Volume 6

Transcription

1 Design Tips for Rapid Injection Molding Volume 6 Real Parts. Really Fast. Proto Labs, Inc Pioneer Creek Drive, Maple Plain, MN

2 Design Tips categorized by topic Page TABLE OF CONTENTS Material selection Design guidelines Quality assurance Understand the process 3 Word processing 4 Rapid Injection Molding, an overview 6 Building better bosses 7 Temporary attachments 9 Corn, it s not just for breakfast anymore 10 First impressions 12 Things we ve learned in the last decade 13 We reexamine cammin 15 Automated and live help: The best of both worlds 16 Sometimes you need to be square 17 When thin is not in 18 Out of many, one 2010 Proto Labs, Inc. All rights reserved. Volume 6 DESIGN MATRIX 2

3 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. 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 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. 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.015 works best 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 need to do so, submit the part with the smaller text for a quote and we ll let you know, or you can contact a Protomold customer service engineer at to discuss. Figure 3: The L is a larger font size and allows room for the tool to mill Finally, if text is located at the top of a tall feature a tall rib, for example there is a tradeoff. If we mill, the text may have to be larger. If we EDM, we can do small features at the top of the rib, but there is an additional charge to make the mold. In summary, for best results when incorporating text, design your parts: with raised text using a bold sans-serif font of 20 points or more and stay away from the tops of tall features If you are wondering whether you ve designed your text properly, simply upload your 3D CAD model for a free ProtoQuote. If there are any problems you ll know by the next day Proto Labs, Inc. All rights reserved. Volume 6 WORD PROCESSING 3 Figure 2: The O is too small to mill

4 Rapid Injection Molding, an overview Over the last several years, our Design Tips have addressed many details of design for rapid injection molding, but we ve never really looked at the overall process. In its simplest form, the injection molding process works as follows: 1. Injection molding resin in pellet form, is loaded into the hopper. 2. The pellets flow into the heated barrel, where the material is melted. 3. A ram-driven screw injects the molten material into the closed mold. 4. After the material cools and solidifies, the mold opens and the part is ejected. Figure 1: The Injection Molding Process Add side-actions and the process gets a little more complicated. If you ve already received one of our free demo molds, you ve seen how a two-part mold with a side-action cam works. If you haven t received one yet, go to our website to order one. Meanwhile, here s a 2D version: Figure 2 shows the closed mold. Note that the colors of the mold halves correspond to those in an online ProtoQuote : green for the cavity or A-side, blue for the core or B-side, and red for side actions. Continued on next page Figure 2: Protomold s Demo Mold 2010 Proto Labs, Inc. All rights reserved. Volume 6 RAPID INJECTION MOLDING, AN OVERVIEW 4

5 Lift the green A-side mold half (figure 3, a ), and you ll see the yellow injected part with its runner, sprue, and edge gate (all of which will be trimmed off) and the red side-action. Note the features indicated on the face of the green A-side mold half, particularly the gate, where resin enters the mold cavity, and the drafted sliding shutoffs, which form the outside of the hook on the part (figure 3, b ). Withdraw the red side action (figure 3, c ) and note the hole and raised lettering it forms on the side of the part. These are undercuts that can not be formed in a simple two-part (straight-pull) mold. Once the mold is fully open the white ejector will rise to push the part off the core on the blue B-side mold half (figure 3, d ). Since the mold halves are normally mounted horizontally in a press, this will cause the part to tumble free from the mold. Note the features indicated on the face of the blue B-side mold half, particularly the core, which forms the hollow center of the part. For more information on Protomold s processes, visit our web site. To request your own Protomold Demo Mold, visit Figure 3: Demo Mold Exploded View 2010 Proto Labs, Inc. All rights reserved. Volume 6 RAPID INJECTION MOLDING, AN OVERVIEW 5

