Moving moulds | moulding masterclass
when it ran in the other moulding machine’. So how can this issue be resolved? A very important
pre-requisite is to accurately scope the moulding process when the baseline process conditions are originated to ensure that a fully detailed setting sheet is created, for its absence can cause no end of problems. Use of a setting protocol, such as that included in the PRO-OP system from G&A and shown in Figure 1, logs all recorded data as each stage of the process is optimised and this provides useful reference data at a later stage. In the introductory article of this series (Part 1:
Perfecting the process) we discussed how process parameters can be segregated into two types of variable: controllable (meaning settable) and conse- quential (the outcomes). Both variables are essential to achieve optimum moulding performance and therefore need to be fully captured. Furthermore, the values recorded for insertion into the setting sheet need to be both the set and (actual) measured. Quite often disparity between set and actual values
occurs for reasons such as lack of equipment calibra- tion, poor equipment maintenance, age of the equip- ment or its inability to sustain a consistent performance or demands from the moulding process in excess of what either the moulding machine or the associated ancillary equipment can cope with. Setting sheets need certain parameters listed, such
as melt temperature, injection time, holding pressure; holding pressure time, cycle time; cooling time; component weight (with/without holding pressure/ holding pressure time); mould surface temperature; clamping force; component ejection temperature; product weight capability assessment; barrel tempera- ture settings; melt cushion; screw recovery time; screw rotation speed; and screw stop position, to name a few. As highlighted in the second instalment in this series
(Part 2: Four steps to perfect mouldings), the variables given in the above list are then segregated into catego- ries such as temperature, time, speed/velocity, volume (distance) variables, as well as product attributes. For reasons of clarity, some setting sheets itemise the process parameters applicable to each stage of the process, for example: Conversion of the polymer from its solid state to a liquid of uniform temperature and fl owability;
Filling of the mould cavity(s); Packing of the polymer melt inside the impression to control the dimensions (shrinkage) and visual appearance of the resultant moulding;
Solidifi cation of the molten core to a value that enables the component to be removed without being misshaped. Whatever approach is used, it is extremely impor-
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tantly to capture the actual or specifi c values for all variables wherever possible. Specifi c values offer the moulding technician/setter greater opportunity to replicate what has previously been set. Although the servoelectric injection moulding machine is a popular choice today, with widespread acceptance of its enhanced performance attributes, the hydraulically-actuated injection moulding machine is still the more common choice and type of machine used for plastic component manufacture. As highlighted in the fi fth article of the series (Part
5:Determining optimum screw back pressures), the relationship between hydraulic pump operating pressure, the diameter of the screw and barrel assembly, and the diameter of the piston of the injection cylinder have a signifi cant infl uence on the conversion of hydraulic to specifi c pressure value, whereas the values created when using the servoelectric machines are solely specifi c and no conversion is required. This discussion of how to manage transfer of tools
from between processing machines will be continued in the next 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.
The fi rst 32 articles published in the Moulding Masterclass series by John Goff between 2009 and August 2013 have now been compiled into one convenient volume. You can now benefi t from all his experience and practical advice by keeping a copy on your desk or the shopfl oor. Re-read early articles on the infl uence of screw design, and the choice of
injection time, holding pressure, gate sizes and much more. To fi nd out more, and to purchase a copy of this essential guide to process optimisa- tion and high quality cost-effective moulding for just €60, £50 or $75, follow the link:
http://bit.ly/1dM2Yhx November/December 2013 | INJECTION WORLD 27
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