Liquid Silicone Injection Molding: A Versatile Solution for Demanding Conditions

Liquid Silicone Injection Molding

A Versatile Solution for Demanding Conditions

Liquid silicone rubber (LSR) can be molded via custom liquid silicone injection molding. Liquid silicone rubber is often used to make gaskets, seals and other products where difficult conditions are an issue.

The fast cure and high performance properties of liquid silicone rubber make it an ideal candidate for small molded rubber components. What makes liquid silicone rubber (LSR) a good solution, is its flexibility and elasticity at -70°F. It also retains its properties up to 4050°F.

Liquid Silicone Injection Molding

An advantage of liquid silicone injection molding process, is its reduction of labor, a major cost in molded components. Additionally, the elevated temperature and molding pressures of the liquid silicone injection molding enhances the cure of the process. Because we receive our liquid silicone rubber supply in airtight containers, humidity is not a concern. Therefore, generally speaking, the liquid silicone injection molding process is more consistent than the rubber compression molding.

Although liquid silicone injection molding is the most common term in the industry; it is also known as liquid silicone rubber molding, liquid silicone rubber injection molding, liquid silicone molding, silicone liquid molding, liquid silicone injection moulding, silicone injection molding, LIM molding, silicone rubber molding and silicone molding all used frequently and mostly interchangeably.

Silicone Prototyping: Chemical Compatibility of LSR

Silicone Prototyping

Chemical Compatibility of Liquid Silicone Rubber

The chemical composition of Liquid Silicone Rubber (LSR) makes it the perfect candidate for silicone prototyping. It is unique in comparison to many common elastomers. The inherent properties of  LSR make it an ideal choice for uses in specific chemical environments as well as for silicone prototyping.

Silicone Prototyping

The silicone-oxygen backbone of LSR has a higher bond strength than that of polyethylene or of a carbon-based material, and it is therefore mostly chemically inert. This inertness, as well as its natural hypoallergenic properties, make LSR a prime candidate for food, medical applications, and silicone prototyping.

In comparison with other rubber materials, LSR is exceptionally compatible with many diluted solutions of inorganic acids and bases (e.g., acetic acid, arsenic acid, boric acid, sulfuric acid, tartaric acid). Extending the variety of uses of LSR products, such as hosing and seals to the medical, food manufacturing, and automotive industries, LSR can be used as a propellant in food products, as filler for vehicle airbags or for silicone prototyping. The extensive list of LSR-compatible materials also includes ammonium hydroxide, ammonium phosphate, and alcohol bases, which are common ingredients of many household products.

In addition, LSR is highly suitable for use with water and ozone (which can be used in small quantities as a treatment for drinking water). This broadens the potential uses for LSR hoses, bellows, seals, and other components for municipal water systems or even agricultural irrigation systems. While this application may not seem exceptional, many other seal and hose materials expand over time, age, and crack under differential flow conditions or are unable to maintain mechanical integrity under varying internal and external temperatures. This same concept extends to the automotive industry, where LSR’s compatibility with many industry-standard oils and high-temperature air makes it an ideal candidate for gaskets, bellows, and electrical connectors among other applications.

Open Nozzle System or Valve Gate?

Choosing the right injection technology for LSR molding is key to a successful part, especially in the case of directly gated components (no sprue).  The two most popular systems involve open nozzle systems and valve gates.

For larger parts (weighing 200 grams or more), a valve gate system offers the advantage of a clean gate area. For open nozzle systems, the gate location is visible at roughly 0.7 mm in diameter and 0.5 mm in height.  Although on smaller parts, the gate size could be closer to 0.25 mm and 0.3 mm for open nozzle systems.

With high cavitation molds (16 cavities or more) the valve gates tend to have greater imbalance during the injection process due to the friction of the many moving parts. Maintenance on high cavitation molds that use the valve gate technology can be very time-consuming whereas open nozzle systems are seen as virtually maintenance free.  But, depending on the application, the open nozzle inserts may need to be reworked after roughly 1-2 million shots.

On any multi-cavity LSR mold, filling imbalances result in short shots or excessive flashing which can be controlled with adjustments to the hold-pressure times for instance. A significant difference between these two gating technologies occurs while the LSR is vulcanizing in the mold. For instance, on an open nozzle system allows the material to flow back into the cold runner during vulcanization when the LSR expands, acting as an equalizer.  Since the valve gate has to be closed before the LSR solidifies, any filling imbalances cannot be corrected after the needle is closed.

Molding paper-thin silicone membranes can be a challenge for any gating system and are often molded with sprues/ sub runners due to pin holes, wrinkles, and other surface defects. All LSR gating systems except UV curing systems have a thermal transition area between the hot cavity and the cold nozzle or valve gate body. Fig. 1 shows a valve gate system that has the potential of harboring vulcanized particles in the thermal transition area.  These then may dislodge on the next shot and become trapped in the thin areas of the part; creating pinholes. Alternatively, on open nozzle systems (see Fig. 2) the particles in the thermal transition area will remain attached to the parts themselves.  This prevents any vulcanized particles from entering the cavity and creating pinholes or other surface defects.

SIMTEC uses both systems in full-scale production runs and can assist with selecting the gating system that is right for your application.

 

© SIMTEC Silicone Parts, LLC

The information provided herein is to the best of our knowledge and it is believed accurate and reliable as of the date compiled. No representation, warranty or guarantee expressed or implied, is made as to the accuracy, reliability or completeness of the information provided herein. It is the user’s responsibility to determine the suitability and completeness of such information for the intended use. We do not accept liability for any loss or damage that may occur from the use of this information. Nothing herein shall be construed as a recommendation for uses which infringe valid patents or as extending a license under valid patents.