6 Building better bosses Princeton s WordNet defines emboss as raise in relief, and that s exactly what a boss is: a feature raised above a surface. In plastic parts, bosses are typically used to assist in assembly, as a receptacle for a screw or threaded insert or as the locator for a mating pin on another part. Because of its function, a boss must have sufficient strength to do its job. This dictates a minimum size for the feature. At the same time, because a boss rises from a surface, it thickens the surface at that point raising the risk of sink or development of voids as the part cools. The challenge: bosses should be big enough to do their job but not big enough to cause avoidable sink in the surface from where they rise. Figure 1 A typical boss is an open-topped cylinder, essentially a round rib (see figure 1a). Standard guidelines suggest that its wall thickness be between 40 and 60 percent of the thickness of the wall from which the boss rises. If your design requires more strength than this guideline would provide, you should consider ways to strengthen the boss without thickening its walls. The most common of these is to surround the boss with gussets to support and strengthen its walls (see figure 1b). If a boss is part of a vertical wall, it should not create a thick area in the wall. Figure 1c shows an example of how this can be done. Similarly, if a boss is located close to a vertical wall, it may be tempting to tie the boss to the wall by filling the space between the boss and the wall, resulting in a thick area (see figure 1d). A better way to tie the boss to the wall is with one or more ribs (see figure 1e). Sometimes, the wall of the boss may be too thin to mill with Protomold s conventional process. In some of these cases, we can use a steel core pin to form the inside of the boss. To aid this process, the end of the boss should be square, rather than having fillets or chamfers. There are two additional points to consider in designing bosses. As mentioned earlier, a boss is a circular rib, and like any rib, its walls both inside and out must be drafted to facilitate ejection. Depending on the height of the boss, this draft can be anywhere from.5 to 3 degrees. If we use a steel core to form the inside, it will be un-drafted. If we don t use a steel core, because a boss is formed by a blind hole in the mold body, we may have to add vent pins to the rim of the boss to allow the escape of trapped gas during mold filling. Otherwise shorts or burns may form on the rim. The Protomold Sample Cube Protomold s free 3D sample cube shows various boss configurations along with many other design capabilities and features of our molding process. If you don t already have yours, you can request one at Proto Labs, Inc. All rights reserved. Volume 6 BUILDING BETTER BOSSES 6

7 Temporary attachments There are plenty of ways to attach plastic parts to one another. They can be permanently welded or cemented. They can be semi-permanently screwed together, with or without inserts. And, for attachments designed to be easily and/or frequently opened, they can be connected using clips or latches. Clips and latches take many forms. Examples include: 1. The back cover on a cell phone (see figure 1), which slides open to reveal the battery. A small notch at the end of the cover engages a protrusion on the underside of the handset shell to latch the cover in place. The elasticity that allows the notch and protrusion to engage and disengage is provided by the body of the cover, which flexes to allow the latch to slide over the hook protruding from the shell. Figure 1: Cell phone battery compartment cover 2. The battery cover on a TV remote (see figure 2), which swings outward to reveal the batteries. A clip at the top of the battery cover engages a blade on the body of the remote shell to hold the cover in the closed position. Pulling in on the end of the clip disengages it from the blade, allowing the cover to swing open. In this case, the shaft of the clip is folded into a U allowing the necessary flex to be distributed over a long shaft that fits into a small space. Figure 2: TV remote showing battery compartment lid latch 3. A molded plastic tool box (see figure 3) held shut by two rectangular latches that hook over protrusions on the body of the box. Flexibility for these latches is provided entirely by bending of the latches and the part, determined by the resin properties and their geometry. The lid of this tool box is retained by a living hinge, a web of resin thin enough to allow repeated bending as the lid is opened and closed. Figure 3: Box structure with folding cover Depending on the type of closure you use, there are issues of stress and flexibility to consider. Both clips and living hinges have been addressed in previous design tips, but some key considerations are worth repeating. Clips Because clips move when operated, they must be flexible. Choice of resin helps determine flexibility, but there are several other contributing factors. The first of these is the length of the flexing arm and the second is thickness clearly if the arm is too thin it will be fragile (see figure 4). Continued on next page 2010 Proto Labs, Inc. All rights reserved. Volume 6 TEMPORARY ATTACHMENTS 7

