|Liquid silicone rubber (LSR) injection molding is in existence for a long time. It finds applications in medical, automotive, infant care and general industrial markets, aerospace, electronics and many other specialized industries. LSR, a thermoset can outperform TPE in performance properties. LSRs are low viscosity, shear thinning, thermosetting polymers that exhibit characteristics exceeding those of comparable materials in terms of biocompatibility, mechanical properties, and thermal, chemical and electrical resistances. LSR has high heat resistance as well as extreme low-temperature flexibility, and an inherent lubricity. LSR is crosslinked utilizing a platinum-catalyzed reaction that produces no by-products. The strong bonds between the silicon and oxygen atoms offer LSR a high heat resistance. Its ability to withstand sterilization processes makes LSR an ideal alternative for many medical uses as well as for baby care.
The injection moulding equipment looks very similar to that used for thermoplastics. Liquid silicone molding is very similar to conventional plastic injection molding, with one major difference: in LIM�, the barrel is cooled and the mold is heated. The injection barrel should be water-cooled to keep the temperature below the point where curing begins. The major difference occurs within the mold. While thermoplastics must be heated in the injection barrel to their melting point and then cooled in the mold back to a solid state, the reverse occurs with LSR. It starts out as a fluid and is then heated within the mold to initiate curing. The mold is heated normally at 180-200 C. LSR requires cold-runner systems to keep the material cold until it enters the hot cavity. LSR is in the form of a paste. It generally is supplied with two components of catalysts and curing agents. They must be mixed together in order to allow the curing reaction to occur. To get to the injection barrel, these components must be pumped into the injection barrel at a 1:1 ratio and they must be thoroughly blended prior to injection. This is usually accomplished with a water-cooled static mixer in a chamber prior to the injection unit. Most LSR injection machines also use an injection screw that contributes additional mixing. LSR materials in a hot mold tend to have very low viscosity, which requires careful attention by toolmakers and molders to parting lines and venting in order to prevent flash. This permits LSR to fill very long or thin sections easily.
Since the parts are injection molded, they are clean and hopefully flash-free. This offers several benefits such as lower cost per part, better quality control, and contamination free parts.
Processing of Liquid Silicone Rubber (LSR) materials via Liquid Injection Molding (LIM) finds that LIM process is ideal for high volume production requiring efficiency and tight tolerances; and extreme detail is required in tool design and development to maintain quality in the final LSR molded product. Injection can take as little as 3-10 seconds, whereas molding and vulcanization take from 10-90 seconds or more (depending upon shot weight and ultimate section thickness of the part). A second major difference between LSR and thermoplastic molding lies in the method that the raw materials are introduced to the injection barrel. LSR is manufactured with a paste-like consistency, and is commonly sold and handled in two separate parts (called �A� and �B� in the industry). Both components are required for the curing reaction of the LSR: one part promotes crosslinking (A) while the other is a platinum catalyst (B) to decrease reaction time. The components are fed to the injection machine at a ratio of 1:1 after being thoroughly blended by a water-cooled static mixer in a chamber. Most injection units feature a reciprocating screw to further mix the material, but plunger style units are also available for the processing of sensitive materials unable to withstand the shear heating introduced by the screw method. The vulcanization properties of the LSR are dependent on the ratio of the two components and typically tolerate variation in the mixing by +/- 10%.
Fundamentally, both types of equipment have feeding systems that convey material into an injection barrel, which then moves it into the mold with the result being a finished part. However, one of the most profound differences in the two processes and their respective equipment is found in the thermal configuration of the barrel and mold systems. In thermoplastic injection molding, solid pellets are fed into the barrel, are then plasticized by shear heating from the screw and induction heating from the heater bands, and finally injected into the mold where cooling takes place. In contrast, LSR molding calls for the material to be pumped into the injection barrel in liquid form while being kept water-cooled during metering and injection to inhibit curing, and then heated in the mold after injection to promote curing and part formation. The mold is typically electrically heated, held between 180°C and 200°C, and contains a cold runner system to ensure no LSR begins to cure before reaching the mold. As a result of the crosslinking reaction, LSR parts experience thermal expansion. Thus, a packing stage, as found in thermoplastic injection molding, is not required. However, an LSR part does contract in a uniform fashion as it cools. To compensate for the contraction, the LSR molding process utilizes a hold pressure in conjunction with molds designed to take both the expansion and the shrinkage rate into account for final desired dimensions. Further, most LSR molded parts require an oven curing operation following vulcanization. Oven curing (also called post baking) assists in the removal of by-products and volatiles. Typically, post curing is done in a standard air-circulating oven for 2 to 4 hours at 200°C.
Injection molding machines have proven to be successful for processing LSRs, and many processing hurdles related to the nature of silicone materials have been overcome by the precision control capabilities of current molding equipment. However, the ability to accurately control two variables-injection pressure and shot size-still eludes many processors of LSRs. Electric injection molding machines, which achieve precise control of the volume and speed of injection through the use of a direct-current servomotor, offer tighter control of the process over hydraulic machines.
Due to their low viscosities, high flow characteristics, fast cure cycle times, clean operation and automated manufacturing processes, LSR has enjoyed increasing popularity.
The three methods of molding silicone rubbers are compression, transfer, and LIM. Both compression and transfer molding are suitable for low-volume production and both methods typically utilize HCRs. Compared with LIM, cycle time for compression or transfer molding parts is longer (approximately 3 minutes). However, the initial cost of production is relatively lower because they both enable the use of less-expensive equipment and less mold design and prototyping work. When looking at higher annual volumes, LIM provides shorter cycle times (typically under 1 minute), nominal scrap loss, consistent part quality, and minimal risk of contamination. It also reduces labor costs by eliminating material pre-handling and minimizes required cleaning. LIM is the ideal molding process when volume, speed and precision are critical. The ability to operate with a lower injection pressure as a result of the low viscosity of LSR materials means less variation, tighter tolerances and consistent quality. LIM brings faster cycle rates that fill high-volume orders quickly. And, because all of this can take place within a closed system, the process is extremely clean. In addition, LIM lines have the potential to be integrated with fully automated robotic machines and semi-automatic equipment to create repeatable precision and manufacturing efficiencies. However, LIM has some disadvantages due to the use of complicated molds and the need to balance cycle time with cavitation. LIM requires extensive mold design during prototyping, expensive equipment investment, and large upfront tooling capital expenditures. Also, LIM equipment has traditionally been limited to processing lower-viscosity LSRs only. Some modern machines, however, can mold high-viscosity LSRs and even HCRs.
(References: Jesse J. Pischlar)