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Description: | Design analysis Polypropylene chair by Richard Carr This analysis describes the development of a chair whose designer set out to create an ttractive, comfortable and cheap product in a new and untried material. The articledescribes the history of the Hille polypropylene chair, from the requirements set by the designer and the opportunities and problems imposed by the material, to the marketing of the Mark 1 and the subsequent design of the Mark 2. It also compares both chairs with the 3S recommendations for office chairs-the only set of dimensions which gives any guidance for chairs of this kind - and for this comparison ,advice was given by Brian Shackel and David Barkla, both of whom are ergonomists. Polypropylene chair Mark 2, designer Robin Day, maker S. Hille & Co Ltd. Available in three colours. Prices: standard version (see page 36) £3 3s. Version with adjustable - height swivel base (opposite) £111 8s 6d; upholstered stacking versions from £10 1 2s. A variety of other bases available. When Robin Day set out to design a chair Dr Hille, his aim was to repeat the kind of fundamental changes in design thinking and se of materials achieved by the glass fibre hair designed by Charles Eames in 1956. "The problem I set myself", he says, "was to design a chair that was attractive, comfortable and low in cost. I also wanted it to be strong and practical - tough enough to tand up to really hard wear." And he wanted chair that could be mass produced. Fortunately for Mr Day, there was a new material at hand which could enable him to do this. Under development by the Shell chemical Co Ltd. it was called polypropylene, material produced by the low pressure polymerisation of propylene gas using catalysts discovered by Professor Natta in 954. Polypropylene is the lightest plastics known to man. It is also extremely strong and resilient. Other factors that made it a good material to use are that it cannot craze or chip, has good resistance to scratching, will only discolour under extremely strong light, and can easily be washed, even in boiling water. It is also warm to the touch and free from the waxy feel associated with a number of other plastics. "This plastics", says Mr Day, "made possible a one-piece seat and back, fully shaped for comfort, and gave me a choice of integral colours." Having defined his problem, Mr Day's next step was to design the seat and back. Here there were both advantages and disadvantages. The advantages were that enough anthropometric data have already been published to give a idea of the dimensions needed to make a chair both comfortable and suitable for a large number of people to use (see, for example, Measurements for Comfortable Sitting: a Short Guide to Chair Designers, issued by The Furniture Development Council). In addition, once the material had been chosen, it was believed that the behaviour of polypropylene itself could be accurately forecast by the Shell chemists. (In fact, this turned out to be only partly true, as explained later on in this article.) The main disadvantage facing Mr Day was that, once he had decided to use polypropylene, a mock up had to be simulated in some other material. This is because a polypropylene shell can only be made in an ejection moulding machine, and once the moulds have been made, little can be done to alter them. Given this situation, Mr Day set about drawing the kind of shell he wanted and then made full size models in plaster of parts. Then, having settled on a final shape, the next stage was to produce it as a glass fibre mock up. This mock up was the only means by which Mr Day could judge whether or not his design was satisfactory, and although it could not reproduce exactly the colouring, texture, feel and resilience of the polypropylene shell which was to follow, at least it could be used to test comfort and give a clear idea of what the proposals on the drawing board would look like in three dimensional form. The mock up was also used to give the chemists a better chance of evaluating the behaviour of a polypropylene shell of similar dimensions, and after a number of alterations had been made, the shape of the seat and back were finalised and moulds put in hand. Making the mould The process of injection moulding is in many ways similar to die casting. It consists essentially of melting a metered charge of the moulding material in a heating chamber and forcing the charge under high pressure through an orifice into a relatively cool mould from which the component can be ejected after it has set. The mould, which is of high quality steel, is subject to specialist design considerations and has to be machined to extremely fine tolerances if a quality product is required. The job of moulding the shell was given to Thermo Plastics Ltd. and the tool itself was made by Talbot Ponsonby & Co Ltd. machined to extremely fine tolerances if a quality product is required. The job of moulding the shell was given to Thermo Plastics Ltd. and the tool itself was made by Talbot Ponsonby & Co Ltd. (caption) 1 and 2 An elevation and a plan showing the shock load test carried out by the Furniture Industry Research Association on the Mark I polypropylene shell. The shell was screwed to a rigid base frame and a 14 lb weight fixed to the end of a cable attached to the top of the back. The weight was then dropped from a height of 1230 inches to test distortion between the base and back of the shell. (caption) 3- 6 show how BS recommendations for back (dotted fine a-D) and seat (dotted line B-C) of office chairs compare with the Hille chairs (solid line.) The Mark 1 is 3, the Mark 2 is 4. 