“Thin-wall” in relation to portable electronics parts are those with a wall thickness less than 1 millimeter. For larger automotive parts, “thin” may mean 2 mm. Whatever the specification, thinner walls call for modifications to the manufacturing process, including (but not limited to) higher pressures, faster speeds, decreased cooling times, as well as modifications to part-ejection and gating set-ups. These process changes have also led to changes to molds, machinery, and part design.
Conventional molding machinery can be used for many thin-wall applications these days, something that couldn’t be said 10 years ago. That’s because the capabilities of today’s injection molding machines are far beyond the capabilities of older machines. Advances in plastics, gating design and the controls on the machines themselves also increase the ability of a standard machine to make thinner parts.
But there are limits to what these machines can do, as well. As wall thicknesses continue to shrink, a more specialized press with higher speed and pressure capabilities may be required. For example, with portable electronics parts of less than 1 mm thick, fill times of less than 0.5 sec and injection pressures greater than 30,000 psi are not unheard of. Hydraulic machines made for thin-wall molding typically have accumulators affecting both injection and clamping cycles.
To withstand the high pressures, clamp force should be a minimum of 5-7 tons/sq in. of projected area. Moreover, extra-heavy platens are needed to reduce flexing as wall thicknesses drop and injection pressures rise. Thin-wall machines commonly have a 2:1 or lower ratio of tiebar distance to platen thickness. Also, with thinner walls, closed-loop control of injection speed, transfer pressure, and other process variables can help to control filling and packing at high speeds and pressures.
Speed is one of the most important factors of successful thin-wall molding. In fact, faster filling and higher pressures are absolutely necessary to force molten thermoplastic material into thinner cavities at a high enough rate to prevent freeze off. If a standard part is filled in two seconds, then a reduction in thickness of 25% could possibly require a 50% drop in fill time — in this case, to just one second.
One side benefit of thin-wall molding is that there is less material to cool. Cycle times can plummet by 50% with aggressive wall-thickness reduction. Scrupulous management of the melt-delivery system can keep runners and sprues from lowering that cycle-time advantage. Hot runners and heated sprue bushings are routinely employed in thin-wall molding to help minimize cycle time.