Liquid Silicone Rubber Injection Molding

Liquid Silicone Rubber Injection Molding

Liquid silicone rubber injection molding is a thermosetting molding process. In the processing, LSR is the main application material. As an efficient production process, it can produce many useful parts. This article will provide you with a detailed explanation from several aspects, including material characteristics, process advantages, equipment, processing procedures and applications. If you want to customize silicone products, this article can help you understand this process better.

What is liquid silicone rubber injection molding?

Liquid silicone rubber(LSR) injection molding is a thermosetting molding process. By using base materials and catalysts, liquid silicone rubber is turned into a solid.First, the two compounds need to be mixed at low temperature, and then injected into the high temperature mold through the injection molding machine under high pressure, and the elastic silicone parts are formed by heating and curing in the mold.

This process is usually used for mass production of complex and delicate silicone components. The products produced are highly consistent and very durable, and are applied in a wide range of industries.

Properties of Liquid Silicone Rubber Material

Odorless

LSR has extremely high transparency and a high light transmittance. Moreover,LSR has no peculiar smell, which is the reason why it is widely used in the food industry and baby products field.

Temperature Resistance

LSR can keep its functional integrity and good elasticity and physical properties for a long time in extreme temperature environments (-50℃ to 250℃). This is where it is superior to rubber and plastic.

Biocompatibility

This is the most notable advantage of LSR. LSR is non-toxic, odorless, not easy to cause allergies, and does not irritate human tissues. Moreover, it has passed multiple certifications such as FDA, LFGB, USB Class VI, etc. It has become the preferred material for medical and food-grade applications.

Low Viscosity

Low-viscosity LSR means it has excellent fluidity. During the injection molding process, it can precisely fill every detail of the mold, achieving high-precision silicone components.

Chemical Stability

LSR is resistant to water, ultraviolet rays, ozone, alcohol, and weak acids and weak alkalis. However, it has relatively poor resistance to gasoline, concentrated acids and concentrated alkalis.

Advantages of LSR Injection Molding

High Efficiency and High Automation

This is the significant advantage of LSR injection molding in industrial production. It can achieve fully automated operation throughout the entire process, including raw material supply, mixing, injection, demolding and part removal. This can greatly reduce labor costs and minimize the possibility of human operational errors.

Additionally, the addition reaction of LSR is extremely rapid. If the product you customize is a thin-walled part, its reaction cycle can even be as short as a few seconds, making it extremely suitable for large-scale mass production. Moreover, LSR injection molding does not require secondary vulcanization molding, and the simplified production process also greatly improves production efficiency.

Consistent Quality

In some high-end application fields, the consistency of products is of vital importance, such as the electronic industry, the automotive industry, the medical industry, etc. LSR injection molding uses precise metering and mixing equipment. It can guarantee from the source that the chemical source of each batch of the product is exactly the same. The stable curing process also guarantees extremely high dimensional stability of the product. The excellent fluidity of LSR makes sure that the product can perfectly replicate the details of the mold, achieving consistent high precision of the product.

Less Waste Production

The material utilization rate of the LSR injection molding process is high, which reduces material waste and saves raw material costs. What’s more, the addition reaction of LSR does not produce water or other substances, which means there is no need to deal with the pollution problem caused by the by-products of the reaction. And the waste LSR can also be processed and recycled to manufacture other items.

Strengthen Safety

LSR itself has excellent bio-compatibility. The material is non-toxic, odorless, and free of allergens, and has passed multiple food and medical-grade safety certifications. The biological inertness of LSR also means that it does not react with human tissues. Moreover, silicone also has certain antibacterial properties, making it highly suitable as the preferred material for food and medical products. Besides, the entire process of LSR injection molding is produced through automation, which largely verifies the safety of the operators.

Durability

LSR products can withstand extreme temperatures. That is, it still keeps flexible in low-temperature environments and will not become brittle or crack. It can make sure that mechanical properties do not fail, and it will not melt or age even in high-temperature environments. LSR products also have excellent weather resistance and anti-aging properties. LSR outdoors products have a long service life and can effectively resist ozone and UV rays.