8 A longer arm pivots through a smaller arc to move a given distance, and the flex can be distributed over a greater length, reducing the stress at any given point along the arm. In other words, longer arm length allows more motion with less stress. Figure 4: Good Clip: Longer, slender, flexible. Fillets at base to reduce stress concentration. Generous through hole for core. Well drafted. Clip head not too large. efunda has a page on spring clip design. Jordan Rotheiser s book, Joining of Plastics, published by Hanser Gardner Publications in 2004, has an excellent chapter on snap fits and spring clips. Living hinges (see figure 5) For living hinges, you should consider both material and design. Polyethylene and polypropylene, coupled with proper design, are excellent materials for latches requiring living hinges. Thickness of the hinge is a key consideration. Make it too thick and stress created when the hinge is bent may crack the hinge; make it too thin and it will not withstand repeated use and may not fill properly during molding. The following geometry (from efunda.com) works well for hinges made of either of the resins mentioned above. Keep in mind that a hinge is a thin area that can be challenging to fill during resin injection. A single gate that forces resin through the hinge area in a mold increases the strength of the hinge but can lead to voids or sink downstream from the hinge. Multiple gates can prevent sink but may leave weak knit lines at the hinge. You can avoid these problems by allowing Protomold to choose gate placement for your design. A well-designed living hinge can be flexed millions of times. For more information on designing with living hinges, see Penn State University Erie s Behrend School of Engineering site or online resource efunda.com. There are a number of sources of information on spring clips and ways to analyze stress on them: Some CAD packages include simple Finite Element Analysis (FEA) programs. If you use a lot of spring clips, consider buying a separate, more sophisticated FEA package; it could save you lots of time and money. BASF offers a snap-fit calculator. Figure 5: Living hinge (courtesy of efunda.com) 2010 Proto Labs, Inc. All rights reserved. Volume 6 TEMPORARY ATTACHMENTS 8

9 Corn: it s not just for breakfast anymore Normally, when nature produces polymers, they are strings of protein like keratin or polypeptides. Now, however, with help from companies like NatureWorks LLC, a subsidiary of Cargill, starch from corn is being turned into a different kind of polymer: plastic. Unlike typical plastics, however, this one is biodegradable. Figure 1: Corn-based Biodegradable resin pellets used for injection molding to create plastic parts The multi-step process begins with the transformation of starch, by hydrolysis, into the sugar dextrose. Dextrose, in turn, is fermented to produce lactic acid. Lactic acid cannot be directly polymerized; it is first processed into a lactide monomer from which the polymer polylactic acid or PLA can be made. The result, sold under the brand name Ingeo, is a compostable thermoplastic made entirely from renewable resources. The process turns about 2.5 pounds of corn into a pound of plastic. The resulting resin is suitable for applications that might otherwise call for polyolefins, polystyrene, or cellulosic resins. According to the manufacturer, Ingeo can be clear or opaque and flexible or rigid. It offers gloss and clarity similar to that of polystyrene, tensile strength and modulus comparable to hydrocarbonbased thermoplastics, and is recommended for packaging applications. Unlike conventional plastics, it can be broken down using commercial composting methods. Technical data sheets for the resin can be found at the manufacturer s web site, Protomold has run Ingeo resin NatureWorks 3051D in small quantities with satisfactory results. The resin can be selected by your account manager as a special order option. Keep in mind the selection of the proper material is crucial to your part production. Designers should consider the mechanical characteristics, molding properties, and cost of the resin used. Application-specific requirements will always drive the need for particular material properties like tensile strength, impact resistance or ductility. As you may already know, successful designs for injection molded parts are also built on an understanding of process-related issues such as the ability to fill the mold, tendency to flash, ease of part ejection, and the potential for warp, sink or void creation. In short, if you are looking for biodegradable injection molded parts, Ingeo brand PLA may be your solution. Rest assured that there s plenty of it available; if the existing supply should get used up, we assume that the folks at Cargill will just grow more. For a full list of stocked Protomold resins and resin information, visit the Protomold Resin Information page at DessignGuidelines_ResinInformation. aspx 2010 Proto Labs, Inc. All rights reserved. Volume 6 CORN: IT S NOT JUST FOR BREAKFAST ANYMORE 9

