PM DesignTip Essentials

Print. Bind. Save.

PROTOMOLD DESIGN TIPS

12 ESSENTIALS FOR MOLDABILITY

Real Parts. Really Fast.

www.protolabs.com

Proto Labs, Inc. 5540 Pioneer Creek Drive, Maple Plain, MN 55359 877.479.3680

Essentials for Moldability

Page

TABLE OF CONTENTS

3

Prototyping

5

Materials

6

Cosmetics

8

Draft

9

Wall Thickness

11

Radii

12

Text on Parts

13

Living Hinges

14

Surface Finishes

16

Cams

18

Metal Injection Molding

20

LSR Molding

© Proto Labs, Inc. 1999–2014

Ten years ago we began publishing Protomold Design Tips to help product developers create better injectionmolded plastic parts. We’ve talked texture, addressed draft, mulled over materials and been through thick and thin (wall sections). More than 100 tips later, we remain steadfast in our effort to continue informing designers and engineers in all areas of injection-molded part design. As a quick reference guide, we’ve gathered some of our most useful manufacturing advice from the past decade in a collection of essential tips.

RESOURCES Our online library of content includes our complete catalog of Protomold Design Tips along with white papers, case studies, a glossary of terms and much more. We also staff experienced Customer Service Engineers who can discuss any design questions that may arise. Find us online at protolabs.com or call us at 877.479.3680.

Essentials

TABLE OF CONTENTS

2

Design Tips Best of Compilation

PROTOTYPING

Bridging the Gap Between Prototypes and Production In the traditional approach to product development, there is a fine line between development and production. Development begins with a light bulb over someone’s head, proceeds through napkin sketches and CAD models, and ends, ultimately, with prototypes. At one or more points in the development process there may be input from the market, be it someone’s best guesses, one or more focus groups, or actual market tests. And from start to finish there is always pressure to “get on with it,” either because you need to catch up with a market leader or because you are the leader and someone may be catching up with you. But then, when you have reached your goal — a fully developed, marketable product — everything comes to a screeching halt and the drawings and/or models disappear into the “production machine,” from which, weeks or months later, a whole lot of deliverable product appears and the rush begins again as it heads off to market. In plastic molding, as in most other technologies, some aspects of this transition are unavoidable. Production molds are costly, and they take time to manufacture. It would be risky to begin producing them before the design had been fully proven in development, when even a small change could turn tens of thousands of dollars’ worth of molds into doorstops and boat anchors. Traditionally, this has always presented manufacturers with a dilemma. They could keep development and manufacturing sequential and live with the resulting delay. Or they could treat

© Proto Labs, Inc. 1999–2014

Figure 1: Tensys Medical used Protomold for prototyping and pilot production on this T-Line® Tensymeter component, which allowed Tensys design engineers to significantly shorten the product design and development program time cycle.

them in parallel, starting on production molds before the end of development, cutting their time to market but running the risk of having to go back and start tool-making over again. It is a painful choice because today’s competitive global markets reward both speed and low cost. Manufacturers already recognize that quickturn injection molding as a prototyping method can reduce both cost and delay in the design of plastic parts. They are now beginning to see that it can also help reduce the post-development delay in bringing a product to market. While quick-turn injection molding is not identical to traditional production tooling, it is similar enough in process and technology to solve several problems and help speed up

production (see Figure 1). First, in addition to proving the design itself, it confirms that a part can actually be molded. Second, while production-tool molding can incorporate capabilities that quick-turn injection molding can’t — internal cooling lines or sophisticated venting, for example — adapting a part to the demands of quick-turn injection molding by equalizing wall thicknesses and maintaining draft can actually simplify and accelerate the manufacturing of production molds while reducing their cost. In other words, quick-turn injection molding doesn’t just produce prototype parts; it prototypes the production method that will produce those parts, allowing avoidable problems to be eliminated before the start of final mold making. Perhaps even more important in today’s fastmoving markets, tools made for quick-turn injection molding can also be used to mold parts in actual production resins and in production volumes — thousand or even tens of thousands of parts — while the “official” production tools are being made. Basically, yesterday’s prototyping molds can produce today’s goto-market parts while tomorrow’s high-volume molds are being made.

Continued on page 5

Essentials

PROTOTYPING 3

PROTOTYPING

In fact, once it becomes clear that you can take prototype parts to market, you may actually find reasons to simply postpone the production of steel tools. One reason might be the ability to reduce up-front expenditures by ordering parts in smaller quantities than you commit to when you turn to production molds. This makes particular sense if there is any uncertainty about market demand for your new product. It’s a way of going beyond mere market tests and actually releasing your product to the market and gauging response before committing to full-scale production.

