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Issue 06/2016

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Consumer Electronics so

Consumer Electronics so it was decided to conduct flow simulations. This would enable defects in the moulded parts to be swiftly identified, thus minimising mould iterations, and enabling significant advantages and cost savings related to the avoidance of further defects in geometry to be achieved. The simulations showed that, due to the unequal diameter of the runners, the cavities could not be fill uniformly. The casing parts displayed distinct sink marks. To eliminate these, a higher holding pressure was needed. Graduation of the injection profile and adjustment of a switch-over point eliminated the sink marks. However, it then became impossible to eject the buttons properly. Moreover, both mouse buttons displayed flash, which indicated additional problems. The purpose of the puzzling valve blocking half of the cavities was now clear. To run the process automatically, the mould could only produce two of the four components at a time. This general outcome was confirmed in practice by validation tests using different PLA materials. An X-rayed mouse New challenges related to the design manifested themselves after the first components were manufactured and analysed. Repeated tightening of the screws of the casing at a defined torque revealed screw boss failure due to the thin-wall design, which could not withstand the torsional loading. Also, the fixing struts of the scroll wheel were found to wear out unusually quickly. The wheel could not tolerate permanent rotation under a constant pressure for an extended time period, so X-ray 3D Computer Tomography (CT) scanning was performed to identify the design flaws in a close-up and to determine the specific structures of the components. Also, an assessment of material shrinkage and warpage after conditioning was made. The dimensional stability of the casing was crucial to ensure the correct placing of the PCB, at the heart of a mouse. The above flaws were corrected through mould optimisation. First and foremost, the runner system was balanced. The gate diameters were optimised, i.e. their size was adapted to the cavity volume and the material flowability. The screw boss was reinforced to withstand the torsional loading. The scroll wheel axis was adjusted, after CT scanning revealed this was skewed. The biobased mouse Having optimised the mould and the design of the mouse, the FNG team could finally implement the purposedeveloped semi-crystalline PLA compound. Compared to amorphous PLA, this material offers a substantially better performance in terms of thermal (HDT-B > 115 °C) Figure 3 CT scan and geometry validation. Source: Fraunhofer WKI @Florian Bittner Figure 4 Mould-fill simulation. Source: IfBB 26 bioplastics MAGAZINE [06/16] Vol. 11

Consumer Electronics and mechanical properties. Superior properties were achieved by means of compounding PLA homopolymers and additives. Even though the material required a mould temperature of 100 °C to achieve proper crystallinity, the cooling time was shorter compared to amorphous PLA. Further optimisation was accomplished through targeted modification of the material. Use of a different nucleating agent in this case enabled a further reduction of cycle time of 20 %, and improved both the impact strength and the material flow. Eventually, the efforts to achieve a balance between material properties, durability and economic production were successful. The product-specific development of a PLA blend with pre-defined properties and the specific modifications to the material were successfully implemented; the injection molding tool was adjusted based on injection simulation tests to identify the mould’s weak spots and CT scanning was performed, which excluded warping of the material and enabled specific components’ flaws to be swiftly pinpointed that would otherwise have been difficult to detect. Subsequent modification of the tool eliminated all critical flaws and precise adjustment of injection parameters has made automatic production feasible. Finally, targeted modification of the compound contributed to the improvement of moulding cycle time. A renewable content level of 80 % was achieved in the casing. Biobased IT-world – a worthwhile goal Starting in late 2015, the computer mouse casing has been sustainably produced from a PLA blend developed at the IfBB at a CO 2 emission-free injection molding shop. The material will be further improved with respect to its biobased content and the fair production of the basic materials and additives. It is estimated that there are about 2 billion PCs used worldwide. Let’s assume that about three quarters of them are used with a mouse. An average mouse casing weighs around 50 grams. A simple calculation reveals that a total of some 75,000 tonnes of material are consumed in the worldwide production of the computer mouse devices. And as the production of PCs continues to rise and with no feasible replacement for a mouse as yet in sight, the transition to a biobased, ‘fair’ product makes all the effort worthwhile. Figure 5 CT scan and geometry validation. Source: Fraunhofer WKI @Florian Bittner Figure 6 Components of the PC mouse. Source: IfBB bioplastics MAGAZINE [06/16] Vol. 11 27

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