10 First Impressions The word prototype comes from the Greek, protos meaning first and typos meaning impression. Like many words, its meaning has changed over time, so that a single finished product can be preceded by a number of first impressions. And while we re often told the importance of making a great first impression, not all prototypes need to be great; some need only be adequate for a specific task. (Of course before committing to production, you might want at least one great first impression.) Prototype plastic parts serve a variety of purposes. They can be used to test: Form: Appearance, including overall shape, surface texture, and color. Fit: The ability to interconnect with other parts of an assembly. Function: The ability to withstand various kinds of stress under varying conditions, such as mechanical fatigue, heat, radiation or chemicals. Manufacturability: The ability to be made using standard high volume production methods such as machining or injection molding. Viability: The ability to appeal to the market. This means getting a productionequivalent part into the hands of the consumer for testing. Because product development is an iterative process, it can include multiple prototyping steps, each serving a different function. 1. Virtual Prototyping Virtual prototyping is supported by advanced 3D CAD software and actually produces a simulation of the part being designed. It is ideal for early conceptualization. Pros: It allows parts to be designed, revised, virtually fitted together, and tested under simulated stress using finite element analysis. It lets a designer create and revise a design in real time at no cost except that of the software. Cons: It is entirely digital, so going directly to high volume production from this point is very risky. 2. Stereolithography Apparatus (SLA) SLA is an additive process that uses a computer controlled laser to cure layers of photopolymer resin. The process is suitable for making concept models or prototypes to support presentations or trade shows. Pros: The process is a relatively inexpensive and fast way to make a single part, produces a good surface finish, and can reproduce very complex (even unmanufacturable) geometries. It is a good choice for testing the form and fit of a part. Cons: It only works with a very limited range of proprietary resins and produces a fragile end product whose dimensional stability suffers over time. 3. Selective Laser Sintering (SLS) SLS uses a computer-controlled laser to fuse powdered material. As is the case with SLA, SLS is suitable for making initial prototypes for demonstration purposes. Pros: The process is relatively quick and inexpensive, produces more durable parts than SLA, and can also reproduce very complex geometries. It is a good choice to test form, fit and, to some extent, function. Continued on next page 2010 Proto Labs, Inc. All rights reserved. Volume 6 FIRST IMPRESSIONS 10

11 Cons: It works with a very a limited range of materials, and the resulting parts have a rough finish. Although the parts tend to be more durable than those made using SLA, they are weaker than injection molded or machined parts. For these reasons it is not a good choice to test manufacturability or viability. 4. Fused deposition modeling (FDM) FDM uses a thermal print head to deposit and fuse layers of resin. The process can produce both demonstration parts as well as low volume production parts for some applications. Pros: It is a relatively inexpensive way to make prototype parts that are stronger than either SLA or SLS, and can also produce very complex geometries. It can be a good choice for testing form, fit and sometimes function. The material properties are better than SLA or SLS. Cons: The process is much slower than the other additive processes and suffers from the same stair-stepped surface finishes (although this can be addressed to some extent with post-processing). It also only works with a small number of proprietary materials and cannot produce parts with the standard mechanical properties of CNC machined or injection molded parts. It is usually not a good choice for testing, manufacturability or viability. 5. Three dimensional printing (3DP) 3DP uses a print head to lay down a plasterlike material. As is the case with SLA, it is well suited for producing conceptual models during the early stages of design. Pros: This is the fastest and least expensive of the additive processes. It allows the production of colored models and is ideal for testing form. Cons: Parts have a rough surface finish and are very fragile. The material choices are even more limited than with other additive processes, and for these reasons it is not a good choice for testing fit, function, manufacturability or viability. 6. Polyjet (PJET) PJET is similar to SLA, using computer controlled UV light to cure layers of photopolymer. As is the case with SLA, PJET is used primarily as a concept modeling process. Pros: The process offers the same advantages of SLA, but the process is less expensive to operate and is more office-friendly. Cons: It has the same disadvantages as SLA, and is much more limited in the size of parts that can be made. 7. CNC machining CNC machining uses standard computer controlled equipment to cut parts from a solid block of material. With the advent of First Cut s automated toolpath generation technology, the process is useful for demonstration parts through low volume production. Pros: It is as fast as the additive processes, and because it uses standard materials as feedstock it produces parts comparable to injection molding. For these reasons the process is a good choice for testing form, fit, function and viability. Cons: The process is generally not well suited for production quantities in excess of hundreds of parts (see rapid injection molding). 8. Rapid injection molding (RIM) RIM involves the use of proprietary software to automate the process of quoting, designing and manufacturing injection molds. It is useful in the production of small to medium quantities of parts for testing or bridge tooling prior to production. Pros: RIM produces real injection molded parts in as little as one business day at a fraction of the cost of conventional injection molding. The parts are ideal for testing function, manufacturability and viability. Cons: The non-recurring cost to manufacture the mold can make this process more expensive than additive prototyping processes for low volumes. For a free, detailed report on prototyping processes, including detailed information on material strengths, surface finishes and process selection, visit our website Proto Labs, Inc. All rights reserved. Volume 6 FIRST IMPRESSIONS 11

