Finding the limits of micromolded 
medical parts

With little to no published rheological data for plastic parts less than 0.040 inch thick, but growing demand for them, medical products designers should look to specialized micro testing techniques for empirical guidance prior to product development.

Today, the presence of micromolded components in medical devices is expanding rapidly. And it’s not only in medical devices. As mobile phones, digital cameras, and many other products become ever smaller, the demand grows proportionally larger for sophisticated plastic parts weighing fractions of a gram or measuring less than 0.040 inch at the widest point.

Yet for many design engineers, crossing the boundary into the realm of micromolding plastic parts is like entering the great unknown. For one thing, rheological properties of thermoplastics often change dramatically under micromolding conditions, and with little or no published empirical data available for part thicknesses less than 0.040 inch, engineers are often left wondering if the product they want to create even can be molded with the first-choice material.

“On paper, you can design the perfect part with 0.005-inch wall thickness, for example, but actually molding that part in the real world is another story,” says Isaac Ostrovsky, an engineer for medical device giant Boston Scientific (Natick, MA).

Fortunately, there are micromolding companies that can and will work directly with engineers and product managers, providing proven testing techniques that will help ensure a good outcome for the micromolded part. This testing reveals the specific behavior of the material under micromolding conditions, and confirms its findings with empirical data, even before prototyping begins.

“This is an opportunity for us not only to tell our customers what we think, but also to show them scientific results,” says Dennis Tully, president and owner of Miniature Tool & Die (MTD; Charlton, MA), one company that specializes solely in micromolding.

The tests include flow rate analysis, tensile strength testing, and other properties of such engineering plastics as Ultem, PEEK, PPS, LCP, shape memory polymers, polycarbonates, and even resorbable polymers—along with special combinations and formulations.

According to Tully, these are a few of the available tests that can help with micro-molded parts:
• Test plaque molds. A test plaque mold allows for the creation of small samples in the desired material, measuring 0.002-0.009 inch thick. The result is an appreciation of the relative properties of that material at that thickness; for example, a relatively stiff material reduced to 0.002 inch may no longer be as stiff as what’s needed.

“A test plaque mold allows us to adjust the thickness of a small plaque in increments of 0.001 inch,” says Tully. “That will give us a determination of how far a particular material will flow, and at what thickness a particular material will fill the cavity.”

• Micro spiral flow testing. Most material manufacturers can provide data on flow properties based on standard spiral flow testing used in the macro world. Such tests are typically performed using an injection molding machine equipped with a test mold and the results are determined by measuring the length and weight of the material flowing along the path of a spiral cavity.

Such data is reliable for a sample that’s 0.040 inch thick, for example. However, for micromolding, the spiral flow mold used to evaluate different grades of a particular material is only 0.008 inch thick.

The micro spiral flow test is useful to help resolve a choice between two or more similar materials, pointing out the rheological properties for each as well as the distance that the material can flow.

• Tensile strength testing. A micro dog bone 0.008 inch thick molded in a specific material can be used for tensile testing, to confirm or deny that the mechanical properties are going to be as strong as what is expected.

• Materials database. In some cases, the expertise of the micromolder might actually preclude the need for testing. Dedicated micromolders with many years of experience have been accumulating testing information and empirical data over time for a broad variety of materials, and if they already have it, there is no need to reinvent that wheel.

“Although we offer testing tools and methods, often we are already familiar with a material and what it can do,” says Tully. “Over the past 10 years, we have built up a substantial database of materials that we have tested in many configurations. With this data, we can often predict and demonstrate material performance without additional testing.”

According to Tully, data about known materials can also be used to predict the performance of similar combinations of polymers or specialty compounds for which there is no available data.

“There’s no way for a micromolder to have every possible combination of compounds in its library,” explains Tully. “So what we do in that case is to try to get as close as possible to some known material, and we perform a mold flow analysis. We would use a test plaque mold, for example, to determine the thinnest cavity that we can fill, and also record the amount of speed and pressure that it takes for purposes of rheology.”

If a material has not been tested previously, a complete evaluation kit with all the aforementioned tests is available for a few thousand dollars—a nominal cost when the alternative often includes abandoning a project, compromising on the size of the component, or having to redo the choice of materials.

To be sure, prototype
With all the issues that must be considered, following testing with prototyping is highly advisable. According to Don Wilson, former senior director of research and development for Teleflex Medical Inc., what he wanted was a micromolding supplier that was more partner than vendor. “We really didn’t want someone that we would simply direct,” says Wilson. “They needed to take ownership of the project and walk us through the process, because micromolding is not the same as the conventional injection molding we typically use.”

With this in mind, Wilson and Teleflex selected MTD to fabricate a specific part: an implantable polymeric ligating clip so small that a shot glass could hold 1000 of them. Among other procedures, the clip can be used in a coronary artery bypass graft (CABG) procedure to clip off branches of arteries that subsequently will be used for grafting. Of the several methods that normally are used for this procedure, doctors prefer clipping, suturing, and several others.

For performing this procedure, the choice of available clips is between a metal type and the polymeric clip. Metal clips present a challenge in that, following the procedure, they interfere with the use of CT and MRI scans and other types of imaging. The polymeric clip doesn’t provide any such interference, so post procedures can be done that ensure the operation was a success.

A single-cavity prototype mold for the ligating clip was initially created for Teleflex, and because of its extremely small size and simultaneous need for accuracy, three iterations were required, with very minute adjustments in the design. “The adjustments had to be done quickly and accurately,” says Wilson. “The prototype really helped us out tremendously, and we could not have done the project without it.”

Once Teleflex was satisfied with the prototype, a two-cavity production mold was made. In addition to the process, special machinery had to be designed and built simply to ensure that the clips manufactured were not lost or spilled, a common problem with tiny parts.

Miniature Tool & Die’s specialized experience and assistance was key in allowing Teleflex to successfully manufacture its ligating clip. “I will definitely say that MTD and their capabilities enabled us to get this clip out on the market,” Wilson said. —[email protected]