Design guide
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Studying guides and sliders Clive Maier of Econology takes a closer look at the outsert moulding design process So far in this Design Guide series on outsert moulding, we have covered the basic principles and the way in which designs are built up by variations on key components that are combined on a base plate. We have discussed many of these key components and this month it is the turn of guides and sliders. Outsert mouldings are frequently used for mechanical and electro-mechanical assemblies and there is often a need to transfer motion from one point to another; for example, from a front panel to a switch. This can be accomplished by a mechanical push rod or slider but it needs to be accurately guided and supported. Fortunately, this is easy to arrange in
outsert moulding. There are two main variants – the slider can travel parallel or perpendicular to the base plate. In either design, the slider may be of metal or plastics construction. A metal slider is likely to be ribbed or U-shaped for stiffness, so the guide surfaces will have to be designed to conform. A plastics slider often works better if it is made of a different material to the guides. Acetal is the most commonly used material for outsert moulding, and therefore for guides. Sliders of polybutylene terephthalate (PBT) work well with these. Parallel sliders can be supported by opposed pairs of guides that provide horizontal
Parallel sliders can be supported by pairs of guides
and vertical bearing surfaces. Of course, it is necessary to arrange a small running clearance on the vertical surfaces. As always in outsert moulding, we have to be aware of problems that can arise because the moulded parts shrink while the base plate does not. For this reason, the vertical bearing surfaces on the guides should coincide with an anchor point. This ensures they remain fixed relative to the base plate, and therefore to each other. It is possible to have a one-piece guide that is recessed in the top to accept the slider, but this should only be done if the overall length of the guide can be kept short, to avoid shrinkage and distortion problems.
Perpendicular sliders have only one guide
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A number of pairs of guides can be arranged to give better support and alignment to a long slider, and it is also possible to provide snap-fits to hold the slider in a running clearance against the horizontal bearing surfaces. Perpendicular sliders work at right angles to the base plate and the simplest guide design takes the form of a frame, with a central hole that provides the bearing surfaces. However, the frame will shrink slightly away from the base plate, so we sacrifice some positional accuracy. If positioning is crucial, the frame design should be extended to include three anchors, in the style of an outsert boss. Another problem with perpendicular sliders is the bearing
Designer’s notes ■ Plastics sliders run best against dissimilar materials ■ Design parallel slider guide bearing faces to coincide with anchors ■ Perpendicular sliders are more difficult to position and align
length is limited, and it is usually not possible to provide more than one guide. The situation can be improved by extending the guide on a convenient side of the base plate. However, the need for moulding draft on the guide hole limits the value of this idea. Clive Maier, Econology
Improved guide for perpendicular slider
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