5 and 6 compare the Hille shell seats with the BS minimum width. The continuous white line shows the Hille sitting area, the dotted line is the BS area. Here, the Mark 2, 6, is better. But the Hille shells are dished, whereas recommendations apply to a flat seat. (caption) 7 shows the rear bosses of the Mark I polypropylene chair with and without web reinforcements. In the unreinforced boss, A and C, distortion occurred to the left of the screw hole. The first task was to make epoxy resin moulds from the glass fibre prototype. From these moulds, the steel billets of the tool were machined and then tested on an injection moulding machine. After this, a stress engineer was called in to examine the first polypropylene shells. On his advice, the tool was altered to make the shell stronger and reduce flexibility, an operation which had to be done by hand, a toolmaker literally gouging out the sides of the tool with a chisel to thicken the sides of the shell at the expense of its interior dimensions so that the sitting area of the seat, and the radius of the back of the chair, became smaller than was originally intended. Indeed, so great were the changes that even at this early stage the original design had been radically altered. The modifications to the tool also meant that its surface had to be re - engraved, and this, too, had to be done by hand-although a few weeks later a machine was imported into Britain which can engrave a tool of this kind by a new photo-acid process. When all this had been done, further mouldings were then sent to the Furniture Industry Research Association for strength tests. Testing the shell As has already been said this was the first time(except for a very brief and ill - fated French attempt) that a chair seat and back had been moulded in polypropylene, and the Shell chemists were still uncertain about the flexing properties of the plastics, despite their elaborate analyses and calculations. Because of this, it was decided to test the flexing properties of the moulded shells, in addition to subjecting them to the standard performance tests on chairs. From previous work carried out on similar chair shapes, it had been found that persons of about 16 stone in weight can exert short term loads of up to 56 Ibs by leaning heavily backwards. Consequently, FIRA decided to develop a repeated loading test whereby the chair back could be horizontally strained while at the same time a suitable vertical restraining load was applied to the chair seat. To give a wide margin of safety, the horizontal load was stepped up to 100 lb (an overload of about 80 per cent), applied 60 times a minute for some 250,000 applications. (The rate of 60 times a minute was reduced to 30times a minute after 30,000 applications as it was found that the load applying mechanism bounced at the higher speed.) The result of this test was that the shell showed slight distortion around one of the rear bosses provided for the attachment of the underframe. The distortion appeared initially as a white crease, but developed so much during the test that eventually the underside of the shell would have rested on other parts of the underframe. In addition to the distortion, microscopic examination of the shell showed that small voids had appeared in several places where it seemed that a change had occurred in the plastics' molecular state, causing greater pliability. As a result of this test, the rear bosses of the shell had to be reinforced by small webs, and for the second time a modification was carried out on the injection tool. User test problems Having thus arrived at a shell that was believed to be satisfactory, and in the meantime having developed legs for it, Hille then decided to put 600 chairs on the market for user testing. However, because the tool had been re - engraved by hand so that the shell acquired an uneven texture in the process, it was discovered that there were small whiskers on the shell which snagged stockings, and the chairs were immediately withdrawn. To overcome this, a flaming device had to be developed to remove the plastics whiskers on the shell and restore it's smooth, even surface. This, however, was not the end of the trouble. Since the rear bosses had been strengthened and the shell had passed all of FlRA's other tests satisfactorily, more shells were moulded which those concerned with the development of the chair expected to be the final version. A number of chairs were then sold, but some of them were returned to the manufacturer showing fractures to the shell around the rear bosses. It appeared that these fractures had been caused by sudden impact, and a new test programme was devised to examine the behaviour of the shells under this particular condition. A complete reappraisal By this time, however, the situation had become complicated as a result of developments in the preparation of polypropylene itself. On the one hand, one supplier of the raw material claimed that it could be made more resistant to sudden shock by annealing the polypropylene after the moulding had taken place; and on the other, there was the possibility of using a different grade of polypropylene altogether. Thus the revised FlRA tests were used not only to evaluate the effects of impact upon the chair shells, but also to select the grade of polypropylene, and its possible processing, that would finally be used. The first test was to measure the effects of impact. This was done by placing chairs on a thick, hard rubber sheet and then dropping a 14 lb weight on them from a height of 6 inches, and at a rate of 20 times a minute, in such a way that the impact occurred 1 1/2 inches from one of the rear bosses and towards the seat's centre. This test was varied by altering the position of impact, the height of the drop and the number of times it occurred in a minute. The result of these tests was that fractures occurred when the drop height reached 21 inches, and this happened both to shells that had been annealed and to those that had not. The next test examined impact from a different approach. It appeared that some of the chairs returned to the manufacturer had been fractured by someone standing behind the chair, holding the back of it with both hands, and then leaning heavily, and perhaps suddenly, upon it. As it was also known that fractures had occurred as the result of someone leaning diagonally backwards in a chair to reach something behind him, it was decided to simulate these types of strain by fixing a chair firmly to the floor, attaching a cable to the top of the back of the chair at one corner 5 inches from the centre position, and then dropping a 14 lb weight attached to the end of the cable in such a way that it acted as a