Testing the Boundaries of Miniaturized Molded Plastic Parts
With little published rheological data available for plastics molded under 0.040 in. in thickness, design engineers turn to specialized micro testing techniques for empirical guidance prior to product development.
Today, the requirement for micro-molded components is rapidly expanding. Medical devices, cell phones, digital cameras, and many other products increasingly require sophisticated, miniaturized plastic parts, weighing just fractions of a gram or measuring less than 0.040 inch at the widest point. However, for many design engineers, crossing the boundary into the realm of miniaturized, micro-molded plastic parts is like crossing into the great unknown.
Rheological properties of thermoplastics often change dramatically under micro-molding conditions. And with little or no published empirical data available for thicknesses under 0.040 inch, engineers are often left wondering if the product they want to create can even be produced with the first choice of material.
"On paper, you can design the perfect part with five thousandths of an inch (0.005 in.) 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 (www.bostonscientific.com), Natick, Massachusetts.
Fortunately, there are some micro-molding vendors that work directly with engineers and product managers by offering proven micro testing techniques to help assure the outcome of the desired part. Such testing reveals the behavior of the material under micro-molding conditions, and confirms its choice with empirical data before prototyping even begins.
"This is an opportunity for us not only to tell our customers what we think, but to show them scientific results," says Dennis Tully, president and owner of Miniature Tool and Die (MTD), a Charlton, Massachusetts company that has specialized solely in micro-molding.
These tests include flow rate analysis and other relative properties of engineering plastics, such as resorbable polymers, Ultem™, polyetheretherketone (PEEK), polyphenylene sulfide (PPS), liquid crystal polymer (LCP), shape memory polymers, and polycarbonates, along with special combinations and formulations. According to Tully, a few of the available tests for micro-molded parts include test plaque molds, micro spiral flow testing, and tensile strength testing.
Test Plaque Molds
A test plaque mold allows for the creation of small samples in the desired material, measuring 0.002 to 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 stiff.
"A test plaque mold allows us to adjust the thickness of a small plaque in increments of 0.001," 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 utilized in the macro world. Such tests are typically performed using an injection molding machine and a test mold. The results are determined by measuring the length and weight of the material flowing along the path of a spiral cavity.
Such data are reliable for a sample 0.040 inch thick, for example. However, for micro-molding, the spiral flow mold utilized to evaluate different grades of a particular material is only 0.008 inch thick.
A micro spiral flow test is a useful tool to help resolve a choice between two or more similar materials, by pointing out the rheological properties for each, as well as the distance that the material can flow.
Tensile Strength Testing
A micro dog bone of 0.008 inch, molded in a specific material, can be used for tensile testing, to confirm or deny that the mechanical properties are going to be what is expected.
In some cases, the expertise of the micro-molder might actually preclude the need for testing. Dedicated micro-molders with many years of experience have been accumulating testing information and empirical data over time for a broad variety of materials.
"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 are no available data.
"There's no way for a micro-molder to have every possible combination of compounds in their 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."
The Final Step: Prototyping
Don Wilson, former senior director of research and development for Teleflex Medical Incorporated (www.teleflexmedical.com), says that he was looking for a micro-molding 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 micro-molding is not the same as the conventional injection molding we typically use."
With this in mind, Wilson and Teleflex selected MTD to fabricate the part--an implantable polymeric ligating clip so small that a shot glass could hold a thousand of them. The clip can be used in coronary artery bypass graft (CABG) operations, for clipping off branches of arteries that will be used for grafting.
A one-cavity prototype mold for the clip was initially created for Teleflex, and because of its extremely small size and need for accuracy, three iterations were required with very minute adjustments in the design. "The adjustments had to be done quickly and accurately," said 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 fabricated. Also, special machinery was designed and built simply to ensure that the clips manufactured were not lost or spilled. This specialized expert assistance allowed 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.
Ultem is a trademark of SABIC Innovative Plastics.
Vertically Integrated Firm at Forefront of Medical Device Development
Founded in 1972 by Richard Tully as a manufacturer of injection molds for miniature electronic connectors, Miniature Tool & Die is today one of the relatively few companies in the U.S. to offer both mold-making and micro-molding services. The company designs and machines precision micro molds with micron-size features, but also uses these molds to manufacture high-tolerance parts for makers of medical, micro-fluidic, and electronic devices. For the medical industry, MTD develops and manufactures micro-molded parts used in minimally invasive devices, such as surgical tools and vascular ligating clips, as well as in biomedical plastic catheters and implants. Many of MTD's customers include Fortune 100 medical device manufacturers that seek assistance with the creation of new products.
According to MTD (mtdmicromolding.com), its understanding of material properties at the micro scale is key to the development of reliable and scalable micro-molding manufacturing techniques. As the sole licensee of a technology that permits mechanical manipulation of polymers, the company can micro-inject material at what it calls “extremely low pressures and temperatures." Thanks to a recent grant from the National Institutes of Health (NIH), MTD was able to research and study the flow of various materials in micro cavities and to improve the capability to fill thin-walled cavities. The company's research was instrumental in the development of a diabetic cannula for an automatic insulin pump. According to MTD, the one-piece cannula was molded to a length of 0.530 inch and a wall thickness of 0.005 inch, a length-to-diameter ratio that had previously been considered impossible to produce.
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