12 Things we ve learned in the last decade In May 2009 we celebrated our tenth anniversary at Proto Labs nothing fancy, just donuts and pop and a little reminiscing but it got us thinking, so we thought we d share a few things we ve learned and see if there s a tip or two in there somewhere. Somebody finally got around to counting all those aluminum molds we ve made in the last ten years. Would you believe over 22,000? After one too many Red Bulls, some guy decided to figure out how many aluminum cans that would be, but since the size and weight of molds we ve been able to make has increased over the years, he s still working on some of the finer points of the calculation. Our tip: if you re going to store that many molds, get a whole lot of warehouse space. Did somebody ask how many people-hours ProtoQuote has saved by generating estimates and design analyses automatically? It s hard to say exactly, but we re thinking something like 50 people-years. Our tip: If you re going to automate quoting and analysis, get yourself a really big, really powerful compute cluster. Proto Labs compute cluster A while back, we heard a lot of people saying that American companies couldn t compete in world manufacturing markets. We found out that we could do more than just compete; we could export our technology and have European and Asian companies beat a path to our doors. Our tip: Keep innovating. They say it s a sound-bite world, and that attention spans are shrinking to the size of a tweet (140 characters or less). Our experience based on web content, Design Tips, 19 issues of the Proto Labs Journal, and more is that if you respect your reader s intelligence, people will listen. Our tip: Trust your customers. How many times have we heard If it ain t broke, don t fix it? Well, our processes never seemed broke but we kept tinkering with them anyway. That s how we got bigger molds, taller and narrower parts, side actions, and First Cut s automated CNC machining. Our tip: You re never too good to work at getting better. Finally, over the last 10 years we ve learned (over and over again) that customers like getting more for less. No surprise there, but we ve sometimes been surprised at ways we ve found to reduce the cost of real injection molded parts. On May 18, 2009, we announced a new, lower minimum cost of $1495, down from the previous low of $1795. Our tip: When you re ready for injection molded parts suitable for functional testing, bridge tooling, or low-volume production, call Proto Labs. Protomold's manufacturing facility mold storage area 2010 Proto Labs, Inc. All rights reserved. Volume 6 THINGS WE VE LEARNED IN THE LAST DECADE 12

