Moulding masterclass | processing
Moulding expert John Goff continues his discussion of moulding machine clamping force selection in the latest instalment in his Moulding Masterclass series
Clamping down on variation
The previous instalment in this series looked at how incorrect selection of clamp force could impact on part quality. In this instalment, we will discuss the different factors that can affect clamping force and opening force during the moulding process and explore what can be done to minimise the induced process variation. Clamping force will be affected by certain moulding
process variables in association with the quality of mould tool manufacture and attributes such as the geometry of the runner and gate employed and/or the performance of any hot runner system installed. Opening forces are, in turn, affected by the pressure
value created within a mould cavity. The pressure value and consistency from cycle-to-cycle is infl uenced by the viscosity of the molten plastic and the speed at which it fi lls the impression. For this reason, it is essential to achieve a homogeneous melt and select an appropriate speed of fi ll. A homogenous melt results from adopting correct principles to ensure effective conversion of the solid granules into the molten liquid within the screw and barrel assembly. Speed of fi ll should be suffi ciently fast to avoid viscosity changes through material batch changes or colour changes and to ensure minimal cavity pressure variation during each cycle. Large variations in melt viscosity will induce
different opening forces from cycle-to-cycle due to changes in cavity pressure, often leading to the
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clamping force value being set higher than necessary. This, in turn, results in dimensional and weight changes to the moulded component. Such changes may go unnoticed in single-cavity tooling but can be very notice- able with multi-impression mould tools, where the greater number of impressions results in a reduction in the latitude of process parameter selection. This reduction in latitude makes the process less robust, often described as the process “being on a knife edge”. More importantly, although all cavity and core sizes in the mould tool are measured as the same, the size differential across a critical dimension of a set of mouldings (shot) can be such that the variation may take up a large proportion of the applied tolerance, causing Cpk issues. For this reason, correct melt plasticisation is critical for the manufacture of compo- nents at fast cycle times using high cavitation mould tools. Although clamping force is one of the last elements to be considered when optimising the moulding process, it is greatly affected by how well the plastic material is converted into a molten liquid. Clamping force optimisation can be undertaken by
monitoring the change in shot volume when selecting different values. Upon each clamping force value selection, either the collective weight of all mould impressions or the weight of a designated impression or set of impressions is measured to a resolution that
October 2012 | INJECTION WORLD 19
Following a structured clamp force optimisation
plan will ensure maximum
process stabilty and is particu- larly benefi cial in multi-cavity mould tools
PHOTO: ENGEL/ZAO
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