Moving moulds part 4 | moulding masterclass
Make use of specifi cations
Moulding expert John Goff rounds off his tips on moving moulds
between machines with an explanation of how to use manufacturers’ specifi cations to ensure precise process replication
Moving moulds between different machines is a task every moulder needs to deal with. Over the previous three instalments (Part 1, Part 2 and Part 3) we considered some of the basic considerations, then looked in detail at the example of a moulder with two four-cavity tools for production of a PP box and lid. We previously considered the transfer of the box tool between two machines (A and B), which had the same clamp force and screw size but different injection unit classifi cations, and saw that the processing parameters could not be replicated. In this fi nal instalment we will look at the lid moulding. The basic injection unit specifi cations for machines A
and B are shown in Figure 1. When we consider the manufacture of the lid component using Machine B, the inherently shorter melt fl ow length to fi ll the component results in an injection time of 0.36 seconds to corre- spond with the injection pressure of 118 bar hydraulic. Converting the 118 bar hydraulic pressure into the corresponding specifi c pressure value gives 1,588 bar. The maximum available specifi c injection pressure
for Machine A is 2025 bar, meaning that the actual pressure needed to produce the lid component is well within its capability. To achieve the required 1,588 bar specifi c pressure on Machine A, a hydraulic value of 141 bar must be selected. So we can see that both Machine A and B are capable
of achieving the same quality standards and output requirements for the lid component, whereas we had previously determined that the box component could only be produced on Machine B if we wished to avoid quality and productivity issues. Naturally, the process parameters selected for Machine A will need to be referenced to the 1.196 : 1
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ratio for the other hydraulic pressure settings, as demonstrated above for the injection pressures. Furthermore, the injection speed settings for both Machine A and Machine B will require adjustment in accordance with their corresponding available injection rate values to achieve an identical fi ll time of 0.36s. The plasticising rate will also need adjustment, as the rate for Machine A is 32 g/s in PS and the corresponding rate for Machine B is 52 g/s. The PP material used for the lid and box components has a lower melt density than PS, therefore, the actual plasticising rate for each machine is reduced by approximately 14%. This means Machine A has a reduced plasticising rate of 27.43g/s. Knowing that the total shot weight for the four lid
components is 36.32g, with the derived cooling time of 3.6s and the screw recovery time of 2.8s, an equivalent plasticising rate value of 12.97g/s is needed. Reviewing the data for Machines A and B shows both can achieve the required rate, even though there is a large disparity between machines. Returning to the scenario of our processor, we can
see the company has a dilemma. It has acquired two perfectly good 2,100kN moulding machines, but only one of them has the specifi cation fl exibility required to produce both the lid and box components. We will now consider the consequences when the
same two mould tools are to be used in a moulding machine of a completely different make but with the same screw diameter. Machine C is a 2,000kN clamping force machine with a 40mm diameter screw and barrel assembly (the specifi cation is detailed in Figure 1). The attributes of Machine C will allow both the lid and box components to be readily moulded to the desired visual,
April 2014 | INJECTION WORLD 29 Above:
Manufacturer’s plasticising system data
can be used to accurately replicate process
settings on a different machine
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