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Opening new doors - innovative door latch features an all-plastic operating mechanism

Distrupol : 28 May, 2003  (Application Story)
A combination of accurate materials selection and design expertise enables Distrupol to be a key partner in the development of a new style of door latch with an all-plastics mechanism
UK architectural ironmonger Allgood is introducing the building industry’s first door latch featuring an all-plastic operating mechanism. The latch relies exclusively on four grades of engineering polymers, manufactured by DuPont and distributed in the UK by Distrupol. It uses three Zytel nylon resins and one Delrin acetal resin, with only the outer casing being made of metal. Lutterworth based Dudley Associates produced the mouldings for the latch.

Features of the new style of door latch and its all-plastic mechanism include easy installation – with the contractor needing no more than a drill and screwdriver – and no loss of efficiency if a door’s usual, inward-opening action is reversed. The latch meets all relevant European and US building industry standards.

The Leicester-based design consultancy of Atkinson Design has been a partner with Allgood, Dudley Associates and Distrupol in the two-year project to develop the door latch. All four have been involved from the very early stages of the development, with John Brenchley from Distrupol's application development team confirming the initial material selections of Dudley Associates’ sales director Peter Wright, and advising Atkinson Design’s Jon Law on specific design considerations for the final material grades that he specified.

“We have a long-standing relationship with Distrupol,” says Peter Wright, “and always appreciate their rapid and informed comments on all aspects of material selection and component design.”

The three nylon components of the latch are the multiplier, tendon and the latch itself. The multiplier needs to be a tough, lubricated material with good wear resistance when used in conjunction with Delrin. The specified grade is a 30% glass reinforced compound, Zytel 70G30 HSL, coloured red.

Strength and shock absorbency are the key mechanical properties demanded of the door latch tendon, making it a natural application for the recognised market leader in toughened nylons, Zytel ST (Super Tough). The grade being used is Zytel ST801.

All the door latch components are contained within the door, apart from the latch itself. Therefore, it is vital that the latch has excellent flame retardant characteristics so that the door maintains its integrity and performance characteristics for as long as possible in a fire. This is achieved in practice by moulding the latch in Zytel 72G25 VO, a 25% glass reinforced, black compound that meets the UL94 V0 flame retardancy standard.

When a door fitted with the new Allgood latch assembly was subjected to an EN1634 fire test for 60 minutes the Zytel latch component held the door closed and did not flame. A flame duration of more than 3 seconds is considered a failure. At the end of the test the 60 minute fire door had almost burnt through but the Zytel latch was still there almost unaffected.

The three acetal components – the tendon retainer, the follower and rack and pinion – all use the same grade of acetal resin, Delrin 111P, in white. This resin provides the correct combination of strength and stiffness coupled with low wear that the functionality of the parts demands.

Distrupol and Dudley Associates have been partners on a range of high tolerance, development projects where rapid prototyping, accurate polymer specification, short lead times and quick materials supply have been integral to their success.

The Allgood door latch is an excellent example of how the two companies have worked together from the outset of a project to achieve the ultimate goal in the shortest possible time. And their collaboration on the door latch may not be finished. They are in discussions currently with Allgood and Atkinson Design on the possibility of contributing to the design and prototype tooling for the outer metal casing of the door latch to be replaced by a suitable grade of Zytel nylon resin.

Allgood plc is the UK's leading architectural ironmonger in the supply of products for commercial and public buildings. The company specialises in unique design and high performance products. Allgood is a holder of certification to BS ISO EN 9001.2000 quality systems. Its products are tested by UKAS approved test houses to meet the latest EN and BS product performance and fire test standards. Many items are also UL listed and conform to American (ANSI) standards.

Based in Lutterworth, Leicestershire, Dudley Associates was formed in 1995 as a one-stop-shop to specifically meet the demand for high quality, technical injection moulded parts on the shortest possible lead times. It has comprehensive design, tooling and moulding facilities, and its use of special aluminium alloys for the tooling allows production of components in engineering thermoplastics to very high tolerances. Customers are mainly blue chip, first- and second-tier suppliers to the medical, automotive, defence, electronics and telecommunications industries. For more information

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