to withstand external impacts as well as internal impacts from objects thrown up by the spinning blade. The problem is that a material like polystyrene is brittle. But if rubber particles are added to the polymerisation stage, a material with much improved properties results. The polybutadiene particles increase the toughness of the material and if a mixture of styrene and acrylonitrile monomer is used for the reaction the resultant polymer is known as acrylonitrile butadiene styrene or ABS. This is how the raw material arrives in the factory in a one-ton container. This bin is actually worth £1,200. In its virgin state, the raw material comes in the form of granules. It's worth twice as much as polypropylene and over five times as much as mild steel. So at this cost, what are the advantages? The big advantage to the user is that the part is much lighter and he's able to manoeuvre it over the lawn very much easier. The advantages to us, the manufacturers, of course, are even greater. The processing is much easier with ABS. As you can see, all these fairly intricate details can be moulded into the hood design to enable the assembly to be made cheaply and quickly. We use a stylo acrylonitrile to give us transparency in this lens application here. There are other polymers such as polyamide 6 and 66, polypropylene of various types, high-density polythene, polycarbonate and a number of other minor ones. So Flymo have chosen a particular set of polymers for their specific properties in their main product. The main component is the ABS hood and it is essential to know how the finished component behaves under in-service conditions. The most critical test is to withstand flying objects thrown up by the blade onto the inside of the hood. Steel ball bearings are loaded into a pipe which leads to the underside of the mower. 50 such balls will be shot successively into the spinning blade by compressed air. Production testing is conducted on an audit basis from regular production runs and a small sample are tested to destruction. If the material behaves in a brittle fashion and the ball penetrates the hood, the failure will mean that the entire batch of that particular run will have to be rejected. This is the result of the test. And on the inside edge of the hood you can see the indentations produced by the impact from the ball bearings. There's also quite a lot of damage to the metal blade of the impellor. On the outside edge of the hood you can see the phenomenon known as strain whitening which is produced by a ductile response of the ABS. So we've looked at the advantages of polymers in design for function and how these designs are tested. But what about the process they chose to produce the ABS hood? They needed precise detail and rapid production which they felt was only provided by injection moulding. This is one of 57 injection moulding machines used by Flymo to make their products. At the moment it's making one of their largest volume products, the hood for the Minimow. The projected area of the hood is relatively large so they need a 500-ton locking force machine to make it. Granules of ABS are added to the hopper. From there it is both mixed and heated to temperature by the shearing action of the screw. Aided by electrical heating, it reaches about 130 degrees centigrade above its glass transition temperature where its consistency is toffee-like. As the material moves down the barrel, it pushes the screw back until there is enough material ready for injection. At that point the screw behaves as a ram. The polymer viscosity drops to that of a thin syrup. The gate is opened and the molten polymer pushed through into the cool, steel mould. After cooling it can be removed ready for the next stage in the cycle. Such a machine can mould a variety of different shapes simply by changing the tool. It's this facility which allows a company to make a whole range of different products. This is one of the largest mouldings produced by Flymo, in this case for a two-wheeled, rotary mower. There's some intricate detail, particularly around the support for the impellor blade and this particular moulding also incorporates a re-entrant angle at the start of the grass-collecting duct. Yet the whole moulding only weighs 20 kilograms. It replaced an aluminium die casting which weighed over three times as much. This is the metal mould from which the hood was made. Hot molten polymer is squirted down the central runner and subdivided via 12 gates to the periphery of the mould itself. The grass-collecting duct forms a re-entrant angle at this point where a massive retractable core can be operated by a joint motor at the outside edge of the tool. The combination in improved production rate using injection moulding together with the weight saving led to a plastic moulding which was less than a quarter of the price of its aluminium equivalent. This tool is actually quite simple in concept. A much more complex problem arises when a moulder wishes to produce more than one component from just one tool. At Flymo they solved this problem using a technique known as computer aided design to produce three components in this one tool. Essentially the problem is one of achieving a uniform flow of polymer into the three quite different shaped cavities in the tool. The solution to the problem was to calculate the right gate size to feed polymer into each of the three different cavities. The success of this technique applied to this particular problem can be judged by the fact that the very first production run was entirely successful. But of course tool design is only one aspect of injection moulding. Injection moulding machines have to work efficiently and effectively throughout 24 hours of the day in a much larger manufacturing environment. Production runs from each mould may vary from 2,000 to 20,000 or more. So an important aspect of smooth production is the ability to change moulds relatively quickly and adjust the conditions for each moulding. Once a machine has been set up for a specific moulding, by fine tuning of the temperature, the position, the pressure, the timing and all the other variables which affect the mould cycle, conditions can be reset automatically using this program card which can be inserted into the microprocessor control console like this.