Plastic Injection Molding Design Guidelines in 2017

Designing Your Plastic Part

When designing parts for injection molding, the manufacturing process is an important consideration. Injection molding is a process in which solid thermoplastic resin pellets are melted, injected into a mold, and then cooled back to a solid state in a new form. During Boss both the injection and cooling stages of the manufacturing process, there are several factors that may affect the quality of the final product and the repeatability of the manufacturing process. Although it is not always possible to follow all recommendations, outlined on the following pages are some of the most fundamental guidelines when designing parts for injection molding.

Radii

An inside radius should be at least 50 percent of the nominal wall thickness. An outside radius should be the nominal wall thickness plus the inside radius (150 percent of nominal wall). Sharp corners at the base of bosses and ribs can be stress concentrators. The edge where a boss meets the nominal wall should be radiused to reduce the sharp corner without increasing the wall thickness enough that it creates a sink problem. The radius at the base of a boss should be ¼ of the nominal wall with a minimum radius of 0.015”.

 

 

Design Recommendations:

Wall Thickness

  • Maintain a wall thickness of less than 5mm because thick walls can lead to long cycle times and poor mechanical

properties.

  • Avoid large variations in wall thicknesses in order to simplify flow pattern and minimize variations in shrinkage that can lead to warpage.
  • Avoid abrupt changes in wall thickness, as this can create stress concentration areas that may reduce a part’s impact strength. Wall thickness changes should have transition zones that reduce the possibility of stress concentrations, sinks, voids, and warp.
  • Avoid gating near an area with a large variation in wall thickness because hesitation and race tracking can create non-uniform flow and shrinkage.

 

Ribs

  • Maximum rib thickness should be 0.5 to 0.75 of the nominal wall to avoid creating areas of sink.
  • To avoid thin sections of steel in your mold, the distance between ribs should be at least two and a half times the nominal wall thickness.
  • Ribs should have a draft angle of at least ½˚ per side in order to accommodate easier ejection from the mold.
  • Maximum rib height should be no greater than three times the nominal wall thickness in order to avoid large variations in wall thickness.
  • Balance ribs on both sides of the nominal wall to avoid non-uniform shrink that can lead to warpage.

 

Bosses

  • Stand-alone bosses should be designed following the design guidelines for ribs

(see more information under the “Ribs” section).

  • Use connecting ribs and/or supporting gussets if possible to stiffen structural parts. Connecting ribs should be 0.6 times the nominal wall thickness at their base to avoid sink.
  • To maintain uniform wall thickness, bosses should be cored to the bottom of the boss.

 

Threads

Plastic threads used for joining parts can be machined or molded-in

  • When designing molded-in threads, avoid feathered edges and include radiused roots in order to minimize stress concentrations and to keep the walls uniform.
  • Sharp edges can be stress concentrators in plastic parts. Thread designs should consider this.

 

Draft Angle

  • Design parts with a minimum of ½˚ per side draft in order to accommodate easier ejection from the mold.

 

Amorphous Versus Semi-Crystalline Materials:

In amorphous materials, molecules are randomly oriented and intertwined. Polymer molecules have no ordered structure. These materials have no identifiable “melting point” but progressively soften through a broad temperature range. Unfilled amorphous materials are typically isotropic, shrinking equally in the flow and transverse directions. Even fiber-filled amorphous materials typically have low shrink and good dimensional control.

How to Choose the Right Plated Coating for Improved Mold Performance

Many moldmakers and molders have had great success with one particular coating and have used it for all of their applications. Unfortunately, there is not a magical coating out there that works in every situation. Many times, excellent coatings are used in the wrong applications. Speaking from a plater’s viewpoint, one of the major challenges is correcting problems caused by another plating company applying the wrong coating for a particular application. When plating is used properly, the positive results can be stunning.

There are many causes of wear and corrosion in plastic molds that can contribute to greatly reducing mold life. Plating can solve many of these situations. However, these coatings also can improve the performance of molds in many other ways. A basic knowledge of plating is necessary to make an educated choice for properly solving your particular problems. There are more than eight different plated coatings for molds. It is the intent of this article to help molders and moldmakers take advantage of these benefits.