Two Shot Injection Molding: Efficient LSR Manufacturing

Two Shot Injection Molding

Efficient LSR Manufacturing

Modern LSR injection molding manufacturing reflects innovation and efficiency that are the results of decades of experience. Today, incredible cost-efficiency can be achieved without sacrificing time or quality, so long as one employs appropriate materials, selects a fitting manufacturing process and sets high standards.

two shot injection molding

Fig 1: Injection Molded LSR Over a PC Housing with Steel, Threaded Inserts

One advancement in manufacturing technology that has greatly improved the possibilities for high-efficiency and cost-savings is LSR two shot injection molding.

SIMTEC is a pioneer in the emerging field of Liquid Silicone Rubber (LSR) two shot injection molding and has a remarkable record of customer satisfaction, efficiency and insight which allows us to improve upon engineering and part design in ways that only a qualified team, such as SIMTEC’s can!

LSR two shot injection molding is a fantastic technology that bring several benefits. It not only combines LSR and a thermoplastic together, but is a flexible method which can be engineered to incorporate additional materials.

One example, seen in Figure 1, is a housing made out of a self-adhesive LSR and steel threaded insert within a polycarbonate (PC) housing. The self-adhesive LSR functions as: (1) a soft-touch illuminated button pad, (2) a gasket and, (3) protection for the PC housing to which it adheres. The housing was designed to provide a sturdy frame which also securely holds the threaded insert. Integrating these three functions into one component drives down not only the initial investment by only requiring one opposed to four tools, but also lowers inspection and other quality assessment costs.

Steel inserts, pins or terminals are incorporated within the initial manufacturing steps in insert molding. Material compatibility is determined before production. The finished housing, pictured below, does not require additional finishing or manual labor (such as flash or tail removal). Depending on tooling design, components are completed in seconds with SIMTEC’s LSR 2-Shot expertise!

At SIMTEC Silicone Parts, a leading company in manufacturing high precision parts and components, we are exclusively focused and specialized in the production of LSR and 2-Shot (LSR/Thermoplastic) components.

© SIMTEC Silicone Parts, LLC

The information provided herein is to the best of our knowledge and it is believed accurate and reliable as of the date compiled. No representation, warranty or guarantee expressed or implied, is made as to the accuracy, reliability or completeness of the information provided herein. It is the user’s responsibility to determine the suitability and completeness of such information for the intended use. We do not accept liability for any loss or damage that may occur from the use of this information. Nothing herein shall be construed as a recommendation for uses which infringe valid patents or as extending a license under valid patents.

Two Shot Molding | Multi-Station Molds: Transfer Molds

Two Shot Molding

Multi-Station Molds: Transfer Molds

For Two Shot Molding, a multi-shot mold is needed. There are several types of multi-shot molding available, such as over-molds, core-back, and multi-station molds. The right mold selection depends on part geometry, volume, quality, and molder capacity/capability.

In a transfer injection process, a plastic insert is molded in one cavity and then transferred to a different cavity to be over-molded by the second polymer filling. The cavity of the second shot is defined by the surfaces of the insert as well as the tool. The two materials can be mechanically, thermally, or chemically bound together, or bound by a combination of those processes. The mold requires insulation between the heated LSR and cooled thermoplastic cavities due to the respective processing temperature of each.

Normally the insert is transferred from a lower to an upper part of the mold or vice versa. The transferring process can be done manually or it can be achieved automatically by robots. With automated transferring, the cycle time is decreased while placement precision is increased. The use of a robot system is recommended when other functionalities like automatic trimming, quality control, etc. are needed.

Two Shot MoldingFig. 1: Multi-Station Rotational Mold

In addition, over-molding allows the LSR to be injected directly to critical function zones on the part. The reduction in material used for the part increases the cost effectiveness of the part and decreases cycle time. The process also eliminates the additional assembly that may have been required.

Following the trend of this emerging technology and assuring its position as leader in the Two Shot Molding industry, SIMTEC Silicone Parts added two additional 250 ton machines in the first half of 2011.

Another multi-station injection mold is a rotating mold. Find out more about rotating mold presses!

Additional information can be found in our “Tech Info” section.

[1] Schmachtenberg, E. and Johannaber, F., 2007, “Montagespritzgießen – Verfahrensprinzipien und Definition,” Technical Conference Montagespritzgießen, Institute of Polymer Technology, Erlangen, Germany, p. 1-18

[2] Schmachtenberg, E., Schuck, M., and Kuehnert, I., 2007, “Forming and Assembly in One Process,” Kunststoffe International, p. 8-13

[3] Kazmer, D.O., 2007, Injection Mold Design Engineering, Hanser Publishers, Munich

At SIMTEC Silicone Parts, a leading company in manufacturing high precision parts and components, we are exclusively focused and specialized in the production of LSR and 2-Shot (LSR/Thermoplastic) components.

© SIMTEC Silicone Parts, LLC

The information provided herein is to the best of our knowledge and it is believed accurate and reliable as of the date compiled. No representation, warranty or guarantee expressed or implied, is made as to the accuracy, reliability or completeness of the information provided herein. It is the user’s responsibility to determine the suitability and completeness of such information for the intended use. We do not accept liability for any loss or damage that may occur from the use of this information. Nothing herein shall be construed as a recommendation for uses which infringe valid patents or as extending a license under valid patents.