electrical and electronics | Processing
as low shrinkage and fast cooling performance without having to accept the disadvantage of poor weld line strength.”
Engel has achieved good results using X-melt on moulding machines with screw diameters of up to 40mm. Pokorny advises that dosing strokes should be maintained in the range of 1-3 times screw diameter – the typical range for injection moulding of technical parts. Remembering that filling of the mould is driven by the expansion of the compressed melt in the barrel, the total shot volume will be at least 10 times greater than the volume of the part to be moulded. No special mould features are required, according to
Figure 1: graph showing screw position and melt pressure during the X-melt process cycle
source: engel
conventional injection moulding results in a peak in injection pressure at the end of filling, in the X-melt process the pressure diminishes as the melt expands during filling so the risk of over-packing the mould is much reduced. Pokorny says Engel’s work to date has shown that
Below: X-melt was used to produce these 0.39g bearing cages in 50% glass rein- forced pa6,6. a 01g shot was employed
the technique can be applied to most thermoplastics. It is necessary to consider the thermal stability of the polymer because the process delivers only around one tenth of the dose volume in each cycle, which consider- ably extends thermal exposure of the resin. And the very high flow into the cavity also means the shear sensitivity of the resin should be considered. However, good results have been achieved with a wide range of polymers. “So far, excellent results have been achieved with
polystyrene, LCP, polyamide (including filled polya- mide), PBT and other materials,” he says. “LCP is especially interesting. These polymers traditionally have poor weld line strength, but X-melt significantly improves this property of the material. This means users can take advantage of the positive attributes such
Pokorny, although due to the very fast fill times it is essential that the cavity is well vented. The system can be used with single or multi-cavity moulds but not with family moulds (moulds with cavities of differing volumes or geometries) due to the uneven flow this sets up. It is also possible to use X-melt with a needle valve type of hot runner providing it offers reasonably fast activation.
The X-melt technology has been used to produce
polycarbonate battery covers for mobile phones with wall thicknesses of between 0.25mm and 0.8mm. The shot weight in these applications is typically around 6g and requires injection speeds of around 2,600mm/s. Other application examples include gear racks with a 0.3mm wall produced in a four cavity mould using a 50% glass reinforced polyamide. Shot weight for this application was 10g.
Alternative options While Engel is the most prominent exponent of melt expansion moulding technology today, the company is not the only company to have explored the potential. Swiss machinery maker Netstal also spent some considerable time developing a melt compression technology. Unlike Engel’s X-melt system – which keeps the injection screw static throughout mould filling - Netstal’s engineers used compression of the melt to provide an initial surge of polymer into the mould and followed that by driving the screw forward to complete the fill. It holds a patent covering this approach. The Netstal solution was intended to elevate the
pressure in the melt to the level required to fill the mould, eliminating the delay in the onset of mould filling caused in conventional thin wall moulding as the screw accelerates. However, despite reporting some positive results with the filling of thin wall components, the company no longer offers the system. According to a spokesperson for the company, the benefits of the melt compression technique were obviated by the company’s development of a high power injection solution for its
32 INjECTION WORLd | April 2012
www.injectionworld.com
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