© Proto Labs, Inc. 1999–2014

If the market’s reaction to your product suggests the need for iterations, you can make changes quickly and be back in the market in days with an improved product. If necessary, you can repeat the process several times, each at modest cost, before committing to mass production. In a sense, this sort of bridge tooling lets you treat a physical product in much the same way that software developers treat theirs, with versions tumbling onto the market one behind another as features are added. There’s really no reason that releases of Widget Mark I, Mark II and Mark III have to be years apart if

the market demands an improved product. For those accustomed to traditional methods, this may be a novel approach, but if it eliminates the lengthy wait while production tools are being made, it may be worth a try. And if you’re able to avoid sending thousands of parts and costly molds to the landfill because the market wants something slightly different, so much the better.

Essentials

PROTOTYPING

4

MATERIALS

Res Solutions There are more than 85,000 commercial options for plastic materials listed in materials databases, and within those, there are 45 polymer families which can be broadly classified into two categories: thermosets and thermoplastics. We primarily focus on the latter category, offering hundreds of stocked engineering-grade thermoplastic resins through Protomold. In addition to our stocked supply of resins, we have the capabilities to support many more customersupplied resins as well. Narrowing down that extensive list of materials can sometimes seem like a formidable task, so we’ve prepared a quick reference guide that outlines the benefits and applications of some of our most commonly molded materials. By choosing the right material, you can improve the form, fit and function of your parts.

clear grades. PC has good heat resistance and accepts high cosmetic finishes well. Applications include lenses, indoor and outdoor lighting, cell phone housings, electrical components and medical devices. PPA (Aliphatic Polyamides) There are many types of PPAs, or nylons, each with its own advantages. Generally, nylon parts provide high strength and high-temperature strength, especially when reinforced, and are chemically resistant, except to strong acids and bases. Applications include thin-walled features, combs, spools, gears and bearings, screws, structural parts (with glass) and pump parts.

ABS (Acrylonitrile Butadiene Styrene) ABS is a tough, impact-resistant plastic that is widely used across many different industries. It has low shrink and high dimensional stability, and it has good resistance to acids and bases. It’s also relatively inexpensive. Applications include cosmetic parts, handheld devices, housings and moldings for electrical tools, and remote controls. PC (Polycarbonate) PC is strong and extremely impact resistant with low shrink and good dimensional stability. It’s a transparent plastic that is available in optically

© Proto Labs, Inc. 1999–2014

POM (Polyoxymethylene) Commonly called acetal, it is characterized by high toughness, stiffness, hardness and strength.

Acetal has good lubricity and resistance to hydrocarbons and organic solvents. It also has good elasticity and is slippery, so it works well for bearing surfaces and gears. Applications include pumps and pump impellers, conveyor links, soap dispensers, fan and blower blades, and automotive switches. PMMA (Polymethyl Methacrylate) PMMA, aka acrylic, has good optical properties, high gloss and is scratch resistant. Acrylic also has low shrink, and has less sink in geometries with thin and think sections. Applications include light pipes, lenses, light shades, optical fibers and signs. PP (Polypropylene) PP is an inexpensive resin option with higher impact resistance in some grades; propylene homopolymer can be brittle in cold temperatures, with copolymers more resistant to impact. PP is wear-resistant, flexible and can have very high elongation. It’s also resistant to acids and bases. Applications include integral hinges or living hinges, fans, snap-over lids (i.e. shampoo bottle tops) and medical pipette tubing. Protomold has many additional stocked resin options including PBT, PPS, TPE, TPU, LCP, HDPE, LDPE, PPSU and PSU as well as highperformance materials PEEK and Ultem®. We offer a detailed Materials Matter white paper at protolabs.com that provides a technical overview of the materials selection process.

Essentials

MATERIALS

5

COSMETICS

A Guide to Looking Good First, the bad news: Injection-molded parts are subject to a host of cosmetic flaws including sink, gate vestiges, ejector pin blemishes, drag marks, texture flaws, knit lines, burns, flash and inconsistent coloring. The good news is that adherence to basic design rules will eliminate many of these problems. Thoughtful planning will further reduce the risk. And judicious prototyping will allow you to eliminate virtually any avoidable problems that remain. The first step is to determine just how important cosmetics will be to your design. In a faceplate, looks may be your first consideration. In an internal part, they may be your last, freeing you to focus on other matters like function and cost. In some cases, improving appearance will require tradeoffs, but this is not always the case. Good function and good manufacturability often go hand-in-hand with good appearance. For example, proper attention to draft doesn’t just ensure ease of ejection; it prevents unsightly drag marks (see Figure 1). Maintaining uniform wall thickness helps prevent functional weakness due to incomplete filling and poor fit due to warp, but it also helps prevent unsightly sink in thick areas, and flash and burn that can result from overly high injection pressures needed to force resin though thin sections. In short, following standard design guidelines will help prevent many cosmetic flaws.

© Proto Labs, Inc. 1999–2014

Figure 1: Without draft (left), the part drags along the entire vertical side, but by adding draft (right), the part falls free from the mold upon ejection.

