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Five Things You Need to Know About Micromoulding

25/07/2012


Micromoulding is allowing designers to meet extreme challenges. Aaron Johnson 
of Accumold outlines the five things it is important know about the process to make 
this possible.

There is little doubt the world is getting smaller. Technologies of all types have enabled faster communications, made life more convenient and have even allowed us to be more creative. This miniaturisation trend is not stopping anytime soon. Our own desires to improve, grow and push the limits will continue to strain the design and product development process. So what does this mean for the designer, product developer or mechanical engineer?

In part, it means they will be asked (if they haven’t been already) to perform miracles. They will be asked to stretch beyond their expertise and become masters of all, not just some. The demand for smaller, faster, cheaper will be the mantra and the focus will be on them to perform. But how?

One of the enabling technologies of recent times is microinjection moulding. Although technically it has been around for a while, only in recent years has demand for it exploded. Micromoulding has played a huge role in all kinds of product development. Medical applications especially have benefited from this technology. Devices for less-invasive surgical tools, diagnostics, metering devices and delicate life-saving instruments are just some of the applications of micromoulding. But what exactly is micromoulding? How can you use it? What do you need to know?

The approach: Design for micromoulding
As we begin this discussion we need to ask, What is micromoulding? This is a common question that has no simple answer. There have been countless articles in magazines and on the Internet about micromoulding, but there is no textbook definition.
When defining micromoulding, the first notion, of course is part size. Figure 1a shows a tiny moulded part that is only 800 microns in length. It has full dimension and shape and is considered to be a critical component for its application. In generic terms it definitely fits into the category of micromoulding by size alone, but is part size all that matters?

The part in Figure 1b measures approximately 8 cm in diameter. With this particular part there are micro features and tolerances that make this a difficult part to mould. Its size in relative terms is small, but not really what one thinks of as micro. However the features and tolerances contained on this part are not considered to be run-of-the-mill details.





Figure 1a:
A moulded part 800-microns long




Figure 1b: This part is 8 cm in diameter with micro features and tolerances that make this difficult to mould















Some have defined micromoulding by the moulding press used in the process. Many press manufacturers have developed their own moulding machines dedicated to moulding small parts. In some respects press size does matter. However, just about any moulding press can produce small parts, but you may not want to use a 500 tonne press for a part that is only 800 microns long. The machine has to be appropriate for the job, but by itself it does not define the moulding category.

With the above parameters in mind, micromoulding is defined in the range of approximately 8 cm in diameter with micro features on the moulded parts, which are measured on the micron scale. In simple terms micromoulding is about micro size, micro features or micro tolerances and in many cases it involves all three of these.

Now that the basic scope and scale of micromoulding has been defined, are there guidelines for micromoulding? The answer is, "Yes, but …” Because every project is unique and has a different set of requirements this discussion can be wide open depending on your situation. The geometry, material selection, tolerances etc. can all have a profound effect on part performance especially at this scale.

That said, there are some good starting points when designing for micromoulding. Obviously the first rule is, "It is still an injection moulded process.” You still have to heat the plastic, it still has to flow into the mould somehow, and the mould still has to be able to open and eject the part. If you have designed larger injection moulded parts the same basic design rules still apply. However, here are some additional guidelines to keep in mind when designing micro plastic parts:
  • You can design gate sizes as small as 0.1 mm in diameter
  • Ejector pins can be as small as 0.25 mm in diameter
  • Thin wall sections near 0.1 mm or better are achievable
  • Feature aspect ratios around 6:1 are a standard rule of thumb for most materials.
Keep in mind that these are general guides especially in terms of feature sizes. The remainder of this discussion will demonstrate where these rules can be broken and where they may fail.

Prototyping: Challenges in development
If your project is like most, the prototyping stage is a critical step in your development process. As in any development there are form and function tests, material tests, cosmetic standards and manufacturability assessments that will need to be performed as product development progresses. The main question here is how. If your moulded part is only 800 microns long, what prototyping method will you choose?

To answer this question a study was conducted to find out which of the standard prototyping methods would be best for producing a micro sized part. The part in Figure 2 was designed to represent the size and feature requirements seen on an average micromoulded part. The part is approximately 5 mm2 with small features including a 0.25-mm through-hole, 0.025-mm tall artwork and other micro structures.




Figure 2:
The part is approximately 5 mm2 with small features, including a 0.25-mm through-hole, 0.025-mm tall artwork and other micro structures











The goal of the study was to produce this part with as many prototyping methods as possible. Eleven different methods were chosen: stereolithography (SLA), 3D printing, PolyJet, fusion deposition modelling, selective laser sintering, laminated object manufacturing, cast urethanes, machining or rapid tooling, rapid injection moulding, and standard hard tooling.