strain at an angle of 45ø to the back of the chair. The drop height varied from 12-30 inches and continued until the shell showed failure. The result of this test was more revealing. The original grade of polypropylene, both in its annealed and nonannealed state, fractured, the former when the drop height was 12 inches, the latter when it was 15 inches, although resistance could, in fact, be increased by lengthening the webs around the rear bosses. The new grade of polypropylene, however, did not fracture at all, although creases or folds around the rear bosses did occur after a drop of 12 inches and appeared to be affected by the number of drops rather than by their severity. At the same time, the flexing tests originally carried out by FIRA were repeated and these, together with a test for stiffness, confirmed first, that annealing made little or no difference to resistance to suddenly applied stresses, and second, that the new grade of polypropylene avoided fracture but was less rigid than the polypropylene originally used. Putting the chair in production By now, the chair had been through so many tests and the polypropylene shell had been submitted to such a thorough examination that it was decided to put the chair into production. "This was," says Mr Day, "an extremely harrowing decision to be involved with. I knew that all the modifications had made the dimensions of the shell unsatisfactory and I would not have gone through with its manufacture had it not been agreed that a new tool would be invested in to make a completely new version of the chair, drawing upon all the experience we had gained in the original exercise." Indeed, anyone sitting in the chair knew it was unsatisfactory, mainly because people of average to large size found that they were gripped by the sides and the back. The situation is diffficult to evaluate because, for example, there are no British Standards for shell chairs. Nevertheless, given that the chair is intended for general use by a large number of people, it seems reasonable to compare it with the BS recommendations for non adjustable office chairs since these recommendations give positions in space for the seat and backrest relationship. If this is done, it will be found that although the seat of the first version of the Hille chair is only 4 inch higher than the BS recommendation of 17 inches (this measurement refers to the outside edges; it is less in the middle), it is at least 1 inch narrower than the minimum width of seat (which is 16 inches). In addition to this, the radius of the curve of the back of the Hille chair is only 11 inches, whereas the BS minimum radius is 12 inches and the recommended radius 16-18 inches. Briefly, this comparison means that this version of the Hille chair was too high and too narrow across the seat and back. It was, in fact, comfortable for less than 25 per cent of the adult male and female population (differences in leg length between men and women being compensated by women's heels), and the chief characteristics of those who could sit in it comfortably were that they had long legs and long, narrow bodies. Other people found that it gripped them tightly around the hips, and that the narrow curve of the top of the back dug into them just below the shoulder blades. It should also be said that even when the first version of the chair (the Mark 1 as it is now called) was produced, it was not free of problems and the second batch suffered from cracking so that the front and rear bosses had to be extended to increase their strength. A completely new design It must now be said that throughout the development of the Mark 1 version of the Hille chair, a very considerable effort was made to get its dimensions, and the behaviour of the plastics shell, right, and Hille certainly suffered from using a new material in a way in which it had not been used before. Besides Mr Day's own reservation about putting it into production, a great deal of money had been sunk into the venture, including about £20,000 for the development (the tool alone cost £8,500) and another £75,000 invested by Thermo Plastics Ltd in the injection machine (which can, of course, be used for other operations besides the Hille moulding). Nevertheless, despite the heavy outlay, the continual setbacks and the disappointments, as soon as the Mark 1 was established, Mr Day set about the new design which, although it had aesthetic similarities to the original version, was otherwise a return to square one. (caption) 8 and 9 are views of the Underside of the Mark I, 8, and Mark 2, 9, showing how the angle of the bosses of the latter has been changed to bring It into line with the angle of stress imposed on the chair when someone sits in it. (caption) 10 and 11 Front views of the Mark I and Mark 2 taken from the same position. They show clearly how the Mark 2, 11, is more generous in its overall dimensions, and also has the advantage of a less pronounced front - to - back slope of the seat-although the angle of slope is still greater than the BS recommendations' preferred figure. The Second version In an analysis of the Mark 2, one faces exactly the same problem as posed by the Mark 1, namely that there are no standards laid down for shell chairs and therefore no accepted criteria for evaluation. Nevertheless, criticism of the new chair can be made on two counts. The first concerns the designer's approach to the problem. Having failed to achieve the result he wanted with the Mark 1, he then redesigned the shell and called in an ergonomist to give a quick evaluation of the new design. However, since an ergonomist had been consulted and therefore an ergonomist's approach to the problem (and the kind of standards an ergonomist would refer to) had been accepted, it was a pity that the designer was not advised to test his new design under laboratory conditions. The second count returns to the BS recommendations for non-adjustable office chairs already referred to because, since the designer must have known that some of his chairs would be used at desks and tables, these recommendations should have been Followed for spatial dimensions. If this is done, it will be found that the overalI width of the