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Liquid Silicone Rubber (LSR) Injection Molding Process: A Comprehensive Guide

By 8.1 min readPublished On: May 22nd, 2023Last Updated: May 22nd, 2023Categories: Silicone industry knowledge

Liquid Silicone Rubber (LSR) injection molding is a versatile and widely used manufacturing process in the silicone industry. It allows for the production of complex, high-quality silicone products with excellent precision and efficiency. Whether you’re new to the world of LSR or looking to enhance your understanding of the process, this comprehensive guide will provide you with valuable insights into LSR injection molding.


In this article, we will delve into the key aspects of the LSR injection molding process, its advantages, and its applications. We will explore the composition and properties of LSR, as well as its unique benefits compared to other silicone materials. By understanding the fundamentals of LSR, you will gain valuable knowledge that will help you make informed decisions in your manufacturing processes.

I. Understanding Liquid Silicone Rubber

Liquid Silicone Rubber (LSR) is a two-part platinum-cured silicone elastomer that is widely used in various industries such as healthcare, automotive, electronics, and consumer goods. Its unique properties make it an excellent choice for applications that require flexibility, durability, and heat resistance.

A. Definition and Composition of LSR

LSR is composed of two components: a liquid base polymer and a cross-linking agent. The liquid base polymer consists of dimethyl siloxane, while the cross-linking agent typically contains vinyl-terminated siloxane with a platinum catalyst. When the two components are mixed, a chemical reaction occurs, resulting in the formation of a three-dimensional cross-linked network.

B. Properties and Benefits of LSR

LSR exhibits a wide range of desirable properties, including excellent heat resistance, low compression set, superb electrical insulation, and biocompatibility. These properties make LSR suitable for a diverse array of applications. For example, in the healthcare industry, LSR is used for manufacturing medical devices such as catheters, implants, and seals due to its biocompatibility and durability.

Additionally, LSR offers outstanding resistance to extreme temperatures, making it ideal for applications in automotive and electronic components. Its ability to maintain its properties in both high and low temperatures ensures optimal performance and longevity of the end products.

C. Applications of LSR

LSR finds applications in various industries, including healthcare, automotive, electronics, and consumer goods. In the healthcare sector, LSR is used for producing items like pacifiers, baby bottle nipples, and surgical instruments. Its biocompatibility, easy sterilization, and softness make it highly suitable for these applications.

In the automotive industry, LSR is utilized for manufacturing components such as gaskets, seals, and connectors. The excellent heat resistance, chemical resistance, and durability of LSR ensure reliable performance even under challenging conditions.

LSR is also used in the electronics industry to produce keypads, seals, and connectors. Its exceptional electrical insulation properties, thermal stability, and resistance to environmental factors make it an ideal material for these applications.

II. LSR Injection Molding Process

LSR injection molding is a precise and efficient manufacturing process that involves injecting liquid silicone rubber into a mold cavity and allowing it to cure into the desired shape. Understanding the various steps involved in the LSR injection molding process is essential for achieving high-quality and consistent results.

A. Mold Design and Preparation

To start the LSR injection molding process, careful consideration must be given to mold design and preparation. The mold material selection plays a crucial role in achieving desired results. Typically, molds made of hardened steel or aluminum alloys are preferred due to their durability and heat conductivity.

Factors such as draft angles, gate placement, and parting lines should be considered during mold design to ensure proper moldability and easy ejection of the final product. Computer-aided design (CAD) software can be employed to create precise and optimized mold designs.

B. Material Preparation and Mixing

Before the injection molding process begins, the LSR material needs to be prepared and mixed. LSR is available in various grades and durometers to meet specific application requirements. It is crucial to select the appropriate LSR material based on factors such as hardness, chemical resistance, and temperature resistance.

The LSR material is mixed with a cross-linking agent and additives to achieve the desired properties. The mixing process should ensure proper dispersion of the components and the removal of any air bubbles or impurities. Vacuum-assisted mixing or dynamic mixing equipment can be used to achieve optimal results.

C. Injection Molding Machine Setup

The setup of the injection molding machine is a critical step in the LSR injection molding process. The machine should be selected based on factors such as clamping force, shot size, and the capability to control temperature and pressure accurately.

Barrel heating and temperature control are essential to maintain the LSR material at the desired temperature during the injection process. The LSR material must be heated to a temperature suitable for injection without degrading its properties. Accurate temperature control ensures consistent results and reduces the risk of defects.

