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Knowing elements of style for injection-molded parts aids R&D efforts

Jul 28, 2011 11:30 AM, Ted Kucklick, author of The Medical Device Handbook and CEO Cannuflow Incorporated, San Jose, CA, www.cannuflow.com

This is the first of a two-part series. Next month: Letting the mold do the work.
With CAD solid modeling software, advances in CNC and CAM milling technology, and the ability to easily transmit CAD data becoming mainstream, reliable and inexpensive aluminum prototype injection molding is within easy reach of R&D designers at a fraction of the cost of steel and on, at a price point of 1/5 to 1/10 the cost of a steel tool just a few years ago, and lead times of 1/8 to 1/12 that of conventional steel tooling. CAD has also made steel tooling faster and more predictable as well.

The best R&D designers know that thinking ahead to how something will be molded, assembled, and manufactured can help prevent design and manufacturing-related headaches down the road when making multiple prototypes for clinical builds, and when the project is moving quickly.

Rule #1: Achieve consistent wall thickness

The first rule of designing an i njection molded part is to design the part with consistent wall thickness. Plastic is melted and shot into a mold hot under high pressure. It shrinks when it cools. The thicker the wall, the longer to cool, and the more the material shrinks. Thick and thin walls in the same part result in uneven shrinkage, warping, potential internal voids, and a potentially bad part.

The next consideration is determining nominal wall thickness. This depends on the plastic. Each plastic has its own optimal nominal wall thickness. A starting point nominal wall for smaller parts (the scale of handles, and small to mid-size enclosures that are typical of medical device parts) is about 2mm (.08 in. ) Smaller parts can be thinner, and larger parts may need to be thicker. Some plastics such as acrylic are more tolerant of thick sections; others like polycarbonate and ABS are not. Table 1 shows minimum and maximum nominal wall thicknesses for common plastics.

ABS	0.045 - 0.140
Acetal	0.030 - 0.120
Acrylic	0.025 - 0.500
Liquid crystal polymer	0.030 - 0.120
Long-fiber reinforced plastics	0.075 - 1.000
Nylon	0.030 - 0.115
Polycarbonate	0.040 - 0.150
Polyester	0.025 - 0.125
Polyethylene	0.030 - 0.200
Polyphenylene sulfide	0.020 - 0.180
Polypropylene	0.025 - 0.150
Polystyrene	0.035 - 0.150
Polyurethane	0.080 - 0.750

Table 1. Minimum and maximum nominal wall thickness ranges for common plastic resins. (Courtesy of Protomold and www.manufacturingcenter.com)

The second part of the wall thickness rule is to avoid thick wall sections. Overly thick wall sections consume excessive amounts of material, shrink more, take longer to mold and cool, and are prone to molded in stress and internal voids. Using the thinnest wall section will help result in a structurally sound part.

Figure 1. "Cored out" handle on medical device to avoid thick sections (Cannuflow, Inc.)

Figure 2. Rib example: Rule of thumb is that rib thickness “B” is 60% of nominal wall thickness “A”.

Conversely , ribs and bosses—useful features for stiffening parts and generating assembly features— generate a wall thickness issue that may not be immediately apparent. Where a rib joins a wall, there is an increase in the nominal wall at that intersection. This thick spot will take longer to cool, and it will shrink more than the material around it. This can result in molded-in stress, and undesirable “sink marks.” Figure 3 shows how proper design can avoid these problems.

Another use for ribs is for flow channels to direct the flow of plastic around a hole feature, and to reduce knit lines. Ribs can also be used to “core out” thick sections.

Rule #2: Design in adequate draft

Figure 3. Drafted and un-drafted part. (Courtesy of Protomold)

Designing draft into a part takes a bit of strategy and understanding how the mold will fit together to form the part. One important concept here is the “A” and “B” sides of the mold. The “A” side is the cavity side, the one that forms the outside of the part. The “B” side is the “core” or the part of the mold that forms the inside of the part. One way to begin to understand how mold tools work is to take a molded plastic part and press modeling clay on to the part. This will produce a negative of the part and give you an idea of what the mold that made the part looked like. Look at how the part is drafted so that it could be removed from the mold.

If a surface is textured, it requires more draft. Textured surfaces typically require at least 1.5 degrees of draft per .001 in. of texture depth.

Rule #3: Radius corners

Figure 4. Radius corners for consistent nominal wall thickness. (Courtesy of Protomold)

When thinking of plastic parts, think in terms of avoiding stress risers, notches, molded in stress and shrinkage. Sharp inside corners are harder to fill and make weak spots in a molded part. Figure 3 shows h ow square corners form a local thick section and a place for shrinkage and molded in stress to occur. In CAD it is very easy to design parts with sharp interior corners, and with milling them can be difficult or impossible. It’s worth taking the time to radius these corners wherever possible. Here is a simple rule of thumb for radii:

Inside wall thickness=1/2 of nominal wall thickness
Outside radius= 1.5X wall thickness

Note: The author gives special thanks to Glenn Beall and Mort Blumenfeld FIDSA both notable educators in the field of plastics, whom he says “taught me some of these ‘Elements of Style.’”