Most importantly, LSR products have an excellent rebound rate. It has an excellent ability to resist permanent deformation and can quickly return to its shape after repeated compression, stretching and bending. Additionally, it also has excellent electrical insulation. Therefore, it is highly suitable for use in products such as sealing parts and electronic components.

Equipment of LSR Injection Molding

Metering Units

The metering pump simultaneously conveys liquid materials of two components, and guarantees the synchronization of the two pumps’ conveying and keeps the consistency of the ratio. The constant proportion of the two liquid material components is the key to product consistency.

Supply Drums

The supply drums are mainly used to store two-component liquid materials. The supply drums are usually equipped with a temperature control system and a stirring device to guarantee the uniformity of the material and prevent pre-curing and sedimentation of the material during the storage stage.

Mixer

The two components are sent into the static mixer after leaving the metering unit. They need to be thoroughly and evenly mixed inside the mixer. The uniform mixing of liquid materials at the molecular level is achieved through the rotating blade structure inside the machine. Well-mixed materials are the key to high product consistency. Uneven mixing can lead to partial curing of your customized product or differences in product strength.

Injectors

The mixed LSR material is transported to the feed cylinder. The cylinder is usually kept in a cooled state to prevent the material from vulcanizing prematurely due to heat inside the cylinder, which could lead to blockage and material waste.The cylinder pushes the material directly and smoothly to the mold.

Nozzle

The nozzle needs to connect the cylinder and the mold to help inject the mixture into the mold. Nozzles are usually equipped with controllable shut-off valves that open during material injection and close immediately after the injection is completed. Such nozzles can effectively prevent the leakage of materials. Nozzles are usually in a heated state to facilitate the curing process of materials after they enter the tool.

LSR Injection Molding Process

Preparation of LSR Injection Molding

Mold Selection

First of all, the forming mold for liquid silicone rubber needs to be designed according to the product you customize.According to your customized products can be used to treat the surface of the mold in different ways. Molds are usually processed and manufactured by CNC router machines. It should be noted that the mold must be resistant to high temperatures in order to withstand the injection molding process. Before production, make sure the mold is clean and the surface temperature is uniform.

Material Preparation

Before production, it is necessary to guarantee that the states of the two components of LSR are stable. Before using the feed cylinder, temperature restoration should be implemented to avoid inaccurate material measurement caused by temperature differences.

Meanwhile, the sealing performance of the feed cylinder is also kept well to prevent moisture from affecting the performance of the materials.What’s more, materials that have been placed for a long time need to be rolled slowly to prevent the sedimentation of fillers, which could lead to an imbalance in the mixing ratio. Ultimately, it may lead to insufficient product strength or partial non-curing.

Equipment Inspection

Before production, it is necessary to make sure that the temperature of the feed cylinder is controlled within an appropriate low-temperature range, and to confirm that the output ratio of the metering unit is precisely 1:1.

Primary Injection Molding Process

Material Metering and Mixing

The materials are pumped into the static mixer by the metering pump. The helical blades inside the static mixer evenly mix the liquid materials of these two components. If the product you customize has unique color requirements, the manufacturer will add pigments or other additives before the materials enter the mixer.

Material Injection into Mold

The well-mixed LSR is injected into the high-temperature cavity insert at high speed and low pressure through the cooled cylinder and heated nozzle. Pressure expels the air inside the cavity, and the material fills every detail of the cavity.Ultimately, it will form the shape of your customized product. The injection volume and push pressure of the injector need to be precisely set. Otherwise, it may cause the product to burn or result in flash.

Curing/Vulcanized

After the mold filling is completed, the high-temperature mold will cause an addition reaction in LSR, and the material will change from liquid to solid elastomer. The length of vulcanization time usually depends on the wall thickness, complexity of your customized product and the temperature of the mold.

