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Moulding masterclass | processing

ture and productivity, while consequential process variables are those upon which the overall stability and robustness of a moulding process is assessed. Particu- lar consequential variables can be attributed to each of the six main steps (see Table 1), with the added advan- tage of being presented in tabular form from the data collected each cycle by the computerised control within the injection moulding machine. These values are clearly displayed, allowing comparison with previous cycles, and highlight the deviation (range) between the values as well as the average value for a pre-selected number of consecutive cycles. Consider the following example. A decrease in the mould surface temperature resulted in a shorter gate seal (freeze off) time causing a non-uniform surface finish, slight sinking and/or dimensional issues due to ineffective holding pressure application. The wrong response would be to increase the holding pressure to pack out the moulding as such an increase then uses more material, resulting in increased volume to achieve the same part dimensions/surface finish as well as greater inherent stress in the moulding. In this example, changing the holding pressure masks the true cause of the defect, which is the decrease in the mould surface temperature. If the temperature of the part had been measured as per the initial process optimisation exercise, the decrease in temperature would have been noted and would have resulted in the correct investigation and conclusion. Alternatively, an increase in the mould surface

temperature due to either an issue with the mould temperature controller (MTC) or the cooling circuitry being wrongly connected can result in a variety of problems such as: longer gate seal time, easier cavity filling, over-packing of the cavity leading to ejector pin marks, surface finish or texture issues, burn marks and/or dimensional variation. Each of the above faults may be resolved using different approaches, while the main culprit (part temperature) is overlooked. Consider a further example of a small reduction in the holding pressure applied when moulding semi- crystalline polymers, which results in changes in component shape and size and leads to alterations being made to cooling time, holding pressure time, mould tool and/or melt temperatures. Reference to the component weight created during the process optimisa- tion exercise would have identified a loss in part volume/weight.

Adopting the optimised process settings as the base

line for comparison with components from subsequent runs ensures consistent component manufacture at the correct quality standards. When mouldings deviate from the desired quality standard, objective assessment can

Table 1: Typical controllable and consequential process variables for the six main process steps

Process step

Conversion of solid granule to melt

Controllable process variable

Screw rotation speed Barrel temperature settings Feed/throat temperature Screw back pressure Decompression distance Utilised shot capacity

Manipulation of molten material to fill the mould cavity

Compaction of molten material within the mould cavity

Movement of the mould halves and related speeds

Solidification of the molten material in the mould cavity

Removal of component from the mould and subsequent collection

Injection pressure Injection speed Screw stroke Mould tool temperature Changeover position

Holding pressure Holding pressure time

Consequential process variable Screw recovery time

Injection pressure Injection time Changeover position Screw stop (end) position

Melt cushion

Mould open/close distances Mould open/close speeds Clamp force application Mould too sensing speed/ pressure

Cooling time

Mould open/close time Side core actuation time

Mould temperature Inlet/outlet coolant temperature

Side core movement, speed and pressure

Ejection stroke, speed and pressure

Robotic movement and speed and placement position

Gravity part collection Conveyor speed/dwell time

Ejector forward time Ejector return time Part extraction time Dwell time on conveyor Part weight Part temperature on ejection Part temperature on conveyor Part temperature in bulk container

be made using these reference settings and compo- nents previously derived. This discussion will continue in next month’s Moulding Masterclass instalment.

About the author: John Goff is a chartered engineer (CEng), a Fellow of the Institute of Materials, Mining and Metallurgy (FIMMM), and managing director of injection moulding process consultancy and moulding process optimisa- tion software developer G&A Moulding Technology ( This is the 25th instal- ment in his Moulding Masterclass series of injection moulding process optimisation articles. You can read the most recent instalments here, here and here. If you want to make sure you don’t miss the next

instalment in the Moulding Masterclass series, you can subscribe to InjectionWorld for free here.

January/February 2013 | INJECTION WORLD 33

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