Water-injection technology and DuPont™ Zytel® for BMW dipstick guide tube

For producing three-dimensionally bent guide tubes for dipsticks, Schneegans Silicon GmbH, an Austrian supplier to the automotive industry, uses cost-efficient, innovative water-injection moulding together with a grade of DuPont™ Zytel® nylon developed for this technology.

This resin combines optimised flow characteristics with superior surface quality, stiffness and non-warping properties, plus resistance to heat, fuels and lubricants, as required by motor-compartment applications. The guide tube, developed by Schneegans Silicon in co-operation with DuPont, moulding-machine maker Engel and tool-maker Kendrion Werkzeugtechnik, is used in BMW’s 2-litre diesel engines.
The processing characteristics of this type of Zytel® ensure that the guide tube has the same wall thickness over its entire length and that warpage and dimensional deviations remain within strictly defined tolerances. Its surface quality, both interior and exterior, meets high requirements, so that the dipstick can be withdrawn and reinserted easily, and the tube’s exterior needs no additional finishing.

Commenting on the use of water-injection technology, Rudolf Gattringer, head of the process and tooling technology section at Schneegans, says: “Conventional dipstick guide tubes of steel need costly finishing steps as well as additional clips and screw-on parts to hold them in place in the engine compartment. This makes manufacturing expensive and the part is relatively heavy. Guide tubes of this kind, which are bent in three dimensions, could not be economically produced until now, because the cores could not be withdrawn by conventional means. But now that water-injection moulding is available for long production runs, together with matching machines and suitable resins, we have made a real breakthrough.”

“The reinforced Zytel® nylon 66 DuPont has developed for this technology meets all the processing requirements and the tough quality criteria which BMW sets for its components” Gattringer continues. “DuPont also helped us to optimise processing parameters. Our new all-plastics guide tubes are substantially lighter than conventional steel ones; we meet very close production tolerances and show a cost advantage over steel. Moreover, as these tubes do not corrode even after years of use, the O-rings will not jam in the entry funnel.”

Background information on water-injection technology (W-I-T)

In water-injection technology (W-I-T) the tool is first filled with melt, as in conventional injection-moulding. As soon as the melt on the tool walls has cooled sufficiently and starts to solidify, a valve at the end of the cavity is opened and water pressure is applied on the feed side. The water forces the still-molten plastic out of the part’s core into a secondary cavity located behind the valve, thereby forming the required hollow space in the moulded part. W-I-T technology’s main advantages are that it can produce long, three-dimensionally shaped hollow bodies without using cores, and it yields a big productivity gain, because the injected water provides additional cooling for the plastic; this makes shorter cycle times possible, i.e. you make more parts per unit of time.
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