This is where the tradeoffs begin, where gains in one area may require sacrifices in another. Start by ordering your priorities. Certain requirements may impact appearance by limiting your choice of resins. Glass-filled nylon, for example, is very strong but cannot be given a smooth finish regardless of how much the mold surface is polished. Similarly, a TPE like Santoprene™ provides excellent elasticity, but will have, at best, a matte finish. Acetals like Delrin™ offer excellent resistance to solvents and

wear due to friction, but can develop an “orange peel” finish as they cool. If your requirements are very specific and limit your choice of resins, they may also limit your cosmetic options, at least as far as finish is concerned. If, on the other hand, your material choice is less constrained, resins like ABS and polycarbonate are easy to mold and will give you precisely the finish designed into your mold.

Continued on page 7

Essentials

COSMETICS

6

COSMETICS

Many cosmetic decisions cross the boundaries of material and design. Knit lines form where cooling flows of resin meet, typically after parting to flow around a core in a mold or where flows from multiple gates meet (see Figure 2). Choice of resin can impact the likelihood of knit lines, but even if they do not weaken the part, they can leave an unsightly line. How visible it is may depend on the color of the resin. Light reflection can make knit lines in black resin very apparent, while the exact same line in white resin will be far less obvious.

There are other addressable issues that affect specific types of resin. Delrin, as mentioned, can crinkle as it cools, a problem that can be minimized by avoiding excessive or inconsistent wall thickness. As polycarbonate cools it can shrink and leave internal voids where thick walls meet. In an otherwise well-designed part these may not cause functional problems, but they can be a cosmetic problem in clear resin in which the bubbles are visible. If you must use a clear polycarbonate, the mold can be redesigned to shorten the distance resin must travel, allowing the press to pack the mold before the resin cools too much to fill the void.

Figure 2: Knit lines may form when the resin flow is divided by an obstacle or hole and then meets again on the other side.

© Proto Labs, Inc. 1999–2014

Resins that are good at filling small details in a mold can also have an increased tendency to flash — to force their way into the parting line of the mold, leaving material that must be trimmed in a secondary operation. A mold in which material must be forced through thin areas will require higher injection pressure to fill the mold, which, in turn, will increase the likelihood of flash. The problem can be addressed either by eliminating the mold features that require higher injection pressure or choosing a resin that is less prone to flash. Finally, there are issues of mold design — gate and ejector placement — that impact a part’s final appearance. Protomold’s designers will work to minimize the afore-mentioned problems, but nearly all molds require gates and ejectors, and these will leave their marks on the finished parts. If this could be a problem, you should bring it to the attention of our engineers before your order is finalized. There are many issues that can impact cosmetic outcomes. Some may be identified by the design analysis included with your ProtoQuote®. Others involve cross-coupled variables, which are discussed in more detail in our free cosmetics white paper available at protolabs.com. Still others may be difficult to anticipate and will only show up when prototypes are made, allowing you to try other resins or, if necessary, redesign your part before committing to full-scale production.

Essentials

COSMETICS

7

DRAFT

A New Slant on Draft Imagine a piston in the cylinder of an engine. It seems like a tight fit, yet the parts slide smoothly with no damage. There are several reasons that a piston moves so smoothly. First, there is space between it and the cylinder; second, both have hard, polished surfaces; and third, the surfaces are liberally lubricated. Eliminate the space or the lubricant and engine damage is guaranteed. The same thing can happen when you pull a straight-sided plastic part out of a straight-sided mold, because there is neither space nor lubricant, and plastic isn’t very hard. To prevent damage or drag marks on the part, surfaces that are parallel to the line of mold opening may have to be drafted — angled away from the line along which the part will be ejected. This causes the part wall and mold wall to move apart during ejection.

Your CAD package will not tell you which surfaces should be drafted, but it may indicate which surfaces are drafted (or reverse-drafted). Many CAD packages have a draft analysis module, which you should use liberally if it’s available. When you upload your design for a quote, however, Protomold’s online ProtoQuote® software identifies, in colors, surfaces that need additional draft and/or thickness. Most features require draft to facilitate ejection, and many features need draft for Protomold to be able to machine the mold. Also, textured surfaces have a particular tendency to stick to mold walls, so 3 degrees of draft is recommended for light texture (T1), and at least 5 degrees of draft for heavy texture (T2).

A protruding arm of the bottom mold half (the telescoping shutoff) that forms the face and hook of the clip, for example, must be drafted by 3 degrees both to protect the part and to prevent mold wear due to rubbing of the shutoff against the top mold half. Finally, proper drafting allows the cutting of deeper mold geometries at lower cost. A good rule of thumb in part design at Protomold is: “When in doubt, draft it.”

