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processing | Optical moulding Single-Layer Double-Layer Triple-Layer


Sprue


Sprue – second shot


Sprue – second shot


Sprue – fi rst shot


Sprue – fi rst shot SCHEMATIC SHOWING OPTIONS FOR PRODUCTION OF THE BMS INJECTION MOULDED POLYCARBONATE LENS COMPONENT


ourselves, we found that the approach of completely overmoulding a premould is the most benefi cial with regard to lens precision and cycle-time reduction. This approach also provides the greatest benefi t for the quality of the moulded parts,” he says. “We were able to achieve mould surface replication with very low deviations of approximately 10 microns after very few moulding trials, while one-layer geometries usually require hours and days for evaluation of processing parameters.” The BMS overmoulding technique effectively


encapsulates the premoulded part produced in the fi rst shot within the second shot to create a three layer PC component. While the use of multi-layer technology is not new in the optical sector – Engel published details of its work in this area last year – BMS has fi led a number of patents covering certain material and processing aspects and the quantitative simulation of cooling behaviour it has developed to model the multi-layer moulding process. This proprietary modelling technology allows the BMS engineers to gain a valuable insight into what is going on during the cooling in both the premoulding and overmoulding stages of production. Klinkenberg explains that BMS research has found that, within a certain range, the surface quality of the premoulded part is not critical to the performance of the fi nal part because many defects are eliminated during the overmoulding process as the interfacial surface is melted by the molten second shot. “The temperature at the interface between premould and second shot allows for remelting of the involved plastic, so smaller surface imperfections such as sink marks, fl ow marks, stick-slip grooves or a certain surface roughness can be compensated,” he says. The premoulded part plays an important role in the


overall cooling of the part. Moulding the fi rst compo- nent at a low temperature means it is possible to shorten the cooling time. A cooler premoulded part also


26 INJECTION WORLD | October 2012


contributes to extraction of heat from the second shot, which enables an overall reduction in cycle time. This can be substantial. Klinkenberg says laboratory trials on its test part showed using its triple-layer technique with a hot premould (a mould temperature of 120˚C) resulted in a 20% cycle time reduction over a standard single-layer part; with a cold premould part (mould temperature of 70˚C) the saving is more than 40%. A further benefi t of using a multi-layer approach is


that the reduced layer thickness helps to easily prevent sink marks and often enables holding pressures to be reduced, with an associated reduction in the level of stress moulded in to the part. Klinkenberg says work it has carried out on its test geometries shows that multi-layer parts display a less sensitive relationship between holding pressure and part surface deviation than a single layer approach. This helps the stability and reproducibility of the process. The multi-layer approach also makes it possible to


avoid the use of the large cross section gates typically found in optical parts. In addition to the opportunities this presents to reduce the level and duration of holding pressure, this also simplifi es post moulding operations. Precise and clean removal of the thick gates used in optical part production can be a complex process. Klinkenberg says it has been possible to produce mouldings with acceptably low levels of internal stress on fast processing cycles for all of the part geometries studied in its trials. However, he acknowledges that this may not be possible for all geometries it may be presented with by customers, so the programme has included a number of annealing trials. These indicate that internal stresses can be relieved in the multi-layer components using an integrated annealing process. Aside from the reduced cycle times, the use of the


multi-layer moulding process also contributes to a reduced risk of process related optical problems such as yellowing and black specks. “Yellowing of transpar-


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