Design Tipsfor Rapid Injection MoldingVolume 9Real Parts. Really Fast.www.protomold.comProto Labs, Inc. 5540 Pioneer Creek Drive, Maple Plain, MN 55359 877.479.3680

Design Tips for Rapid Injection MoldingDesign Tips Categorized by TopicPage Proto Labs, Inc. 1999–2013TABLE OF CONTENTS3Tips on Clips (Part 1)5Tips on Clips (Part 2)7Bigger and Better (Part Size Overview)8Cut to the Bone10When NOT to Draft11It’s Never Too Early for ProtoQuote 13Getting into Gears14Parts on a Budget: The Top Five Money-Saving Tips16Word Processing17Resin Selection — Check the Data Sheet19Sand Castles and Plastic Parts20Bridging the Gap Between Prototypes and Production22Crush Ribs — Getting to the anceUnderstandthe Process Volume 9 Design Matrix2

Design Tips for Rapid Injection MoldingTips on Clips (Part 1)One of the many benefits of plastic resin isthe ease with which it can be molded intocomplex shapes. This often allows a singlepart to replace two or more parts made ofother materials. Among the complex featuresthat can be molded into a plastic part area variety of integrated snap connectors,which can also eliminate the need for stillmore parts, such as screws, or for secondaryprocesses like adhesive bonding.The first consideration in snap connectordesign is material. In order for a snapconnector to work, some area of the partmust flex. This is why snap connectors canwork in plastics (though not all plastics) butnot in rigid materials like glass or ceramic.Resins that are especially suited for snapfitted parts include ABS, polycarbonate,unfilled nylon, polypropylene, andother resins with similar properties.The most familiar type of molded-inconnector, the hooked cantilever clip (Figure1), will be addressed in this design tip. Otherconnector types, including annular snap fitsand torsional snap fits, will be addressedin Part 2 of this design tip next month.Figure 1: Cantilever clip with 90 hook faceCantilever clips are used in a variety ofapplications (e.g., access panels in electricaldevices) and can take many forms. Two keyquestions in designing such clips are: Do you want the connection to lockor to release with a pull? Do you want it to release at all or tobe permanent?If the clip’s hook face is at 90 to the directionof connection, the connection will lock andcannot be undone by a simple pull (unless youpull hard enough to break the clip). If, however,the latching face of the hook is angled (FigureFigure 2: Cantilever clip with angled hook faceto facilitate removal2), a simple pull will release the connection.If you want a locking but non-permanentconnection, say for an access panel, you canangle the hook face at 90 but allow thehook to be pushed manually out of its slotto unlock the connection. This is simple ifthe hook is positioned on the outside of thepart. If the hook is located behind a wall, thedesigner can provide a “window” throughwhich the hook can be accessed (Figure 3).Design of a cantilever clip determines itseffectiveness and durability. The clip’s arm mustflex enough to allow it lock and unlock withoutbreaking or deforming. This ability toContinued on next page Proto Labs, Inc. 1999–2013Volume 9Tips on Clips (Part 1)3

Design Tips for Rapid Injection MoldingFigure 3: Clip positioned in a “window” toallow unlockingflex depends on several factors including thematerial’s Young’s modulus, the angle throughwhich the clip must deflect, determined by thedepth of the hook, and the shape and lengthof the clip’s flexing arm. (Detailed formulae forclip design can be found at Theyare also incorporated in many CAD programs,eliminating the need for separate calculation.Finite element programs can also be usedto adjust the clip design to avoid breakage.Additional information on clip design can befound in a previous Protomold design tip.Because the length of the clip’s flexing arm iscritical and some designs offer limited space,there are several ways to increase the effectivelength of the arm.Figure 4: Clip folded to increase the flexingarm’s effective length in limited space The arm can be folded into a Ushape, as is often seen in batterycompartment covers (Figure 4). The wall from which the arm extendscan be notched, making that segmentof the wall an extension of the arm. The wall from which the arm extendscan itself be made flexible, reducing theamount by which the arm must flex.Figure 5: Clip with hole at the base to allow useof a simple two-part mold The simplest is to use a sliding shutoffextending through a hole at the base ofthe clip to form the bottom of the hookand one face of the flexing arm (Figure 5).This allows use of a simple two-part mold. A side-action cam can form the hook andthen withdraw before the mold opens. Thisis an effective, but more complex approach. A pickout can be inserted manually intoBecause a clip is, by its nature, designed tocatch, it can, depending on its orientation, act asan undercut in a two-part mold. There are threeways to deal with this.the mold to form the clip and thenmanually removed from the finished partand reinserted into the mold for the nextcycle. Get more information on pickouts.For further tips on clips, check outthis Video Design Tip on Spring Clips: Proto Labs, Inc. 1999–2013Volume 9Tips on Clips (Part 1)4

