Medical Device Link.

MICROMOLDING

These switch pins are an example of how micromolding saves time and materials. They have the same weight, 0.017 g. But the ones on the left, made by conventional molding, required a shot weight of 0.23 g, a sprue/runner weight of 0.165 g, and a cycle time of 6.3 sec. Whereas the ones on the right, made by micromolding, required a shot weight of 0.109 g, a sprue/runner weight of 0.044 g, and a cycle time of 2.7 sec.

Over the years, micromolding has become a hotbed for solutions to problems that have been plaguing medical device companies. As implants and other devices get smaller and smaller, they have tinier parts that need to be molded. However, some in industry have a faulty assumption that normal everyday injection molding machines can mold microparts. They may try, but there’s a price to pay. You simply cannot apply the same principles of injection molding for regular-sized parts to injection molding for microsized parts. This article will explain why.

Different Principles

We often see people trying to use oversized machinery to injection mold microparts. Sometimes we are asked to take tools that were fabricated for larger machines and try to incorporate them into a micromolding system. There are many issues with doing that. The end result is unfortunate: a frustrated customer who is late on delivering product to market and has a variety of problems to figure out.

A 30-ton machine attempting to mold a part that is micro in size presents a plethora of issues. Yet some people apply the same principles of injection molding to microsized parts. They fail and often don’t understand why. Typical symptoms are inconsistent part weights or short shots, yield issues, resonance problems, excessive material waste, and parting line flash. All are serious problems and can be solved by micromolding.

There are complexities with both conventional injection molding and micromolding. Much tooling and processing work goes into each system. However, there are misconceptions as to the complexity and detail that needs to be addressed when creating a tool that produces a part half the size of a human hair.

On the left are two photos comparing shot weight, sprue/runner weight, part weight, and cycle times. These photos clearly show differences based on the technology applied and truly will depict the savings in time and cost savings from not wasting material. The conventionally molded part took more than twice the shot weight and almost quadruple the sprue/runner weight than the micromolded part. Yet the micromolded part was produced at a rate almost three times faster.

Micromolding and small part molding are not the same thing. Here are the definitions that will be used for the purposes of this article.

• For micromolding, part weight is 0.001 g or smaller, and part size of 0.075 in. diameter or smaller. These parts require specialized machines for molding, specialized tooling, and custom part handling for part extraction and part packaging.
• A small part might have a part weight of 0.1 to 1.5 g or a diameter of 0.25 in. or larger. These are produced using a conventional molding machine and standard tooling practices with common-part ejection and handling.

Microtool Design

Designing a microtool can be broken into four different categories for discussion. They are: tool design and simulation, or “mold flow”; tool fabrication; tool assembly; and test shots.

Once a tool is complete, simulation will help you understand material flow for your particular tool design and part. This may aid in such a way that you will identify problems before tool fabrication occurs. You might be able to identify potential voids, sink marks, too-small gate sizing, freezing off prematurely, or even incomplete filling of the cavity. Any of these issues will affect your part quality and dimensions. Some design software may provide condensed versions of this technology, but they are limited in options. They may not have enough options as far as rheology data for specific materials, gate size options, material types, location of gate, and cooling or heating of tool. A simulation system does have a full range of those options, and so it will certainly help you create a tool that is near-perfect before you even cut steel.

Fabricating your tool using conventional machining practices such as machining centers, mills, lathes, grinders, wire electrical-discharge machining (EDM), and sinker EDM technology is good, and in most applications, sufficient.

Some applications require an alternate process to be used, such as when the resolution of the EDM is not good enough to capture the fine detail of a cavity feature. In this case other options include laser, x-ray lithography, electroplating, silicon etching, and photolithography. All are acceptable means of creating fine detail for your cavity geometry.

Potential Problems

Typical problems of injection molding a micropart with an incorrect machine are material plasticizing (or plastification) and melt homogenization. This can be due to a number of issues. One may be that the part and runner are sized such that the screw of the molding machine might not have enough material to move (by screw rotation) before it has to switch over from injection to holding.

A micromolding system (bottom) has a smaller melt cushion, a lower resonance time, a shorter flow length, and much less runner loss than a conventional molding system (top).

In many cases where this occurs, the screw may move as little as 0.01 in., and then switch from injection over to holding. This is very typical when using a large tonnage machine to mold a micropart. When this occurs, the next few shots typically are good, but then the tool might flash, and the process starts all over. This is evidence of an inconsistent process due to using the incorrect machine for the application.

Most solve the problem by making the runner diameter large enough to allow the machine to control the dosing or shot size. This is one solution that does work. However, it is not ideal and it is extremely costly to the customer. Additionally, it does not solve the resonance problem that occurs due to excess material sitting idle in the screw. But it does allow the machine to function and mold continuously.

The issue surrounding processing and resonance of material is nonexistent for a micromolder. They are well equipped to control small amounts of material while being very sensitive to shot size, runner size, and resonance time. This becomes more of an issue when utilizing engineering materials that cost hundreds of thousands of dollars per kg. Implantable, absorbable, or resorbable materials cannot withstand the exposure to heat, nor can the medical device manufacturer afford the material waste.

Additionally, it is difficult to properly control the correct amount of material to be injected into the cavity while holding extremely tight or close tolerances. The problems that result can include inconsistent or irreproducible shot sizes, material freezing due to extremely small mass, material degradation, melt homogenization, and static electricity issues.

Economic Justification

(click to enlarge)

There are many reasons to move towards micromolding from standard injection molding, especially when considering large-volume runs where material costs and cycle times are extremely important. In many situations involving high-volume parts, the material savings alone justifies the new tool expenditures. In some cases, the cost savings in material usage justifies the cost of retooling even if you already have a tool created for a larger system.

Why? Raw material costs are half as much for micromolding as they are for standard injection molding. Standard machines have a significantly larger part-to-runner ratio than micromolding machines. That translates into increased cycle times, material usage, and costs to manufacture.

Conclusion

When considering a micropart to be molded, there is more to it than meets the eye. Using an oversized machine to mold a micropart can create process problems and part issues, including inconsistent shot size, degradation of material, and excessive flashing of the tool – which is ultimately a process that won’t work.

Even without process and part problems, there is a cost associated with using the wrong machine. A significant difference in tooling, material usage, part handling, and cycle times all add costs to the part.

If you have a microsized part, don’t assume that conventional injection molding is the best way to make it. For a better process at a better cost, micromolding may be the answer.

Scott Herbert is the president of Rapidwerks Inc. (Pleasanton, CA).