Editor's note: This is the second of a two-part series. Part one appeared in the October issue (page 43) and discussed Rules 1-3. This second part begins with Rule #4.

Rule#4: Specify the least expensive and "friendliest" material to mold that will accomplish all design objectives.
When designing medical parts it is tempting to use an exotic engineering plastic when an easier-to-process and more available commodity material would work just as well. High-performance engineering plastics tend to run at higher temperatures and can be several times the cost of a commodity plastic. For example, there is no need to mold the handle of a medical device out of a high-performance engineering resin like polysulfone unless it is meant to be steam sterilized or has to withstand high temperatures. Often, ABS or a PC/ABS blend works just as well, is easier to mold, costs less, and provides a better surface finish. Over-specifying material does not make for a better part. It can result in a part that is harder to produce, with a higher cost per part.

The other risk of using an exotic material is availability. If the resin is discontinued by the supplier, this can be a real headache, especially if you are producing a device under an FDA PMA (premarket approval) and you are locked in to using a specific material. Changing a material can mean re-opening the PMA submission, triggering a "Supplemental Application" to get the change approved. This can take several months for the FDA to review and involve significant expense. It’s better to be sure your material is readily available in the future, which may mean using an engineering resin if needed and that may meet a requirement that a commodity plastic cannot. For example, PEEK (polyether ether ketone) is a high-performance plastic often used for spinal implants, is highly biocompatible, and is specified where very high stiffness and heat resistance is needed. Just make sure that the decision to use a high-performance engineering resin adds value and is not just over-engineering the part.

Sometimes you will want to modify the properties of a stock resin in order to increase its stiffness, to make the material radiopaque, or increase its surface lubricity. Plastics can be compounded with additives to achieve this. Two common fillers:

• Glass is added typically to increase stiffness. Polycarbonate is a common plastic to load with glass. The drawback to a glass or mineral-filled resin is that it can be abrasive and cause wear to the mold tool.
• Barium is added to give radiopaque properties to plastics, and also in catheter tubing to increase lubricity. Tungsten may also be added to make a plastic part radiopaque. Talk to your medical plastics compounder to discuss your options.

Note: There is greater sensitivity to plastic additives. Two of these are pthalates in PVC Bis(2-ethylhexyl)phthalate, or abbreviated DEHP, a common plasticizer in flexible PVC, and Bisphenol A (BPA) commonly found in polycarbonate. So as your development program moves forward, check to see if your device materials contain these substances, if they are allowable in your device, or if a DEHP or BPA free alternative must be found. In the EU, devices containing DEHP must disclose this on the label.

Rule #5: Use straight parting lines whenever possible
Sometimes a part is designed with what is called a "stepped" parting line. This is where the two halves of the mold are not flat to each other. There are some features that can only be done with a stepped parting line, however a stepped parting line results in a more complex (and expensive) mold tool, and one that is more prone to wear. Angled mating surfaces can rub against each other in a way that two flat mating surfaces do not. This can result in galling in aluminum tools, shortening tool life, or causing unwanted "flash."

Rule #6: Design in the mold
One simple design principle to avoid labor costs, design features "in the mold" even if the tool is more expensive (within reason). These can be snap fits, features to capture components, or combining features that would be assembled from parts into one part. Labor costs never go away, and unnecessary labor leads to process variability. Tooling costs can be amortized, and if a feature is "in the tool" it is less likely to suffer from variability. Do a quick financial model to see if your expected consumption volumes and labor savings offset the cost of a more complex tool. Anything saved past breakeven is "found money."

One money-saving trick is to gain two housing shells from one molded housing shell

This technique relies on a concept known as "rotational symmetry." In this example, two identical molded shells fit together to make a complete case for a surgical power supply. A number of complex features were incorporated to capture circuit boards and electrical connectors. Not all housings lend themselves to this. However, when it works, it can cut your tooling costs in half.

The two part-series presented here and in the October issue (page 48) can serve as a guide to some of the most important considerations in designing a successful part for a medical device. Injection molding, especially for R&D is cheaper, faster, and more available than ever before. Further, it is a critical tool for developing innovative medical devices.

Note: The author thanks 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.'"