Today, polymer injection molding is one of the largest contributors to the emergence of global manufacturing. As the backbone of many industries, it has brought countless conveniences to everyday life. One might think that, after 100 years of development, new ideas have been exhausted.
Along with this push to make everything plastic, manufacturing has demanded everything should get smaller. So where do you turn when your ability to meet the demands for plastic and small size reach the limits of your own expertise, or your supplier’s? What exactly are those limits?
One response to these demands has been micro molding. This is defined as not just molding tiny plastic parts, but molding and processing them with appropriately sized tooling, appropriately sized shot volumes, and appropriately applied efficiencies. It may be odd to say that a 30-year-old industry is still in its infancy, but demand for this technology has skyrocketed only during the last decade. Today there’s a plethora of opportunities to find micro molding, but no real standard definition or textbook procedure to guide design and development. While numerous micro press manufacturers and a growing number of molders offer their versions of micro molding, most of these did not exist a decade ago. This leaves an experience gap in the marketplace that can make the designing and building of micro plastic parts difficult and confusing.
Molded parts from Accumold’s Material/Thin-Wall study, which demonstrated that producing micro parts and features may have more to do with material choice than design. The study’s example was designed with a 76 micrometer (0.003 inch) wall thickness at a 42:1 aspect ratio. These three micro molded parts are made of some of the resins that filled out the full aspect ratio at that thickness. They are (from left) Delrin 900P, Vectra E130i, and Fortiflex HDPE T50-200.
What is micro molding?
In attempting to define micro molding, certainly small part size is a factor, but typically there’s also a precision factor or geometric requirement that separates a tiny part from a micro molded part. In most cases the difference lies in all three: size, tolerance, and features.
A starting baseline defines micro molding as plastic molded parts under one cubic centimeter with complex geometries or challenging feature sizes such as thin walls, long aspect ratio, and small-diameter holes, and with tolerance requirements of ±25 micrometers or less. It’s possible to identify parts larger than one cubic centimeter with looser tolerances or simpler shapes as micro molded. However, the farther away from these defined extremes the part is, the less likely it would be categorized as micro molded.
Micro molding, especially in part design, is a challenging process that requires a certain expertise. Not every molder does it. The micro molder has typically made thousands of pieces of a part considered “unmoldable” by injection molders. To expert
micro molders, the exact definition of micro molding is not only a frustratingly vague range of parameters, but also lies somewhere between the difficult and the impossible.
Understanding the distinctions between micro molding and general injection molding can make all the difference when designing a micro-sized part or choosing a molder. The size and complexities of a part design should drive the choice of vendor. Any experienced micro molder should be able to demonstrate fully its expertise, going beyond the spec sheet of the presses it owns.
These distinctions are also becoming a bigger factor in product development and manufacturing. As the demand for smaller products has not stopped, more molders see increasing opportunity. While this is good news for OEMs, the risk arises when there is a mismatch between the complexities of a part’s design and the supplier’s actual expertise.
The prototyping distinction
Another challenge is an expectation that the micro molding process is the same as the general molding experience, especially for prototyping. For example, when prototyping larger, less complex parts there’s a variety of rapid prototyping processes from which to choose. There’s also an expectation that these parts can be made both fast and inexpensively. But for producing a micro-sized part with micro features and extremely tight tolerances, there are often very few options, if not just one: hard steel tooling.