Top View

Undrafted

Drafted No draft results in sliding parallel mold surfaces

© Proto Labs, Inc. 1999–2014

Top View

Drafted shutoffs minimize wear and extend mold life

Essentials

DRAFT

8

WALL THICKNESS

The Incredible Shrinking Wall (and Other Problems) In theory, injection molding is a simple process. You create a mold with a cavity the size and shape of the part you want. You inject molten resin, which fills the available space. The resin cools and hardens. You open the mold and remove the finished part. Unfortunately, nothing is ever that simple. First, materials expand when heated and shrink when cooled, and plastic resins are no exception. The shrink of the resin must be taken into account when designing the mold. The cavity that forms the part is designed slightly oversize to allow for shrinkage. How much oversize? That depends primarily on the resin. Some resins shrink uniformly in all directions and some, like glassfilled resins, have different shrink rates in the direction that the resin flows in the mold versus the direction of cross flow.

Minimizing variations in wall thickness helps to reduce variations in shrinkage.

Second, certain shapes seem to invite mischief. One of these is the overly thick wall, which is subject to several types of problems. ■■ Because shrinkage is proportional to resin

depth, and because molded parts cool from the outside in, thick walls are susceptible to sink marks — low areas that can be both structurally and cosmetically problematic.

■■ Resin pellets are heated in a barrel and

injected into the mold with a screw at pressures of up to 20,000 psi (pounds per square inch). Air and gas can be dissolved in the resin melt. As the parts cool and the resin shrinks, this gas can come out of solution. The result is either bubbles hidden within the part or voids visible at the surface. Both cause structural weakness. Voids at the surface create cosmetic problems as well. This problem is more pronounced in thick areas of the part.

In short, excess wall thickness creates problems, adds little value to a design, increases weight and adds to material cost (see Figure 1). Fortunately, designers have come up with elegant ways of eliminating unnecessary thickness without compromising performance. The accompanying diagrams demonstrate some of these techniques. Figure 1 shows a part that, due to excess wall thickness, is susceptible to all the risks mentioned above. The right side is surrounded by thick walls, and the left side is one thick wall. (Note the six screw holes, presumably for attachment to some assembly.)

■■ In some cases, particularly where wall thick-

nesses vary significantly, uneven shrinkage can cause parts to warp as they cool.

Figure 1: Excess wall thickness can cause a range of problems.

© Proto Labs, Inc. 1999–2014

Essentials

WALL THICKNESS

9

WALL THICKNESS

Figure 2 shows the same part redesigned, demonstrating a variety of techniques for eliminating excessive wall thickness. The three walls at the far right have simply been reduced in thickness. Each of the two angled walls at the center have been cored out, leaving two thin walls in place of one thick one. The “slab” on the left side of the part has been cored out in several sections, leaving a network of ribs to maintain strength. The diagonal rib in the section at the lower left provides torsional stiffness to help prevent twisting of the part. (The triangle, as you probably know, is an inherently stiff form.)

The revised design also shows several ways of reconfiguring the screw holes in the original design. At the right, two screw holes have been configured as bosses attached to the wall. Note that the wall thickness of the bosses is consistent with that of the part walls. In the center, two screw bosses have been made part of the rib formed by the coring of the slab.

Fortunately, designers have come up with elegant ways of eliminating unnecessary thickness without compromising performance.

Finally, uniform wall thickness is important in good injection-molded part design. Minimizing variations in wall thickness helps to reduce variations in shrinkage. This results in less warpage and better dimensional tolerances. When you upload your 3D CAD model to ProtoQuote®, it’ll provide a useful check on wall thickness from the standpoint of moldability. Areas that are too thick and thin will be highlighted. But since ProtoQuote doesn’t know your application, it cannot take into account the use to which the part will be put or the forces to which it will be subjected. That is for the user to determine, which, of course, is why you do prototyping in the first place.

At the left edge of the part, the screw bosses have been separated from the wall, but are tied to it by ribs to add support for the bosses.

Figure 2: Eliminating excessive wall thickness can reduce weight, material cost and more.

© Proto Labs, Inc. 1999–2014

While part walls should not be unnecessarily thick, they need to be thick enough to allow the molten resin to flow and fill the part. They should also be thick enough to provide the strength necessary for your application. A chart showing the acceptable range of wall thickness for various resins can be found in our Resources section online at protolabs.com.

Essentials

WALL THICKNESS 10

RADII

Reduce Your Stress With Radii Whenever possible, a part to be injection molded should be designed with generously radiused corners to enhance its quality and moldability. Corners designed without radii can cause stress concentrations. These, in turn, may reduce the ability of the part to withstand load and/or cause warping in its geometry. Sharp corners can adversely affect the flow of resin during molding, potentially causing incomplete fill. They also tend to cause the part to stick to the mold during ejection, which can cause a variety of problems. The judicious use of fillets can also help to improve mold life by helping to minimize corner stresses at the bottom of tall, thin cores in the mold (at the entrance to deep, thin holes in the plastic part). These fillets also help to enhance the ability of the mold to fill and further reduce internal part stresses. Now, let’s distinguish between inside and outside corners, both of which should be radiused but for slightly different reasons.

