Moulding masterclass | processing feature
Making clamping force effective In the second part of his discussion
The use of an incorrect clamping force – whether too low or too high – has a detrimental impact on the quality of injection moulded parts, making it important to give its selection proper consideration during process setting and optimisation. The use of too low a clamping force results in fl ash
on the resultant moulding. From a mould tooling viewpoint the presence of fl ash should be avoided at all cost, especially if the mould cavities/cores are produced from a pre-toughened steel or aluminium. The pres- ence of fl ash will cause the edge at the split line of the mould tool to become blunt and lose its integrity/ sharpness so the edge of the moulding becomes less defi ned. This problem increases with each occurrence. A common practice within the injection moulding
industry is to reduce the clamping force value until slight fl ashing occurs on the moulding and then to increase the value again until fl ashing disappears. However, this is not recommended as it will often reduce the life of the mould tool. The surface hardness of the shut-out faces of the
mould tool is a critical consideration during design and manufacture. The selection of a clamping force which is suffi cient to prevent fl ashing or visually poor mouldings but which may be inadequate for consistent component
www.injectionworld.com
on effective clamping force, moulding expert John Goff explains how incorrect selection impacts on part quality
manufacture is not always noticed in the initial stages of a production run. Sometimes it is only during or upon completion that dimensional problems, visual issues (surface imperfections, ripples, gloss, surface fl atness and non-uniformity of surface texture) and overall weight changes are noticed. By this time, machine production hours have been wasted, material used and labour expended in investigating the problems, resulting in additional costs and loss of productivity. However, selecting too high a clamping force (often
more typical) can also cause problems as it can mask the extent of inherent variation within an unstable moulding process. This usually results in inconsistent part quality, which may often go undetected until parts fail in service or have been running in production for an
September 2012 | INJECTION WORLD 33
PHOTO: SIPA
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