Melt expansion supports micromolding on electric machine

Consistency is critical for a DVD player component. Technology that compresses and then expands the melt provided the repeatability needed.

Conventional wisdom says it is best to mold micro parts weighing 50 mg on a specialized machine with a screw diameter in the range of 15 mm. HTP High Tech Plastics of Neudörfl, Austria is taking a different path. In fact, the company says that using the 55-ton Engel

E-Motion all-electric machine with a 25-mm screw was the only way it could mold two parts of a lens holder for the laser head in Philips DVD players.

The two parts weigh 115 mg and just less than 50 mg, respectively, with highly complex geometry, thin segments, and projections. The material chosen for this supercritical assembly is a 30% glass-filled LCP (liquid crystal polymer from Ticona). HTP knew that feeding that material using normal technology, even with a relatively small screw, was sure to cause problems with melt homogeneity and shot size consistency. In addition, the company didn’t own a dedicated micromolding machine.

The key piece of enabling technology, besides the precision of the electric machine’s screw movements, is Engel’s X-Melt expansion molding technology. X-Melt compresses the shot in front of the screw tip, or alternatively in the hot runner, reducing its mass by 10% based on a pressure of about 2000 bar (29,000 psi). The mold cavity is then filled in a fraction of a second through what amounts to a controlled explosion when the needle-valve nozzle is opened. Rather than the explosion overheating the melt, the part actually cools a bit faster than normal. The melt expanding into the cavity consumes heat energy.

No Variation

HTP is a multinational supplier to the automotive, electronics, and aviation markets. It anchors its expertise squarely on the concept of precision, building its own molds as further assurance. Despite the tiny dimensions of this lens holder, it must be true-to-spec to ensure that the laser beam is directed accurately onto the spinning DVD.

X-Melt also contributes here because injection relies on the physical properties of the compressed melt, not on forward movement of the screw. If the screw moved forward it would overfeed the mold. Engel’s electric machine positions the screw precisely as desired, and holds it in position during the rapid melt expansion.

HTP and Engel say reproducibility is considerably superior to conventional injection molding, which is not only good but also in this case absolutely necessary. Owing to the complex geometry, inline quality control was deemed impossible with available monitoring systems. HTP relies on the precise repeatability of the machine, and the system has already proved itself more than 5 million times.

Harald Schermann, production manager at HTP, says that working with a molding machine whose screw didn’t move even a hundredth of a millimeter took some getting used to, but the X-Melt technology always works. The Neudörfl plant has three Engel E-Motion machines, and HTP has begun using X-Melt for other high-precision micro parts. A specialized micro machine could not be used for the larger parts that the Engel electrics also produce.

HTP has been rapidly growing in the last few years. Sales for 2003 were a81.5 million ($99.9 million), and besides several plants in Austria, the company now has facilities in Germany, the U.K., Slovakia, Italy, and the United States.