Outside corners The first thing to keep in mind is that an outside corner of your part is created by an inside corner of a mold, and vice versa. One reason we don’t make parts with sharp, outside corners is because our molds are made by a vertical milling process that cannot cut a sharp, inside corner. The radius of our inside corner (your outside corner) cannot be smaller than the radius of the cutter, which will vary somewhat with the depth of the cut. Inside corners Our milling process can produce sharp, outside corners when making a mold, so you can have sharp, inside corners. The problem is that sharp, inside corners can create serious stresses in a part as it cools. The reason is simple: The rate of resin cooling is proportional to surface area. Any corner will have more surface area on the outside of the curve than on the inside. (Think about the advantage of the proverbial “inside track.”)

On a radiused corner, there is always a difference between the two surface areas, but if the inside of the corner is square, it essentially has a surface area of zero, which maximizes the difference between the inside and outside surface areas. Sharp corners versus radiused corners If the part consisted of two walls meeting to form an L shape, the part may tend to warp as it cools, reducing the angle between the two walls. If, however, the corner is in, say, a box whose shape keeps the walls from moving in relation to one another, instead of warping, they’d merely become stressed. The result could be cosmetic problems; a fracture or buckled floor. In addition, because they are sharp, the outside corners of a mold half can grab the part within which they are forming a core, either making ejection difficult or risking damage to the part or mold. And finally, sharp corners can contribute to sink and weakened knit lines. So, radius those corners.

Radiused Corners

Outside Corners

Inside Corners

Figure 1: Corner radii can enhance part quality, moldability and mold life.

© Proto Labs, Inc. 1999–2014

Essentials

RADII

11

TEXT ON PARTS

Word Processing There are lots of reasons to add text to a part. It could be an assembly instruction, a part number, a legally advisable warning or simply a logo (see Figure 1). Whatever the reason, text characters tend to be the smallest features of a part and, as such, deserve the designer’s careful attention.

lines of the letter itself, making them too small to mill. Instead, use a sans-serif (non-serif) font like Century Gothic Bold (the default font in SolidWorks). Other common sans-serif fonts are Arial and Verdana. In general, remember that while most 3D CAD programs allow you to use standard Windows fonts, you should resist the temptation to get cute without a good reason.

Figure 3: The “L” is a larger font size and allows room for the tool to mill.

need to do so, upload the part with the smaller text for a ProtoQuote® interactive quote and the quote will show any required changes or advisories. Figure 1: Off/On switch instructions on a part.

The first thing to keep in mind is that it works much better if text on a plastic part is raised above, rather than recessed into the part (which means it will be milled into the mold). Raised letters on a part are easier to read , and recessed text in a mold allows for polishing, whereas raised letters in a mold make it difficult to achieve a good finish. The second issue is consistency of wall size in your lettering. Avoid serif fonts, the ones with the little squiggles at the ends of uprights. The serifs are typically narrower than the primary

© Proto Labs, Inc. 1999–2014

Figure 2: The “O” is too small to mill.

The third issue is the size of the letters themselves (see Figure 2). Text doesn’t need to stand very tall above the surface of a part — 0.02 in. is plenty — but even so, the rules for thin ribs apply. You don’t need to measure the thickness of every line of each letter; just stick to font sizes of 20 points or more and use the Bold version of the font, and odds are excellent it can be milled (see Figure 3). In some cases, we can mill smaller fonts. If you

Finally, if text is located at the top of a tall feature — a tall rib, for example — the text may have to be larger. In summary, for best results when incorporating text, design your parts: ■■ Design your parts with raised text. ■■ Use a bold sans-serif font. ■■ Size text to 20-point type or larger. ■■ Stay away from the tops of tall features.

Essentials

TEXT ON PARTS 12

LIVING HINGES

Hinge Worthy Take a look at a door and the hinges on which it swings. There are probably three or four hinges, each of which consists of three separate parts and four to six screws. Do the math, and you’ll see that the hinge you take for granted in your daily comings and goings consists of at least 21 separate components. Life would be so much simpler if the number of parts needed for a hinge could be reduced. The good news is that, at least in the design of plastic parts, it often can.

When a hinge bends, tensile forces are transmitted to the material along the outside of the bend. The thicker the hinge, the greater the stress in the outside surface, so the hinge should not be too thick or it may crack when it is bent. On the other hand, if the hinge is too thin, it will not be strong enough to withstand any tearing forces, especially at the ends. Also, be careful that, when the hinge is bent fully, there won’t be interference from thick edges along the hinge.

A reduction to three or two or just one part for a hinge would be notable. A reduction to zero is truly impressive, and that’s exactly how many additional parts a living hinge requires. Quite simply, a living hinge is a thin strip molded into a plastic part to create a line along which the part can bend. Properly designed and executed, it can be closed and opened over the life of the part with little or no loss of function. But simple though it may be in concept, a living hinge must follow certain guidelines if it is to work properly.