Cooling and Demolding

When the vulcanization of the product is completed, the manufacturer will open the mold, loosen the clamping device, and smoothly remove the product from the mold. The products produced rarely have flash.This is due to the inherent characteristics of LSR.

Post-processing

After the molded part is taken out, post-molding may be required according to the requirements of your customized product. Post-molding involves a series of processes such as printing and assembly. What’s more, molded parts may also have requirements for post-curing and deburring. Not all molded parts require post-curing, but for some high-end applications, the post-curing process is very important. Post-curing can bring the tear resistance, compression set rate, optical performance or electrical performance of the product to the best state.

Quality Control and Inspection System

Raw Material Inspection

Raw material inspection is the first step in product quality testing. For LSR materials, it is necessary to check the viscosity, specific gravity and catalyst activity of the two components, and guarantee the consistency of the raw material batches. Secondly, it is also necessary to check whether the color paste and additives are compatible with the basic raw materials. And whether the color paste and additives contain impurities that could cause catalyst poisoning. Whether the raw materials are stored in accordance with the requirements to guarantee that they are within the validity period.

Monitoring of Production Process

During the production process, a metering system will monitor the mixing ratio to guarantee that it is always precisely controlled. The temperature of the feed cylinder and the mold will also be monitored in real time to guarantee that the curing time of your product is appropriate. Real-time monitoring of production process parameters can largely guarantee the outcome of the product.

Product Inspection

After the product is produced, it is still necessary to check the size, appearance and so on of the product. For instance, whether the product has bubbles, impurities or uneven color. Whether the surface of your product is smooth and free of scratches. Does the size of the product match your product requirements? The most important thing is whether the product can meet your usage requirements. Therefore, it is also necessary to conduct hardness tests, sealing performance tests and more needed tests on the products. If you are customizing medical products,it also needs to conduct bio-compatibility tests on the products.

Data Recording

The production of products requires the full correlation of material batch numbers, equipment parameters, production process data, and inspection results. Establish complete production batch records. If there are any defects in your product, it can trace the entire product life cycle through the recorded data.

Application of LSR Injection Molding

Healthcare

Products made from liquid silicone rubber have excellent bio-compatibility, and they also possess antibacterial properties, chemical inertness and flexibility. These features make it the preferred material for medical products. For instance, some implantable drug delivery pump tubes, artificial tissue engineering stents and other implantable and long-term contact devices. Breathing masks, drainage tubes, various catheters, sealing rings and other diagnostic instruments can also be produced through LSR injection molding.

Infant Care

For baby products, the primary considerations are the softness, safety and resistance to disinfection of the product.For baby products, the most classic application of LSR is undoubtedly the pacifier. The pacifiers produced by LSR are transparent, soft, odorless and can withstand repeated high-temperature steaming and disinfection. Similar ones include teething rings and teething picks. Some silicone toys for infants and toddlers are also usually made by LSR injection molding.

Automotive Industry

LSR products can be applied in the automotive and transportation fields mainly due to their resistance to high and low temperatures, aging, insulation and excellent sealing performance. High-temperature components in the engine compartment of the car, such as spark plug caps and sensor seals, etc. Moreover, some interface seals and insulating components inside the car are usually LSR products as well.

Food and Kitchenware

The safety, high-temperature resistance and aesthetics of LSR have promoted its application in food and kitchenware. The sealing rings of some kitchen appliances, such as rice cookers, pressure cookers and coffee machines, are all LSR. They are usually resistant to high-temperature steaming and cooking and are not prone to adhering to food. Some baking molds, non-stick spatulas, spoons and LSR water cups are all common items.

Final Thought

As a multi-functional and efficient manufacturing process, LSR injection molding is an ideal choice for many large-scale productions and complex product shapes. If you also want to customize silicone products, Legensilico can provide you with a comprehensive silicone customization solution. As a professional silicone manufacturer, Legensilico has always been committed to providing you with high-quality silicone products. Contact us now!