When designing a new device, component, or complex injection-molded part, world-class OEM engineers and rookies alike come to the same choke point when selecting the perfect micro mold material. Your instinct may be to opt for a familiar or even high-flow resin but dealing with microscopic tolerances completely re-writes material selection.
Every OEM designer and engineer is being required to create more functionality in smaller designs these days. While your initial Moldflow analysis may lead you to select a high-flow resin to completely fill your mold cavity and achieve microscopic tolerances, there are far a lot more considerations.
Molding Considerations Beyond High-Flow Resin
Selecting the perfect high-flow resin is less a puzzle and more a Rubik’s Cube. One minor change in micro mold material selection to accommodate one aspect of your project can compromise a component’s dimensional stability or your manufacturing process entirely.
Some high-flow resins might not withstand your manufacturing process due to extreme heat, or excessive torque from your equipment. Others can’t stand up to steam, gamma or various sterilization methods. Micro molding materials can be sensitive to environmental conditions like humidity, and others unfit for uses include friction. Delrin, for example, is a really good self lubricator. The “skin” on the plastic stays smooth.
The chemicals a micro molded component may come in contact to play a major role as well. If a polycarbonate component comes in contact with acetone, for example, you will see it turn hazy and become brittle. ABS on the other hand, will be completely eaten away in a very short amount of time.
It’s vital to understand the entire process a component will go through, even after it reaches the end-user, before selecting the best high-flow resin or material for your project. Ideally, alongside or even before the design process.
Additives for micro molding
There are times when a project calls for a specific high-flow resin or material, but that material isn’t ideal for the injection molding process. In these cases, we may select an additive to a given material like metal or glass.
Nylon is a popular material for hearing aid devices, but because it absorbs moisture, we often add high glass content, around 30% to combat the resulting problems, adding to its overall dimensional stability.
We’ve added glass and stainless steel spheres to polycarbonates; carbon to various materials and high-flow resins; tungsten in LCP and polycarbonates; and we’ve added IR absorbing materials as well. Even color can completely alter material properties. Injection molding a red component, for example, can pose a challenge because at time the component may be too thin for the pigment itself!
At any given time Accumold is running a few hundred materials and over the course of our history, we’ve likely run around 10,000 different materials though our micro mold machines. There are 100’s of varieties of polycarbonate alone, and we have over 60 variations and that doesn’t even include the different colors of a grade.
Three Core Questions When Consider Micro Mold Materials
When you consider the full spectrum of materials there are tens of thousands of material options and when additives are factored in, there are millions of combinations depending on the job at hand. That’s why I typically ask three core questions to narrow down the choices before any projects start.
Question 1: What is end-use?
Knowing the end-use narrows down the options fairly quickly. If the device is a medical component, options start to narrow quickly because FDA regulations determine what can be used. We do a lot of work in medical micro molding. Class VI materials, for example, have undergone testing and are approved for body contact in medical devices but are not implantable or to be used next to tissue or blood.
If your end use is for a fully implantable device, your choices narrow even further, as the FDA has defined certain materials as safe. Class 10993 materials are generally the best for short term implantable uses under 30 days or so, and resins to be used over the 30 days mark include specific grades of PEEK, costing anywhere between 200 to 1,200 USD per pound.
Some plastic manufacturers don’t want to have anything to do with medical because of the legal ramifications, so options become slim when medicine and the FDA get involved. This is one reason most vendors want to know where their materials are going. (This may also explain the high cost, to proactively compensate for any legal issues down the road.)
Question 2: What kind of environment is this going to see?
As stated above, environmental conditions play a huge role in the selection of high-flow resin or materials. Some materials like an ABS might not be able to withstand temperature, others if exposed to UV light may become brittle in less than one year.
Other materials, like LCP are great for thin wall micro molding but also notorious for having a bad knit line, potentially snapping from excessive torque in later manufacturing processes.
Question 3: What volume are we running?
Volume may not seem like an important question, but the higher the volume the more room we typically have in cost to fully customize a material. At lower volumes, we may look for an off-the-shelf option, ensuring the customer’s budget is respected.
Higher volumes, however, give us a bit more room to customize precise materials and additives that fulfill all the customer’s most demanding requirements like a conductive high-flow wear-resistant black component for example. (Yes, we made that.)
Choosing a high-flow resin or materials isn’t a simple decision. Whether your “baby” needs to bend and flex, needs translucent materials, absorbs lights, or needs to be transmissive we’ve truly seen it all. My team, just like the parts they make, hold their shape even under the most intense pressure.
Tom Murphy has been a certified professional in supply management since 2014 and holds a BA from Buena Vista University. He consults with major resin suppliers to test experimental materials pre-commercialization to ensure effectiveness in micro applications.
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