Finally, a thin spot in a part (which is what a hinge is) can be challenging to fill during resin injection (see Figure 1). Success depends on proper gate placement. A single gate that forces resin through the hinge area in a mold increases the strength of the hinge; however, this approach can lead to sink in areas downstream from the hinge.

First, only certain resins are flexible enough to support the degree and frequency of bending required of a hinge. The best resins for parts with living hinges are polyethylene and polypropylene.

© Proto Labs, Inc. 1999–2014

Multiple gates may eliminate the problem of sink, but if resin flows meet at the hinge (which they will tend to do), they will usually cause cracking. When you order a mold, Protomold will propose gate location(s) to optimize filling of the part including any living hinges.

Figure 1: The arrows point to the very small section where resin would be forced through to fill the other side of the body of the part. The resulting web of resin is thin enough to allow repeated binding.

If this all seems like a lot of trouble, remember that experts suggest that a well-designed living hinge can be flexed millions of times. That’s more times than most of us will walk through doors in a lifetime.

Essentials

LIVING HINGES 13

SURFACE FINISHES

Talkin‘ Texture Surface finish on a plastic part can serve many functions, from improving grip to hiding fingerprints to facilitating paint adhesion. Protomold offers eight finishes on molded parts: six polishes and two textures. The six levels of polish are created using manual mold polishing techniques; the two levels of texture are achieved by bead blasting the mold surface after applying a manual base polish. Protomold’s available textures are light bead blast (T1) and medium bead blast (T2). Many factors can influence your choice of texture. The most obvious is the intended function of the finished part. Either texture can provide an attractive, non-reflective surface. If grip is an issue, that may also influence your choice between light and medium texture. Other factors may include secondary processes to be applied to the part and the part’s aesthetic fit with other components of your finished product.

Another factor linking resin choice and surface texture is sink. In many cases, a slight sinking of the part surface as the part cools may be no more than a minor cosmetic issue. A textured surface, however, will create shadowing, a visible darkening of areas with sink across a surface. This makes an even slightly sunken area far more apparent, creating a significant cosmetic problem. In such cases, there are three possible solutions: 1. Keep the texture but reduce the thickness of the feature to reduce the likelihood of sink. 2. Keep the texture but choose a resin that shrinks less to reduce sink. 3. Reduce or eliminate the surface texture to reduce shadowing.

The last and potentially most serious issue with a textured surface is the possibility of the part sticking in the mold. A textured surface that is perpendicular to or significantly angled away from the direction of mold opening will not present a problem. On a surface that is parallel to the direction of mold opening, however, each little valley in the textured surface acts as a tiny undercut. Forcibly ejecting such a part from the mold can create drag marks on the part surface as the textured mold face scrapes off the high areas of the textured surface. To eliminate drag marks, Protomold suggests that surfaces with T1 texture be drafted by at least 3 degrees and surfaces with T2 texture be drafted at least 5 degrees. (Surfaces that are created using sideactions need not be drafted away from the direction of mold opening; rather, they must be drafted away from the direction of side-action cam withdrawal.)

The resin being used can also affect texture choice. For example, olefin resins such as polypropylene can have a waxy feel and, in conditions of high humidity, can become slippery. An appropriate texture can reduce these problems. In such cases, the ability to texture the part’s surface can expand your range of usable resins.

© Proto Labs, Inc. 1999–2014

Continued on page 15

Essentials

SURFACE FINISHES 14

SURFACE FINISHES

For a variety of reasons, Protomold’s ProtoQuote® design analysis does not look for or identify surface texture. Instead, you will select surface finishes — polish or texture — after you have received your quote. Once you recieve your interactive quote (along with moldability analysis), you can select the surface finish within the quote and immediately see the resulting effect on cost in real time (see Figure 1).

Figure 1: Pricing is immediately updated as you make selections on your quote.

© Proto Labs, Inc. 1999–2014

While ProtoQuote does not recognize texture, it does identify surfaces that fall short of the draft required for light or medium texture by highlighting them within the quote (see Figure 2). You should either revise your design to increase draft or skip texturing this area entirely in any potentially troubled areas that are noted in the design analysis.

There are two additional points to keep in mind regarding texture. First, the quote will have provisions for texturing either the entire A-Side, entire B-Side, or both sides. Partial texturing a side can be done in some cases, but it requires a conversation with a Customer Service Engineer at Proto Labs. Second, areas that cannot be accessed by the bead-blasting stream cannot be textured. Customer Service Engineers can help address either of these issues as well as any other questions you may have regarding surface finish.

Figure 2: Areas highlighted in red have less than three degrees of draft.

