Medical Plastics News editor Laura Hughes sat down with Patrick Haney, research and development engineer, MTD Micro Molding, to find out what manufacturers need to know about injection and micromolding.
1. What are the main differences between micromolding and conventional injection molding?
Micromolding has many aspects that differ from conventional injection molding. These differences may vary from the way that the processor may optimize molding parameters, to the subtle differences that molding different material types may present.
Typically, conventional process parameters are compartmentalized and optimized in stages. Most molders would think of the fill/pack/hold/cool molding cycle to be as fundamental as elementary grade mathematics.
However, influences from thermodynamics and ultra-high shearing mechanics make compartmentalizing the molding process a lot more difficult in micromolding. The shot’s small volume surrounded by so much relative tool steel forces a micromolder to invent creative ways to ensure that each processing parameter is set with not only part quality in mind, but also material preservation.
In short, micromolding requires an unprecedented attention to detail that is not typically required in more conventional molding settings.
2. What do you think are the key things manufacturers involved in the injection and micromolding of plastics need to know?
It is really important for manufacturers to understand the differences between micromolding and macromolding.
Micromolding is a thermal form-change method for creating very small, three-dimensional, components having smaller features and dimensional tolerances. Micromolding is not a slightly altered version of macromolding, made to fit the microworld. The materials used in micromolding behave very differently in the microworld, requiring specialized expertise and technologies.
The micromolding world is governed by principles of scalability which state that as the physical size of an object is reduced, the volume and surface area are also reduced, but not linearly with the size. Therefore, the same materials used in macromolding applications yield far different results when scaled to micromolding applications.
Plastics in general do not scale linearly. This is in terms of properties and material behaviour such as flow or any type of non-Newtonian behaviour. In a conventional molding world those characteristics are more or less understood and systematically manipulated. But in the microworld, predictable non-Newtonian behaviour rarely occurs. This idea applies to more than dimensional or mechanical things like strength or shrink - it also influences things from the way material processes to the macromolecule behavior.
Design, processing, polymer science, shear sensitivity, crystallinity, flow dynamics, etc. often behave in ways the conventional molding world would label as unexpected.
3. Which factors must be considered by manufacturers when micromolding plastics for medical and drug delivery devices?
You need to think about manipulating material differently in micro than you would in a macro environment. For example, it is important to consider:
- Optimizing velocity conventionally: This can be done in micromolding, but if we go through the steps in the conventional way, the results will say our optimized velocity is extremely high which can be detrimental to materials, due to the high shear exposure. So, we need to come up with other ways to optimize injection velocity in the microworld in order to ensure a robust process and maintain material integrity.
- Widely understood decoupled molding process: The latter half of this technique is primarily based off of gate freeze time in the macro world. In micro, gate freeze is nearly instantaneous so parameters cannot be optimized based on something like gate freeze time. The micromolding process does not give an operator the data to even determine those parameters conventionally. This, however, is not to say that those parameters cannot be optimized for a micro process. Rather, it means that we must think critically about the capabilities of the micro process and determine new ways to scientifically optimize those parameters based off of what is best for the material integrity and application.
Additionally, medical microinjection molding requires much more specialized equipment than traditional microinjection molding.
In conventional injection molding machines, the screw performs four basic actions: Melt, feed, convey and inject the polymer. MTD’s advanced medical micromolding machines utilize a screw-over-plunger design, where the screw only melts, feeds, and conveys the polymer into the plunger cavity.
As you can see in the table above, this has several advantages, but the most basic are that we can control residence time at high heats, and we do not shear all of the material in the barrel. The advantages lead to very tangible results for our customers such as cost savings from reduced material usage, and faster product development timeframes.
4. What do you believe are the main challenges associated with micromolding plastics?
In micromolding, it is extremely important to realize that this is not just scaled down macromolding. A processor not only needs to understand the basics of how to optimize a molding process, but also how each decision effects things like part functionality, material morphology, degradation mechanics, and flow characteristics.
The key to success lies in understanding every possible avenue of your polymer, and how you may manipulate and control those avenues to shape them into a fully functional world class product.
5. How do you see the industry evolving in the future?
Specifically, with bioabsorbable resins, these materials will be tailored to perform specific functions because we are considering the polymer science aspect of things too. Also, things like customized molar ratios and optimized crystallinity are going to lead the way to allow bioabsorbable materials to have the ability to break down at a very precise tissue regeneration rate.
In the future, electronics will incorporate micromolding technology more and more. We are already seeing this today. As technology becomes increasingly more digital, the micromedical device industry will continue that trend, which will in turn require powerful electronics to be incorporated into plastic implants and devices.