Design Guides For Plastics - Tangram
April 2009Design Guides for PlasticsClive Maier, Econology LtdTANGRAMTECHNOLOGY
This publication is made up of a series of articles published in Plastics and Rubber Weekly as a piece work. Thekind assistance of the author and PRW is acknowledged in the publication of the work.The publication will be updated in a regular basis as new sections of the guide are published by PRW.The design hints in this booklet are given in good faith and represent current good practice. The short nature ofthe hints means that not all information can be included. No responsibility can be taken for any errors orconsequential damages resulting from using these hints.This publication may be freely reproduced except for sale or advertising purposes. It may be hosted on web sitesfor free downloading providing that it is used in it’s entirety and that reference is made to the original publication. Clive Maier 2004Typeset by Tangram Technology Ltd.
ContentsPreface . 1Introduction . 2Injection moulding. 4Basics1.2.3.4.5.Wall thickness . 5Corners. 6Ribs . 7Bosses . 10Design for recycling . 13Special features6.7.8.Living hinge . 18Bearings . 20Gears . 23Assembly9.10.11.12.13.14.15.16.17.18.Press fits . 28Snap-fits . 29Hot air staking . 33Ultrasonic welding. 38Hot plate welding . 40Spin welding . 41Friction welding . 42Induction welding . 43Laser welding. 44Adhesive and solvent bonding . 45Special techniques19.20.Design for outsert moulding . 50Design for gas assist injection moulding . PlannedExtrusion.21.Design for profile extrusion . PlannedBlow moulding.22.Design for extrusion blow moulding . PlannedThermoforming.23.Design for thermoforming . PlannedDesign information24.Design information sources. 63
April 2009
April 2009PrefaceThis set of hints and tips for plastics product designers is intended as a source book and an 'aidemémoire' for good design ideas and practices. It is a source book for plastics product designers at alllevels 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 limitations to nonplastics specialists.The book covers each topic in a single page to provide a basic reference to each topic. This spaceconstraint means that each topic is only covered to a basic level. Detailed plastic product design willalways require detailed knowledge of the application, the processing method and the selected plastic.This information can only be provided by raw materials suppliers, specialist plastics product designersand plastics processors but there is a need to get the basics of the product design right in the firstinstance.Using the hints and tips provided in this guide will enable designers to reduce initial errors and will leadto 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 willhave served it’s objectives.Clive MaierECONOLOGY Ltd.11
April 2009INTRODUCTIONGood design is important for anymanufactured product but for plastics it isabsolutely vital. We have no instinct forplastics. Most of those we use today havebeen around for little more than twogenerations. Compare that with thethousands of years of experience we havewith metals. And plastics are more varied,more complicated. For most designs inmetals, there is no need to worry about theeffects of time, temperature or environment.It is a different story for plastics. They creepand shrink as time passes; their propertieschange over the temperature range ofeveryday life; they may be affected bycommon household and industrial materials.The philosopher Heidegger definedtechnology as a way of arranging the worldso that one does not have to experience it.We can extend his thought to define designas a way of arranging technology so that wedo not have to experience it. In other words,good design delivers function, form andtechnology in objects that meet the needs ofusers without making demands on them.The well-designed object gives pleasure orat least satisfaction in use, and does what itshould do without undue concern.In these Design Guides we will set out thebasics of good design for plastics. The rulesand recommendations we give willnecessarily be generalisations. They willapply often but not invariably tothermoplastics, frequently but notexclusively to injection moulding. The basicadvice will be good but because plastics areso complex and varied the golden rule mustalways be to consider carefully whether theadvice needs adjusting to suit your particularapplication.Good design combines concept withembodiment. Unless the two are consideredtogether, the result will be an article thatcannot be made economically or one thatfails in use. This is particularly important forplastics. It is vital to choose the rightmaterial for the job. When that is done, it isequally important to adapt the details of thedesign to suit the characteristics of thematerial and the limitations of the productionprocess.Plastics come in a bewildering variety. Thereare a hundred or more distinct generictypes. On top of that, advanced techniqueswith catalysts and compounding are creatingnew alloys, blends and molecular forms. Allof these materials can have their propertiesDESIGN CONSIDERATIONSmodified by control of molecular weight andby additives such as reinforcements. Thenumber of different grades of plasticsmaterials available to the designer nowapproaches 50,000. The importance - andthe difficulty - of making the right choice isobvious.Plastics can be grouped into categories thathave roughly similar behaviour.Thermoplastics undergo a physical changewhen processed; the process is repeatable.Thermosets undergo a chemical change; theprocess is irreversible. A key distinctionbetween thermoplastics relates to themolecular arrangement. Those with randomtangled molecules are called amorphous.Those with a degree of moleculararrangement and ordering are called semicrystalline. The difference is significant. Forexample, most amorphous materials can befully transparent. And the more crystalline amaterial is, the less likely it is to have a wide'rubbery' processing region, so making itless suitable for stretching processes likeblow moulding and thermoformingDesigners must design for process as wellas purpose and material. In single-surfaceprocesses for example, there is only indirectcontrol over the form of the second surface.Design must take this limitation into account.2