13 We reexamine cammin Since 2003, Protomold has been offering the option of side actions (often called side pulls or cams or cam actions ). Starting with a few here and there, Protomold now produces thousands of molds with side actions each year, allowing us to mold more complex parts and support your geometry even better than before. The purpose of side actions is to enable undercut geometry; in other words, to mold parts that could not otherwise be made in a straight-pull mold. In straight-pull injection molding, the A-Side and B-Side of the mold open and the completed part can be removed by pulling it straight out of either side. An undercut is a feature in the part design that would prevent the part from releasing out of the mold (part of the mold cavity is undercut in such a way that it grips the molded part). To release a part whose design includes undercuts, the mold surfaces that create the undercut geometry in the part must be pulled out of the part before it is ejected otherwise the part will be stuck in the mold. A cam device in the mold is used to pull the side-action mold surfaces away from the undercut features allowing the part to be released from the mold (see figure 1). Protomold produces parts using linear side actions that move perpendicular to the mold s opening and closing axis. Angle pins (cams) Figure 1: Side pulls must be perpendicular to the primary (A-B Sides) pull direction on the A-Side guide the cam carriage closer to the mold cavity on the B-side of the mold as the mold closes. When opening the mold, the angle pin pulls the cam carriage away from the cavity and out of the part, allowing the ejector system to advance and push the part off the B-Side of the mold. Side actions must be on the exterior parting line of the part, and there are some restrictions to the size of the cam feature and length of travel. These size requirements can be found in our resources section on our website. Cams will increase the cost of the mold, but they also increase your design options and reduce secondary operations, thus potentially reducing the cost of your overall project. While side actions are primarily used for forming undercuts such as holes for cables, vent holes for internal fans or slots for assembly, there are some additional applications for them such as making tall, thin parts with a core a test tube with minimal draft, for instance. If you were to mold a test tube using just a core and cavity (see figure 2), the mold might require additional draft and wall thickness to permit it to be milled and to allow for ejection. On the other hand, if you lay the test tube down, placing the parting line down the length of the test tube (see figure 3) and form the inside core of the tube with a side action, you can eject the part on its side and reduce the required draft. This also allows the gate to be placed on the closed end of the test tube allowing for a uniform fill of the cavity and balanced pressure around the side action core pin. Figure 2 Continued on next page 2010 Proto Labs, Inc. All rights reserved. Volume 6 WE REEXAMINE CAMMIN 13

14 Additional uses of side actions may include creating sharp edges on outside corners and reducing draft requirements. You can also add text or logos to the exterior shell of your part, or add recesses for decals. Side actions can be a valuable addition to your parts design tool chest, allowing you to produce parts with more function. Protomold uses software that automatically helps to decide if a side action is needed for your part, and how to design it, so you need only upload a part with your desired geometry you don t need to know how to make a side action to take advantage of this capability. For general information on side action cam requirements or Protomold capabilities in general, go to our Design Guidelines page, or call your Proto Labs account manager at with questions. Figure Proto Labs, Inc. All rights reserved. Volume 6 WE REEXAMINE CAMMIN 14

15 Automated and live help: the best of both worlds If you ve had much contact with Proto Labs our products, our quarterly Journal, monthly design tips, online design guides and the like you know about our automated processes. Available 24/7, FirstQuote and ProtoQuote interactive quotes can supply free detailed design feedback and pricing overnight based on your uploaded 3D CAD model. Both are incredibly smart systems, running on the industry s largest compute cluster, but sometimes you just need to talk to a human expert. That s what our customer service engineers (CSEs) are here for. Nobody knows your needs better than you do, but our CSEs eat, sleep, and breathe machined and injection molded parts, so there s hardly a parts-related question they can t answer. Is there something in your FirstQuote or ProtoQuote design analysis that you d like to talk about? Call us. Wondering about the right resin for a specific need, whether a sharp corner will cause stress in your finished part, or the possibility of sink in a thick wall? That s what we re here for. Wondering how to eliminate or minimize draft requirements? If there s a way, we ll know about it. What about the possibility of knit lines weakening a molded part? Our CSEs can run a computerized ProtoFlow analysis that you can download to see how resin will flow into the mold and whether it will cause potential problems. Once you get a layout for approval, we can work with you on matters of gate and ejector pin placement to make sure that they don t affect your part s functioning or cosmetics. And once you ve received your parts, we re still here to help. Prototypes are often just one step in a process, so if you re thinking about further modifications, our CSEs are available to help with those critical decisions or just to be a second pair of eyes. It s all part of our commitment to you, our customer. We re here to get you prototypes that work and to do it in days or weeks (rather than months) and at prices you can afford. The final decisions are yours, of course, but we ve got smart systems and smart people standing by to help. For automated assistance, information, and quotes go to com. For live help, call us at Proto Labs, Inc. All rights reserved. Volume 6 AUTOMATED AND LIVE HELP: THE BEST OF BOTH WORLDS 15

