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moulding masterclass | Moving moulds part 2


There’s more to an injection unit than screw size, says John Goff. Failing to understand the differences between different injection unit classifi cations can lead to process setting problems


PHOTO: SUMITOMO SHI DEMAG Understanding injection units


Main image: There’s more to injection unit selection than matching screw size


In last month’s edition we began to discuss the challenges moulders face when having to move a mould from one machine to another. Over the next few months we will take a closer look at these, beginning with the transfer of a mould tool between two injection moulding machines of the same make and clamping force but with different injection units. This was touched on briefl y in an earlier instalment but will be considered more fully in the following article. For some time now, injection moulding machine


manufacturers have offered the industry moulding machines of a specifi ed clamping force with the option to select a particular screw and barrel assembly to accommodate required component weights. Figure 1 shows the typical array of injection units and their associated screw and barrel assemblies that can be selected for one particular moulding machine model of 2,100kN clamping force capability (in this case a 2,100kn Systec 210/580 model from Sumitomo SHI Demag). Let us consider a scenario where a polypropylene


(PP) box and lid assembly is to be manufactured from two four-impression valve gated hotrunner mould tools. Both of the four-impression mould tools are designed to fi t and run in a machine of 2,100kN clamping force capacity. The component weight for the box and lid are


26 INJECTION WORLD | January/February 2014


19.86g and 9.08g respectively. The moulding company, when considering the


purchase of the two moulding machines, needed to be able to manufacture both lids and boxes in either moulding machine for ease of planning and production call off. However, due to urgent product supply require- ments, the moulder decided to buy two 2,100kN clamping force capability machines, each installed with a 40mm diameter screw and barrel assembly having the same shot capacity but each with a different injection unit specifi cation. Machine A possessed an injection unit of 430 classifi cation; Machine B has a classifi cation of 600. Figure 2 shows the injection unit specifi cations for Machine A and Machine B (Sumitomo SHI Demag 210/580 models equipped with standard motor and pump and no accumulator). To many processing personnel, the relevance of injection unit specifi cations is either overlooked or they are unaware of its implications. This is particularly important if the moulding machine is hydraulically actu- ated (although the same issues are encountered with servo-electric machines, which will be covered in later articles). More often than not, the mould shop staff’s main daily duties are to install the mould tool in the


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