Even luxury goods such as watches increasingly contain tiny components made of thermoplastic—for example, cog wheels in watch movements or clips—and silicone for tiny seals. It is the latter material, in particular, that is gaining increasing market share because of its special properties. Due to trends toward electromobility and autonomous driving, the automotive industry will also require larger numbers of micro-parts made of liquid silicone rubber (LSR) in the future.
What Makes Micro-Molding so Challenging
Micro-injection molding means working with part weights of significantly less than one gram. Right now, some manufacturers are already achieving part weights below one milligram—although it makes a big difference here whether the raw material is a thermoplastic with a low specific weight or a liquid silicone with higher density (typically 1.10 to 1.50 g/cm3). Micro-parts made of silicone are generally heavier than thermoplastic micro-parts, but they can be smaller—and more challenging—in terms of dimensions.
But before we turn to the differences between LSR and thermoplastic in processing, it’s worth taking a comprehensive look at the special challenges of micro-injection molding. These challenges exist independently of the raw material used, because some of the engineering challenges in micro-injection molding arise solely from the part—and part feature—dimensions and extremely low masses processed.
Given a component 1.7 mm long and 0.9 mm in diam., the part weight made of LSR is 0.0005 g (Fig. 1). If you have 32 cavities, the total shot weight (including sprue) is then 0.125 g. This is the equivalent of about 125 grains of sugar. For comparison’s sake, a sugar cube contains about 20,000 to 30,000 grains.
Such small quantities as in this example—connecting elements used in instruments for ophthalmic diagnostics—now must be uniformly and repeatably distributed over all the cavities. The micro-injection unit, the sprue channel and thermal management are enormously important here in terms of quality, precision and repeatability.