An empirical study of gas penetration in full-shot gas-assisted injection moulded parts

Abstract

Full-shot gas-assisted injection moulding has increasingly become one of the most important methods used to produce plastic components. It has the advantage of eliminating the switchover mark, which usually occurs on the surface of short-shot gas-assisted injection moulded parts. This paper is devoted to an investigation of the eVects of diVerent processing parameters on the length of gas penetration in full-shot gas-assisted injection moulded parts. The ®rst part of this report shows how the gas penetration of moulded parts is optimized. An L 0 18 experimental matrix design based on the Taguchi method was conducted to investigate the processing factors that aVect the length of gas penetration in full-shot moulded parts. The second part of this paper identi®es the relative signi®cance of each processing parameter on the gas penetration of moulded products. The materials used were general-purpose polystyrene and polypropylene. Experiments were carried out on an 80 ton injection-moulding machine equipped with a high-pressure nitrogen gas injection unit. For the factors selected in the main experiments, melt temperature, gas injection delay time and gas hold time were found to be the key processing parameters aVecting the length of gas penetration in full-shot gas-assisted injection moulded parts. In addition, the sink mark of full-shot moulded parts decreases with the length of gas penetration. Bending strength of full-shot gas-assisted injection moulded parts is higher than that of short-shot moulded parts.

Simulation of the gas-assisted injection moulding process using a viscoelastic extension to the Cross-WLF viscosity model

“…where T is the current temperature, T S a reference temperature, and c 1 and c 2 the empirically determined constants. CÀWLF can be used as the polymer viscosity ( p ) in equation (3). This model allows the correct steady-state shear viscosities to be used for current temperature and shear conditions, but does not incorporate viscoelastic effects such as progressive stress rise under an applied steady flow or stress relaxation.…”

“…trace s ð Þ ¼ 0 because trace L ð Þ ¼ 0 in incompressible flow). Upon obtaining this polymer viscosity, p , the concentration-dependent viscosity, , can then be calculated using equation (3) and used directly in equation (4). Noting that the right-hand side of equation (4) is the divergence of the total (Cauchy) stress tensor, :, and combining equations (3), (4), and (10) allow the total stress tensor to be written explicitly in terms of " as…”

“…Structural rigidity and dimensional stability are also good compared with conventional injection moulding (CIM) [1]. Tensile strength has been shown to benefit from low gas pressures (GPs) [2] and the factors affecting gas penetration have been analysed [3]. Many attractive features of GAIM in polymer processing has led to the development of GAIM for rubber processing [4].…”