Types of Plating

There are two basic types of plating – electrolytic and electroless. Electrolytic plating requires electricity to make the process occur. There is a positive and negative charge. The positive is called the anode and the negative is the cathode. The part to be plated gets the negative charge so it becomes the cathode. The anode is made of conductive metal – such as lead – and becomes the source of the positive charge. Hard chrome plating is one of the oldest electrolytic plating processes. The chromic acid solution is the medium by which the current transfers. When the plating process occurs, the negative and positive ions transfer in the solution causing a metal (chrome) to reduce onto the base metal of the part being plated. A good analogy would be to look at it as the reverse of EDM. Because the process uses electricity, the plater is constantly fighting against the laws of nature.

The old rule still holds true: electricity travels in straight lines and goes to the closest point. On sharp corners there will be a heavy buildup of plating. In the recesses, the plating will tend to be thinner.

Electroless plating, such as electroless nickel, is just the opposite of electrolytic plating. Additives in the solution take the place of electricity. These additives are known as reducing agents. Since all metals have a charge, when the reducing agents detect the base metal charge in the plating bath, they start to react. This causes the metal in the bath solution – in this case nickel – to reduce onto the base material of the part being plated. No electrical current is required. Wherever the plating solution touches the base material, the plating will adhere. This gives the plated part a very uniform deposit and the plating thickness can be controlled within .00005-.0001, even on complex shapes.

Within these two families you have several types of deposits. These different types include composite and alloy deposits. As a generality, if you need a perfectly uniform deposit in a complex shape, electroless nickel deposits are best. However, hard chromium is the hardest deposit and has excellent release.

How to Choose

When choosing plating for your mold or mold components, you must first identify the problem to be solved or the problem to be prevented. The first question you should ask is “What is the cause of this problem? Is this problem the result of corrosion or wear?” If the component has previously been plated, this may be difficult to determine. Have your plating vendor look at the component to determine if it has been plated and if so, what type of plating was used.

Abrasive wear can occur on compression molds that use mineral or glass-filled materials. These materials can cause a scouring action on the mold surface. In transfer and injection molding of thermosetting materials, wear often is detected in the high flow areas such as in the sprues, runners, gates and portions of the cavities and cores that are directly opposite the gates. In injection molds for thermoplastics, wear most commonly appears on the surface opposite the gate.

Most damage results from continuing to run the mold after flashing occurs. However, there are other sources of damage of which to be wary. These sources can include water contacting unplated surfaces, causing corrosion; water condensing in the molds; seepage through porous metals; and leaky pipe fittings and “O” rings. Where chillers are used for mold temperature control, condensation of moisture on the mold surfaces can sometimes occur even while they are in full operation. Careless handling of hoses and feed lines during hookup can leave water on the mold surface. Corrosion is progressive and even if the molds are stored after being sprayed with an antioxidant, a few drops of water or condensation can cause tremendous and costly damage.

Another source of damage is attacks from acids. These acids may form after exposure to corrosive materials generated by thermoplastics decomposition (often due to overheating). Overheating can occur in the platicizing cylinder, the hot runner system or in the mold cavities as the result of too small gates, or inadequate venting or cooling systems. During the molding of PVC, a small amount of hydrochloric acid is formed, which is extremely corrosive to the mold cavity.

Electroless nickel, by its very nature, is an excellent corrosion barrier for most mineral acids, whereas these acids will attack chrome. Stainless steels also can be susceptible to attack by chlorine or fluorine containing plastic, leading to pitting or stress corrosion cracking. This can be eliminated with a high phosphorous electroless nickel deposit over the stainless steel.

With so many different types of molding and even more types of materials available, there is no easy answer as to which coating will best enhance performance. An investigation into these coatings is worthwhile because if your molding operation is left unprotected, it can create corrosion, erosion, materials flow or release problem. The proper plating can make all the difference. Before you make that critical decision, call your plating vendor of choice and ask which coating will work best for your application.

How to Make Your Mold Shop Stronger

In March of 2001 the U.S. economy slid into a recession. By the time the powers that be decided to make the announcement in 2002 it was old news. Many organizations, including those in the moldmaking sector, had already taken a hard hit from the economic downturn, and were taking measures to survive long before it was made formal. It’s now almost 2004 and they are still in a survival mode.  There’s no need to announce that the economy is taking longer to recover than anticipated.

Today’s companies are facing issues and problems exacerbated by the economic downturn.

