After more than a hundred years of development, injection moulding has been improved and enhanced and has become the most popular and widely used processing method in various fields ranging from daily consumption to medical care, and even to extreme industries (aerospace and aviation). Generally speaking, some application materials for injection molding include most thermoplastic and thermoset polymers, and metal substrates. Liquid silicone rubber (LSR) is a member of the silicone rubber family. Due to LSR's unique and shorter molecular chain structure, it has excellent rheology, such as low viscosity, high thermal expansion coefficient, and shear thinning. The excellent combined properties make it the first choice for injection molding applications.

Liquid silicone rubber can be processed using different molding methods, such as injection, casting, and coating. In this article, we will describe the processing technology most often used: Liquid silicone rubber injection molding. It is also called Liquid injection molding (LIM), and they are often used interchangeably.

Today we will introduce in detail the definition of liquid silicone rubber injection molding, how it works, applications, etc. From this article, you will learn more about LSR injection molding and some of the unique advantages of this process.

What is liquid silicone rubber(LSR) injection molding?

A brief history of liquid silicone rubber injection molding

From the late 1970s to the early 1980s, as the significant advantages of this processing method were highlighted and the cost benefits it brought, it received great attention from various countries and the demand for it surged, especially in Europe and the Americas. During this period, although it began to be used in industrial production, certain technical constraints (how to coordinate fluidity and strength, operation timeliness, complex mold design level) led to the slow development of the LSR process and hindered its promotion in more fields.

From the late 1980s to the 1990s, entered a stage of comprehensive global expansion, especially in emerging market countries. The introduction of MQ silicone rubber, the use of methyl hydrogen siloxane and alcohol inhibitors, and the innovation of cold runner technology in molds promote its application in more fields. Its application range ranges from narrow military fields to a wide range of commercial fields, including automotive, power, consumer, electronics, electrical, and medical.

Since the 21st century, it has entered a systematic and intelligent stage. This technology has evolved from single-piece molding to two- or multiple-injection molding, making it possible to batch-assemble original parts with extremely complex shapes and different properties into one component.

How does liquid silicone rubber injection molding work?

Liquid silicone rubber injection molding is a continuous process in a closed system. How does it use liquid silicone rubber to achieve high-precision injection molding? The following is its basic process as follows:

Step 1: Metering and Pumping

Using a metering device, components A (containing platinum catalyst) and B (containing cross-linking agent) of the same or different viscosities are extracted and transported from 20KG or 200KG drums with extremely high ratio (1:1) accuracy and stability. If the product needs to add pigments or other additives, an independent additive metering pump can be added to meet the specific performance and color requirements of the final product. Generally, a two-way pump is used in the pump body to ensure that component A and component B are continuously pressed out simultaneously.

The two components (containing other additives) are passed through a mixing device and mixed under shear to completely disperse all ingredients. This ensures the integrity and high consistency of all parts after molding. Since the static mixing system has a simple structure is easy to clean, and can meet general mixing needs, so most injection machines currently use static mixers. If you need more uniform mixing, you can choose a dynamic mixer with a complex structure.

Step 3: Injection

The mixed material is delivered to the barrel through a tube under pressure and pushed directly into a sealed preheated mold cavity using a screw at a predetermined pressure and speed. The polymer will first pass through a valve-controlled needle valve cold nozzle and then evenly distribute into each mold cavity. Since the mixture is thermosetting, it needs to be cooled by the cooling device of the injection machine barrel to ensure that it remains below 30C° before being injected into the mold.

Step 4: Mold clamping

After the material fills the cavity, the mold clamping device is linked with each functional device to provide tight closing pressure to fix the mold. LSR after mold clamping still has good fluidity and a high expansion coefficient when cured. Therefore, the mold clamping system needs to provide continuous and stable pressure to ensure that the liquid silicone rubber does not overflow or leak.

Step 5: Curing and molding

Increase the heat of the mold and reach the highest temperature required for curing (generally 130C°-180C°), allowing the compound to undergo a Polyaddition reaction inside the mold, and finally forming parts consistent with the final design shape and prefabricated mechanical properties. The time required for the reaction to complete generally ranges from seconds to minutes.

Step 6: Demold

The formed final product is pushed out through an independent liquid system. The mold opening pressure of this process increases from small to large.

Why Choose Liquid Silicone Rubber Injection Molding?

Injection molding with liquid silicone rubber offers a variety of unique advantages in applications where hygiene, dimensional accuracy, and automation are critical. Compared with the traditional solid silicone rubber molding process, it eliminates some labor-intensive operations (mixing, preforms) and reduces upfront capital and plant area requirements. It is a fully closed system, that ensures that the material does not come into contact with any substances that may cause defects in the parts, it is widely used in mass manufacturing of multi-functional precision components of medical devices. This process is particularly suitable for production in ISO Class 7 or ISO Class 8 clean rooms to meet certain special requirements of customers.