Essentials

SURFACE FINISHES 15

CAMS

Getting to the Core of Cams By moving in directions perpendicular to the direction of mold opening, side-action cams allow the production of parts with undercuts that could not be successfully made in two-part, straight-pull molds. But there are parts without undercuts that can also benefit from the use of cams. The Protomold process (vertical milling) requires increased wall thickness and draft as the part depth increases. Using cams, we can reduce the need for draft and wall thickness in some instances. Imagine, for example, a thimble, essentially a cup with tapered sides (see Figure 1). The obvious way to mold such a part would be in a twopart mold in which the outside is formed by the A-Side mold half and the inside core is formed by the B-Side mold half (see Figure 2). Resin would be injected through a tab gate placed along the parting line at the rim of the cup. If the walls of the part were thick enough, this might work. In a thin-walled design, however, there could be problems.

First, in a thin-walled design, the resin could cool quickly enough to result in a short shot, that is, incomplete filling of the cavity. Obviously, this would not be acceptable. Even if the cavity did fill completely there would be two flow fronts meeting on the side of the core opposite of the gate, creating knit lines that could substantially weaken the resulting part. One possible solution would be to use a hot-tip gate and inject resin at the closed end of the cylinder — the bottom of the cup or top of the thimble. This may not be possible due to the height of the part, resin compatibility with the hot tip or other unforeseen issues. A better solution might be to lay the design on its side and use a cam to create the core (see Figure 3 on page 17). Figure 1: A thimble with tapered sides.

Continued on page 17

Figure 2: The B-side of the mold half forms the inside core.

© Proto Labs, Inc. 1999–2014

Essentials

CAMS 16

CAMS

In this case, the core of the thimble is formed by a retractable cam while the outside of the part is formed by two straight pull mold halves. As shown in the diagram, the parting line runs across the closed end of the cylinder and down the sides. For the sake of symmetry, a tab gate is located along the parting line at the closed end of the cylinder. Resin is injected through the gate and the resin flows down the length of the cavity uniformly.

Figure 3: A cam is used to create the core of the thimble.

© Proto Labs, Inc. 1999–2014

Basically, cams aren’t just for undercuts. Need a flat face to bolt up against a mating part? How about texture? How about your company’s logo or part numbers? Adding a cam to the mold may be just what you need.

So how do you identify parts that can benefit from the use of cams? Tall, thin parts with a core, like our thimble, are prime candidates. Another candidate might be a part that, in a straight-pull mold, would require more draft than the designer is able to provide. In that case, side-action cams may completely eliminate the need for draft as the outside face is pulled perpendicular to the part instead of being pushed or pulled from the mold. And, finally, cams can allow production of parts with texture on faces parallel to the direction of mold opening. In simple straight-pull molds, such texture acts essentially as a field of undercuts that can prevent the clean ejection of parts. If there is a design need and we can get a cam on that face, why not? It just may be the face that makes your part.

Essentials

CAMS 17

METAL INJECTION MOLDING

Designing for Metal Moldability Metal injection molding — often referred to as MIM — is a molding process that was developed in the early 1970s and subsequently embraced throughout the manufacturing industry at the start of the next decade. It’s a particle materials technology that has existed for the last 40 years, but since the early days of MIM, the molding of metal parts has been a process typically only used when large volumes were needed due to high tooling costs. A mold that is only needed for a few thousand parts is usually too labor intensive and expensive for traditional manufacturers to consider. Using the same methodology that allowed for cost-effective manufacturing of low-volume thermoplastic parts, Protomold provides product designers and engineers the option of 25 to 5,000+ affordable stainless steel (316L and 17-4PH) parts in less than three weeks. To better understand a how to create a welldesigned MIM part, let’s start with the process itself. Metal injection molding is a multi-step process that begins very similarly to plastic injection molding. A metal powder suspended in a binder matrix (or feedstock) is injected into a tool where it is heated just like plastic, and the resulting molded parts physically resemble plastic parts. Now in a fragile green state, parts

© Proto Labs, Inc. 1999–2014

are moved from the injection-molding press into an oven where a catalytic debinding process chemically removes about 80 percent of the plastic polymer. Once the debind process is complete, we’re left with even more fragile brown parts that are transitioned from the debind oven to the sintering furnace (see Figure 1). Brown parts are heated to approximately 1,400 degrees Celsius in the furnace, where the remaining binder is removed to form a fully dense metal part.

Stainless steel parts produced in our MIM process tend to be small and are limited to certain constraints. The mass of most MIM parts does not exceed 150 grams with a typical mass well below that. Our current maximum part size limits are approximately: ■■ 4 in. by 4 in. by 4 in.

(10.1 cm. by 10.1 cm. by 10.1 cm.)

■■ No deeper than 2 in. (5.1 cm.)

from any parting line

■■ Maximum projected mold area

of 10 sq. in. (64.5 sq. cm.)

■■ Maximum part volume less than

1.25 cu. in. (20.48 cc.)

Under some circumstances, it may be possible to extend these limits, but there are tradeoffs. If you need larger parts, please discuss your design and application with a Customer Service Engineer at Proto Labs. Continued on page 19 Figure 1: A Proto Labs technician unloads a tray of metal parts from a sintering furnace during the MIM process.

