Mould design: part 2 | DFM
cooling time by implementing optimal cooling channels in the tool structure and taking care to achieve good heat transfer from the cavity into the steel and out of the tool via the cooling circuits. The biggest mistake in this area is to jump from large cooling channel diameter to a smaller diameter along the cooling channel ans this will create fl ow restriction. A simple mistake that is seen very often is to make
bubblers or risers too small compared to the main cooling lines. Cutting a 10mm diameter riser hole in half will leave only 5mm diameter per side as fl ow channels. This mistake will leave areas of the tool with insuffi cient cooling and this will show up in poor component quality and dimensions, as well as extended cycle times. Figure 4 shows appropriate relationships between cooling channel and bubbler diameters. To add cooling to the structure the most traditional
and most often used technique is to drill into the tool steel. Unfortunately using this technique it is not possible to reach all areas of the tool as placement needs to be balanced with other tool features such as ejectors, gating, sliders, screw holes etc. Manufacturing an insert in slices and milling the
cooling channels in is quite an old and established technique. The individual steel slices are soldered together to form a solid insert and in this way it is possible to place cooling lines quite close to the component geometry. The downside is that additional witness lines may occur on the moulded component. Better results in terms of short cycle time and high
component quality can be achieved using laser sintered inserts. Laser sintering is a highly fl exible manufactur- ing method that creates a 3D structure by fusing together layers of metal powder using a laser beam. It allows small cooling channels to be established very close to the cavity wall of the component geometry and the savings on cycle time can be phenomenal. AST has been involved in one project where a cycle time reduction up to 75% was achieved while also improving the dimensional and visual quality of the product. However, there is also a risk attached to part cooling. Sometimes, high cooling rates or extended cooling periods are used to overcome warpage problems by
Figure 4: Bubblers are often incorrectly sized to achieve proper cooling. This table shows some suitable diameter recommendations
freezing tension into a part. While this may reduce immediate post moulded distortion, it is an attempt to process around a component geometry issue such as a wall thickness difference along the fl ow path, incorrect gate point location or other root causes. And tension frozen in to the component by longer holding and cooling time may be released later on in the manufacturing process, such as where heat treatments are used. Figure 5 shows a mobile phone cover that was
treated in this way. A longer cooling time did lead to a fl atter part but after the plating process, which applies a lot of heat to the component, the internal stress was released and showed up on the visible surface. Several component design changes were needed to modify the fi lling behaviour, as well as additional assembly and plating tests. The cost to modify the four production tools was calculated at close to €80,000 but the total fi nancial damage caused by new testing slots, addi- tional travel and labour costs, and delayed ramp up would have been much higher. However, as cooling needs are determined by
component geometry, this means that the component designer can infl uence this area very effectively. Analysis tools within CAD programs such as thickness analysis, cooling time plots and fl ow simulations will show the weak areas on a component design where cooling improvement is needed. Removing thick areas is the most important target to get the overall cooling time down, as well as improving the part quality. The fi nal instalment in this three-part discussion of mould design will be published in the May edition of Injection World.
About the author: André Eichhorn is general manager of Germany-based AST Technology. This is the latest in a series of articles presenting a step-by-step discussion of the Design for Manufacturing (DFM) process. Previous instalments in this series can be viewed here, here, and here.
www.ast-tech.de
www.injectionworld.com
Figure 5: Release of frozen-in stress can cause prob- lems in post moulding operations such as plating
April 2013 | INJECTION WORLD 49
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