Of the 11 processes, only five produced parts. Most of the prototyping companies that quoted for the project said their service was not capable of producing such small features. Of the five, 3D printing ended up more like a sugar cube maintaining only the basic cube shape. The other four processes faired quite well: SLA, PolyJet, machining, and standard hard tooling managed to give at least a decent representation of the desired part.

Figure 3 shows the results of the four best methods. Their success is based on what you may actually need for a prototype. SLA and PolyJet are good options for some situations. Machined parts can be a viable solution in many cases. However, if, to be of value, your prototype requires extreme features, tolerances or other fine details, then standard hard tooling may be your only option. It will not be the fastest or cheapest of the choices, but to date there are limited options when it comes to micro part prototyping.





Figure 3:
The results of the four best prototyping methods, from left to right, SLA, PolyJet, machining, and standard hard tooling









Material selection: Beyond the data sheet
The third thing you should know about micromoulding relates to material selection. Choosing the right resin is an important step in product development and that by itself can be challenging. You often have strict parameters in which your part has to function. Environmental factors such as temperature, chemical resistance or biocompatibility and durability are only some of the restraints one may have when choosing material. Where do you begin?

Most often the place to start is the data sheet. Unfortunately there can be some confusion when looking at the data to try to determine whether a specific resin will work for your micro part. Data sheets are based on much larger part parameters and this can be misleading when trying to apply the information to your project. Even the stated "easy flow” characteristic of some materials often does not make much difference when moulding micro parts.

To shed some light on this predicament a second study was conducted. Figure 4 is a drawing of a part that was designed to demonstrate how material selection can affect part performance. The part has a thin, high aspect section to show how different materials would fill. The mould was built to accommodate the best chance to fill the thin 76-micron section, but was not modified for each resin. The goal was simply to show the variety of responses based on the material alone.





Figure 4:
Drawing of a part to demonstrate how material selection can affect part performance








Eleven common engineered thermoplastics were chosen for the study, some natural and some filled. The materials chosen were polyethylene (PE), polypropylene (PP), polyamide (Nylon), polycarbonate, polysulphone, polyoxymethylene (POM), polybutylene terephthalate, polymethyl methacrylate, polyetheretherketone (PEEK), polyetherimide (PEI/Ultem), and liquid crystal polymer (LCP).

Of the 11 different materials, four filled completely (PE, PP, POM and LCP), several travelled a fair distance and PEI and PEEK travelled the least. Keep in mind that the study was not designed to show how far a material such as PEEK could be pushed, in some cases it can do better, but to show how resin selection alone can make or break the desired outcome (Figure 5).



Figure 5: Material study results from left to right, HDPE, LCP, PEI/Ultem and 30% glass filled nylon

Remember, the rule of thumb for aspect ratio is 6:1. Here is a case where in some instances the ratio was above the guideline (42:1) and in some cases below (3:1). The overall geometry and other part features such as surface finish or cosmetic requirements can also affect the outcome. Where the part can be gated or ejected may have an adverse effect on the mouldability of a part. Just because it is a small part does not mean there will not be any issues with end-of-fill. Always consult your micromoulder when there may be a question about materials and feature performance.

Beyond the part: Metrology, handling and packaging
So you have made a small part. "Now what?” Because you are probably not designing a small part just for fun there must be some kind of end in mind. What is that end? Often with micromoulded parts the handling after moulding can be just as difficult. Understanding how parts need to be measured, handled or packaged is the fourth thing everyone needs to know about micromoulding.

For the part in Figure 1a, the challenge is packaging not moulding. The process to orient and package a part only 800-microns long needs to be carefully determined so that there are no surprises once full production begins. Inspection, quality control and any special packaging requirements can be production stoppers if not thoroughly thought through. The first question is always, "Can I make this part?” But don’t forget the endgame or you may be stuck with parts your assembly team cannot use.

Size and features: I didn’t know you could do that
The fifth and last thing everyone should know about micromoulding is simple. Don’t lose your creativity! Despite all of the challenges in micromoulding do not lose sight of your objective, which is to design and make a great product. It is easy to get frustrated if you cannot find a prototype method to start the design and development process rolling; or if a resin supplier does not think he has a material that can achieve what you need; or in the end it seems overly complex to manufacture the part. Always start with your ideal. Take your design to your experienced micromoulder and let him react. Yes, there are some impossible projects, but you may be amazed at what can be accomplished. Push the limits and you can be the hero.

To download the full versions of the studies mentioned in this article visit www.accu-mold.com

Aaron Johnson is part of the Accumold technical sales team. The company specialises in microinjection moulded plastic parts and components, 1711 SE Oralabor Road, Ankeny, Iowa 50021, USA,
tel. +1 515 964 5741,
e-mail, ajohnson@accu-mold.com




   

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