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Mould project for the mass production of disposable syringes

Сase studies

When producing disposable syringes highest precision in mould making is required to guarantee the perfect function of the finished product. Werkzeugbau Ruhla GmbH in Seebach, Germany now has successfully entered this demanding business segment by building 10 full hotrunner multi-cavity moulds for disposable syringes with filling volumes from 1 ml up to 20 ml. The close cooperation with EWIKON ensured that during this project mould making and hotrunner expertise complemented each other in an ideal way.

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24-drop half with HPS III-MH nozzles in radial version for the barrel of the 1 ml syringe.

The customer runs a modern, automized production plant for medical components in Russia and is a supplier to the Russian Federation as well as to the international market. The new mould project was started to further increase the productivity and the efficiency when producing disposable syringes with filling volumes of 1 ml, 2 ml, 5 ml, 10 ml and 20 ml according to the medical DIN EN ISO 7886 standard. The total production capacity is 375 millions of syringes per year. When working with the Werkzeugbau Ruhla GmbH the customer benefits from the company's long-term expertise in the production of injection moulds for medical applications as well as from the profound knowledge of the Eastern European markets with a considerable sales volume there.

However, designing and building moulds for syringes was a new challenge for the company. "Moulds for the syringe production have their own special requirements", explains Udo Köllner, technical director at Ruhla, "they must be built with particularly high dimensional precision with special focus on optimal cooling of the mould inserts as well as on even filling of the cavities. The latter is important to eliminate deflection of the inner cores by the melt pressure and thus to avoid ovality caused by uneven wall thickness of the syringe barrel. All this especially applies to two-piece syringes as produced with the new moulds."

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Internal pre-production test of the 32-drop mould for the plunger of the 10 ml syringe (right). 8 linear HPS III-MH nozzles in linear version are arranged in a row.

Two-piece syringes do not feature an additional plunger seal made of a soft component which due to elastic deformation can compensate inaccuracies in production. "Even dimensional deviations of 2/100 of a millimetre are enough to cause problems with leak tightness and the required force to push the plunger and thus can affect the functionality to such an extent that the finished syringe is reject“, says Köllner, „for this reason we have invested in a special measuring and testing machine especially for this project. Now we can check these parameters inhouse during the pre-production tests and refinish mould components when necessary."

Considering the high production volume direct gating with a full hotrunner solution was the method of choice to optimise cycle times, save material and avoid subsequent work steps. Each syringe size requires two moulds, one for the PP barrel and one for the plunger which is made of PE. The mould design is based on the planned production volume for the respective version. This resulted in 48 cavities for the most frequently used 2 ml and 5 ml versions, 32 cavities for the 10 ml version, 24 cavities for the 1 ml version and 16 cavities for the 20 ml version. Barrel and plunger are both gated from the side in a 90° angle to the demoulding direction. The shear action during demoulding is used to achieve a perfect surface quality without gate marks which, for several reasons, are not acceptable for the medical industry.

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A special testing machine checks the proper function of the finished syringes during the pre-production tests.

Considering the long-term successful cooperation in side gating Ruhla decided to go for EWIKON hotrunner technology. "When we have to realise a project of this scale within a very tight time frame we cannot afford to take any risk when it comes to technical solutions.

So we have to fully trust in our supplier", says Köllner, "after all we are talking of total mould costs of more than 2 million Euros with a share of more than 500.000 Euros for the hotrunner technology. And up to now our experiences with EWIKON side gating solutions have been positive without exception." EWIKON supplied the hotrunner technology for all moulds as a complete package consisting of the hot half and the matching touch screen control system. For the production of barrels and plungers different radial and linear versions of the HPS III-MH side gating nozzles are used. The syringe barrels are positioned in the mould in demoulding direction and are arranged in compact circles, each one with a radial nozzle in the centre.

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Final assembly of moulds at Ruhla. In the foreground: mould for the 10 ml syringe barrel.