April 2009SOME COMMON PLASTICSCOMMON PLASTICS FORMING PROCESSES3
April 2009Part 1Injection moulding4
April 20091 WALL THICKNESSParts that might be made as solid shapes intraditional materials must be formed quitedifferently in plastics. Moulded plastics do notlend themselves to solid forms. There are twoprincipal reasons for this. First, plastics areprocessed with heat but are poor conductorsof heat. This means that thick sections take avery long time to cool and so are costly tomake. The problems posed by shrinkage areequally severe. During cooling, plasticsundergo a volume reduction. In thicksections, this either causes the surface of thepart to cave in to form an unsightly sink mark,or produces an internal void. Furthermore,plastics materials are expensive; it is onlyhigh-speed production methods and netshape forming that make mouldings viable.Thick sections waste material and are simplyuneconomic.So solid shapes that would do the job well inwood or metal must be transformed to a'shell' form in plastics. This is done byhollowing out or 'coring' thick parts so you areleft with a component which regardless ofcomplexity is composed essentially ofrelatively thin walls joined by curves, angles,corners, ribs, steps and offsets. As far aspossible, all these walls should be the samethickness.It is not easy to generalise what the wallthickness should be. The wall plays a partboth in design concept and embodiment. Thewall must be thick enough to do its job; itmust be strong enough or stiff enough orcheap enough. But it must also be thinenough to cool quickly and thick enough toallow efficient mould filling. If the material isinherently strong or stiff the wall can bethinner. As a general guide, wall thicknessesfor reinforced materials should be 0.75 mm to3 mm, and those for unfilled materials shouldbe 0.5 mm to 5 mm.Ideally, the entire component should be auniform thickness - the nominal wallthickness. In practice that is often notpossible; there must be some variation inthickness to accommodate functions oraesthetics. It is very important to keep thisvariation to a minimum. A plastics part withthickness variations will experience differingrates of cooling and shrinkage. The result islikely to be a part that is warped anddistorted, one in which close tolerancesbecome impossible to hold. Where variationsin thickness are unavoidable, thetransformation between the two should begradual not sudden so instead of a step, usea ramp or a curve to move from thick to thin.Thick sections and non-uniform walls causeproblemsSolid shapes must be redesigned as ‘shells’Gradual transitions between thick and thinsectionsDESIGNER’S NOTEBOOK Keep wall thickness as uniform as possible. Use gradual transitions between thick and thin sections. Wall thickness 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 materials5
April 20092 CORNERSWhen the ideas of correct and uniform wallthickness are put into practice the result is aplastics part composed of relatively thinsurfaces. The way in which these surfacesare joined is equally vital to the quality of amoulded part.Walls usually meet at right angles, at thecorners of a box for example. Where the boxwalls meet the base, the angle will generallybe slightly more than 90 degrees because ofa draft angle on the walls. The easiest way,and the worst, to join the walls is to bringthem together with sharp corners inside andout. This causes two problems.The first difficulty is that the increase inthickness at the corner breaks the rule ofuniform wall thickness. The maximumthickness at a sharp corner is about 1.4times the nominal wall thickness. The resultis a longer cooling time accompanied by arisk of sink marks and warping due todifferential shrinkage.The other problem is even more serious.Sharp corners concentrate stress andgreatly increase the risk of the part failing inservice. This is true for all materials andespecially so for plastics. Plastics are said tobe notch-sensitive because of their markedtendency to break at sharp corners. Thishappens because the stress concentrationat the corner is sufficient to initiate amicroscopic crack which spreads rightthrough the wall to cause total failure of thepart. Sharp internal corners and notches arethe single most common cause ofmechanical failure in moulded parts.The answer is to radius the internal corner,but what size should the radius be? Mostwalls approximate to a classical cantileverstructure so it is possible to calculate stressconcentration factors for a range of wallthicknesses and radii. The resulting graphshows that the stress concentrationincreases very sharply when the ratio ofradius to wall thickness falls below 0.4. Sothe internal radius (r) should be at least halfthe wall thickness (t) and preferably be inthe range 0.6 to 0.75 times wall thickness.If the inner corner is radiussed and the outercorner left sharp, there is still a thick point atthe corner. For an internal radius of 0.6t, themaximum thickness increases to about 1.7times the wall thickness. We can put thisright by adding a radius to the outside corneras well. The outside radius should be equalto the inside radius plus the wall thickness.This results in a constant wall thicknessaround the corner.Properly designed corners will make a bigdifference to the quality, strength anddimensional accuracy of a moulding. Butthere is another benefit too. Smooth curvedcorners help plastic flow in the mould byreducing pressure drops in the cavity andminimising flow-front break-up.Good and bad corner designStress concentration factors for cantileverloadingDESIGNER’S NOTEBOOK Avoid sharp internal corners. Internal radii should be at least 0.5 and preferably 0.6 to 0.75times the wall thickness. Keep corner wall thickness as close as possible to the nominalwall thickness. Ideally, external radii should be equal to theinternal radii plus the wall thickness.6
