There is a world of difference between how a product looks on a computer screen and reality. At the beginning of the design stage of product development, it is vital to give due consideration to the scale of micro molded part — especially the scale of some of its internal features. Put simply: parts may look great conceptually but can be impossible to make when it comes to micro mold tooling.
The Design for Manufacturing (DfM) process and Design for Micro Molding (DfMM) address the unique challenges of micro molding and determine the success or failure of a project. DfMM has a massive impact on the tool build, and will therefore have a pivotal effect on product development time, cost, and the quality of final product.
Micro mold tooling and multi-disciplinary expertise.
Success in micro molding is truly the result of multi-disciplinary expertise. Micro mold tooling is only one of a number of critical areas where your chosen micro molder must show experience and knowledge. The best micro molders are truly vertically integrated. Teams from design, tooling, molding, validation, and automated assembly must work collaboratively to optimize the outcomes of product development on behalf of customers.
It is important that individuals within specific departments have a rounded view of their own area of focus. For example, my career in toolmaking started at a die shop. I left that shop for Accumold, where I have worked for 17 years, perfecting the art of micro mold tooling.
At Accumold, I began by applying my experience to the development and maintenance of dies and molds in lead frame systems. While working with these systems, I added automation experience and mold maintenance and repair.
I learned to program and cut electrodes that were used on all my repairs as well. This led me to program and cut electrodes full-time for about 2 years before returning to the micro mold tooling bench. Exposure to many different areas of the business has given me a unique perspective which has proven valuable when working with DfMM and tool fabrication.
This rounded view of our world is something many of us at Accumold share. Our depth of knowledge and experience in all areas is the key driver for product development success.
Micro mold tooling and specific considerations for DfMM
When the team at Accumold start discussions about a potential project, and micro mold tooling, we discuss things such as gating, ejection, mold splits, and direction of pull. This is where our multi-disciplinary expertise really shines.
Micro mold ejection and gating
Often when designers miniaturize their parts, ejection and gating is not included in the design consideration phase. Because our engineering team understands the crucial role this plays, gating and ejection are at the forefront of our discussions. This optimizes the DfMM process and micro mold tooling process.
Consider this scenario: having a small ejector pin on a thin and deep feature can make burning a start hole difficult or impossible. Meanwhile, gating in a thin area when there are other thick areas on the part can cause non-fill and sink. Therefore, as we plan for gating and ejection we anticipate gate vestige or flash. An assembly of parts also requires additional planning.
Micro Mold Split Lines
The micro mold tooling perspective can often seem odd at times, especially when we consult with customers on split lines. Many split lines are originally designed to reduce flash, but we’ve been known to move them to areas that make designers scratch their heads.
A project comes to mind where the preferred split from the tool making perspective would have been a saddle shutoff. Due to flash concerns, however, the shutoff was placed on a slide to make the flash direction perpendicular rather than parallel. The slide created some cosmetic areas on the part, so this caused us to shutoff on the cosmetic surface.
The consequence of this was more flash than anticipated in the windows in the preferred flash direction. In addition, there was some mismatch between the slide and the rest of the cavity.
Doing a saddle shutoff, in this case, would have put all the cosmetic surfaces on one side of the tool. While the potential for flash would still have existed (it always does) the shutoff would have been in the direction of the clamp rather than on a slide pull. This would also have eliminated a split line on the cosmetic surface and would have improved the overall part appearance.
A slide with a large surface area presents the possibility for injection pressure to flex the mold so if you can have a shutoff in the direction of pull it is always a more robust shutoff. The direction of the clamp holds a shutoff together as a direct result of clamp tonnage, while alternatively, a slide is a factor of the tonnage reduced by the heel angle and strength surrounding mold steel.
Micro Mold Feature Size
Another aspect of micro mold tooling that can influence a build is feature size. Several examples come to mind where the part tolerance was used to increase the thickness of an area in order to fill the cavities.
When working with micro molded parts, it is best to remember small changes represent a large percentage of the feature size. Consider a part with a 0.004” (0.1 mm) wall thickness: changing that wall thickness by 0.0005” (0.013 mm) is a 12% change and may be the difference between filling the cavity reliably.
When it comes to creating electrodes to burn the cavity, the amount and difficulty can be greatly affected by the design of the part as well. Consider, if there is a square counterbore with sharp corners, we may have to use four different electrodes to create this feature. If the part had radii added at the corners this may be possible using only one electrode.
The depth of the feature is also something to think about when adding radii. An extremely small radius on a deep feature may not be possible to cut, so if the radius can be increased slightly it can change the difficulty of the electrode significantly.
Part Release in Micro Mold Tooling
When it comes to ensuring a part will release from the cavity, an important factor — especially when dealing with very small features — is draft.
Often with small parts, draft is an oversight because it’s such a small amount. It is important to look at percentages instead of actual sizes, however. Adding 0.5 degrees draft on a 0.10” (0.25 mm) tall feature may not sound important, but that action can add 0.0009” (0.023 mm) of draft, so on a thin 0.010” (0.25 mm) feature, it is almost 10% of the feature’s thickness.
Micro Mold Assembly
Consideration of assembly of several parts is also important in terms of final product optimization, micro mold tooling, and ease of build. When building a mold, it can be easy to believe you are in a situation where everything must be machined exactly to size. I prefer to hedge my bets and add clearance when possible to minimize the amount of areas that need to be accurate.
Adding clearance allows the choice of a less accurate method to manufacture for certain parts, leaving the accurate machining to areas where it is needed which saves time. Clearance on the parts being molded can be approached the same way. This also applies to the part tolerance on a plastic part.
Our team at Accumold involves the technical team very early on in the process of product development, which isn’t common. We know this involvement from project kickoff through production ensures our consistent view of DfMM will not only improve the reliability of the mold but also reduce the time it takes to manufacture.
We’re focused on finding solutions for issues that may seem insurmountable and optimizing project outcomes.
This article originally appeared in Worldwide Engineering Magazine in their December issue. Read the latest issue here.