Better optical parts, faster through overmolding

Engel has introduced new injection molding technology specifically aimed at the production of optical components, offering machines with plasticizing unit components optimized for running transparent polymers and an overmolding process to create thick lenses. Called optimelt, the process starts with the production of a pre-molded part that is recoated with additional layers (usually of the same material) in one or more subsequent stages.

Engel pursued the technology at a time when it says more and more high quality optical components are switching from glass to polymers, propelled by greater cost efficiencies and more flexibility in product design. "The challenge now is to strike the right balance between superior optical quality and high cost effectiveness," Engel noted in a release.

Injection molded optical components, optimelt.

Utilizing injection molded plastics does pose its own challenges, however, including microstructures on the surface of light-guide elements that must be precisely molded across the entire flow path, and the need to minimize residual stress in molded parts, especially where a subsequent coating will be applied. Finally, the optically effective surfaces on lenses with large differences in thickness must be accurately contoured.

Engel says a primary advantage of its multi-layer overmolding process compared to single-layer designs is that the recoating covers sink marks and other defects on the surface of the previous layer, helping ensure high optical quality. The company says this particular benefit makes the process especially attractive to manufacturers of thick- section components such as LED lenses. Engel also notes that studies on multi-layer components in the area of lighting technology have shown that boundary layers between coatings have no effect on the function of illumination optics.

Thermal simulation of the cooling process proves that the multi-layer process also offers benefits in terms of cost-effectiveness, driven by cycle-time savings. This was demonstrated by comparing three different triple-layer variants and the conventional single-layer process in the production of a 30-mm thick lens.

More layers, more cost effectiveness

In the first multi-layer prototype, the three layers were applied in turn at three stations of a rotary table. For the second variant, two thick outer layers were injected into separate cavities, and in a subsequent step, these layers were brought together in a single cavity and joined by a thin layer of plastic. For the third variant, the inner layer was produced first in an index platen mold, and this was followed by two outer layers that were produced simultaneously.

For an objective comparison, an identical number of cavities was used for each of the four processes examined, and in the multilayer variants, layer thicknesses that ensured the same cooling times for the individual layers were chosen. A lower mold temperature was set for the inner surfaces in each case, i.e. poorer surface quality of the pre-molded part was deliberately accepted. This was possible because these surfaces are covered in the following step, by the next layer. The reduction in mold temperature was found to significantly reduce cycle times.

The triple-layer process with index plate, where a middle layer is injected followed by two outer layers, performed the best overall. Since two layers are produced simultaneously in one station, the productivity boost compared to conventional single-layer injection molding amounts to 180%. In addition to greater efficiency, the new process also means that sink marks can be corrected on both sides. Engel has applied for several patents on the technology, in the hopes it can give its customers a competitive advantage in the optical components markets.