April 20093.1 RIBSSo far in this design series we have seenthat plastics parts should be made withrelatively thin and uniform walls linked bycorner radii, not sharp corners. Both ideasare important in the design of ribs.When the normal wall thickness is not stiffenough or strong enough to stand up toservice conditions the part should bestrengthened by adding ribs rather thanmaking the whole wall thicker. The principleis the familiar one used in steel girderswhere 'I' and 'T' sections are almost as rigidas solid beams but are only a fraction of theweight and cost.A thicker section is inevitable where the ribjoins the main wall. This rib root thickness isusually defined by the biggest circle (D) thatcan be inscribed in the cross-section, and itdepends on the rib thickness (w) and thesize of the fillet radius (r). To avoid sinkmarks, this thick region must be kept to aminimum but there are constraints. If the ribis too thin it will have to be made deeper togive adequate rigidity and then it may buckleunder load. There are other problems too;the mould becomes difficult to machine andfill. And ribs filled under high injectionpressure tend to stick in the mould.The fillet radius must not be made too smalleither, or it will not succeed in reducingstress concentrations where the rib joins themain wall. Ideally, the fillet radius should notbe less than 40 percent of the rib thickness.The ribs themselves should be between ahalf and three-quarters of the wall thickness.The high end of this range is best confinedto plastics that have a low shrinkage factorand are less prone to sink marks.A simple comparison shows the benefit ofgood rib design. A rib that is 65 percent ofthe wall thickness and has a 40 percent filletradius, results in a root thickness that isabout 1.23 times the wall thickness. Bycontrast, the root thickness soars to 1.75times the wall thickness when the rib is asthick as the wall and has an equal radius.Ribs of course must be extracted from themould, so they must be placed in thedirection of draw or provided with movingmould parts to free them. Ribs should betapered to improve ejection; one degree ofdraft per side is ideal. If the rib is very deepthe draft angle must be reduced or the ribbecomes too thin. For this reason ribs areoften limited to a maximum depth of fivetimes the rib thickness. So far, so good. Buthow many ribs are needed to make a partstrong enough and how should they bearranged? We will examine that in the nextDesign Guide.Ribs create thick sections at the rootHow rib root thickness increasesDESIGNER’S NOTEBOOK Rib thickness should be 50 - 75% of the wall thickness. Fillet radius should be 40 - 60% of the rib thickness. Rib root thickness should not be more than 25% greater thanthe wall thickness. Rib depth should not be more than 5 times the rib thickness. Taper ribs for mould release.7
April 20093.2 RIBSRibs are used to improve the rigidity of aplastics part without increasing the wallthickness so much that it becomesunsuitable for injection moulding. In theprevious guide we looked at the basics of ribdesign. This time we will see how to put ribsinto practice.Usually we want a part to be equally rigid inall directions, just like a solid plate. We canget almost this effect by running ribs alongand across the part, so they cross at rightangles. This creates a thick section wherethe ribs cross but if we follow the designrules for ribs and fillet radii the increase iswithin acceptable limits - about 1.3 times thewall thickness at the worst. This can bereduced almost to the basic wall thicknessby forming a cored-out boss at the junction,but a better solution is to use a normaljunction with ribs that are less than 0.75times the wall thickness.But how many ribs do we need and howdeep should they be? Rigidity is a functionof the moment of inertia of the rib section.This tells us that the stiffening effect of a ribis proportional to its thickness butproportional to the cube of its depth. Sodeep ribs are structurally much moreefficient than thick ribs.A common task is to develop a relatively thinribbed plate that has the same rigidity as athick solid plate. Standard engineering textbooks provide the basic formulae to makethe calculation but the mathematics can be achore to manage manually. To minimise thework a number of ‘ready reckoners’ havebeen devised, including an elegant cross-ribsolution developed by DuPont. Most ofthese reckoners or calculators are based ona particular set of assumptions so use withcaution if your design varies.For example, the DuPont ribbed platecalculator assumes the ribs are the samethickness as the wall. To see how it works,let’s imagine that we want to design a crossribbed plate with a 2.5 mm wall (tB) that willbe as stiff as a solid plate of 5 mm thick (tA).Calculate tB/tA – the value is 0.5 – and findthis value on the left-hand scale. Rule a lineacross to the right-hand scale and read offthe value which is 1.75. This v
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