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Modelling the ejection friction in injection moulding
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Abstract(s)
The quality of parts produced by injection moulding may be affected during the
ejection stage of the moulding cycle. At this stage the parts are mechanically
forced to separate from the moulding surfaces. The ejection force depends on
the shrinkage of the polymer onto the core and on the friction properties of the
contacting surfaces at the moment of extraction. As during moulding there is a
replication of the part on the mould surface the ejection process is also
dependent on the plastic deformation of the moulded material. Ejection takes
place in a very short time, hence the static coefficient of friction must be
considered for modelling the ejection process.
To understand the contribution of the mechanisms involved in friction during
the ejection stage, a mixed approach was developed: analytical simulation for
the ploughing component, numerical simulation for the deformation
mechanism, and an experimental inference for the adhesion. The study was
based on the observation of three materials that are commonly used in injection
mouldings: polypropylene, polycarbonate and a blend of polycarbonate/acrylonitrile-
butadiene. The friction behaviour was studied with two testing methods:
a prototype tester that is fitted to a universal testing machine, and an
instrumented mould for the characterization of the friction force.
The relevance of roughness, temperature and contact pressure on friction was
evidenced, on the actual value of the static coefficient of friction that applies in
the demoulding of thermoplastic mouldings.
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Pedagogical Context
Citation
Correia, M.S., – Modelling the ejection friction in injection moulding, Guimarães: Universidade do Minho, 2013. Ph.D. thesis. Co-orientation: A.S. Pouzada; A.S. Miranda, Universidade do Minho; C.A. Capela, Instituto Politécnico de Leiria, Portugal.