seat has been increased from 16 1/8s to 7 1/4 inches. This has enabled the eternal width of the seat (measured 3 inches from the seat's front edge) to increase from 14 9/10 inches to 16 4/10 inches. But this is still only 1/4O inch more than the BS recommended minimum width for a chair with a flat seat, and does not take into account any area lost by the seat's dishing. Similarly, the depth of the seat has been increased from 14 1/2 inches to 15 inches. (he increase however, is unnecessary as he maximum depth recommended by British Standards is 1 inch more hen the BS preferred depth. In addition, Ithough the radius of the back has been increased from 11 to 13 inches, it is still less than the recommended radius of 16-18 inches. The seat also slopes quite sharply down from the front edge to the join etween seat and back (a drop of 5ø - the referred drop is 3ø which is, in fact, provided by the pedestal version of the Hille chair). And the saddle effect of the reverse backrest curves, although a definite dvance, still seems to give insufficient umber support. In other words, as far as the se of the Mark 2 chair for offices is concerned, the main points of criticism are hat its seat is barely wide enough, its seat slope is too steep, its lumbar support is inadequate and the radius of the backrest is still too small. The matter of seat slope could probably be rectified by lowering the front legs slightly, in which case the support given for an upright posture might also be improved. But, and this is a qualification which must always be borne in mind, the above analysis refers to one use only. For other uses (and even in the office), the chair has received considerable approval while certain of its characteristicsthe seat slope, for example- may make it particularly suitable for lecture halls and theatres, where an upright position is not required, although the chairs' lack of width is a disadvantage. With an all-purpose chair like this, no one standard is applicable. On the other hand, in these circumstances a designer or manufacturer who has called in the expert opinion of an ergonomist should logically define the purpose of this chair and relate it specifically to its different bases. This is, of course, calling for the highest possible professional standards, but there is no doubt that these are the standards aimed at by Hille in the design of the polypropylene chair. Other points This analysis has been almost exclusively concerned with the design of the shell, the main problem in the development of the Hille chair, and little has been said about the legs that can be added to it. The variety of legs has been steadily increased during the past 12 months and the method of fixture has been greatly improved on the new shell by altering the angle and position of the bosses so that they are now in line with the direction of stress imposed on the chair when someone sits in it. Another important change has been made to the caps to the legs. The legs themselves are cut off at an angle to the floor, so that all the weight of the chair is taken on one edge if the cap sits on the leg without any modification (as was the case in the 600 versions of the Mark 1 sent out for sample testing). Later on, however, polyethylene caps were designed to fit on the legs but at the same time sit squarely on the floor, and in fact they are sufficiently pliable to adjust to variations in the distribution of weight of different people and different postures. It is the attention given to details like these which very often serves as an accurate guide to the design approach to the product as a whole. Conclusion At the beginning of this article it was said that Mr Day hoped to repeat the kind of fundamental changes in design thinking and use of materials achieved by the glass fibre chair designed by Charles Eames. How far has he managed to do this ? Insofar as the use of materials is concerned, there is no doubt at all that the Hille chair is a technological breakthrough, not only because it uses a new plastics which enables a chair to be mass produced and sold for under £3, but also because the shell of the chair is self supporting. A measure of the success of this breakthrough, indeed, is given by the number of other firms and manufacturers anxious to jump on to the polypropylene band - wagon. For the first time since glass fibre, a new material for chairs has been introduced whose colour variations, pleasant texture and strength are beyond dispute, while the kinds of shape a designer may give to it are still almost completely unexplored. With regard to the design, Mr Day's greatest achievement was to make the shelI self supporting. From an ergonomic point of view, however, the analysis has shown its weaknesses. Since the chair is being produced in very large quantities and at a low price, it will be within the reach of a wide public and therefore it cannot be argued that the chair is in any sense designed for a specialised group of people. In these circumstances, it seems only right to try to design the chair to fit the maximum possible range of human body sizes and all the evidence, resources and experimental techniques of human factors experts should have been used to do this. This analysis of the chair has adopted the only other method when experiments cannot be made, which is to compare the dimensions of the chair with the only available data on chair sizes for adult use in this country, namely the British Standard Recommendations for the Dimensions of Non - adjustable Office Chairs (BS3079). These recommendations are compiled from expert evidence and in departing from them, even allowing for the fact they that are for on e specific type of chair, the changes in the shell dimensions should be based on experimental findings, rather than personal preferences. Thus in this respect, the design of the Hille chair leaves something to be desired, though in all other respects it must be emphasised that it represents a major achievement which deserves high praise. | Source: | Vads | Creator: | Artist: Richard Carr | Identifier: | http://www.vads.ac.uk/large.php?uid=8289... | Go to resource |
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