processing | Moulding masterclass
Figure 1 (on left above): Table showing example data collected as part of a clamp force optimisation protocol Figure 2 (on right): Graphical analysis of clamp force and part weigh identifi es the optimum value
suits the shot weight under consideration. Selection of the measurement resolution is very
important, when undertaking this optimisation exercise, to ensure that any minor changes in the component weight are detected. As a guide, for components weighing up to 1kg a measurement resolution of 0.1g is necessary, whereas above 1kg a typical resolution would be 0.5g-1.0g. For components of less than 20g, a resolution of 0.0001g will be required. In addition to weight monitoring, both the melt cushion and injection pressure values should be recorded for reference purposes. Figure 1 highlights a typical set of results from this simple and effective systematic procedure. In summary, the procedure for determining the
correct clamping force is as follows: Set the moulding machine at maximum clamping force and weigh parts produced; Reduce the clamping force in increments of 5-10 tonnes or 50-100 kN, weighing parts at each incremen- tal change (for much larger machines the incremental change should be 100-200 tonnes or 1,000 to 2,000kn); Continue to reduce the clamping force and weigh the parts produced using same force reduction intervals until there is a signifi cant change in weight but insuffi cient reduction to induce fl ash; Plot the clamping force against weight value at all times to produce a curve such as shown in Figure 2. Within the determined clamping force range, there is
often a set of values that will create consistent compo- nent manufacture and allow entrapped air within the impression to escape to marginally increase the weight of the moulding. In other words, the mould impression will be fi lled to its full extremities without detrimentally affecting the visual quality of the mouldings. Above this range, further reduction of the clamping
20 INJECTION WORLD | October 2012
force value increases the component weight to a point which induces variability from cycle-to-cycle. It will eventually cause fl ash on the component as the gap between the mating faces of the mould tool becomes too great. Low viscosity materials are more critical than high viscosity materials when optimising clamping force. For the same size of gap created between the mould halves, the low viscosity materials will penetrate further to give minute traces of fl ash. By undertaking this simple, systematic protocol when trialling and setting mould tools, the most effective and optimum clamping force for a mould tool/ material combination can be effectively determined. This improved understanding of the actual clamping force required can result in component cost reduction and increased fl exibility in planning production schedules. For instance, it may reveal that a smaller moulding machine can be used. Transfer of a mould tool from one machine to another will be discussed in a later article as size and geometry of the mould tool are not the only factors to be considered; screw size and other factors also need to be taken into account.
About the author: John Goff is a chartered engineer, a Fellow of IoM3 (Institute of Materials, Mining and Metallurgy) and manag- ing director of injection moulding process consultancy G&A Moulding Technology (
www.gandamoulding.co.uk). This is the 24th instalment 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 Injection World for free here.
www.injectionworld.com
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