Technology | thin wall moulding
Moulding thin wall sections requires high pressures and speeds as well as modified part- ejection and gating arrangements. Mark Holmes learns more about some of the latest developments
Meeting the thin wall challenge
Sustainability, production and raw material costs, and functionality are the key trends driving developments in thin walling, according to Nigel Flowers, Managing Director of the UK-subsidiary of Sumitomo (SHI) Demag Plastics Machinery. “With thin wall thicknesses typically less than 0.6mm, containers and components are lighter, cheaper to produce, and easier to transport and store,“ he says. “Food packaging is by far the largest end-use category of thin walling. However, there are also non-food applications such as pharmaceuticals, cosmetics, paint and adhesives, as well as the automotive sector.” On the technology front, Flowers says that producing thin wall parts requires machinery that supports the higher pressures and speeds as well as tooling with appropriate part-ejection and gating arrangements. “To be successful in the thin walling arena it is vital for manufacturers to examine every potential application to ensure that the selection of materials, machine and tooling give the optimum blend of speed, quality and consistency,“ he says. “One of the key challenges with thin moulding is the uneven filling of moulds. Parts with multiple gates, or thin wall areas at the end of the flow-path, and products which are very small in relation to their sprue weight can all too often incur unaccept- able reject rates.” Sumitomo (SHI) Demag’s activeFlowBalance
technology is designed to overcome this problem and is now widely used by customers running its all-electric machines to address the uneven filling of moulds. The
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technology is a control system feature that exploits the precision of all-electric machine drives and comes into play at the transition from injection to hold pressure. At the switchover point, the screw is held at a fixed position for a pre-selected time prior to the standard hold pressure setting being applied. This allows the compressed melt in the injection system to expand to complete the fill of part-filled cavities without the application of additional pressure on fully-filled cavities. By allowing the pressure to stabilise naturally across the mould the melt automatically flows into the cavities that are under-filled, reducing imbalance and producing a more even fill pattern. And, because there is a more homogeneous distribution of pressure between the cavities, Sumitomo (SHI) Demag claims narrower variation can be achieved in areas such as shrinkage, warpage, weight consistency and optical characteristics.
Precise control The precise control of all-electric machines provides other benefits to producers of thin wall parts. “Because moulds for thin wall parts are made with a very tight tolerance, mould maintenance needs to be done frequently,” Flowers says. “Any build-up of residue across the venting surfaces will result in part quality issues especially on multi cavity moulds. Using closed loop clamp force control, the clamp force can be optimised to the minimum required level – helping to avoid damage to vents.”
January/February 2017 | INJECTION WORLD 53
Main image: Arburg and Mould & Matic demonstrated production of thin wall petri dishes on a 3.2s cycle at the recent K fair
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