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1、Design guides for plasticsClive Maier, Econology LtdTANGRAMTECHNOLOGYConsultingEngineersconologyDecember 2021This publication is made up of a series of articles published in Plastics and Rubber Weekly as a piece work. The kind assistance of the author and PRW is acknowledged in the publication of th
2、e work.The design hints in this booklet are given in good faith and represent current good practice. The short nature of the hints means that not all information can be included. No responsibility can be taken for any errors or consequential damages resulting from using these hints.This publication
3、may be freely reproduced except for sale or advertising purposes. It may be hosted on web sites for free downloading providing that it is used in its entirety and that reference is made to the original publication. Clive Maier 2004Typeset and edited by Tangram Technology Ltd.ContentsPreface 1Introdu
4、ction 2Injection moulding 4Basics1. Wall thickness 92. Corners 103. Ribs 114. Bosses 145. Design forrecycling 17Special features6. Living hinge 227. Bearings 248. Gears 27Assembly9. Press fits 3210. Snap-fits 3311. Hot staking 3712. Ultrasonic welding 3913. Hot plate welding 4414. Spin welding 4515.
5、 Friction welding 4616. Induction welding 4717. Laser welding 4818. Adhesive and solvent bonding 4919. Outsert moulding 5444PrefaceThis set of hints and tips for plastics product designers is intended as a source book and an aide mmoire for good design ideas and practices. It is a source book for pl
6、astics product designers at all levels but it is primarily aimed at: student designers carrying out design work for all levels of academic studies; non-plastics specialists involved in the design of plastics products; plastics specialists who need to explain their design decisions and the design lim
7、itations to nonplastics specialists.The book covers each topic in a single page to provide a basic reference to each topic. This space constraint means that each topic is only covered to a basic level. Detailed plastic product design will always require detailed knowledge of the application, the pro
8、cessing method and the selected plastic. This information can only be provided by raw materials suppliers, specialist plastics product designers and plastics processors but there is a need to get the basics of the product design right in the first instance.Using the hints and tips provided in this g
9、uide will enable designers to reduce initial errors and will lead to better and more economic design with plastics.I hope this short work will improve the basic design of plastics products and if it can do this then it will have served its objectives.Clive MaierECONOLOGY Ltd.6INTRODUCTIONGood design
10、 is important for any manufactured product but for plastics it is absolutely vital. We have no instinct for plastics. Most of those we use today have been around for little more than two generations. Compare that with the thousands of years of experience we have with metals. And plastics are more va
11、ried, more complicated. For most designs in metals, there is no need to worry about the effects of time, temperature or environment. It is a different story for plastics. They creep and shrink as time passes; their properties change over the temperature range of everyday life; they may be affected b
12、y common household and industrial materials.The philosopher Heidegger defined technology as a way of arranging the world so that one does not have to experience it. We can extend his thought to define design as a way of arranging technology so that we do not have to experience it. In other words, go
13、od design delivers function, form and technology in objects that meet the needs of users without making demands on them. The well-designed object gives pleasure or at least satisfaction in use, and does what it should do without undue concern.In these Design Guides we will set out the basics of good
14、 design for plastics. The rules and recommendations we give will necessarily be generalisations. They will apply often but not invariably to thermoplastics, frequently but not exclusively to injection moulding. The basic advice will be good but because plastics are so complex and varied the golden r
15、ule must always be to consider carefully whether the advice needs adjusting to suit your particular application.Good design combines concept with embodiment. Unless the two are considered together, the result will be an article that cannot be made economically or one that fails in use. This is parti
16、cularly important for plastics. It is vital to choose the right material for the job. When that is done, it is equally important to adapt the details of the design to suit the characteristics of the material and the limitations of the production process.Plastics come in a bewildering variety. There
