Water Assisted Injection Moulding: a Study on the Influence of Melt and Process Parameters on the Residual Wall Thickness and the Occurrence of Part Defects for Pa6.

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  • Topic: Nylon, Polypropylene, Injection molding
  • Pages : 27 (8974 words )
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  • Published : January 22, 2012
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KATHOLIEKE HOGESCHOOL LIMBURG
Departement Industriële Wetenschappen en Technologie

Water Assisted Injection Moulding: A Study on the Influence of Melt and Process Parameters on the Residual Wall Thickness and the Occurrence of Part Defects for PA6.

Gielen Bjorn, Heynickx Lien, Van den Brink Dominique, Vandersteegen Jochen

Water Assisted Injection Moulding: A Study on the Influence of Melt and Process Parameters on the Residual Wall Thickness and the Occurrence of Part Defects for PA6.

Gielen Bjorn, Heynickx Lien, Van den Brink Dominique, Vandersteegen Jochen

Received 7 March 2011
Abstract
Although water assisted injection moulding is a relatively new and advanced technique to produce hollow parts, several problems which reduce the product quality can occur. Namely, water inclusions in the wall, severe fingering and double wall defects reduce the product part stiffness. This study investigated the effect of the process parameters water volume flow rate, water injection delay time and melt temperature on the part defects to obtain a better understanding of the formation of these part defects. The effect on the residual wall thickness and the weight of the products was also investigated. Three materials were used: two polyamides and one polypropylene as a reference. The experiments were carried out on an injection moulding machine equipped with a water-injection unit to produce hollow handles. The parts were weighed before and after drying, their wall thickness was measured and their percentage of defects was calculated in order to determine the influence of the processing parameters. IR images and pressure profiles ware also used to investigate possible detection methods for part defects. It was found that all the investigated parameters have a significant influence on the residual wall thickness and part defects. Their ‘ideal’ settings depend on the material and the desired properties since small wall thicknesses and few part defects do not always correspond. IR images can give an indication for the presence of defects, but the internal structure of the surface can not be examined using this technique. Key words: water assisted injection moulding, polyamide, processing parameters, residual wall thickness, fingering, double wall

1.
1. Introduction
The development of the water assisted injection moulding (WAIM) technique has led to a breakthrough in the manufacturing of hollow or partly hollow plastic parts, which is not possible with the conventional injection moulding process [1] [2]. These hollow parts are characterised by the lighter weight of the products, relatively lower resin costs per part and less shrinkage and warpage compared to conventional moulded products [3]. Several techniques are available for the production of hollow parts. Two of the most important techniques for hollow parts are gas assisted injection moulding (GAIM) and water assisted injection moulding (WAIM). The principle of

water assisted injection moulding is basically similar to gas assisted injection moulding, but the use of water has some interesting advantages over gas assisted injection moulding such as a shorter cycle time and an average smaller residual wall thickness of the products. This is due to the fact that the thermal conductivity and the heat capacity of water are respectively 40 and 4 times greater than those of gas [1], providing a better cooling of the part and therefore a faster production. Water assisted injection moulding is better to mould larger parts with a smaller residual wall thickness. Another advantage of water assisted injection moulding is the absence of the foaming phenomenon in the internal surface which can occur by using gas assisted injection moulding , since water does not dissolve or diffuse in the polymer melt during injection moulding. Disadvantages of water assisted injection moulding are part defects such as fingering and double wall arising in the internal surface. Also, the...
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