16 Sometimes you need to be square Much of today s design aesthetic seems to be about swooping curves. Nevertheless, sometimes you just need a nice square corner. When it comes to molded plastic parts, inside corners are easy. The inside corner of a part is formed by the outside corner of a mold, and the mold is formed by machine tools, which are great at cutting straight lines. Where two of those lines cross, you get an outside corner that is as sharp as the material will allow. If you want a sharp outside corner on a part, however, that s another matter. The problem is that you ll need to cut a corresponding inside corner in the mold, and the lines that define that corner can t cross, they must simply meet. Since cuts are typically made by a rotating mill, the corner cannot be sharper than the radius of the cutter. Large diameter cutters produce larger radius inside corners than thinner cutters, and deeper cuts generally require thicker cutters. As a result, milled inside corners of molds will all have rounded radii (see figure 1), and deeper square holes will have more rounded corners. The rounded inside corners of the mold will produce rounded outside corners of the resulting part. If you think you need a square outside corner on a part, there are ways to do it, but you first might want to consider whether you actually need one. Rounded corners are aesthetically pleasing and can reduce the likelihood of warp Figure 1: Cavity in the mold with rounded inside corners and internal stress as molded parts cool. And if you are forming a square peg simply to fit into a square hole, the corners of the peg don t actually have to be sharp. The flat sides of the peg will fit against the sides of the corresponding hole in your assembly and keep the mating parts from wobbling or rotating even if the corners of the peg don t fully fill the hole s square corners. If, however, you actually need molded, square, outside corners on your part, there is a way to produce them. In many cases, we can use a selective EDM (electro-discharge machining) process to produce sharp corners in the mold, which in turn produces sharp corners on the resulting part. This process uses CNC machining (which, as we said earlier, is good at producing outside corners) to produce a graphite electrode in the desired sharp outside corner shape. The electrode is then used to cut a sharp inside corner in the mold, and the mold produces parts with sharp outside corners. Side actions can also be used to create sharp outside corners. In summary, if you think you need square outside corners on a molded part, first consider whether somewhat rounded corners will do. Keep in mind that Protomold s process can produce sharp-ish corners, the radius of which will vary with the depth of the required cut in the mold. In figure 2, the portion of the corner shown in gold has a radius of The deeper cut, shown in red, has a radius of Finally, if you need sharper corners than can be milled by our normal process, talk to a customer service engineer to see whether EDM or side actions are the answer. Figure 2: Square cavity with small radius from Protomold s milling process Proto Labs, Inc. All rights reserved. Volume 6 SOMETIMES YOU NEED TO BE SQUARE 16