It is highly automated, meaning it delivers the highest levels of productivity, precision, and overall quality. The advantages of this production process also include:

Eliminates the risk of assembly errors, deviations, and contamination

Reduces warehouse requirements, turnaround times, and ultimately unit costs

Uses needle valve type cold runner to realize the manufacturing of water-free materials

Achieves venting and better sealing by adding vacuum equipment to the mold

Uses the highest purity silicone and platinum curing agents, this ensures zero contamination, optimal safety, and the fastest molding cycle

Liquid silicone rubber injection molding can not only injection mold a single silicone material but can also perform secondary or multiple injection molding of LSR and different types of polymers or metals. It can be used to develop innovative solutions, for example by combining original parts with different properties into a more compact, more powerful component in a limited space. It provides greater freedom and flexibility in design, allowing R&D researchers to develop more economical and reliable solutions, completely eliminating the hidden dangers and additional expenses associated with assembly, thereby providing component manufacturers with more durable, versatile key components.

Why is liquid silicone rubber (LSR) suitable for injection molding?

With the increasing diversification and application of silicone formulations, stringent emerging industries have posed new challenges and requirements for manufacturing and design, including dimensional accuracy, multiple functions, and service life, especially for medical device seals and baby pacifiers.

Liquid Silicone Rubber (LSR) is a high-purity, platinum-cured two-component system with a range of exceptional properties. It has low viscosity (usually 60 to 1500 Pa.s), which means it has high rheology. This is very suitable for injection molding processes where raw materials are transported through a pump system and helps fill fine parts of the mold, especially Suitable for producing O-rings or other thin-walled parts. It is resistant to extreme temperatures (-50°C to +230°C), resistant to corrosion, and therefore able to withstand the highest temperatures in the injection molding process and contact with different media.

Because LSR uses a unique platinum curing process, it can accelerate the curing reaction by increasing the temperature while maintaining the properties of the final part during injection molding, achieving rapid cycles. It exhibits good tear resistance at high temperatures, giving molded parts excellent release properties and easy removal from the mold. It is high-purity and transparent, and the parts that can be manufactured through the LSR molding process have excellent reactive inertness, high biocompatibility and biostability, and excellent transparency. These inherent properties make it possible to produce medical tubing and food packaging containers with high-quality consistency and high throughput.

It is flexible and can be used alone or combined with other elastomers and substrates through the LSR process. This provides component manufacturers with different solutions and greater choices. Compared with other ordinary plastic materials, it can be molded at lower injection pressure (3-12 Mpa).

The difference between liquid silicone rubber injection molding and injection moulding

Injection molding using liquid silicone rubber is generally similar in process to traditional injection moulding. However, in some processing devices and curing methods, they are completely opposite, as follows:

Equipment

Injection moulding uses the heating device of the injection molding machine barrel to melt the mixture before injection and uses the cooling system of the mold to control the temperature. In contrast, the LSR process uses a cooling device to cool down the compound and keep the mixture at a low temperature before vulcanization. In addition, LSR injection molding uses heating elements (heating rods) to heat the mold.

Vulcanization

Injection moulding usually molds thermoplastic plastics. The principle is to push high-temperature homogenized melt into the mold cavity through high pressure. The solid from this process can be melted and used again. Liquid silicone rubber injection molding uses unheated polymer and solidifies the silicone rubber in a cavity through a combination of increased mold temperature and low pressure applied to the liquid silicone rubber. This molded state is irreversible.

Liquid silicone rubber injection molding application fields

The LSR process is the optimal processing method for manufacturing parts in general industry, infant products, medical equipment, and other fields. Take food-grade liquid silicone rubber using this process to produce pacifiers as an example. By analyzing the production process control of baby pacifiers, the best process parameters can be obtained to create the most efficient and best-performing products.

Molding temperature

The setting of the vulcanization temperature depends on the raw material properties and the material of the mold. Too low a curing temperature will extend the production cycle. Excessively high molding temperatures increase energy costs and even cause mold deformation. Considering the smaller size of the product, generally LSR curing at 150℃~180℃ has the best economic benefits.

Injection pressure

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Summary

Liquid silicone rubber injection molding is valued because it combines non-polar silicone-oxygen bond structure silicone rubber with multiple advantages (fluidity, bio-inertness, temperature resistance, long-term durability) and efficient and stable molding technology which provides the highest yields and lowest unit production costs.

Yuanyi is a leading silicone parts manufacturer capable of providing high-level business support to customers around the world. Our team provides expertise on your project from the concept stage, product design, material grade or hardness selection, production, and quality inspection through its state-of-the-art liquid silicone rubber molding technology to help customers speed up time to market.