Downsizing, reorganization, restructuring, cost cuts, hiring freezes and other measures have been taken to stay in business in a tough economy. Managerial calls to employees to embrace change have sometimes been met with resistance and uncertainty. Low employee morale is prevalent. Although overall competition has always been tough, today competition from foreign outsourcing is fierce.   These are just some of many overwhelming challenges organizations must face. Most companies are trying to figure out what to do until things turn around. But who knows the shape of things to come or the duration of the change?
Although each company has its own issues and challenges, here’s what you can do now to help drive business growth:

Accept that times have changed and what has worked in the past may not work in the future. We must use change to provide the creative tension to keep us moving forward. Think of tough times as opportunities to tap into your creativity; get your organization running like a well-oiled machine and seek new ways of doing business.

Rethink your entire business strategy. Ask these questions:
Have we been complacent for too long?
Where might there be opportunities to expand our services?
What more can we offer?
How can we penetrate new markets?
How can we do more with less?

Take Advantage of Foreign Competition

Business is looking very interesting on an international basis for companies in the U.S., especially in certain emerging markets like Russia and specific markets in Latin America such as Brazil.  Also, the Chinese market continues to do quite well despite the fact that their currency is overvalued.

Speed and execution are the keys to taking advantage of these markets.  Companies in the moldmaking sector that respond to these emerging situations will be the most competitive. This will be especially true if growth accelerates and companies find themselves with inadequate capacity to meet demand. Smart companies are moving forward aggressively in terms of new markets, emerging growth sectors and product opportunities through strategic alliances on an international basis.

You must be receptive to seeking out innovative new products that will enhance your current product line and stimulate business growth.  Learn what new products and technology are available that can help to expand your business. You need to accept that finding products overseas can enhance your current product or add to your product line. Thoroughly explore the alternatives. Some companies have found critical new products and technologies from outside companies. If you are not receptive and highly conscious to outside new technology and overseas markets, you may quickly find yourself acting in a defensive position simply to survive.

Invest in New Technologies

Invest in technology that reduces your operational cost, improves quality and increases your contact with the customer. Pay attention to customer-relationship management regardless of the size of your company.  There are new technologies being created everyday, use this as the driver and the tool for business growth.

Maximize Your Business Strategies

Make this economic downturn a good time to re-examine your core business values. It helps to seek input from your employees by involving them on a day-to-day basis. Asking them for their extra effort allows you to crosstrain people in various positions. Make sure they know that they play a strong role in developing the company’s reputation and establishing customer loyalty. When things do turn around, you will have a competitive edge because your employees will be more supportive and understand the big picture to a greater degree.

Going back to basics means working harder than ever to sustain and develop strong business relationships. Talk with your customers about where they see their business in the future and identify how you can assist them in selling more of their products. No matter how often you’ve heard about the importance of customer service, it always warrants reinforcement. If you want to obtain customer loyalty and remain competitive, your level of service must far surpass your competition. If what you sell or offer is essentially the same as other competitors, differentiate your approach to marketing and sales. Be willing to take a risk. Tap into your creativity and think about what you can do to stand out in the minds of your customers. Innovation will make you sharper, better and bolder by definition.  By giving them the help that they need, you will create quality customer relationships that mean long-term profitability.

Companies that institute smart strategies and believe in their employees, products and services are able to introduce new products and differentiate themselves from their competitors by portraying a higher level of service in every aspect of the customer experience. This can gain a competitive edge until things turn around. But keep in mind, that when they do, business will be very different. Determine your future and its potential.

Bridging the Divide Between Art and Engineering

The terms “artistry” and “moldmaking” have some odd connections. Certainly a finished mold often looks like a work of art – and not just to engineers. There also is the artistry involved in the mold’s design when dozens of components must function with the synchronized precision of a ballet. And there is that cliche – art to part.

But this syntax is not to the point. For years moldmakers have been looking at it in another way entirely. They want CAD/CAM software to help them meet the rising artistic demands of product designers. Frustration has been high because moldmaking is a quintessential engineering task. Software developed for it has so many engineering features that any artistic capability just seems to get in the way.

It’s no help, either, that the initial approach to designing a mold is diametrically opposed to the usual approach of an artist or sculptor. A moldmaker must turn the product inside out, then deconstruct it one surface at a time in order to generate the necessary steps. Artists and sculptors work the other way around – creating lines, colors, perspectives and shadings that reproduce an image from the mind or the physical world.

