Liquid Silicone Rubber Injection Moulding Processes Machine Technology Application

Transcription

1 Liquid Silicone Rubber Injection Moulding Processes Machine Technology Application

2 LIQUID SILICONE RUBBER A very special material On account of their excellent properties, liquid silicone rubbers lend themselves to a vast spectrum of applications. Key features: Physiologically acceptable No odour or taste transfer Unaffected by weather and ageing High-temperature capability (continuous working temperature up to 180 C) Low-temperature capability (remains flexible beyond 50 C) Sterilisable Food industry Medical devices Vehicle and machine building Products from liquid silicone rubber (LSR) are to be found in numerous applications. And its range of uses will no doubt extend as further and new developments come on the market. Especially where thermoplastic elastomers no longer meet requirements, LSR provides the capability profile to fill the need. An added advantage is in the fact that LSR will combine with a host of other plastics and with metals. 2 Low viscosity creates high flexibility Liquid silicone rubbers belong to the group of heat-curing rubbers. A characteristic feature is their low viscosity during processing compared to solid silicones or elastomers. Two-component mixtures increasingly crosslink by the addition process. This means that the reaction takes place without any decomposition product forming. This is a great benefit in injection moulding because there is no concern of deposits forming on the moulds. In order to define processing time until crosslinking starts, one of the two components has an inhibitor added, which is also referred to as a pot-life regulator. The other component has a catalyst added. Crosslinking better known as curing starts due to the influence of the mould temperature which, depending on the mix, is between 160 C and 220 C. But then the reaction proceeds fairly rapidly at approximately 5 s/mm of wall thickness. Consequently, much shorter cycle times can be achieved with LSRs than, for instance, with classic elastomers.

3 Kitchen and plumbing Major fields of application Despite their comparatively high price, liquid silicone rubbers (LSRs) cut across almost all areas of every day life and industrial production. Primary applications are in Vehicle and machine building: O-rings, gaskets, spark plug connectors Medical devices: Mouthpieces for anaesthetic devices, clinical nipples Food industry: Dosing devices, covers, baby bottle teats Kitchen and plumbing: Baking moulds, keypads, seals and gaskets Electricals and electronics: Mobile phone keypads, keyboard mats Sports and leisure: Diving masks, toys Electricals and electronics Sports and leisure Demag machines for LSR processing Basically, all Demag machines have LSR processing capability, if fitted with the appropriate additional equipment. So the processors can take their choice. And can rely on Demag s expertise to assist them in planning for the right equipment. Stability of the process parameters is key to assuring product quality at a high level. Therefore, moulders should be aware of the following pointers in processing LSR: Melt cushion variations not to exceed 0.1 mm Screw stop position variations not to exceed 0.1 mm Injection time variations not to exceed 0.02 s Metering variations not to exceed 5 % The range of pressure variations of the LSR in the feed zone 5 to 10 bar maximum Furthermore, the machine should incorporate the following features: Programmable back pressure Injection by means of Smart pump (closed loop control of speed) Injection speed setting in extremely small increments (0.1 mm/s) to avoid air entrapments Ultrasonic stroke measuring system Move to vacuum gap, wait for vacuum build up, then apply clamp force and inject 3

4 MACHINE TECHNOLOGY Demag s modular system Ranging from dummies and elastic seals to cold and high-temperatureresistant mouldings for automobiles and aircraft the spectrum of LSR-moulded parts and rigid/flexible combinations is of an overwhelming variety. The technology for it comes from Demag Plastics Group: a complete LSR package with its helpful add-on options covers almost every application of liquid silicone moulding. Features included in the LSR package: LSR plasticising unit with liquid temperature control bushes LSR screw with non-return valve and shaft seal Plasticising cylinder connection to mixing and metering unit Water-cooled LSR shut-off nozzle Programmable backpressure Freely programmable pneumatic valve Pneumatic cores for control of brushing equipment or demoulding handling Flexible vacuum program Delayed injection with adjustable delay time For silicone processing, the machines are equipped with an LSR plasticising unit with cooled shut-off nozzle Optionally, there are additional LSR modules available: Mould temperature controller Pneumatic 5/2 way valve or hydraulic valves for control of cold-runner nozzles in mould Water filter and water monitor for cold runner cooling Apart from the modified injection moulding machine, processing of liquid silicones calls for a special mixing and metering plant. Here, Demag Plastics Group cooperates with a number of leading manufacturers. The mixing and metering unit delivers the two components automatically at a mixing ratio of 1 : 1 Process sequence fully automatic and repeatable The mixing and metering unit delivers the two components, which are supplied ready for processing, automatically and at the preset mixing ratio from the containers first into a reservoir. Where necessary, the unit also adds pigment separately from these two components. Next, the components which have been brought together but are still unmixed, go to a static mixer to be thoroughly mixed by means of flow-dividing elements. Non-return valves in the piping prevent back flow of the paste-like components. In response to a metering signal from the injection moulding machine, the mixture eventually passes through an adapter into the feed zone of the plasticising unit. Delivery into the screw 4 barrel is effected during metering at an adjustable, repeatable pressure. Continuing the process, the screw feeds the paste-like material into the space in front of the screw. Simultaneously, the two main components plus any pigment addition once more undergo thorough mixing. Injection then takes place by the screw forcing the material through a cold runner into the mould. Once the mixture in the mould has reached the necessary activation level >170 C, crosslinking or curing starts. A special LSR non-return valve prevents the mixture from flowing back out of the space in front of the screw into the forward part of the screw. Inside the barrel, too, absolute tightness is crucial. Therefore, Demag uses special-purpose screws and barrels in its LSR machines. Thanks to high-precision seals, Demag machines guarantee exceptionally clean and effective LSR processing. A major asset for all moulders in this context is Demag s modular injection moulding machine concept. A flexibly laid out production setup capable of responding quickly and straightforward to new market requirements is of paramount importance. In fact, all Demag machines can be used again for thermoplastics and vice versa within a minimum of time by changing the plasticising unit.