Design Tips for Rapid Injection MoldingTips on Clips (Part 2)Plastic’s ability to flex without permanentlydeforming allows molded parts to incorporatea variety of snap fasteners other than thecommon hooked cantilever clip. These include: annular (round) snaps torsional snaps, which store return forceby twisting rather than bending compressive snaps, which work bycompressing and then returningto hold the fastener in place.Annular SnapsAnnular snaps are used in a variety of everydayapplications, from the tiny snaps that close thewindflaps on jackets to larger ones that fastencaps to pens to the still larger lids on plasticyogurt containers. While these are all designedfor ease of opening and closing, annular snapsare also used, with slight modification, in“childproof caps,” which can be easily openedin one position but are virtually impossible toopen in any other. And while many applicationssupport repeated opening and closing, annularsnaps can be used in industrial applications forpermanent fastening, typically ensured by theangle of the locking edges of the snap.Whatever their application, annular snapsoperate by elongation and recovery, typicallyof the female component. This restricts thematerials that can be used to those withrelatively high elastic deflection limits, thepoint where material fails to fully recover from Proto Labs, Inc. 1999–2013deformation. Maximum permissible strain variesfor different materials, from about 50% of thestrain at break for most reinforced plastics tomore than 70% of strain at break for more elasticpolymers. Detailed information on annular snapfit design can be found at SnapsTorsional snaps store closing force when openedby imparting twist to a torsion bar at the pivotpoint of the arm, as opposed to bending theflexing arm as in a cantilever clip. But in mostother respects—hook design, etc.—the torsionalsnap is similar to the cantilever clip. Rockerclips are typical torsional snaps. For detailedinformation on the design of torsional snap fits,visit SnapsCompression (or interference) snap fits can takemany forms. The compound dovetail snap usedin the Protomold sample design cube (Figure1) is one example. This clip is dovetailed in twodirections to lock together two of the cube’ssix faces. In one direction, the male componentis highly tapered providing an unbreakableconnection between the two components. Inthe other direction, the male connector is onlyslightly tapered providing a connection thatcan be easily undone to disassemble the cube.As the male connector is pushed into position,material in both male and female componentsis compressed and then released as the snapmoves into its locked position.Figure 1: Clip is dovetailed in two directions tolock together two sides of a box.Another example of a compression snap fit isfound in the Reptangles building system. Thechallenge here was to design a connector thatwould mate when the parts came together inany direction within a 90 arc. The patentedconnector (Figure 2) consists of a triangular archthat fits into a corresponding slot. “Fingers” inthe slot walls grasp the hollow under the arch tolink the pieces firmly together while still allowingeasy disassembly.Figure 2: Arch-shaped connectors fit into a slotin an interference fit.Continued on next page Volume 9Tips on Clips (Part 2)5

Design Tips for Rapid Injection MoldingA compression snap fit can create moldabilitychallenges. In some cases the connector canbe formed as a “bump-off,” in which the partis slightly undercut but the material flexes toallow ejection. In the case of Reptangles, bumpoffs were not required, as both arch and slotcomponents of the connectors were formed bysliding shutoffs passing through holes in the partto form the undersides of protruding features.The wide variety of clip and snap options allowsdesigners a great deal of latitude in creatingintegrated connectors. Proto Labs, Inc. 1999–2013Volume 9Tips on Clips (Part 2)6

