Mention micro-molding to most designers and they'll likely think “small parts.” For all practical purposes, the association is correct. But it ignores the complexity of molding parts that are smaller than you might imagine. That's why selecting an experienced contract manufacturer (CM) can help avoid headaches when it comes to micro-molding plastic parts. More on this later. First, a brief description helps in understanding micro-molding.

Micro-molded parts are usually described as 0.5 in³ or less. But part sizes can also run as large as 3-in. in diameter, when they have small features. These might include, for example, 0.004-in. walls or 0.008-in. diameter holes needed for particular functions. Larger components meet trends that call for packing more function into smaller spaces, especially in high-tech medical devices. For example, some diagnostic tools require a 30 micron-wide microfluidic channel to deliver biomaterial to sensors encapsulated in complex plastic housings.

Regardless of size, micro-molded parts have tight tolerances, ranging from 0.001-in. and down. Different-size parts face different manufacturing problems. For example, it's much harder to maintain repeatability and precision on small features for normal-sized components than on parts of a comparable scale. Also, molding micro parts is difficult enough, but imagine parts so small that over 2,000 are molded from a gram of material. Such a part exists, what's more, each has five critical dimensions with tolerances of ± 0.001 in. and the parts are produced by the millions with the same precision.

Today just about anyone can buy an injection press capable of micromolding. But a successful process takes more than equipment alone. Also important is innovation in design, efficient processing (ie., reducing scrap as much as possible), and expert, in-house tool-building. To select a contract manufacturer with all the necessary capabilities, ask questions such as:

Does the CM help improve part design for manufacturability? Many of the same design factors that apply to larger parts also apply to micro parts. Some do not. Computer aided design makes it all-too-easy to draw parts that really can't be made. So which factors make a design feasible? In some cases, parts must include draft angles and specific length-to-width aspect ratios so they can even be ejected from the mold. On the other hand, factors such as material melt flow, ejection options, and flash allowance can also influence micro-molded parts differently than a normal size part. A good CM will assist you up-front with these issues.

How does the CM keep mold tolerances? Obviously high-precision tooling is required, but that's not the total answer. Fewer than expected cavities (for example, less than four) are typically requested to reduce cavity-to-cavity variation. This is also dependent on the end-product tolerances and volume requirements. A CM with micro-molding experience will know how to work though the mold design and layout to maximize productivity and performance.

What about gate location and size? Here, an innovative and experienced CM proves especially helpful. It must decide, for example, how to place a gate in a part where the gate and vestige are often larger than part itself. It is also helpful to keep this in mind when designing parts.