5 Depending on moulded part geometry, the use of a brushing device can be practical for demoulding Key plant components of an LSR processing unit Silicone component A 2. Silicone component B 3. Pigment or additive addition 4. Non-return valves 5. Mixing reservoir 6. Static mixer 7. Shut-off valve 8. Non-return valve 9. Pneumatic/hydraulic shut-off 10. Temperature control 11. Mould heating 12. Vacuum pump Temperature control precise and zone-selective A crucial aspect for LSR processing is precise temperature control for the different elements of the plant. While in the mould or rather in the cavities, required temperatures are between 160 C and 220 C, the plasticising barrel, the shut-off nozzles and the cold runner on the mould should maintain a temperature between 20 C and 30 C. Failing this, the mixture would be liable to undergo excessive and premature heating resulting in premature curing. Consequently, it is important to ensure exact segregation of the temperature zones. With this in mind, Demag uses water cooling to control barrel temperature which as all other control loops as well as the peripheral equipment is integrated in the machine control system. Demoulding quick and reliable with brushes Basically, demoulding of elastic, flexible moulded parts is a difficult undertaking. Ultimately, it depends on the geometry of the part as well as on the specific conditions, that is, whether production is fully automatic or semiautomatic. Classic type ejectors (sleeves or pins) may be used or stripper plates, blow-off ozzles, handling devices or brush rollers. Of great influence in this context is the mould concept which is why demoulding must be given proper consideration in the design stage. 5

6 THE MOULD Cold runner plate with 16 pneumatically operated shut-off nozzles Section of a 128-cavity mould for single-wire terminations (photo: Elmet) C 65 C 90 C 170 C 270 C Temperature distribution in a two-component mould for a thermoplastic/ LSR combination Precision within tightest tolerances Design and build of a mould suitable for LSR moulding are challenging tasks. This is because injection moulding of a low-viscosity material obeys its own laws. Drawing on an extensive body of know-how, Demag Plastics Group will assist customers for best results. For strictly practical, qualitative and also economical reasons, fabrication of the moulded parts should be without sprues and flashes, fully automatically and without the need for secondary operations. These requirements alone underline the need for precision. In order to obtain flashfree parts, extremely close tolerances of the order of mm and less are mandatory. On the other hand, it is also necessary to reliably evacuate the cavities to avoid rejects due to air entrapment, filling problems or so-called white spots. Therefore, it is a good plan to have 6 a vacuum pump integrated in the machine to assist mould venting. Mould design also influences the selection of the machine nozzle. Possible alternatives are: Provide a hydraulic or pneumatic shut-off nozzle on the machine side for hot runner moulds or moulds with an open cold runner Provide either an open nozzle or a shut-off nozzle on the machine side for moulds with a cold-runner shut-off nozzle Process variant: Multi-component moulding Whether for technical perfection or better appear ance, multi-component parts using rigid/ flexible combinations are gaining ground. A com bination of liquid silicone rubber with thermoplastics can extend moulded part properties but from the process point of view poses a special challenge. For instance, the thermoplastic component must be capable of withstanding the curing temperatures of the LSR without degradation. Of course, the process itself must be chosen to fit to the moulded parts. Moreover, prior testing is required to study the adhesion of the different materials. LSR will not bond to all thermoplastics which is why some mechanical connection may be necessary.

