PT_Plastics Today

Plastics Today, September 2015

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InjectIon Molds 48 Global PlastIcs RePoRt 2015 PlastIcstoday.coM Additive manufacturing optimizes conformal cooling Additive manufacturing eliminates the limitations of conventional machining techniques in the design of cooling channels. Mike Godber, rPM international tool & die T he process of designing a mold tool involves a series of compromises between sometimes conflicting requirements. These include: • meeting the customer's part quality expectations, such as flatness, surface finish and an absence of warpage, sinks and blemishes; • splitting the designed part geometry in a way that results in robust tool cavity parts that are readily manufacturable; • filling the cavity with plastic and incorporating runner, gate and venting design; • cooling the tool without hot or cold spots, and without compromising part quality while maximizing the produc- tion rate; • ejecting the plastic part evenly without damage. These five basic design requirements have to fit around the part design in the same space. If one requirement has more flexibility, it leaves greater options in the other areas. For example, the use of spark erosion to create square recesses means that a tool does not have to be split into as many pieces as would be needed if only conventional machining techniques were used. Conformal cooling is an analysis to establish the most effective uniform cooling of the injection molded parts. This generally results in a more complex cooling channel arrangement than can be achieved by conventional machin- ing. Additive manufacturing techniques allow the cooling channels to be made exactly as the analysis demands, without the limitations that normal machining techniques have in following complex cavity geometry. This process results in quicker, more-even cooling of the plastic and, hence, cheaper and more-accurate finished products. Conformal cooling is made pos- sible by using an additive manufactur- ing process, where metal powder is melted by focused laser beams, layer upon layer. These layers are built up and joined together as a solid block via direct metal laser sintering (DMLS). The layer thickness can vary down to 0.02 mm, depending on the finish required, balanced against the time and cost of manufacture. A solid metal part is pro- duced without limitations in its internal or external geometry. This allows cooling channels of any shape to be built into the part (following the cavity shape). The external geometry can be machined and polished to ensure the molding surface has a smooth finish. Conformal cooling offers much greater flexibility in the design of cooling for the mold. With the correct computer flow and cooling analysis of the part, it is possible for mold cooling to be opti- mized in a way that is not possible using conventional drilling, plugs, bubblers and baffles. This can shorten setup time, which is often the longest part of the mold cycle time, while simultaneously improving part quality, especially in the common problem areas of distortion and warping. Cooling channels can be made in any shape, so diameters can vary; baffles can be built in to introduce turbulence and, hence, a cooling effect; and cross sections can be oval or square. The accuracy of cooling channels is not compromised by long drilling lengths that are prone to wandering, so the cooling channels func- tion as they were designed to do. In addition, conformal cooling can leave more space for the other require- ments listed above, which can lead to better optimization of the tool in these areas. For instance, a gate position that would be ideal to fill the part but impos- sible to accommodate with conventional cooling channels may be possible with a laser-sintered insert. Therefore, to maxi- mize the benefits of conformal cooling, the whole tool design needs to be inte- grated around it. Cooling channels are visible in a laser-sintered insert cut in half.

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