Design Tips for Rapid Injection MoldingBigger and Better (Part Size Overview)We’ve posted tips in the past regardingmaximum part size, but since our moldabledimensions have continued to increase, wethought it was time for an update. Some ofthe figures that follow are simple and absolute;others may vary based on multiple factors. Ifyour design seems to be approaching the limits,the best way to determine whether we can moldit is to submit a 3D CAD model for a free onlineProtoQuote . Knowing the guidelines in advance,however, can help speed up the process.VolumeThe simplest and most absolute limit to partsize is the total volume of your part, whichcannot exceed 59 cubic inches (967 cc). That isthe maximum capacity of the barrel from whichour largest presses inject resin into a mold. Thismaximum figure has increased significantly overtime as we’ve added larger-capacity presses toour production floor, and the good news is that ifyou follow standard guidelines for wall thickness,59 cubic inches of resin goes a long way.HeightThis one is also simple: the Protomold processallows parts to be milled to a maximum depthof four inches from the parting line. So, if theparting line falls exactly halfway between topand bottom, total part height can be up toeight inches. Proto Labs, Inc. 1999–2013Mold Area at the Parting LineBecause resin is injected into a mold underpressure, the two halves of a mold must beclamped together during injection to keep themfrom separating prematurely. The pressure thatmust be overcome equals injection pressurein psi (pounds/in2) times projected part area(cross section of the part, in square inches, atthe parting line). Our presses can exert up to600 tons or 1.2 million pounds of clamp pressure,which allows a projected part area at the partingline of up to 175 square inches.OutlinePressure generated by injection is exerted in alldirections and does more than try to force openthe mold. It also presses outward against thesides of the mold. If that force is exerted over alarge enough area and the mold wall is too thin,the pressure of injection can actually bow thewalls of the mold. To prevent this, as the part ismilled deeper into the mold (increasing the areaof mold wall exposed to pressure) the mold wallmust be thicker. Thicker mold walls reduce themaximum rectangular outline into which the partmust fit. In other words, the taller the part fromthe parting line, the smaller the maximum outline(as defined in Table 1).Table 1Allowance for CamsBecause side action cams must fit withinthe allowable mold outline, they also reducethe maximum size of a part. The space a camrequires will vary with the stroke required tocreate the undercut, but it can be significant.The Protomold software will make appropriateallowances for side actions, but designersshould be aware that parts with side actionsmust fit within a reduced outline.Part Wall ThicknessLarger parts generally mean longer resinflow paths, which, in turn, require thickerwalls; how thick depends on the resin.Minimum wall thickness for a long-fiberfilled resin can be nearly 2.5 times that fora nylon wall. Check out our information onrange of thickness for various resins.Volume 9Bigger and Better (Part Size Overview)7

Design Tips for Rapid Injection MoldingCut to the BoneWe’ve all seen cartoons of the caveman with hiswaist-high stone wheel. It literally weighs a tonand never seems to be attached to anything.We’ve seen images of chest-high, solid woodwheels on a peasant’s lumbering cart; betterthan nothing but still far from race-ready. Thereal breakthrough in wheel design came whensome genius realized that a wheel needs a huband a rim but that most everything in betweenis superfluous. It was only with the invention ofspokes that the wheel—suddenly lighter, moreeconomical, and with less angular momentum tospin up and brake down—came into its own.The same principle applies to plastic parts.Plastic is inherently light, generally economical,and relatively easy to form. But that doesn’tmean that it can’t, with thoughtful design,be lighter still and even more economical, allwithout sacrificing performance or complicatingmanufacturing. Accomplishing this requiresthe elimination of unnecessary material,which raises two questions: why and how.Why has several answers, and the first is cost.A recent item in The Plastic Exchange pointedout that buyers were snapping up suppliesof resin in late January 2012 to avoid priceincreases expected in February. The impendingincreases ranged from 02 to 06 per pound onsome relatively inexpensive resins: polyethyleneand polypropylene. If pennies-per-pound savingsare that motivational, any significant reductionin the per-part volume of resin could be even Proto Labs, Inc. 1999–2013more worthwhile, and such savings would beproportionally greater on more expensive resins.The second reason to trim down is weight. Frombicycles and cars to military gear and aerospace,weight reduction has become a kind of HolyGrail. Plastic is, of course, lighter than metal, butless plastic is lighter still. As the saying goes,“You can never be too thin or too rich,” and intoday’s market being lighter can make you richerby reducing cost and increasing functionality.Another argument for trimming is style. Perhapsbecause skeletonization is associated withhigh-tech, sport, and the like, it has developed acertain cachet. Minimalism sells, which brings usto the next question, which is, “How?”One of the simplest ways to lose weight in aplastic part is to core out thick areas. This notonly saves money and weight, but also preventsproblems like sink, voids, and warps. Anotherapproach is skeletonization. The concept hasbeen used for years and applied to all sorts ofmaterials. Wood and metal trusses are designedto maintain strength while minimizing materialrequirements. So is a honeycomb. In many cases,hollow cylinders can replace solid posts, andproperly positioned arches can redirect forcesand replace bulky solid supports. All of thesetechniques can be incorporated into plasticparts. The challenge is to remove materialwithout impairing function. Figure 1 showsan example of the process.Figure 1: This part evolved from a solid disk to onewith cored-out sections to a fully-skeletonized spokewheel with no loss of functionality.Since resins vary widely in their characteristics,the amount of material that can be eliminatedfrom a particular design will depend on thematerial, and choices of material and formwill interact as the designer searches for anoptimal design. Finite element analysis (FEA)software is great during the early virtual phaseof development, but prototypes using actualmaterials are necessary for testing and refiningthe design as the process moves forward.Thoughtful prototyping by rapid injectionmolding can provide maximum information atminimum cost. You can test multiple resins in thesame mold (as long as you remember that resinswith different shrink rates will produce partsof slightly different dimensions). Dependingon what you plan to test, e.g., individual partperformance rather than a final assembly, thismay not be a problem. Also, if you plan to testvarying degrees of skeletonization, rememberthat metal is easy to remove from a mold butVolume 9Cut to the Bone8

