Silicone Rubber Molding Parts

Liquid Silicone Rubber Injection Molding: A Detailed Guide

In the injection molding process, most parts are made from thermoplastics or thermosets. And among thermosets, especially liquid silicone rubber (LSR), it is the primary choice for injection molding applications. Liquid silicone injection molding offers an efficient and economically viable process for producing pliable, impact-resistant, and temperature-resistant parts. It uses liquid silicone rubber to fix the material from an uncured state into a cured part. This process enables us to produce many useful parts—gaskets, kitchen equipment, over-molded components, and much more.

In this detailed guide, we’ll cover everything you need to know about LSR injection molding, including its processing equipment, working principle, applications, benefits, considerations, and more. Let’s dive in!

A Brief Introduction to Liquid Silicone Rubber

As we know, silicone rubber is a thermoset elastomer with a backbone of alternating silicon and oxygen atoms and methyl or vinyl side groups. The presence of methyl-groups makes silicone rubber extremely hydrophobic and suitable for electrical surface insulation. 

What is liquid silicone rubber? Liquid silicone rubber, or LSR, is a high-purity platinum-cured silicone with excellent fluidity. It is often injected into a silicone mold cavity to produce different parts with high precision. In general, liquid silicone rubber has a low compression set, good stability, and resistance to extreme heat and cold temperatures. This material is typically used to create seals, sealing membranes, electric connectors, multi-pin connectors, infant products where smooth surfaces are desired, kitchen goods, etc.

liquid silicone rubber for injection molding

Properties of Liquid Silicone Rubber:

  • Tasteless and Odorless: LSR has no taste or odor and is ideal for manufacturing food, beverage, and baby products.
  • Mechanical Properties: LSR has good elongation, high tear and tensile strength, and great flexibility.
  • Durable: LSR parts can withstand extreme temperatures and are suitable for outdoor applications.
  • Chemical Resistance: LSR can resist water, oxidation, and some chemical solutions like acids and alkalis. It can also be sterilized via various methods.
  • Temperature Resistance: It can maintain excellent mechanical properties at temperatures ranging from -60 °C to 250°C.
  • Biocompatibility: LSR is compatible with human tissue and body fluids, is hypoallergenic, and is resistant to bacteria growth.
  • Electrical Properties: LSR exhibits good insulating qualities, making it useful in various electrical applications. Silicone can function at far higher and lower temperatures than traditional insulating materials.
  • Transparency and Pigmentation: Liquid silicone rubber is translucent by nature. Its native color is white, which lends itself well to coloring, enabling the production of custom-molded items in all kinds of colors.

What is Liquid Silicone Rubber Injection Molding?

Liquid silicone rubber (LSR) injection molding, or liquid injection molding (LIM), is a thermoset process to produce a high volume of pliable, durable, intricate, and precise silicone rubber parts. This process first mixes two compounds (generally consisting of the base-forming silicone and the platinum catalyst). Then, the mixture is injected and heat-cured inside a mold to create flexible silicone parts. Notably, two compounds require intensive distributive mixing while keeping them at a low temperature before it is pushed into a heated cavity and heat-cured. The liquid silicone rubber is cured through heat, producing solid parts or products. 

Liquid silicone rubber injection molding provides ease of processing, high-volume production, consistent part quality, and improved productivity. A low compression set and the ability to resist extreme temperatures make liquid silicone rubber durable and ideal for demanding applications. Liquid silicone rubber injection molding is popular in various industries, including automotive, healthcare, general industrial, infant care, consumer goods, and electronics. These industries produce parts more efficiently and cost-effectively with easy-to-mold liquid silicone rubber.

How does Liquid Silicone Rubber Injection Molding Work?

Liquid silicone rubber injection molding works by forcing low-viscosity LSR into a mold cavity, where it quickly cures into the final part shape. Here is the complete process step-by-step:

how liquid silicone rubber injection molding works

Step 1: Create LSR Molding Tool

Initially, a molding tool for liquid silicone rubber is fabricated using CNC machining. For the purpose to withstand the LSR injection molding process, this tool must possess high-temperature resistance. Also, the tool can be polished to meet different finish options. The finished molding tool is then put into an LSR-specific injection molding press.

Step 2: Prepare Material

You can prepare uncured liquid silicone in two containers connected to the pumping system. One container holds the base-forming material, whereas another container houses the catalyst, typically platinum based. Both two materials are pumped through a static mixer by a metering pump. Before the material enters the static mixer section, an additional color pigment and other additives can be added. The two compounds must be mixed in a 1 to 1 ratio.

Step 3: Inject the Material into Mold

The material is heated and injected into the mold cavity under high pressure through a nozzle. After filling the mold cavities, the material takes the shape of the molds. The mold decides the shape of the liquid silicon rubber products. Injecting the LSR mixture into the mold cavity under high pressure can ensure it fills the needed details and features.

Step 4: Curing Process

After filling the mold, it is heated to begin the curing process. This converts the liquid silicone rubber substance into a solid silicone rubber part. The size and complexity of the parts or products determine the curing time.