Depending on the size four or eight barrels are gated simultaneously with one nozzle. The heat conductive tips are angled by 60°. Thus, the gating point can be positioned as closely as possible to the parting line and the mounting of the inner core so that core displacement caused by the melt pressure is minimised. In the moulds for the plungers the parts lie in the parting line at right angle to the demoulding direction. A slide mechanism is integrated to prevent the parting line from running across the sealing front plate of the plunger and therefore to eliminate the risk of flashing in this sensitive area. All plunger moulds feature linear nozzles. This design allows to arrange the plungers in a row and enables an easy and cost-effective integration of the slide mechanism. The gating point is placed on the plunger head. On the manifold side all moulds are equipped with the HPS III-T technology which uses streamlined direction elements for a fully balanced melt distribution to the nozzles. Since the flow channel layout inside the nozzles is balanced as well, an even filling of all cavities is guaranteed.

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Easy-to-maintain solution. Due to a plug connection the nozzle plate including the complete wiring can be removed easily.

During each stage of the mould project Ruhla and EWIKON worked in close cooperation with the customer. Moldflow analyses for each version of the syringe were conducted in order to check and if necessary optimise the article design with regard to the injection moulding process. Important parameters were the wall thicknesses of the barrels as well as the design of the plungers where necks in the rear area of the plunger rod help to save material. Furthermore, when starting the project 8-drop test moulds for barrels and plungers of the 1 ml and 5 ml version were built within a very short time to determine the optimum dimensions for important mould components. The results were directly incorporated in the design of the production moulds. Thus, the expenditure of time in this project stage could be considerably reduced.

Precision mould making is the core competence of Werkzeugbau Ruhla GmbH. All components are manufactured by using state-of-the-art machinery. One of the highlights is a fully automated manufacturing cell for electrodes which has reduced the production time by 70 % and increased the sinker EDM capacity by 30 %. A further advantage is the highly motivated team. "Many of our employees have worked in the watchmaking industry", Köllner points out, "and thus have an increased awareness of quality and precision. The company really benefits from that." All moulds have a very compact, modular and easy-to-maintain design. "The mould inserts are easily exchangeable if required", explains Udo Köllner, "and the other mould components have been designed with focus on utmost maintainability as well.

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Mould insert for the 1 ml barrel.

The EWIKON side gating systems allow to quickly exchange the tip inserts from the parting line without having to dismantle the mould from the machine. And in case a nozzle itself needs maintenance we have installed them in a separate mould plate which also has the wiring integrated. So the whole nozzle unit can be completely separated from the manifold plate with minimum effort. In general EWIKON could supply a tailored hotrunner solution for each syringe size and the respective mould design." A most compact design for the mould and the hotrunner system was important because the customer plant is equipped with fully electric injection moulding machines. The machine size used only allows a certain mould weight. For this reason Ruhla had to reduce the weight of the larger moulds. This was done by placing additional cutouts in thoroughly calculated positions so that the mould stability was not affected.

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For production of the syringe barrels the HPS III-MH111 nozzle is used. Its particularly slim design leaves enough space in the mould inserts to integrate surrounding cooling close to each cavity.

To guarantee an efficient production with high repeatability Ruhla paid special attention to effective mould venting and cooling. The former is important to ensure proper filling of the cavities especially for the thin walled syringe barrels and thus to avoid reject caused by incomplete filling or air entrapments. In the moulds for the syringe barrels a special cooling concept has been implemented. It allows a cooling layout which surrounds the whole cavity in a close distance although the part is gated from the side. This was feasible because a particularly slim version of the EWIKON HPS III-MH nozzle is used here which has been especially designed for the production of syringes and pipettes. Furthermore, Ruhla managed to integrate cooling for the inner core even for the smallest barrel size. The results were cycle times below ten seconds for all syringe sizes.

After all designs have been approved by the customer the whole project including extensive pre-production tests for each mould in the Ruhla technical centre and checking of the finished parts could be realised in 28 weeks only. When the moulds went into serial production Ruhla and EWIKON application engineers supported the customer on-site to ensure a trouble-free system startup and to conduct final adjustments to the production parameters.

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