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Optical moulding | processing Sizing the barrel


Selecting the appropriate barrel is decisive in fi nal part quality as parameters such as shot weight, stroke utilisation,


residence time and the required injection moulding pressure are of special importance. The shot weight is determined by the part design and material specifi cations and should be used to select a screw size that will provide a stroke utilisation in the ideal range of between 20% and 60% (and certainly no more than 75%). An appropriately sized barrel and screw will,


depending on the cycle and the dwell time of the melt in the barrel, deliver a homogeneous preparation of plastic material. A poorly selected barrel will result in too long a dwell time, which will lead to damage and discoloration of the polymer and a subsequent deterio- ration in optical function.


Injection moulding pressure must also be consid-


ered very closely in the design and confi guration of the system as most transparent materials used in the optical sector are highly viscous. Holding pressure should be assessed accordingly.


The mould environment The mould and its immedidate environment determines the consistent and lasting cleanliness of all compo- nents. Unnecessary hardware and areas that can harbour contaminants should be avoided. In some cases, encapsulated or metal-clad clamping or unloading systems incorporating laminar fl ow cabinets and self-contained peripherals may be used to keep the production area permanently clean. It is also worth considering antistatic paint for the machine. Hydraulic components should be kept well away from the moulding area and from locations where parts are handled or deposited to minimise risk of contaimination. For the same reason, all pneumatic valves should be provided with adequate fi ne mesh fi lter systems. And temperature control hoses should be kept as short as possible while maintaining operational safety.


Machine selection The decision of whether to use full-hydraulic, all-electric or hybrid drive concepts will depend on a number of factors, with shot weight, component geometry, holding times, required precision and cleanliness being among the prime considerations. Generally, all-electric machine axes provide an order of magnitude improvement in precision over hydraulics and are able to provide parallel machine movements with no special modifi cations. The


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latter is an essential requirement where simultaneous


compression moulding processes are operated via the clamping unit of the machine.


Due to the mostly


mechanical power transmis- sion, mould approach positions are maintained in a very precise and stable manner in an all-elec-


tric design. This accurate action is especially effective in production of small prisms or lenses with surface accuracies in the region of 2-3 microns. Flat, thin- walled optical parts carrying Fresnel or micro-struc- tures on their surface can be manufactured very effectively by compression moulding using an electri- cally powered clamping unit.


Production of thick-walled lenses normally does not need such accurate controlled movements and the cushioning characteristic of the hydraulic oil can help to smooth out changes of velocity and pressure within the injection and holding pressure profi le. Slow and smooth movements generally yield better results in optical moulding.


However, the major benefi t of hydraulic injection moulding machines is their ability to maintain long holding pressures high levels. Hydraulic machines also make compression moulding with high embossing nips possible. Both are very useful techniques – and sometimes the only techniques – capable of compen- sating for the enormous shrinkage involved in moulding thick-walled lenses.


Ultimately, it makes sense to make a detailed product-related comparison of potential machine concepts to deter- mine which is best for the specifi c product that will be manufactured. Demand for high optical quality plastic components will continue to grow and machine and process developments will make it easier and more economical to meet the requirements of this market. However, producing high precision optics at the


required quality and cost will remain a challenging task that will continue to test the skills of processors.


About the author: Thomas Brettnich is technology development manager at Sumitomo (SHI) Demag and is based at its facility at Schwaig in Germany.  www.sumitomo-shi-demag.eu


Above: Thick wall optical parts require high level moulding expertise


September 2012 | INJECTION WORLD 39


Left: All-elec- tric machines are well suited to production of fl at optical parts such as Fresnel lenses


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