Step 5: Cooling and Demolding

After the LSR products have undergone sufficient cooling, they should be removed and separated from the molds. This is commonly accomplished through the utilization of automated systems. However, a worker is responsible for removing the parts in manual molding operations characterized by less sophistication and smaller production volumes. The occurrence of flashing in LSR molding is quite infrequent, mostly because of the inherent properties of LSR. 

Step 6: Post-molding Secondary Operations

Post-molding secondary operations may be necessary following the demolding and parts removal process. In general, post-molding secondary operations are conducted after the completion of the molding process. Secondary operations encompass a range of jobs such as slitting, printing, marking, assembling, post-curing, etc.

Benefits of LSR Injection Molding

Injection molding is perfect for producing precise LSR parts. It provides parts with precise dimensions, intricate designs, and superior surface finishes. Below are some main benefits of LSR injection molding.

Durability

LSR molded parts have excellent UV and ozone resistance and superior electrical insulation. LSR molding is ideal for gaskets, cushioning pads in portable communications, ruggedized electronic gadgets, and various other applications.

High Precision Products

Liquid silicone molding enables the creation of complicated parts with tight tolerances. The liquid silicone substance may flow into complicated mold cavities, allowing for precise reproduction of features and dimensions.

Reduced Flash

The nature of low viscosity guarantees the liquid silicone rubber fully fills the mold cavities. LSR molding minimizes flash development. Naturally, the requirement for post-molding trimming or finishing operations is reduced.

Automated Processing

LSR molding may be manufactured automatically without any touch with the outside environment. The liquid silicone rubber substance is readily handled and injected utilizing machines, reducing the possibility of human error.

Consistent Quality

The even distribution of the liquid silicone rubber material within the mold results in consistent quality and minimum variation between individual pieces.

High-volume Production

Advanced LSR materials allow manufacturers to produce complex liquid silicone injection molded parts cost-effectively in high volumes.

Less Waste Production

Silicone molding is considered an environmentally friendly solution since it produces minimal to no waste throughout manufacturing.

Furthermore, unlike metals, which cannot be recycled, silicone may be recycled to make other items in the future.

Enhanced Safety

Because of the automation options, operators are not necessary to enter the molding area. Parts can be removed and transported from the injection molding machine using robots, chutes, or conveyor belts. This eliminates handling and the possibility of burns or other safety issues.

liquid injection molding part

Equipment for LSR Injection Molding

Several mechanical components must be installed to complete the liquid injection molding process. A molding machine often requires a metered pumping device in conjunction with an injection unit to which a dynamic or static mixer is connected. The key components of a liquid injection molding machine include:

Injectors. The injection device pressurizes the liquid silicone to help inject the material into the pumping section of the machine. Pressure and injection rates are adjustable at the discretion of the operator.

Metering Units. The metering unit pumps two main liquid materials: the catalyst and the base forming silicone. This can guarantee the two substances retain a consistent ratio while being released simultaneously.

Supply Drums. The principal containers for mixing substances are supply drums, often known as plungers. The supply drums and a pigment container are connected to the main pumping system.

Mixers. After the materials exit the metering units, they are combined in a static or dynamic mixer. Once mixed, pressure is applied to force the mixture into an identified mold.

Nozzle. A nozzle is utilized to aid in depositing the mixture into the mold. The nozzle frequently has an automated shut-off valve to avoid leaking and overfilling the mold.

Mold Clamp. Mold clamps are used to hold the mold during the injection molding process and to open the mold after completion.

Applications of LSR Injection Molding

Liquid silicone rubber injection molding has emerged as an extremely versatile production method across many industries:

Medical and Healthcare

Because of its great biocompatibility, liquid silicone injection molding is a beneficial method for medical purposes. Thus, injection-molded silicone is safe to use on human skin for long periods of time. Moreover, its softness and flexibility make it a popular material in the medical sector.

LSR is widely used in the production of medical devices, such as seals, gaskets, and surgical instruments, due to its sterilization capabilities. Examples of injection-molded medical parts are drug delivery systems, fluid management systems, biotechnology components, seals, catheters, respiratory masks, lenses, and covers for electronic medical devices.

Automotive Industry

Injection-molded silicone has several applications in the automotive field, owing to its extreme temperature resistance allows it to function near engines and other heat sources.

Some typical automotive parts made from silicone include single-wire, radial, flange seals; connectors; assemblies; electronic covers; and A/C vent cushions.

A more common application of liquid silicone can be found in the soft edge of a windshield wiper blade. It is a suitable material for this application since it does not scratch the windshield surface. In addition, liquid silicone blades also tend to last longer than traditional rubbers since they do not degrade over time when exposed to sunlight.

Industrial Sector

In the industrial field, injection molding liquid silicone suits items such as seals and gaskets. Strain relief devices and grommets are other popular silicone parts used in industrial applications.

