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September 8, 1998

4 Min Read
The Principles of Design:Designing To Avoid Weld Lines

Editor's note: This is the third in a series of articles describing solid solutions to plastics design challenges from Professor Gunter Erhard, formerly with BASF AG, Ludwigshafen, Germany, in thermoplastic applications engineering. Professor Erhard is now an industry consultant, a frequent seminar instructor, and a lecturer at the University of Karlsruhe, Germany.

In the injection moulding of thermoplastics, weld lines are formed when the fronts of two or more melt flows meet up inside the mould. This is inevitable when a component is gated at several points or when the melt has to flow around an opening in the moulding made by a core in the mould. Weld lines can also occur, however, when there are significant differences in wall thickness and dissimilar flow path lengths in an individual moulding. Weld lines are essentially weld seams, which can impair the mechanical and optical properties of the component.

In the weld line area, the orientation of the molecules differs appreciably from that in neighbouring areas and with fiber-reinforced thermoplastics in particular, this can lead to directionally-dependent changes in properties. The change in orientation means that these lines are visible to a greater or lesser extent as unsightly lines on the surface of the moulding, especially with coloured plastics. The mechanical properties are impaired because the melt fronts cannot fuse together adequately when they meet (see Table I). When melt flows hit each other head on, notches also form on the surface of the moulded part in the area of the weld line and these, too, impair the strength properties.

Geometry and flow-related weld lines are basically unavoidable, although their effects can be lessened by modifying the process conditions, altering the mould design or implementing design measures.

Process Conditions

The higher the temperature of the melt fronts when they meet, the greater the strength of the weld line and the better the quality of the surface. The influence of the melt temperature is particularly significant with materials of an amorphous structure. With semicrystalline and glass-fiber-reinforced thermoplastics, the melt temperature, mould surface temperature, and injection rate have no significant influence on the strength of the weld line in the range generally employed in practice.

Mould Design

The position of the weld line is fixed by the positioning of the gate. A computer simulation of the mould filling pattern will provide information on the expected position of a weld line (Figure 1), so that this can potentially be shifted to a noncritical area during the mould construction phase by varying the position of the gate. The mould filling process can also be controlled in such a way that weld lines are moved or even avoided completely. Two processes are available for this, depending on the geometry of the moulding:

  • For components with a large surface area that have to be filled via several gates, sequential injection moulding is the best-suited process. This involves a hot runner in which the needle-valve nozzles can be opened and closed in such a way that the weld lines are moved to non-critical places.

  • For long mouldings, which need several gates in a row, the cascade injection moulding method has proved successful. The injection process begins with the middle hot runner nozzle, and only when the melt front has reached or flowed past the neighbouring nozzles, are they opened. This procedure is carried out step by step through to the end pair of nozzles. The cascade can also be started at one end of the moulding (Figure 2). This then ensures that no weld line occurs.

For both processes, it is essential to use numerical calculation methods to determine precisely the mould-filling process and the required amount of pressure so that the nozzles can be positioned at the correct point.

The melt flow can also be controlled with a flow promoter or flow brake to move the weld line away from a critical point. A flow promoter is an enlargement in the wall thickness which ensures that the melt can flow easier and faster at that point. A flow brake is a narrowing of the cross section so that the melt flow is slowed down. Figure 3 shows a flow brake of this kind in the upper bar of the opening. As a result, the weld line is shifted away from the thin, risky point behind the opening and forms at the side.

An unsightly weld line can be masked by a suitable surface structure or, in the case of decorative articles, by inserting a printed film into the mould.

Design Measures

The most simple design modification for eliminating the weakening effect of a weld line is to increase the wall thickness in the weld line zone (Figure 4). This principle has been implemented on a complex component in the case of the spring-loaded cogwheel shown in Figure 5, for instance. The wheel has to be gated at three places on the hub and three places on the toothed wheel rim. Weld lines inevitably form in the thin arms of the springs. The critical cross section is reinforced by making it thicker in these areas; at the same time, these thicker areas can be gripped by the ejector to ensure safe demoulding of the thin spring arms.

It is thus possible to adapt the design to comply with the requirements imposed on mould engineering or with the loading that acts on the component.

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