17、are a hundred or more distinct generic types. On top of that, advanced techniques with catalysts and compounding are creating new alloys, blends and molecular forms. All of these materials can have their properties modified by control of molecular weight and by additives such as reinforcements. The
18、number of different grades of plastics materials available to the designer now approaches 50,000. The importance - and the difficulty - of making the right choice is obvious.Plastics can be grouped into categories that have roughly similar behaviour.Thermoplastics undergo a physical change when proc
19、essed; the process is repeatable. Thermosets undergo a chemical change; the process is irreversible. A key distinction between thermoplastics relates to the molecular arrangement. Those with random tangled molecules are called amorphous. Those with a degree of molecular arrangement and ordering are
20、called semi-crystalline. The difference is significant. For example, most amorphous materials can be fully transparent. And the more crystalline a material is, the less likely it is to have a wide rubbery processing region, so making it less suitable for stretching processes like blow moulding and t
21、hermoformingDesigners must design for process as well as purpose and material. In single-surface processes for example, there is only indirect control over the form of the second surface. Design must take this limitation into account.DESIGN CONSIDERATIONS7PLASTICS10THERMOSETSTHERMOPLASTICSHDPEHigh d
22、ensity polyethyleneLDPELow density polyethyleneLLDPELinear low density polyethylenePAPolyamide (Nylon)P日TPoltLrtylee terephthalatePEEKPolyether ether ketonePOMPo 1 oxrnet hy 1 b e(Acetal)PRPolypropylenePPPolyphenylene sulphideAB.crylonitrile b.rtadiene styreneCACellulose acetateCABCellulose acetateb
23、.rtyrateCPCellulose prorionatePCPolycarbonateRESPolyether sulphonePETPolyethyleneterephthalatePMMAPoly methylmethacrylateF,POPolyphenylene oxidePPolystyrenePSJPolysLJphonePVCPolyvinyl ChlorideSANStyrene acrylonitrileEPDMEthylee-propylene- diene terpolymerEPTEthylee-propylene terpolymerNBRNitrile but
24、adienerubberPEBPclyether block amideBStyre ne-b.rta dienestyreneTPThermoplasticpolyurethaneDAPDiallyl phthalateMFMelamine formaldehydePFPhenol formaldehydeUFUrea formaldehydeEPEpoxyUPUnsaturated polyesterSemi-crystallineAmorphousElastomericSOME COMMON PLASTICSALL SURFACES DEFINEDSINGLE SURFACE DEFIN
25、EDBATCH PROCESSInjection moulding Compression mould!ngTnmnmrn口uldi旧Blo, moulding rermoforming Rotational mouldingCONTINUOUS PROCESSExtrusion Calendering PultrusionCOMMON PLASTICS FORMING PROCESSESInjection moulding1WALL THICKNESSParts that might be made as solid shapes in traditional materials must
26、be formed quite differently in plastics. Moulded plastics do not lend themselves to solid forms. There are two principal reasons for this. First, plastics are processed with heat but are poor conductors of heat. This means that thick sections take a very long time to cool and so are costly to make.
27、The problems posed by shrinkage are equally severe. During cooling, plastics undergo a volume reduction. In thick sections, this either causes the surface of the part to cave in to form an unsightly sink mark, or produces an internal void. Furthermore, plastics materials are expensive; it is only hi
28、gh-speed production methods and net-shape forming that make mouldings viable. Thick sections waste material and are simply uneconomic.So solid shapes that would do the job well in wood or metal must be transformed to a shell form in plastics. This is done by hollowing out or coring thick parts so yo
29、u are left with a component which regardless of complexity is composed essentially of relatively thin walls joined by curves, angles, corners, ribs, steps and offsets. As far as possible, all these walls should be the same thickness.It is not easy to generalise what the wall thickness should be. The
30、 wall plays a part both in design concept and embodiment. The wall must be thick enough to do its job; it must be strong enough or stiff enough or cheap enough. But it must also be thin enough to cool quickly and thick enough to allow efficient mould filling. If the material is inherently strong or
31、stiff the wall can be thinner. As a general guide, wall thicknesses for reinforced materials should be 0.75 mm to 3 mm, and those for unfilled materials should be 0.5 mm to 5 mm.Ideally, the entire component should be a uniform thickness - the nominal wall thickness. In practice that is often not po
