Liquid silicone rubber (LSR for short), due to its outstanding material and processing properties, has unlimited potential for the production of challenging elastomer parts. The stable single bond of the silica polymer chain is responsible for the well-known excellent chemical and physical properties of this type of rubber. For example, LSR provides nearly constant rubber-mechanical properties over a wide temperature range and very good resistance to ozone, UV light and weather. In addition, the molded article can be freely colored with various colors.
Raw material manufacturers offer LSR as a concentrated liquid, pumpable two-component system. Shipped in containers, ready for processing and fast vulcanization, allowing fully automatic injection molding with very short cycle times. According to the mold temperature, the cross-linking time is about 3-7s per millimeter of wall section thickness of the molded product.
Silicone rubber is suitable for a variety of applications, mainly in areas where both thermoplastic elastomers and general elastomers reach their limit values. Typical uses are various seals, switch pads for electronic devices and a large number of medical devices. ES silicone manufacturers have ISO 13485 medical system qualifications and a 100,000-level clean room.
Compared to thermoplastic elastomers, LSR also shows the same mechanical properties over a wider temperature range. LSR offers many advantages over ordinary elastomers, such as simple processing, complete freedom of coloration, significantly faster crosslinking, and ready-to-use container delivery.
The technical equipment for processing LSR consists of a two-component dosing pump for feeding in a 1:1 ratio, a mixing section with a static mixer, an injection moulding machine, a mould and peripherals suitable for its purpose such as brushes and mould release devices Wait.
The dosing pump feeds the two material components to the plasticizing unit through the mixing section under the specified pressure. Additives and pigments were added as required prior to the mixing stage.
Molding Equipment
The technology available on the injection moulding machine is decisive for the quality that can be achieved in the moulded product. Particular attention must be paid to the plasticizing and clamping devices. Unlike processing thermoplastics, the plasticizing unit is temperature-controlled with liquids between 20 and 40°C. Another important aspect is that all relevant actions are precisely controlled in a closed loop. Even a small difference in the volume of ingredients will lead to insufficient filling, lack of glue or flash products with overflowing glue. In addition to maintaining the dosing end point and back pressure, the design must ensure that no uncontrolled backflow of material occurs at the check loop.
In terms of clamping devices, making a strong mold hot plate prevents undesirable warpage problems. It is also necessary and necessary for the clamping action to meet the technological requirements and to provide a sensitive mold safety signal. Further development of silicones has resulted in significant reductions in cycle times.
This means that the fast movements and synchronisation functions of injection moulding machines have become increasingly important.
The processing of LSR requires special molding techniques. The mold is usually heated by electricity, and the temperature is about 180-220 ℃. Feedback control of mold heaters on injection molding machines is convenient and practical. To avoid flashing problems, the gap size must be minimized, as the LSR becomes very easy to flow at higher shear rates. To effectively avoid the risk of trapped air bubbles and burning, the mold must be emptied before the injection process begins.
The nozzles of most current products are directly connected to the cold runner. An opening or thimble closing system with appropriate temperature separation prevents cross-linking of materials within the communicating system. Therefore, the quality of the gate point is so good that it is difficult to see the spots.
Critical Factors In Process Control
Due to the low viscosity of LSR at the shear rate of the injection molding process, the maximum injection pressure per unit area of 80-100 MPa is usually sufficient. Higher pressures are also possible if a small cross-section cold runner nozzle is used.
The reproducibility of cavity injection and cavity filling at the same pressure level is most critical if microflash molded parts are to be produced. The goal is to fill the cavity with the lowest pressure allowed when volume injection is completed.
Unlike processing thermoplastics, no further injection into the mold occurs during the stage where the pressure is maintained to compensate for the shrinkage of the material. During LSR processing, the main task of maintaining pressure is to prevent the LSR that has started to cross-link through the gate and escape into the system or cold runner nozzle that communicates with the sprue and runner.
Vulcanization time, also referred to as heating time, depends on the melting temperature of the mold during injection molding, the mold temperature, and the geometry and wall thickness of the molded article.
LSR’s press machining of composite injection moulds eliminates steps requiring elaborate assembly and logistical supply operations, while improving product quality, and the production of housings featuring integral sealing is just one example of many.
Various methods are suitable for the production of two-component LSR/thermoplastic composite molded articles. The method of embedding through holes in thermoplastic parts can achieve mechanically fixed and more complex molds.
Where white-bonded LSRs are used, the LSR components are simply bonded to the thermoplastic. This facilitates mold design and provides greater freedom in design.
To efficiently produce advanced molded products, the immature molding technology requires high-precision rapid injection molding machines.
Epilogue
Liquid silicone rubber processing is a production method with many advantages. Because it is easy to store and transport, it is prepared by inexpensive metering devices, and it is a directly processable mixture, which can eliminate the traditional and labor-intensive preparation of rubber materials.
When producing small parts and the quantities are large enough, injection molding will greatly reduce costs due to short cycle times, low injection pressures, and small amounts of flash.
ES has ISO13485 medical system certification and a 100,000-level dust-free workshop, which can meet any of your silicone customization needs. Welcome to contact us.