Design Tips for Rapid Injection Moldinghard to add, and less metal means more plastic.In other words, start with the most trimmeddown version of your part. If it doesn’t standup to testing, you can add plastic for yournext iteration by milling more metal from themold instead of starting from scratch. As withskeletonizing a part, it’s a way of trimming fatfrom your development process, maintainingeffectiveness while reducing cost and effort. Proto Labs, Inc. 1999–2013Volume 9Cut to the Bone9

Design Tips for Rapid Injection MoldingWhen NOT to DraftPlastic part design can be a complex processunder any circumstances, and when you aredesigning entirely new types of product thechallenges can be greater still. In design yourarely know what little trick or idea is goingto make the difficult easy or the seeminglyimpossible possible; that’s why we offer newdesign tips every month. That said, every oncein a while we run across one that seems soimportant that it’s worth revisiting. Here is a“Design Tip Classic” from December 2009 We’ve talked so often about the need fordraft in injection molded parts that you maybe shocked to hear that there are featuresthat not only don’t need to be drafted, butthat work better if they aren’t drafted. It’strue, and the reason we haven’t mentionedit before is this is a new capability addedto our injection molding process.tighter as it was screwed into the tapered hole. Ifit got too tight, it would crack the part. If it wastoo loose it could “strip” and fail to hold.Long, straight screws, tapered pilot holes, andknit lines were a bad combination. If the holeand corresponding screw length were short,the part could be safely produced, but designswith deeper holes unfortunately went back todesigners as no-quoted parts.That’s all in the past. Now, Protomold canproduce high-aspect-ratio small diameterholes using steel core pins in the mold. Say, forexample, you are designing a part with a 3/4"deep, 1/8" diameter hole (see Figure 2). Yousimply include that feature in your 3D CADmodel; our proprietary software will go to workand design the mold with a cylindrical steel corepin for forming the hole (see Figure 3).The features in question are typically screwholes used to connect plastic parts—front andback halves of a plastic shell, for example—withthread-forming screws. The holes are formed byposts in the mold called “cores” (see Figure 1).Previously, we created cores in the mold bydirectly milling them from the aluminum moldbody. Tall thin cores could “stick” to the plasticpart and break off when the parts were ejected.To strengthen these cores and reduce ejectionstresses we required them to be drafted as tallnarrow cones. The resulting tapered screw holescould be problematic. Unless the screw was alsotapered (like a wood screw), it would become Proto Labs, Inc. 1999–2013This innovation changes two things for you. First,we can now mold parts with deeper, narrowerholes. Second—and here’s the shocker—youdon’t have to draft those features. The reasonsare simple. A steel pin is strong enough to handlethe stress of ejection and its surface is smoothenough to release cleanly from the part withoutdraft. And, while there shouldn’t be any cosmeticeffect on the resulting part, if there is, it will beinside the hole where it won’t be seen.The size of the hole in your part will bedetermined by the size of the thread-formingscrew you’ll use for assembly. The hole itselfwill be slightly larger than the minor diameterof the fastener—the diameter of the shaft at theroot of the threads. Typically, the manufacturerwill specify a diameter for the pilot hole intheir screw specs. Finally, note that somescrews will be specified for particular plasticresins, so if you change your resin duringprototyping you should make sure you’restill looking at the right type of fastener.Figure 1: Example of a core used to form a holein a molded part.Volume 9When NOT to Draft 10