The injection unit and process parameters, including injection speed, injection pressure, and injection time, should be set based on the specific requirements of the LSR material and the geometry of the part. Fine-tuning these parameters can help achieve optimal flow and fill characteristics, minimizing the risk of defects.

D. Injection Molding Process Steps

The injection molding process consists of several steps, including mold clamping, injection, curing, cooling, and ejection.

  1. Mold Clamping and Injection Unit Alignment

The mold is securely clamped to prevent any movement during the injection process. The injection unit, comprising the injection barrel, screw, and nozzle, is aligned with the mold cavity to ensure precise and accurate material flow.

  1. Injection and Curing Cycle

The LSR material is injected into the mold cavity through the injection nozzle under controlled pressure. The material flows and fills the cavity, taking the shape of the mold. The curing process begins immediately, triggered by the platinum catalyst in the LSR.

The curing time varies depending on the LSR material and the part’s complexity. During the curing phase, the LSR material cross-links and solidifies, forming a durable and resilient silicone part.

  1. Cooling and Ejection

After the curing cycle is complete, the mold is cooled to allow the LSR part to solidify further. Proper cooling is crucial to prevent deformation or warping of the part. Once cooled, the mold is opened, and the finished LSR part is ejected from the mold using ejector pins or other mechanisms.

III. Optimization and Troubleshooting

To achieve the best results in LSR injection molding, optimization of the process parameters and troubleshooting common issues are essential.

A. Process Optimization

Optimizing the LSR injection molding process involves adjusting various parameters to improve part quality and efficiency.

  1. Shot Size and Injection Pressure

The shot size, determined by the amount of LSR material injected into the mold, should be optimized to ensure complete filling of the mold cavity without excess material. Injection pressure should also be optimized to achieve proper flow characteristics and avoid defects such as short shots or flash.

  1. Injection Speed and Time

The injection speed and time play a crucial role in achieving optimal filling of the mold cavity and preventing flow-related defects. Adjusting the injection speed and time can help control flow front progression and minimize the risk of air entrapment or voids.

  1. Curing Temperature and Time

The curing temperature and time need to be carefully controlled to achieve thorough cross-linking of the LSR material. Proper curing ensures the desired mechanical properties and dimensional stability of the final product. It is important to follow the manufacturer’s guidelines for curing parameters specific to the LSR material being used.

B. Common Challenges and Troubleshooting

Despite careful optimization, challenges may arise during the LSR injection molding process. Understanding and effectively troubleshooting these challenges are key to ensuring successful outcomes.

  1. Flash and Parting Line Issues

Flash refers to excess material that escapes from the mold cavity during the injection process, resulting in thin, unwanted extensions on the part. Proper mold design, appropriate clamping force, and control of injection pressure and speed can help minimize flash. Parting line issues, such as visible seams or mismatched surfaces, can be addressed through mold design improvements and precise alignment of mold components.

  1. Air Traps and Voids

Air traps and voids can occur when air becomes trapped in the mold cavity, leading to incomplete filling or localized defects. Proper venting of the mold and optimization of injection parameters, such as speed and pressure, can help minimize the risk of air traps and voids.

  1. Overcure and Undercure

Overcure and undercure refer to improper cross-linking of the LSR material, resulting in either excessive hardness and brittleness or inadequate curing and reduced mechanical properties. Fine-tuning curing parameters, such as temperature and time, is crucial to achieve optimal cross-linking and avoid overcure or undercure.

In conclusion, the LSR injection molding process is a versatile and efficient method for manufacturing high-quality silicone products. Understanding the fundamentals of LSR, from its composition and properties to the various steps involved in the injection molding process, is essential for achieving consistent and reliable results.

By optimizing the process parameters and addressing common challenges through troubleshooting, manufacturers can unlock the full potential of LSR and harness its unique qualities for a wide range of applications in industries such as healthcare, automotive, electronics, and consumer goods.

As the demand for silicone products continues to grow, mastering the LSR injection molding process becomes increasingly crucial for manufacturers to stay competitive and deliver innovative solutions. With its exceptional properties and the ability to produce intricate designs, LSR offers endless possibilities for creating durable, flexible, and heat-resistant silicone products that meet the demands of today’s market.

By implementing the best practices outlined in this comprehensive guide and staying updated on the latest advancements in LSR technology, manufacturers can achieve remarkable outcomes and unlock new avenues of success in the silicone industry. So, embrace the possibilities of LSR injection molding and unleash the potential of liquid silicone rubber to shape a better tomorrow.