7 APPLICATIONS LSR moulding solutions in practice The processing of liquid silicone rubber involves vulcanisation or curing under heat. This implies three particularities of LSR injection moulding: metering of two different liquids by means of a special metering unit; homogenisation and delivery of the paste-like rubber in the injection moulding machine; and curing in the mould at around 160 C to 220 C. As a single-component material, LSR is used in many technical applications, such as keyboard mats, plug connectors, O-rings and gaskets. Recently, it has also found application for such common consumer products as dummies and dispensers in food packaging articles. Moreover, LSR is used in medical devices, such as tubing, respirator masks, and body electrodes. The most dynamic growth areas today are flexible baking moulds, self-ceramifying products, e. g. sheathing of cables in fire-hazard areas and lift wells and technical parts made by twocomponent injection moulding for use in the automotive and electronic industries. Tips and tricks from LSR pros 1. In the case of thermoplastic/lsr combinations, continue processing the thermoplastic immediately whether direct by transfer into a second injection moulding machine or by overmoulding in a two-component mould. This way you will be able to avoid changes in surface tension on the thermoplastic part by oxidation or water absorption. 2. Try out and adjust new moulds for some time with standard LSR in order to siliconise the surface so that zero-loss demoulding of the parts is ensured in commercial production. 3. You can significantly shorten the time for adhesion build-up between the thermoplastic and LSR by allowing a longer dwell time after injection moulding or by post-curing. This is the way to quickly achieve optimum final bonding. 7

8 Asia Demag (Malaysia) Sdn Bhd 15-E, 5th Floor, Block 1 Worldwide Business Park Jalan 13/50, Section Shah Alam Selangor Darul Ehsan, Malaysia Tel.: Fax: [email protected] Brazil Demag Ergotech Brasil Ltda. Av. Ceci, 608 Galpao B11 Tamboré Barueri (SP) Tel.: Fax: [email protected] China Demag Plastics Machinery (Ningbo) Co., Ltd. No.669, Kunlunshan Road, Beilun District, Ningbo, , Zhejiang Province, P.R.China Tel.: Fax: [email protected] China Demag Ergotech GmbH Shanghai Rep. Offi ce 6F, No. 1221, Hami Road, Shanghai , China Tel.: Fax: [email protected] CIS Mannesmann Demag Plastservice Prombaza OAO Stroitransgaz d.ascherino Leninskiy raion Moscow region Tel.: Fax: [email protected] France Demag Ergotech France sas Zac du Mandinet 9, rue des Campanules Lognes Tel.: Fax: [email protected] Germany Demag Ergotech GmbH Werk Schwaig Altdorfer Str Schwaig Tel.: Fax: [email protected] Germany Demag Ergotech GmbH Werk Wiehe Donndorfer Str Wiehe Tel.: Fax: [email protected] India L&T-Demag Plastics Machinery Limited Mount-Poonamallee Road Manapakkam/Chennai Tel.: Fax: [email protected] Italy Demag Plastics Group Italia s.r.l. Via Bassano Anagni (FR) Tel.: Fax: [email protected] Poland Demag Plastics Group Sp. z o.o. ul. Jagiellońska Częstochowa Tel.: Fax: [email protected] Spain Demag Ergotech España, S.L. Pol. Ind. Can Calderón Avd. Riera de Fonollar Esquina C/Murcia, n 37-A, Nave F Sant Boi de Llobregat (Barcelona) Tel.: Fax: [email protected] United Kingdom Demag Hamilton Ltd. Hamilton House, Broadfi elds Bicester Road, Aylesbury Bucks, HP19 8AY Tel.: Fax: [email protected] USA Demag Plastics Group Corp Alameda Drive Strongsville, Ohio Tel.: Fax: [email protected] The responsibility to ensure that everything runs smoothly Many moulders today operate three shifts, some on 365 days of the year this calls for a maximum of availability of the machines, spare parts, and service support. Backed by highly skilled service teams, advanced spare parts logistics, and multiple service levels to address a customer s specific needs, we provide total support world-wide: from straightforward inspections through comprehensive maintenance, and extended warranties for high capacity utilisation levels to emergency hotline support, and training of your personnel. Full documentation and a digital catalogue ensure that spare parts are delivered to you in a minimum of time, usually within a few hours. Users of older machines can have them upgraded by our retrofit service at fair prices, for instance, by state-of-the-art control software or for specialised injection-moulding processes. In short, the Demag Service provides you with whatever support you need to complete your jobs efficiently and to schedule Flüssigsilikon-GB 1007 he