Design Tips for Rapid Injection MoldingIt’s Never Too Early for ProtoQuote By this time, you probably know all aboutProtoQuote interactive quotes: they’re free,automated quotes for rapid injection moldingthat allow you to interact with the materialchoices, quantities, and design feedback (seeFigure 1). The design analysis suggests whereand how your design should be tweaked toimprove moldability. For most users, ProtoQuotefits into the design process somewhere afternapkin sketches and 3D printing (or otheradditive prototyping processes) and before theinjection molding of production parts. That’sthe tried and true methodology, but we’d like tosuggest an alternative approach that you mightfind useful.On your next project, consider uploading your3D CAD model to get a quote from Protomoldbefore you make your first prototype no matterwhat prototyping process you plan to use. Inother words, whether you start with an injectionmolded or machined prototype from Proto Labs,a layered prototype from an outside servicebureau, or a layered prototype made on yourown in-house equipment, get a ProtoQuote first.The reason is simple. If you plan to use injectionmolding for final production, it is crucial to knowyour design is moldable, and the earlier in thedevelopment process you know this the better.As we’ve often pointed out, Protomold injectionmolded prototypes will tell you whether yourdesign is moldable. (There are, of course, otherreasons to use rapid injection molding in yourdevelopment process—it provides functionality Proto Labs, Inc. 1999–2013free. In return you get valuable information thatcould potentially prevent molding problems orthe need to “go back to the drawing board,”later. If your part has undercuts, ProtoQuote willfind them. Walls that are too thick or too thin foreffective molding? Additive processes won’t findthem, but our proprietary quoting software withdesign analysis will.Figure 1: Dropdown menus allow you to changematerials, A- and B-side mold finishes,and quantities to see the impact on price.information that you can’t get from any of theadditive processes—but we understand that theadditive prototyping processes can be usefulin early stages of development.) What we’resuggesting is that you not wait until you’reready for molded prototypes to get the designanalysis that comes with your ProtoQuote. We’deven go so far as to suggest that you request aProtoQuote even if you have no intention of evergetting a molded prototype before committingto production tooling!The benefit for you is that our quoting softwarecan identify potential moldability issues (seeFigure 2), like missing draft or potential for sink,at any point in development. All you need is a3D CAD model you can upload, and since all ofthe above processes require a 3D CAD model,requesting a ProtoQuote is fast, simple, andFigure 2: The ProtoView 3D viewer allow usersto view a fully rotatable model of the design andzoom in on potential moldability issues.If you plan to use ProtoQuote this way, there area few things to keep in mind. Along with your 3DCAD model, the software will optionally ask foryour resin choice, the number of parts you wantmolded, and your preferred speed of delivery.Your model will then be run through the largestcompute cluster in the industry, and youVolume 9Continued on next page It’s Never Too Early for ProtoQuote 11

Design Tips for Rapid Injection Moldingwill receive, in less than 24 hours, an interactiveprice quote along with your design analysis. Ifyou aren’t ready to order molded prototypes,you can ignore the quote and go straight to theanalysis. Color-coded markups on your CADmodel will indicate potential moldability issues(see Figure 3). If you will be using the Protomoldinjection molding service, these concerns willhave to be addressed. If, on the other hand, youare planning to make prototypes using any ofthe additive—layered—prototyping processes,you don’t have to make these changes, but ifyou plan to use injection molding for productionyou’ll probably want to address them anyway.Once you have made the indicated changesin your 3D CAD model, you can resubmit it to make sure you’ve addressedeverything. As development progresses you’llprobably want to make one or more injectionmolded prototypes to use for functional testing,because real prototypes provide informationthat CAD models, simulation software, layeredprototypes, and even a ProtoQuote designanalysis can’t.Figure 3: The grey area on the rim of the partindicates an area with zero draft, which may causeissues such as drag marks, or distortion fromejection stresses. This is one of the many designconsiderations for optimal performance in theinjection molding process.Watch short videos about ProtoQuoteand Proto Flow (simulates resin flow, predictspressure, and anticipates resin fill issues) at Proto Labs, Inc. 1999–2013Volume 9It’s Never Too Early for ProtoQuote 12

