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News / Innovations
DESMA paper at SKZ on the advantage of cold runner technology when processing silicone elastomers.
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In March 2009 a conference took place in W¨rzburg on "Silicone elastomers - a versatile and unusual material." Organiser was the Süddeutsche Kunststoffzentrum (SKZ), the event was moderated by Dr. Udo Wachtler, of Wacker Chemie AG. Over 80 participants used this opportunity to update themselves on the current status of the entire silicone elastomer value chain.
The agenda covered by the conference was very broad:
- Silicone rubber - challenges for the future
- Silicone rubber - production and characteristics of a high performance elastomer
- Comparing key high performance elastomers with silicone rubber
- Thermoplastic elastomers compared to silicone vulcanised products
- Silicone elastomers in car building - new application possibilities and trends
- Silicone elastomers for LED applications
- Fantastic - Liquid - Fluoro: LSR - the new dimension
- Demoulding silicone rubber - simple, flexible, precise
- Silicone foams - a class of materials for high performance applications
- A fully electric dosing unit - Only a complement to electric injection moulding machines?
- 2-component injection moulding in solid and liquid silicones
- Tool concepts and automation in LSR and 2-component injection moulding
- Economic 2-component production with LSR
- Silicone production and handling possibilities in the injection moulding process
DESMA's paper, entitled "Possibilities in silicone processing in the injection moulding process" provided the audience with information on practical examples on the economical use of cold runner technology, on suitable tools and peripheral equipment in injection moulding of liquid and pasty silicones.
The equipment needed to process high temperature vulcanising silicones (HTV) is in principle the same as for plants used to process more standard elastomers. Because of the low strength of pasty silicones, hydraulic stuffing systems are attached to the injection unit. The stuffing systems allow the silicone compound to be pressurised by a piston and flow through an adapter to the screw. The injector unit (with liquid temperature control) is fitted with an open temperature controlled nozzle or needle nozzle. When using cold runners, an open nozzle is preferred. If the machine is to be used for processing rubber and silicone, then a screw and drive is paramount.
Liquid silicone rubber (LSR) comprises two components which are vulcanised by addition-vulcanisation to form an elastic material. Elastomers are generally transported in barrels of 20 or 200 litres volume. The components are mixed at a static mixer to which the two viscous components are pumped through pipes by a metering pump. The components are mixed in the static mixer and transferred to the injection unit. A spring loaded non-return valve reliably prevents the creation of excessive pressures in the dosing system during the injection phase. A plastifying screw is not required. The nozzle does, however, have to be of needle design.
Tools used to handle silicones do not differ greatly in their design to those used in rubber processing.
Because of the lower viscosity of silicones, the precision of fit needs to be higher. The mould cavities must be hardened in order to ensure a long mould lifetime. In addition, demoulding forces can be positively influenced by tool hardness. Because of the high affinity of silicone to oxygen, ventilation design is particularly important.
The use of cold runners in silicone processing is recommended in most cases due to the high cost of materials. FlowControl needle cold runner technology enables for example the sprueless injection moulding of several exterior terminals within one cycle, thanks to the direct injection of a single cavity. The waste fraction is less than 1.4 % of product volume, due to the need to include flow channels. These are used to connect the vacuum system with the cavity.
Sprue demoulding is waived thanks to direct injection using FlowControl cold runner technology. Expenditure for automated production is limited to product demoulding with simultaneous shortening of cycle times.
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