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processing feature | Moulding masterclass


extended time period. Too high a clamping force can accommodate overpacking and overfilling of the mould cavity, resulting in overstressed parts. This may typically show as cracking of components and/or breakages of bosses, location spigots and poor output yield after decoration. Another common problem is the presence of air/gas within the mould. Upon application of clamping force, air is contained within the sprue, runner and gate system (as well as in the cavity) in a cold runner mould tool, or only in the cavity within a hot runner mould tool. This needs to be able to escape as the cavity is filled by molten plastic. The most common technique to achieve this is to use vents strategically positioned in and around the cavity to ensure effective escape of air. A lack of good venting often results in burn marks (caused by ignition of trapped volatiles under compression), short shots or bubbles in the moulding. Volatiles ignition attacks the surface of the metal cavity and core to cause erosion. Such erosion is more prevalent with mould tools made of pre-toughened steel or soft metals. Selection of too high a clamping force reduces the


effectiveness of air removal. The size and depth of venting on a mould tool is important as it allows air/ volatiles to freely escape from the cavity at the extremi- ties of the material flow. Too deep a vent can result in flash occurring on the moulding, so caution should be employed during mould tool manufacture to prevent the need for rebuilding or welding. The use of too high a clamping force can often cause excessive compression that, over a period of time, reduces the initial vent depths and prevents a free unrestrictive passage for the air to escape. Such compression, therefore, will cause damage to the mould tool through erosion due to air/volatiles being trapped and “crushing” of the venting due to the high compressive forces being applied to the vent areas. A simple and short term solution adopted by many


moulders is to reduce the velocity at which the molten material enters the cavity by reducing the injection speed, which also increases the injection time. This speed reduction reduces the likelihood of combustion, eliminating black burn marks. The potential problem is that it also induces product variability, as previously explained in discussions on selecting the most effective injection time earlier in this series of articles. Further-


Correction: The third paragraph in the July/August instalment of Moulding Masterclass referring to measurement of clamping force was misprinted and may have confused some readers. It should have read: “The clamping force is usually measured in metric tonnes (imperial tons in the US) or kiloNewtons (kN) and is the amount of force used to keep the mould closed and so oppose the opening force.”


34 INJECTION WORLD | September 2012


more, reducing the injection speed does not eradicate entrapped air/volatiles but can force the pocket of air into the bulk of the moulding. If the moulding is opaque, the presence of air bubbles would not be detected. Porosity in the structure of the moulding could cause failure if an external load was applied to the component during, say, a drop test, or if an impact is sustained in the area of the small pocket of trapped air/gas. High clamping forces also provide the basis for using high holding pressures. In certain circumstances, these are necessary to achieve correct dimensional, visual and functioning of the component. However, the higher the compression, the greater the inherent stress and the risk of component failure. As stated earlier in this series of articles, the lower the holding pressure employed the more stable and effective the resultant moulding. Values of less than 20% of injection pressure, in particular, need interrogating to ensure sufficient pressure within the cavity. High holding pressures are sometimes used for


components possessing a textured or etched surface (VDI or grained) to ensure a consistent surface - the pressure forces the frozen layer into the resident grained structure within the cavity. Too high a holding pressure, however, can and will erode the etched surface through friction, causing the depth of the VDI or grained finish to reduce over a period of cycles (typically 200,000 to 1,000,000 cycles). This will necessitate the mould tool surfaces being re-textured/refurbished at some point. The time span for this to occur is often wholly dependent upon the holding pressure value used. The use of too high a clamping force can also result


in wasted energy. The application of clamping force by the injection moulding machine needs both electrical and physical energy. Savings in power consumption of up to 25% can be made, especially when operating larger injection moulding machines of 800 metric tonnes and above, by using the most effective clamping force. This discussion of clamping forces will be continued in the next Moulding Masterclass instalment.


About the author: John Goff is a chartered engineer, a Fellow of IoM3 (Institute of Materials, Mining and Metallurgy) and managing director of injection moulding process consultancy G&A Moulding Technology (www.gandamoulding.co.uk). This is the 23rd 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 wish to be sure you don’t miss the next


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


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