Design Tips for Rapid Injection MoldingGetting into GearsAs design tools and techniques becomemore sophisticated and the arsenal of availableresins grows, plastic continues to replace othermaterials in a variety of applications. Gearsare among the mechanical parts being madeincreasingly of plastic. To effectively replacemetal, the design and material must, of course,be suited to the demands of the application,but moldability of the part is equally important.All injection molding processes have specificcapabilities and limitations, and if you planto have Protomold produce gears it is crucialto understand the requirements of ourmolding process.The primary challenge in the rapid injectionmolding of gears is the teeth. In order to be ableto mold a gear we have to be able to fully fit anend mill into the area of the mold that will form agear tooth. Figure 1.1 shows what happens whena gear tooth is too small for our smallest endmill. The mill cannot reach the end of the taperedtooth, so this gear cannot be produced using ourprocess. Figure 1.2 shows a gear with teeth largeenough to accommodate our end mill.Note that while the mill can reach the end ofthe tooth in Figure 1.2, it cannot reach into thecorners of the squared off tooth. Teeth on thispart will need to have more rounded corners.When you submit such a part for a ProtoQuote ,the inability to produce the squared outside Proto Labs, Inc. 1999–2013Figure 2: Requirements affecting the radii ofoutside corners.Figure 1: The dark blue dot represents the end millthat would be used to form a gear tooth in the mold.corners will be noted in the design analysis, andyou will have the opportunity to decide whetherto modify your design accordingly. Dependingon your design, this rounding could affect theengagement of gear teeth causing unacceptable“slop” in the operation of the gears. Your 3DCAD software can help you determine whetherthe modification will cause problems.Another design consideration for gears is theside-to-side width of the gear. Wider gears maybe needed to handle greater force, and the widthof the gear determines the depth to which amold will have to be milled. Deeper cuts requirelonger mills, and because long thin mills couldwobble during cutting operations, our minimummill radius increases with cut depth. If cut depthis great enough we use tapered mills to preventwobble during cutting. As a result, very deepcuts require draft (see Figure 2).When draft is required on gear teeth, it’s toallow for proper mold milling, not ease ofejection. While the amount of draft required forwider gears is small, as in the case with roundedgear teeth drafted gear teeth can affect theengagement of gears. In some cases, designingcomplementary draft into the teeth of matinggears can eliminate engagement problems.Because they have to move and mate, mostgears are made from a relatively small range ofhigh-lubricity resins. For this reason, our abilityto produce gears is not generally affected byresin choice. Following the above guidelinesshould allow you to design gears that canbe effectively molded using our process. Ifmoldability problems are found, they will behighlighted in your ProtoQuote design analysis.Volume 9Getting into Gears 13

Design Tips for Rapid Injection MoldingParts on a Budget: 5 Money-Saving TipsWe are sometimes asked, “How can I get myparts made for less money?” This design tipshows you the best ways to cut costs from yourProtomold project. A word of warning: most ofthe ways to save involve changing the designof your part. Obviously, if your part needs to bea certain way, it needs to be that way. On theother hand, if you have flexibility in your designconstraints, there are many ways to save.As you consider these tips, you need to keepyour end use of the parts in mind. Two majorconstituent groups use Proto Labs: people whoneed prototype parts and people who needshort-run or bridge production parts. Within theprototyping constituency, there are subgroupsinterested in fit, appearance evaluation, processvalidation, strength testing, and so on. Savingmoney by eliminating a cosmetic finish might bejust fine for a prototype part used for strengthtesting, while it would not make sense for aprototype case used in a marketing sample.For prototypes, minimizing the tooling costusually makes the most sense. For people whoneed production parts, it might make sense t

Real Parts. Really Fast. Proto Labs, Inc. 5540 Pioneer Creek Drive, Maple Plain, MN 55359 877.479.3680 www.proto