INNOVATION | OPTICAL MOULDING
Results from a recent optical moulding application show the differences between Husky’s UltraShot and traditional hot runner systems
Source: Husky
continues to grow globally. With this growth we are seeing the adoption of smart technology that is placing an increasing number of sensors and optical lenses in and around the vehicle. Finally, there is significant growth in the use of payment devices, with areas of the world now completely transferring to electronic fund payment methods. Commercial establishments are all installing various types of payment terminals, many with cameras and sensors. We are seeing this growth globally, but it is particulary present and active in Asia.” Ellis adds that because this is a growing area and these products can be connected to very demanding and technical moulding applications, development of new, advanced hot runners or injection systems that focus on part quality with higher cavitation output have become key. “Look- ing at these applications closely, they are all about moulded-in stress,” he says. “If you think about what is happening to the resin in terms of flow hesitation, filling and packing, the mould has a major impact on part quality, especially optical clarity. The residual stress in the resin will change the optical behaviour of the lens and ‘bend the light’, which means that the lens will become ‘blurry’. Very low residual stress is key for accurate lens moulding. In addition, as cavitation is in- creased, variation across different cavities results in various amounts of moulded-in stress that makes each part different. This process leads to major quality and scale limitations with the moulding of optical parts.” Husky says its UltraShot hot runner technology
can address these issues. Ellis adds that the Ultrashot Injection System is about volumetric and precise filling. With the melt management system specifically, longer flow paths for thin walls typically require higher pressures to fill them. To prevent the
34 INJECTION WORLD | July/August 2021
flow front from freezing before reaching end-of-fill, high injection flow rates are sometimes used, creating high in-mould stress and birefringence in the part. He says using UltraShot eliminates the pressure loss from the machine barrel and a majority of the pressure loss in the distribution network (manifold) of a geometrically balanced hot runner melt delivery approach. This can allow a slower fill rate, less molecular orientation and moulded-in stress, which reduces the effect on the optics of the final moulded product. In addition, the further injection is from the gate, the less control of the resin there is. Being closer to the gate, increases injection precision significantly, which removes much of the cavity-to-cavity variability, while also removing much of the stress on the resin. This can be controlled precisely, which almost removes the compressibility of the melt. Husky says that with UltraShot, by moving closer to the gate it is possible to fill the part precisely with extremely accurate pressure and no concerns of overfilled or underfilled parts. The company adds that moulded-in stress,
scaling, cold runners and expensive resin waste are all specific optical moulding challenges that require new technology solutions. In addition, direct gating onto the lens is an issue connected to clarity. Melt management from a residence time issue on such small parts becomes significant, which is why so many of these applications are still cold runnered. Also, optical grade resins are expensive and hot runner channel surface finish is key. It is necessary to ‘super polish’ the channels, for example provid- ing a surface finish better than Ra 0.3, which many hot runners cannot achieve. The Ultrashot Injection System is a recently launched new product that is well aligned to the needs of optical moulding applications, says Ellis.
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