The conceptual differences between the two approaches dictate fundamental differences in the software packages. For years, the possibilities intrigued Dale Hillesland, owner of Paraflex, Inc. (Tacoma, WA) – a small mold shop, employing three people full time and one part-time. In business for seven years, its bread-and-butter markets are molds for consumer goods ranging from airline food trays and photo slide mounts to toys and industrial printer housings.

Hillesland regularly searched engineering trade shows, art expos and craft fairs. But nothing seemed to bridge the divide between art and engineering. That is, until he discovered ArtCAM Pro software for engraving. ArtCAM, from Delcam International Inc., was created specifically to turn tabletop milling machines into engraving tools. The software has been an instant hit with jewelry manufacturers and firms that produce awards. It also is widely used by engravers, sign makers, wood carvers, packaging designers, sports equipment manufacturers and many others with a need for artistic output.

Based in Windsor, Ontario, Delcam International is the North American operations of Delcam plc (Birmingham, England). Delcam is the developer of ArtCAM as well as software for tooling and complex surfaces – PowerSHAPE and PowerMILL – plus PowerINSPECT for coordinate measuring machines (CMMs) and CopyCAD for reverse engineering.

“As soon as I saw ArtCAM back in May 1998, I instantly wanted to have it,” says Hillesland, whose background includes a stint making molds on pantagraph-type milling machines. “I had never seen anything like it. It was the first time I had seen that kind of hand engraved detail produced by CAD/CAM software. The artistic slant was something that had been sorely lacking from everyday moldmaking.

“I watched ArtCAM develop as a product for about a year while I waited for the right project to come alone to justify the purchase,” he says. ArtCAM was, after all, $7,500 – a significant investment for a company with just three full-time workers, including Hillesland.

A Royal Opportunity

The ArtCAM business opportunity was a chance to bid on lucrative tooling work with a high degree of artistry in product design and packaging. The opportunity came in mid-1999 in the form of a bid package for a set of three plastic injection molds. The tooling produces a plastic castle for sale in pet stores to cat lovers.

mold case study 1

This indoor play space for cats was the brainchild of Michael Stamnes, president and owner of Irving Industries, Inc. (Anchorage, AK). Given the long Alaskan winters (wet or cold or both), pet owners there put a high premium on anything that preoccupies housebound pets.

As pet store owners know, human preferences rather than the pets’ determine what gets to the cash register. “Cute” and “ap pealing” are major factors in these decisions. Part of that appeal, Hillesland notes, is the Kitty Castle’s life-like detail and high quality surface finishes.

Up against two rivals for the work, Hillesland turned to Steve Kidd, owner of Cimtech Inc. (Gig Harbor, WA), an ArtCAM distributor in the Pacific Northwest. Cimtech helped Paraflex win the bid by using ArtCAM to cut Irving’s sample parts in aluminum.

“That sample proved to the customer that Paraflex could make a much more realistic and appealing castle than the competitor,” states Hillesland. The competitors managed little more than a basic-looking castle – attempts that people from Irving later told Hillesland were “cheesy.” Without ArtCAM, they were forced to use traditional etching methods to create a moldable brick texture.

However, winning the bid and doing the work were two different things. When Irving awarded Paraflex the work, Hillesland had yet to buy and learn how to use ArtCAM, so the system’s short learning curve became the critical path to his success. “I learned to use ArtCAM in three days,” says Hillesland. “The only ‘outside help’ that I had was a short on-site training course from Cimtech.”

This was enough to let Hillesland start work almost immediately. Product data in the AutoCAD Drawing Exchange Format (DXF) was read into ArtCAM as into any CAD system. “The DXF files had to be modified and that was a big learning curve,” Hillesland says. “But with that behind us, we are very pleased that Delcam chose DXF as one of its standard CAD input formats. Ninety percent of the files we get are from AutoCAD users. These guys are product designers, not toolmakers,” he adds.

Paraflex’s designer, Richard Ellis – a mechanical engineer with a bachelor’s and a master’s degree – built the model of the castle with ArtCAM. Ellis used ArtCAM’s color modeling plus some vector-based shapes for the basic underlying castle geometry. The ArtCAM color modeling process gave a good indication of what the finished tools would look like. Vector shapes were used to make the turrets of the castle.

mold case study 2

Irving Industries’ Kitty Castle measures about 18 inches at the base and is 22 inches high.

The brick and granite effects were added to the walls using ArtCAM’s texturing tool. “From this point on, Rich just needed a few edits and some smoothing to finish the modeling,” Hillesland explains. Toolpaths were then created to rough and finish the molds for prototyping parts.