These molded parts are long-lasting and have excellent UV, chemical, and temperature resistance.

Electronics

Due to its exceptional electrical insulation properties, LSR produces electronic components, such as keypads, connectors, seals, water-tight gaskets, connectors, buttons, and switch pads.

Consumer Products

Liquid silicone injection molding can be used to create kitchenware, wristwatches, wearable technology, toys, pacifiers, infant bottles, and personal care items, among other consumer goods. This is because the LSR has properties of durability, flexibility, and ease of cleaning.

Silicone Rubber Parts

Design and Manufacturing Considerations of LSR Injection Molding

Some essential tips and considerations can ensure successful LSR injection molding. Below are some of these critical design and manufacturing considerations:

Part Design Rules

The design requirements for liquid silicone injection molding differ slightly from those for thermoplastic injection molding.  Fortunately, silicone parts have simpler design requirements than thermoplastics.

Silicone is simple to remove from a mold because it is flexible. This eliminates the need for designers to consider ejector pin location, as the part may be lifted manually from the mold and allows them to be looser with draft angles. Furthermore, because liquid silicone is shear thinning, it flows freely to all mold sections, eliminating the demand for highly consistent wall thicknesses. Silicone parts can be thicker than thermoplastic parts.

Another design option provided by liquid silicone injection molding is the ability to have protruding undercut areas. Because the material is soft and pliable, the part and its irregular protrusion can frequently be dragged out of the mold without requiring a side action.

The viscosity of the liquid implies it can readily leak at the separating line if not properly sealed, which is a restriction of liquid silicone injection molding. To mitigate this issue, it is critical to ensure that the tooling is correctly sealed.

The following are general rules to follow:

  • Short parting lines help to reduce the impact of flash.
  • For particularly shallow parts, no draft is required; otherwise, one degree per 2.5 cm.
  • Gates should be installed on the underside or in regions where they are not visible.

Metering and Mixing

In contrast to thermoplastic injection molding, liquid silicone injection molding needs two materials, and one of them is a platinum catalyst. These materials are pumped out in a constant ratio via a metering unit and mixed with a static or dynamic mixer.

Working Principle

In the process of thermoplastic injection molding, a thermoplastic is first heated and converted into a liquid, which is subsequently injected into a mold. The material is cooled following this injection to facilitate its reversion to a solid state.

However, liquid silicone does not work like the thermoplastic injection molding process. It is liquid at room temperature, and the mold is heated to accelerate curing after injection. Manufacturers may employ a cooled runner and sprue system to keep liquid silicone from curing before it enters the mold.

Other Considerations

  • Ensure the process and mold design support proper bonding between the substrate and the LSR.
  • Potential variations in dimensions may occur because of curing and material shrinkage. In this case, it is recommended to consult your mold designers to achieve accurate dimensions.
  • Design your parts with the proper draft angles and reduce undercuts to ensure easy ejection.
  • Choose the texture that suits your silicone rubber parts’ functional requirements and aesthetics.
  • Proper mold design, including gate placement, venting, and cooling channels, is vital in achieving optimal part quality and reducing cycle times.

LIM Silicone Molding Vs. Compression Molding: What’s the Difference?

  1. LIM silicone molding is accomplished with an injecting device, whereas the compression force carries out silicone compression molding.
  2. At a higher level, compression molding is regarded as a cost-effective and optimal process. LIM silicone molding, on the other hand, is a pricey approach.
  3. LIM is a more efficient and less time-consuming method, whereas silicone compression molding takes time to complete.
  4. Aside from cost and efficiency, the LIM can manufacture various shapes and sizes quickly and with minimum labor. Conversely, silicone compression molding can make any size and shape with thick and thin walls with the assistance of personnel.
  5. There are thermal, chemical, and electrical applications for SIM molding. Simultaneously, compression molding is extensively utilized in kitchen wares, infant goods, and pet products.

Summary

Liquid silicone rubber injection molding is a versatile and efficient manufacturing process that offers numerous advantages over traditional rubber molding techniques. Its design flexibility, high precision, and consistency make it suitable for various applications across various industries. This process is an ideal choice for mass production and complex shape products. 

Are you looking for a reliable supplier to provide high-quality silicone rubber mold for your projects? LEADRP is here for you. We are a one-stop prototyping manufacturer with various services, including injection molding, CNC machining, vacuum casting, and 3D printing. Whether you want intricate prototypes or precision parts, LEADRP will offer an efficient and cost-effective solution for you. Contact us and get an instant quote.

Reference

Injection molding of liquid silicone rubber – From Wikipedia

FAQs

LSR is the material of choice for a wide range of applications, including automotive and medical equipment, electronic parts, bakeware other household items, and more. LSR is suitable for single- and two-shot injection molding, as well as overmolding onto other components.

Yes, liquid silicone rubber is biocompatible and widely used in medical applications due to its safety and sterilization capabilities.

Yes, LSR exhibits excellent resistance to both high and low temperatures, making it suitable for applications that require heat or cold resistance.

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