moulding masterclass | Moving moulds part 3
Moulding expert John Goff continues his explanation of why thorough machine specification scoping is required when moving moulds between even seemingly similar moulding machines
Under pressure on melt preparation
Most moulders will, at some point, need to
move a mould from one machine to another.
Achieving this successfully, however, is not always
straightforward as seemingly similar moulding machines can, in fact, hide some significant differences.
In last month’s instalment, we looked at why it is important in process setting terms
to consider not only the screw diameter and LD ratio but also the size of injection units
installed on the machines. We continue that discussion this month. Last month’s example
moulding company was produc- ing a PP box and lid assembly on two
Main image: Care must be taken when moving moulds between
machines with different
injection units
four-cavity valve-gated moulding tools designed to run on a 2,100kN moulding machine. The company had two Sumitomo SHI Demag moulding machines. Both were standard 2,100kN Systec hydraulic machines with identical 40mm screws but Machine A had a 430 classification injection unit and Machine B a 600 type. We determined from the manufacturer’s data that
while both machines could provide the same shot volume, Machine A delivered a lower specific injection pressure of 2,025 bar (compared to 2,423 bar from Machine B), a higher maximum injection rate (224 cm3
/s against 187 cm3
the perspective of the injection process. However, they will also impact on melt preparation. The conversion of solid granules to molten polymer within the screw and barrel assembly will be affected by differences in the specific pressure values for the same hydraulic setting as frictional energy has a much more dominant influence than conductive energy for any specific type and grade of polymeric material. Along with the selected screw rotational speed setting, the use of a higher or lower specific screw back pressure for the same selected hydraulic pressure will not only affect the shot volume but may also affect the resultant melt temperature value. For this reason, the use of specific pressure values is imperative when hydraulically-actuated machines are used. Further- more, irrespective of the machine make and type, the derivation of the optimum specific screw back pressure setting is an important prerequisite in obtaining process consistency for every mould tool/material and moulding machine combination. As stated in the previous article, fully scoping the moulding process is essential for effective transfer of a mould tool from one machine to another. Some of the processing details recorded for the manufacture of the box mouldings at the required quality and output standards using Machine B are given in Figure 1. As both Machine A and Machine B possess the same
/s), and a higher plasticising rate
(52 g/s compared to 32 g/s). These differences mean simply transferring hydraulic pressure settings from one machine to the other would not produce the same process conditions. Last month we considered these differences from
32 INJECTION WORLD | March 2014
specification apart from the injection unit, then process parameter selection relating to screw stroke/shot volume, clamping force, barrel temperatures, coolant medium temperature, mould open/closing speeds and distances, ejection distances, pressures and speed profiles, together with other common setting features, would be transferred directly from one machine to the other. In this scenario, both the lid and box components are
ejected using a combination of stripper plates and air poppets, with a side entry robot removing the mouldings
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PHOTO: SUMITOMIO SHI DEMAG
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