There was the usual number of design changes. “The ease of use of ArtCAM made it simple and quick to modify the engineering as Rich went along,” Hillesland continues. “This helped us find a solution which worked technically and fulfilled the aesthetic requirements of the customer.”

Machining was done on P20 steel, commonly used for injection mold tooling. The steel was prehardened to 34 Rockwell C, making the metal hard to machine. Finish machining was done with a relatively large 3/16″ cutter (0.1875″ diameter). “We couldn’t use anything smaller because of the tight deadline,” Hillesland says. “We really had to push the metal. Anything smaller in cutters would not have held up.” Rough machining was done with 3/4″ ball-nosed cutters.

The jobs were cut on a three-axis Giddings & Lewis Inc. Fadal 40 x 20 vertical milling machine with a 22-hp spindle drive motor and a 10,000-rpm spindle. The process was speeded up by the extensive use of tooling inserts for detail. “The inserts allow the molder to quickly make tooling changes,” Hillesland says, “and they make the tool easier to build.”

Most metal cutting at Paraflex uses NURBS – an acronym for non-uniform rational B splines. NURBS programs generate true curves in the machine tool’s control, eliminating minuscule straight-line segments. “NURBS gives us a faster and more accurate way to machine directly from the solid model,” explains Hillesland. He began machining with NURBS in 1996.

“The CAM files got very big – an average of 33 MB per mold tool,” Hillesland says. “We split these into two 18 MB files and cut in two different directions. The parts had thousands of surfaces to be cut and the machine often went so fast it looked like a sewing machine,” he notes. The Fadal 88 CNC controller reads 1,000 lines ahead; Hillesland notes that some CNCs can only read three lines ahead.

“Everyone involved was blown away by the detail and quality of the finish in the first mold made in tool steel,” Hillesland says. “The only other way to achieve such a look would have been hand engraving. That would have been impossible within the budget and deadlines. Our competitors would not have tried, even if they had ArtCAM and knew how to use it.

The Right Tool for the Right Job

“What is really important for a business this size is being able to pay for a lot of software on the first job and still make a profit,” Hillesland says. “Also important is being able to deliver on time – taking into account the learning curve and the many changes that Rich had to make.”

Because ArtCAM is so much more an artistic tool than an engineering tool, Hillesland ran into some fit and function difficulties. Some internal corner radiuses were originally cut too tight and had to be recut. “ArtCAM wasn’t really designed to be used that way,” he explains, “but you can do this with it if you plan the job correctly.

“In doing mold work with ArtCAM, you have to remember that this is an art-type product and not an engineering-type tool,” Hillesland reiterates. “You need to design the part and tool around that fact. What Rich has done is taken ArtCAM up a notch in capability, up to a different level of true moldmaking. This is a lot more than just adding decorative inserts, although we do a lot of that, too.”
Hillesland is confident that he’ll recover his ArtCAM investment in the first year. He is sensitive to this because the last time he bought software – in 1997 – he paid $30,000 for a high-end Unix-based system. “We still haven’t gotten our money back from that,” he notes. “The software can find parting lines and that’s a big help. But a less costly PC-based system would have been just fine for nine out of 10 jobs that we do.”

Though the Unix package was used for some of the Kitty Castle, fine details, textured surfacing and adjustments to make the molds fit properly were done with ArtCAM. “ArtCAM still gives us better finishes than the Unix package,” Hillesland notes. “This has given Paraflex a level of quality and surface finish that other shops just couldn’t match.”

New Opportunities

ArtCAM has opened a whole new world of tooling in the packaging industry for Paraflex – aluminum tooling for vacuum-formed blister packaging. “This is very popular for premium-priced food and candy,” Hillesland says. “They need exotic packaging to help them stand out in the store. We are the first guys on the block to do this and we look forward to exploiting ArtCAM’s technology to go after work we previously were unable to bid.”

Another solid market for Paraflex is construction company logos used on the covers of underground vaults. These are scanned into ArtCAM from photos or business cards and they end up in foam molds. “They buy these foam tools a thousand at a time,” Hillesland says.

“The business is changing,” he summarizes. “We are getting more artistic work all of the time. Pretty soon, thanks to ArtCAM, we will go looking for this kind of work rather than just waiting for it to come to us. Right now, out here, no one’s doing it.”