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Hotrunner mould enhances efficiency for diving mask production

Сase studies
1

The market for stylish sporting goods is a price sensitive one. An efficient production is decisive to profitably and successfully compete here with a product manufactured in Western Europe. For the production of the frame parts for a new diving mask from Tribord, a private brand of the Decathlon Group, the Italian injection moulder AR Group Plastic Division trusts in a new direct side gating hotrunner mould designed by mould maker Linea Stampi.

AR Group Plastic Division is a onestop supplier to a variety of important customers from different industrial sectors. Almost all components of the Tribord diving mask such as frame parts, lenses and the liquid silicone rubber skirts are produced, assembled and packed in the Sulbiate plant in Northern Italy. The strap is the only purchased part. Since the new diving mask is positioned in the lower price range the challenge was to establish a profitable production and at the same time to fulfill the high customer requirements. Here first of all an attractive, upmarket outer appearance is important. The polycarbonate frame comes in transparent or opaque high gloss finish in the up-to-date trend colours which are changed for every season. Flaws like minor air entrapments or flowmarks are not accepted. Furthermore, Tribord demands a high mechanical stability which has to be verified by load tests as well as a flexible just-in-time supply of the quantities and colours needed.

2

The finished frame segments
are picked up by a handling system.

During the project planning stage all production steps were analysed with regard to efficiency and potential cost saving. Experiences with another diving mask version already produced inhouse as well proved to be helpful here. “The end assembly of the soft and hard components of a diving mask requires many manual operation steps”, Fabrizio Gianni, project manager at AR Group Plastic Division explains. “This does not leave much room for reducing costs. So we had to increase the degree of automation as well as the process efficiency for the production of the thermoplastic and liquid silicone parts. Our aims for the frame segments were to produce them with an utmost small scrap rate and short colour change times. At the same time it was important to avoid subsequent work steps and to operate the machine with a minimum of staff.”

The frame assembly is made of Lexan 123R and consists of an upper and a lower segment with different shot weights of 9.5 g and 11.1 g. During end assembly these are irreversibly connected by plugging them together. Since the segments are always required in pairs and considering the medium quantities needed Plastic Division and mould maker Linea Stampi decided to build a family mould. “The other diving mask is produced with a family mould as well but it uses coldrunner technology”, says Giuseppe Lissoni, co-owner of Linea Stampi, “for the new mould we decided to make no compromise and to build a full hotrunner solution. Otherwise it would have been not possible to fulfill the requirements.”

3

The Tribord diving mask comes in
a variety of trendy colours changing every season.

Of course designing a hotrunner family mould is much more complex regarding mould stability and filling of parts. Thus, Linea Stampi decided to involve hotrunner supplier EWIKON already in the early design stage to ensure the best possible integration of the hotrunner components and to define the optimum flow channel layout. Of course, for even filling of the parts the gating points had to be placed in the middle of each segment. However functional and visible surfaces in that area made it difficult to find an appropriate position. EWIKON recommended direct side gating with HPS III-MH nozzles onto an easy-toreach position at the backside of the frame segments. “Till then we were quite sceptical about direct side gating”, Giuseppe Lissoni remembers, “especially as this technology has very seldom been recommended by other hotrunner suppliers. But since we had recently successfully used the HPS III-MH nozzle in another application for a different customer and EWIKON was convinced about the potential even for a difficult-to-process material like polycarbonate we decided to go on with this concept.” Two radial HPS III-MH nozzles with one heat conductive tip insert each in lengths of 135 mm and 165 mm were used. The difference in length is explained by the position of the frame segments in the mould and the special geometry of the lower frame segment where the gating point is placed on the curved nose bridge. Considering the shot weights and the high-viscosity resin EWIKON decided to use the HPS III-MH200 version which is suitable for larger shot volumes. The nozzles are fed by a 2-drop linear manifold. It features the EWIKON element technology with streamlined flow channels without sharp corners or dead spots enabling very quick colour changes and allowing the problem-free processing of sensitive materials.

4

The hotrunner system features two HPS III-MH200 nozzles of different lengths with one tip each.

Since the shot weights differ by 15 % special attention was paid to the balancing respectively the filling behaviour of the system. The high demands regarding the part quality do not allow a high scrap rate neither caused by incomplete part filling nor by overpacking of the cavity with the lower shot weight which increases the risk for the formation of burrs. A Moldflow analysis conducted by EWIKON showed considerable deviations in filling between the two frame segments. This called for a rheological balancing of the system with the flow channel diameter and the gate diameter as correcting variables. The longer nozzle which gates the lower frame segment with the higher shot weight got a flow channel of 10 mm which is the same as used in the manifold. In the shorter nozzle the flow channel is reduced to 6 mm. At the same time the gate diameter of the shorter nozzle is reduced by 0.2 mm. These measures resulted in a perfectly balanced flow channel layout which allows an even filling of both frame segments.

5

View on the mould's ejector side with partially filled parts showing the good system balance.

The very compact hotrunner mould started production in spring 2014 and has been working reliably with a scrap rate close to zero. The frame segments are picked out of the mould by a handling system and put onto a conveyor belt which feeds them into separate transport boxes to be moved to the end assembly area. Compared to the diving mask version which is produced with coldrunner technology it was possible to increase the efficiency in several aspects. It is no longer necessary to remove the frame segments from the sprue by hand and – as often required – to trim and clean the gating points before finally assembling the parts. Further advantages are the material savings of 25 % and the higher efficiency of use of the operating staff. Thanks to the higher degree of automation regarding the parts handling the efforts for machine operation and supervision could be reduced by 40 %.

 

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