32、ssible; there must be some variation in thickness to accommodate functions or aesthetics. It is very important to keep this variation to a minimum. A plastics part with thickness variations will experience differing rates of cooling and shrinkage. The result is likely to be a part that is warped and
33、 distorted, one in which close tolerances become impossible to hold. Where variations in thickness are unavoidable, the transformation between the two should be gradual not sudden so instead of a step, use a ramp or a curve to move from thick to thin.Thick sections and non-uniform walls cause proble
34、msSolid shapes must be redesigned as shellsWRONG-sharp stepRIGHT-gradual transitionby planeRIGHT-gradual transitionby radiusGradual transitions between thick and thinsectionsDESIGNERS NOTEBOOK Keep wall thickness as uniform as possible. Use gradual transitions between thick and thin sections. Wall t
35、hickness must suit both function and process. Wall thickness guide range is:0.75 mm to 3 mm for reinforced materials0.5 mm to 5 mm for unreinforced materials2CORNERSWhen the ideas of correct and uniform wall thickness are put into practice the result is a plastics part composed of relatively thin su
36、rfaces. The way in which these surfaces are joined is equally vital to the quality of a moulded part.Walls usually meet at right angles, at the corners of a box for example. Where the box walls meet the base, the angle will generally be slightly more than 90 degrees because of a draft angle on the w
37、alls. The easiest way, and the worst, to join the walls is to bring them together with sharp corners inside and out. This causes two problems.The first difficulty is that the increase in thickness at the corner breaks the rule of uniform wall thickness. The maximum thickness at a sharp corner is abo
38、ut 1.4 times the nominal wall thickness. The result is a longer cooling time accompanied by a risk of sink marks and warping due to differential shrinkage.The other problem is even more serious. Sharp corners concentrate stress and greatly increase the risk of the part failing in service. This is tr
39、ue for all materials and especially so for plastics. Plastics are said to be notchsensitive because of their marked tendency to break at sharp corners. This happens because the stress concentration at the corner is sufficient to initiate a microscopic crack which spreads right through the wall to ca
40、use total failure of the part. Sharp internal corners and notches are the single most common cause of mechanical failure in moulded parts.The answer is to radius the internal corner, but what size should the radius be? Most walls approximate to a classical cantilever structure so it is possible to c
41、alculate stress concentration factors for a range of wall thicknesses and radii. The resulting graph shows that the stress concentration increases very sharply when the ratio of radius to wall thickness falls below 0.4. So the internal radius (r) should be at least half the wall thickness (t) and pr
42、eferably be in the range 0.6 to 0.75 times wall thickness.If the inner corner is radiussed and the outer corner left sharp, there is still a thick point at the corner. For an internal radius of 0.6t, the maximum thickness increases to about 1.7 times the wall thickness. We can put this right by addi
43、ng a radius to the outside corner as well. The outside radius should be equal to the inside radius plus the wall thickness. This results in a constant wall thickness around the corner.Properly designed corners will make a big difference to the quality, strength and dimensional accuracy of a moulding
44、. But there is another benefit too. Smooth curved cornershelp plastic flow in the mould by reducing pressure drops in the cavity and minimising flow-front break-up.Internal: r = 0.6t Extern al: r = 0.6t + tInternal: r = 0.6t External: SharpInternal: Sharp External: SharpGood and bad corner designStr
45、ess concentration factors for cantileverloadingDESIGNERS NOTEBOOK Avoid sharp internal corners. Internal radii should be at least 0.5 and preferably 0.6 to 0.75 times the wall thickness. Keep corner wall thickness as close as possible to the nominal wall thickness. Ideally, external radii should be
46、equal to the internal radii plus the wall thickness.3.1 RIBSRibs create thick sections at the rootwdp 3q-s =raRib thickness relative to wall thickness (wt)How rib root thickness increasesDESIGNERS NOTEBOOK Rib thickness should be 50 - 75% of the wall thickness. Fillet radius should be 40 - 60% of th
47、e rib thickness. Rib root thickness should not be more than 25% greater than the wall thickness. Rib depth should not be more than 5 times the rib thickness. Taper ribs for mould release.So far in this design series we have seen that plastics parts should be made with relatively thin and uniform walls linked by corner radii, not sharp corners. Both ideas are im