INNOVATION | LIQUID SILICONE RUBBER


A single slotted micro-dosing valve moulded by Arburg is made of post-curing-free LSR with a hardness of 50 Shore A weighs 0.038 g


On Arburg’s new LSR micro injection unit, the LSR is premixed by means of a cartridge and injected with an 8mm screw, on a first-in-first-out basis


strength too. Parts return figures of up to 45 N/mm in the non-post cured state (as per ASTM D 624 B). Generally, conventional liquid silicone rubber only achieves such high values after post-curing, Wacker says. For example, a cured rubber part made of Elastosil LR 3003 with a hardness of 50 Shore A has a tear strength of 24 N/mm without post cure. After four hours of post-curing at 200˚ C, the value increases to 30. The same silicone part made of Elastosil LR 5040 with the same hardness, on the other hand, achieves 40 N/mm even without any additional thermal treatment. Thomas Siegel, an LSR expert in Application


Technology Consulting at injection moulding machine maker Arburg, says the requirements for complete system solutions for processing LSRs are becoming more and more important. “Nowadays, customers want not only an injection moulding machine, but complete turnkey solutions,” he says. “Arburg cooperates with several partners and is able to provide customised solutions.” Siegel also points out that LSR components, especially in medical technology, are becoming smaller and more demanding. “Therefore, the processing of micro components is an important field for further developments in the LSR sector.” He cites a new micro-production cell developed


at Arburg that creates a flexible system solution for the precision manufacture of delicate LSR compo- nents. He notes that Arburg offers OPC UA commu- nications solutions to integrate LSR dosing systems into its Selogica control system. Arburg first presented its special LSR micro


production cell at Fakuma 2018. At its heart is an electric Allrounder 270 A with a clamping force of 350 kN, equipped with a size 5 micro-injection unit and a four-cavity mould with a pneumatic needle-


36 INJECTION WORLD | April 2019


type shut-off nozzle from Rico. It produced four elastic micro dosing valves weighing 0.038g each in a cycle time of around 20s. The post-cure-free LSR is premixed using a


290mL cartridge. Its feed unit is contained in a cooled brass cylinder arranged at 45 degrees to the horizontal injection unit fitted with a special LSR screw with a self-closing 8mm diameter non-return valve. The liquid silicone is continuously fed forwards from the material inlet to the tip. “The first- in-first-out principle is fully implemented, which compensates for the disadvantage of pure piston injection,” says Siegel. Process reliability is ensured by a melt pressure sensor integrated in the feeder, which is visualised and monitored by Arburg’s Selogica control system. Also highlighting its expertise in production of very small LSR parts is KraussMaffei. It demonstrat- ed its new PX 25-55 SilcoSet for LSR injection moulding at Fakuma. The 250kN unit is aimed at articles like precision components for clocks, gears and gearbox elements. At the show, it made a sealing ring with an intricate undercut and a weight of only 0.15g, in a cycle time of just 14s. To make this possible, KM developed a new plasticising process for a 12mm screw complemented with a spring-loaded check valve. Below the cantilever clamping unit there is space for the necessary vacu- um pump or similar peripherals. KraussMaffei Automation contributed the


intricate precision grippers, which are required for the removal of the tiny components. Dr. Boy demonstrated production of LSR seal


rings at Fakuma, on a 10-tonne Boy XS, with the LSR being cured by ultraviolet radiation rather than heat, which it says can cause damage on some small parts. With more elements from outside companies on


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