17 When thin is not in Picture this: it s a holiday weekend. Four lanes of traffic are on the Interstate headed out of town and suddenly everything backs up. The problem, as you will discover in a couple of miles, is a jackknifed truck narrowing the road to one usable lane. With not much else to do for the next few miles, you start watching your fellow drivers and realize that a substantial part of the movement that is occurring is drivers who ve gotten frustrated with the slow pace and are now taking the exits and driving parallel routes. Not only that, but a couple of cars, crawling along on a scorching hot day, have overheated and are now blocking traffic. It s that narrow section on the road that s causing all the trouble, and that s pretty much what happens when you configure a thin spot in an injection molded part. Molten resin, driven by pressure at the injection gate, travels quickly and smoothly until it reaches that thin spot, and that s where the trouble starts. First, as long as there are thicker areas for the resin to flow into, the pressure driving it into the thin region goes down, slowing its flow. See figure 1. (It s like those cars pulling off onto routes parallel to the bottlenecked Interstate.) Second, because the surface-to-volume ratio is higher in the thin area, resin there cools faster. Cooling makes the resin more viscous, slowing flow even more. In the worst case, the resin actually solidifies in the thin area, blocking flow there entirely (like those overheated cars stalled on the freeway). Figure 1: Example of thin and thick area resin flow problems But wait! That s just the start of your problems. If the resin has to travel through the thin area to fill a thick area behind it, there may not be enough flow to fill the thick area and you ll end up with voids (like that light traffic downstream of a blockage on the freeway). Or resin may flow into thick areas surrounding the thin area first it s called race-tracking and then into the thin area from its edges. This traps air in the thin area, resisting the entry of the resin, and you end up with voids. Or cooling flows of resin may meet in the thin area and form knit lines, causing cosmetic or even structural problems. See figure 2. Figure 2: Issues caused by cooling resin flow The obvious solution is to avoid areas of significantly different thickness, but sometimes you need a thin area. It could be for translucence, flexibility, to allow room for something behind the wall, or for a knockout (as in an electrical box). If you must have a thin wall, you ll have to do some planning. You can t put a gate in a thin area, but you may be able to gate in a nearby thick area. You may have to vent the thin area to keep air from being trapped and blocking the entry of resin. Tough decisions, but you can always call Protomold; we re here to help Proto Labs, Inc. All rights reserved. Volume 6 WHEN THIN IS NOT IN 17

18 Out of many, one When the designers of the Great Seal of the United States included the phrase E pluribus unum (Latin for out of many, one ), they certainly weren t thinking about plastic parts. They did, however, recognize the many benefits of unifying separate entities into a single, strong unit, and that certainly holds true for parts. Joining mechanical assemblies into a single injection molded piece can reduce the cost of production in a number of ways. Producing a single molded part is typically a one-step process; producing an assembly is, almost by definition, more complicated. If the parts being replaced were produced by a non-molding process machining for example the molding process will almost certainly be less costly. Because plastic resin is a relatively inexpensive material, there s a good chance that the original materials themselves, if they were not plastic resin, cost more. And then there s the expense of ordering, stocking and assembling multiple parts, which is often the largest cost factor and is eliminated if you mold the assembly as one. Also, there s the future cost of maintenance and, if necessary, replacement. A well-designed plastic part will tend to stay in one piece for life. An assembly, on the other hand, can loosen at the joints or literally come apart at the seams requiring preventive maintenance, repair, or replacement. In addition, there can be collateral damage when connections loosen, systems go out of alignment, or equipment breaks while in service. An excellent example of part count reduction is the redesign of a bearing hanger assembly used to support the grit-roll shaft in a largeformat printer. Six hanger assemblies were used in each machine, and each assembly consisted of seven parts including the screws holding the parts together, for a total of 42 separate parts in each printer. Engineers set out to reduce that total number to six. Figure 1: Example of single molded part designed to replace multiple part assemblies Obviously, the redesign involved more than ordering the same parts in plastic resin. In fact, the new single part doesn t look at all like the assembly it replaces. It is significantly wider and provides two cradles for the shaft instead of one (see figure 1). It does not require a bearing insert because the resin itself RTP 200 AR15 TFE15, a blend that includes nylon and TFE, also marketed as Teflon has excellent wear and low-friction properties. The injected resin also contains aramid fiber (Kevlar ) for added strength and wear resistance. This combination proved to be an excellent choice since wear tests of the part exceeded goals; the plastic bearing wore less in tests than the stainless steel shaft it supports. Finally, the cost of fasteners was eliminated and installation simplified by designing the new bearings to snap into place in the printer s platen. Finite element analysis (FEA) of the new design predicted that the bearing clip would flex during installation without breaking. Even including the cost of tooling for the new mold, over the life of the printer, the cost of the all-in-one part was far smaller than that of the assembly it replaced Proto Labs, Inc. All rights reserved. Volume 6 OUT OF MANY, ONE 18