Moving moulds part 3 | moulding masterclass
from between the mould halves and positioning them into a stacking unit located at the back of the machine. By reviewing the setting data for Machine B in Figure
1, the following technical facts can be deduced: An actual injection pressure of 158 bar hydraulic is required to effectively fi ll the mould cavities to the required fullness in 0.58 seconds. When converting this hydraulic value into a specifi c injection pressure, the corresponding value would be 2,127 bar; An actual injection rate of 136.97 g/sec, or179 cm3
/
sec, is required to suitably fi ll the box mould cavities in 0.58 seconds; A holding pressure value of 88 bar hydraulic is required to suitably pack the box mouldings to achieve the required visual fl atness and overall dimensions. When converting the hydraulic value into a specifi c pressure the corresponding value would be 1,185 bar; With a coolant temperature of 13°C and coolant fl ow rate of 35 litres/min, the resultant component tempera- ture upon ejection would be 82°C for the given cooling time of 3.6 seconds; The plasticising rate to consistently produce the box mouldings is 57.1g/sec, with a screw recovery time of 2.4 seconds. Upon transferring the box mould tool from Machine B (600 injection unit) to Machine A (430 injection unit), we could expect to encounter a number of processing issues in attempting to achieve an equivalent standard of component quality, weight and cycle time. Certain time values will be affected, as well as component weight and fl atness issues. The infl uence of Pressure Control is often refl ected
in the resultant box mouldings being heavier than those manufactured by Speed Control from Machine B. The reason for the weight increase is the increased molecular compression, particularly within the gate area, resulting in an increase in molecular mass per unit volume of component. Also, the longer injection time, due to a slower injection speed setting, will contribute to a heavier box component. The greater the extent of molecular compression,
the greater the tendency for over-packing of the cavity. This results in the component incurring shape/fl atness problems and/or lower mechanical strength properties. From a visual viewpoint the box mouldings may possess ripples, especially in the side walls towards the extremities of the moulding or at the end of the melt fl ow path. To remove such defects, the holding pressure is often increased to a value higher than the original specifi c equivalent. Naturally, any increase in the melt and hot runner
temperatures will lead to more heat energy being transferred into the cavities of the mould tool. Therefore,
www.injectionworld.com Cycle time Injection time Injection pressure Holding pressure (HP)
Holding pressure time (HPT) Cooling time Plasticising
Clamping force Melt temperature
Component temperature upon ejection Total shot weight with HP & HPT Total shot weight without HP and HPT
Percentage fi ll achieved during the injection phase Screw stroke without decompression Changeover position Melt Cushion
Maximum operating pump pressure
for the same coolant temperature and fl ow rate when the box mould tool is run in Machine B, a less effi cient heat removal rate occurs. This can lead to shape/dimensional changes to the box component upon ejection and/or stacking problems due to the component temperature being too high. Depending upon the extent of excess temperature, both the cooling and cycle times may need to be increased to overcome either the ejection, or assembly issues leading to lower productivity. More importantly, the hydraulic holding pressure
value selected for Machine A would need to be higher than when the box moulding was produced from Machine B. For example, where Figure 1 shows the holding pressure for Machine B as 88 bar (or 1185 bar specifi c) the value for Machine A would be 105 bar hydraulic. For this reason alone it is important to record the weight of the box moulding with and without holding pressure and holding pressure time so that such values can be referenced together with other component attributes. This discussion of mould tool transfers between machines will continue in next month’s edition.
About the author: John Goff is a chartered engineer (CEng), a Fellow of the Institute of Materials, Mining and Metallurgy (FIMMM), and CEO of injection moulding process consultancy and G&A Moulding Technology (
www.gandamoulding.co.uk), which provides consultancy services on all aspects of process setting, optimisation and control, including hot runner technology, and developed and markets its Pro-Op process optimisation software tool. This is the 34th instalment in his Moulding Masterclass series. You can read the most recent instalments in this series here, here and here.
March 2014 | INJECTION WORLD 33
11.20 seconds 0.58 seconds
158 bar hydraulic 88 bar hydraulic 2.8 seconds 3.6 seconds 2.4 seconds 1850 kN 243°C 82°C
79.44g 75.63g 95.2%
88.8mm 10mm 6.1mm
180 bar hydraulic Figure 1:
Processing data for
production of PP box
mouldings (Machine B)
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