NetComposites
Juniar Spraybooths

Locking in Repeatable Accuracy with Light RTM Moulds

04 October 2004

Plastech's M24 clamp/dowels bring further technological advances to LRTM.

The need for accurate location on the x and y axis of LRTM matched moulds is often overlooked when building production tooling. It can be argued quite correctly that no location device is necessary in circumstances where a mould with a deep draw cavity is sufficiently deep to provide accurate x, y location. This is due to the voluminous style of fibre employed, and when loaded between the two matched mould halves it creates an equal pressure that is exerted globally. This positions the mould faces in equilibrium providing a natural positional accuracy.

This argument is not true however if the mould’s draft is not equal on all sides, or in another case where it has very little depth of draft. For example: A truck hood shape would have three sides of reasonable draft depth, but the fourth side would be ‘open’. Equally a flat door skin mould would have almost no depth of draft. In the truck hood example, when closing the mould there would always be the likelihood of the back half of the mould sliding out of true registration towards the ‘open’ edge, as there would be little or no fibre resistance to prevent it from doing so.

With the flat door skin example, by the very fact that the mould has little or no draft, the mould could easily be closed out of line, thus creating the prospect of unpredictable peripheral resin fill characteristics.

To overcome the possibility of these scenarios it is wise to design the mould with location pins more commonly known as mould-tool dowel locators to avoid such inaccuracies. We see a variety of such devices and designs for mould location - some are built into the mould flange in the form of moulded, shaped cones, whilst others prefer flange corner angle protrusions locating on the corresponding contra mould half.

Plastech have for many years been manufacturing the more accurate, heavier duty, steel framed RTM moulds and many applications have demanded repeatable accuracy able to hold better than 0.1mm tool location. With this background it has been proven that some of this technology has now found a place in the manufacture of LRTM moulds. The use of Plastech’s proprietary M24 clamp/dowel station is found to be the most cost effective method to not only locate precisely the matched moulds, but also to provide a very effective and fast preinjection clamp fixing.

Unlike RTM moulds that required the clamp/dowel locators positioned every 600mm around the mould edge, it has been found that one set every 3 - 4 metres of mould edge is sufficient. As larger LRTM mould designs are fitted with a light steel edge frame it is a simple task to fit the heavy duty clamp/dowel sets to this frame at the appropriate locations.

As an example of this technique a mould with a peripheral flange length of 7 metres has proved that just two clamp dowels are sufficient to provide both a high x, y location accuracy, and the initial clamping force required before flange vacuum is applied, without the need to fit further latch clamps to the remaining flange edges.

To further provide a service for LRTM mould builders, Plastech now offer the CNC machined M24 clamp/dowel sets at a reduced price, with this proven piece of equipment now costing no more than the simpler latch clamp, which although giving a means of clamping, does not provide any form of mould location. At these low purchase prices the M24 clamp/dowels are highly cost effective, bringing further technological advances to LRTM.

The latest edition of RTM today, Autumn 2004 is now available from the RTM today section of the NetComposites site here
Alternatively, click here to download the latest Autumn 2004 newsletter.