Essentials

METAL INJECTION MOLDING 18

METAL INJECTION MOLDING

An important consideration when designing for metal injection molding is the proper fixturing of your parts during the debinding and sintering phases to account for shrink and sag. An ideal MIM part has a common co-planar surface that can rest flat to support itself, but we realize that in real-world manufacturing, this is not always the case. There are a few ways to support more complex parts:

If you’ve designed for thermoplastic injection molding before, you’ll notice that metal injection molding shares a number of similar guidelines. Design features like ribs, gussets, fillets and radii should be considered to improve the strength of MIM parts; try to observe uniform wall thickness when possible; and although you can occasionally get by with less draft on certain MIM parts — e.g., shallow parts with good surfaces for ejection — a standard 0.5 degrees of draft per inch (2.54 cm.) of depth on the molded part size (not the final part size) is a good rule to follow.

■■ Protomold has a number of trays with built-

in support grooves and channels for common geometries found on MIM parts. If your parts require more fixturing attention, we can machine custom trays for additional support, but that can become costly.

■■ An alternative is to design your MIM part

with support fixturing built right into the geometry. If the added support features do not compromise the final product functionally or cosmetically, they can most likely remain on the part. If they need to be removed, they can be machined off in a secondary process.

There is generally a 15 to 20 percent shrink between a green part and fully dense part. Final dimensions are highly dependent on the geometry of the part, and larger parts present more opportunity for variances in final size, but well-designed parts typically achieve linear tolerances of ±0.005 in. (0.013 cm.), plus 0.001 in. (0.003 cm.) for each inch of dimension.

© Proto Labs, Inc. 1999–2014

Figure 2: From start to finish — fully dense stainless steel parts sit atop a pile of MIM feedstock.

Metal conducts heat very well, so MIM feedstock (see Figure 2) freezes in the mold much more rapidly than plastic due to its composition of metal particles (along with the mold also working as a heat conductor). As a result, long and/or complex parts can be more challenging to fill. Strategic gate placement, larger gates or multiple gates should be considered to better facilitate material into the mold. We also run our ProtoFlow™ analysis on each design to detect any issues with material flow before the MIM process begins.

Learn more about metal injection molding at protolabs.com or call a Customer Service Engineer at 877.479.3680 to discuss any questions you have about MIM.

Essentials

METAL INJECTION MOLDING 19

LSR MOLDING

Molding Flexibility Proto Labs has recently added liquid silicone rubber (LSR) to its expanding list of injectionmolding capabilities. LSR molding shares many similarities with conventional injection molding, but there are a few notable differences. Unlike thermoplastic resin, which is melted before injection, LSR is a two-part thermoset compound that is chilled before being injected into a heated mold and ultimately cured into a final part. Since LSR is a thermosetting polymer, its molded state is permanent — once it is set, it can’t be melted again like a thermoplastic. LSR has certain inherent characteristics. It is a strong, elastic material with excellent thermal, chemical and electrical resistance. LSR parts also maintain their physical properties at extreme temperatures and can withstand sterilization. LSR is biocompatible so it works good for products that have skin contact. Those benefits lend themselves well to automotive, medical and food appliance industries, typically in the form of seals, gaskets, valves and cables (see Figure 1). Designing parts for LSR and thermoplastics are similar, but there are some LSR-specific guidelines to consider:

Size: Parts with a maximum size of

approximately 5.00 in. by 5.00 in. by 2.00 in. can be uploaded. For larger parts, please contact Proto Labs to discuss possible options.

© Proto Labs, Inc. 1999–2014

Undercuts: LSR can be molded to

accommodate parts with undercuts, which are manually removed by a press operator. Mechanical tooling actions to release undercuts are selectively offered at Proto Labs.

Figure 1: LSR molding is a great process for producing pliable, durable parts.

Wall and rib thickness: LSR typically fills thin

wall sections with minimal challenges, and walls as thin as 0.010 in. are possible, depending on the size of the wall and the location of adjacent thicker sections. Rib thickness should be 0.5 to 1.0 times the adjoining wall thickness. LSR is accommodating to variations in wall thickness and sink is almost nonexistent.

Part ejection: Ejector pins are normally not used during LSR molding due to the flashy nature of the material. Thus, parts should be designed so they can be retained on one half of the mold when it is opened at the end of the molding cycle. The part is then manually de-molded, often with air assistance. Protomold offers LSR parts in volumes of 25 to 5,000+ in three weeks or less. To learn more about our LSR molding capabilities — including additional design guidelines on draft, finishes and more — check out our LSR page.

Shrink and flash: The shrink rate on LSR is

fairly high with an expected tolerance of 0.025 in./in. LSR also tends to flash very easily during molding (in gaps as small as 0.0002 in.), which Protomold helps reduce by incorporating additional features into the mold design.

Parting lines: Simplifying and minimizing parting lines in your design will help you get cleaner LSR parts as quickly as possible.

Essentials

LSR MOLDING 20