Comprehensive Guide to Designing for Liquid Silicone Rubber (LSR) Molding

Table of Contents

Liquid Silicone Rubber molding
Liquid Silicone Rubber molding


Liquid Silicone Rubber molding (LSR) has gained widespread adoption as an injection molding method across various industries for several compelling reasons. One key factor contributing to its popularity is its ability to offer strength, lightweight properties, and effortless molding into desired shapes, making it highly sought after in diverse sectors.

Furthermore, unlike thermoplastic elastomers that tend to deform and lose shape under high temperatures, LSR demonstrates exceptional heat resistance, further enhancing its appeal among manufacturers.

Despite its numerous advantages, it’s essential to consider certain limitations associated with liquid silicone rubber molding. However, all these factors will be thoroughly examined in this post. So, without further delay, let’s explore these aspects in detail.

silicone rubber pacifier
silicone rubber pacifier

Section 1: Understanding Liquid Silicone Rubber (LSR)

Liquid silicone rubber (LSR), a type of silicone known for its two-component composition and high purity, is widely utilized in industries requiring products with exceptional quality, strength, and resistance. Typically manufactured through a process called liquid injection molding (LIM), LSRs are versatile materials commonly used in the production of various goods, including those in consumer goods, automotive, food, appliances, and textiles.

Transitioning to its advantages, let’s delve into why silicone rubber is highly favored:

Firstly, its remarkable thermal stability ensures that it maintains its shape, hardness, and flexibility even in extreme temperatures, both hot and freezing. With the ability to withstand temperatures up to 180°C without melting or distorting, LSRs are increasingly becoming the material of choice in industrial and automotive applications, such as for seals and gaskets.

Furthermore, silicone rubber boasts excellent environmental resistance, making it resilient against harsh elements like UV rays, open flames, high temperatures, and corrosive chemicals. This durability makes it the ideal choice for applications requiring prolonged exposure to adverse conditions.

Additionally, silicone rubber is incredibly versatile, offering flexibility in molding to meet diverse requirements. Its formulation can also be customized to emphasize or minimize specific properties like electrical conductivity, flexibility, and toughness, catering to a wide range of applications across industries.

Again, it allows for easy sterilization, crucial for medical-grade applications while offering vibrant color options, making it versatile in aesthetic designs.

Moreover, its superior electrical properties make it ideal for electrical components and high-voltage applications. Furthermore, its durability and reliability ensure longevity, particularly in critical applications.

Lastly, its low compression set ensures resilience under continuous tension, maintaining elasticity even after prolonged use and exposure to varying temperatures.

silicone rubber material
silicone rubber material

Section 2: Material Selection for LSR Molding

Choosing the appropriate LSR material is essential to achieve the desired characteristics in the final product. Liquid silicone rubber (LSR) injection molding relies on a variety of materials to fabricate durable, high-quality parts.

Among the most frequently used materials in Silicone LIM is high-purity platinum-cured silicone, capable of crafting robust parts suitable for diverse industries. For instance, liquid silicone rubber is widely favored in the medical sector due to its biocompatibility, precision, and reduced risk of contamination. Another emerging material in LIM is fluorinated liquid silicone rubber, particularly for automotive and precision parts.

To enhance properties such as strength and stiffness, fillers like glass, mica, and silica are introduced into silicone polymers. These fillers not only improve dimensional stability but also minimize shrinkage during curing. Adjusting the filler content enables the attainment of specific product properties.

Additionally, pigments are incorporated into silicone polymers to introduce color and enhance aesthetics. Whether aiming for a transparent or opaque finish, pigments offer versatility in achieving the desired appearance. Moreover, pigments enhance UV stability, ensuring long-term protection against discoloration and degradation.

Given the critical role of materials in the LSR process, it is imperative to collaborate with a seasoned LSR injection molding partner like TDL Mold Co., which can provide expert guidance in selecting the optimal material for your unique application.

Design for silicone rubber parts
Design for silicone rubber parts

Section 3: Key Design Principles for LSR Parts

When it comes to crafting parts for LSR injection molding, several key factors demand careful consideration to ensure a seamless production process. These encompass material selection, mold design, part geometry, and post-molding operations.

First and foremost, the selection of materials is paramount. Liquid silicone rubber presents a spectrum of options, each with distinct properties. Choosing the suitable material is essential for attaining desired characteristics like resistance to temperature and chemicals.

Following this, meticulous mold design is indispensable. The mold must undergo optimization to realize the desired part geometry while taking into account factors such as material flow, cooling, and ejection. Adequate ventilation and cavity configuration are pivotal for achieving high production efficiency.

Equally significant is the optimization of part geometry. Fine-tuning wall thickness and geometry ensures that the final product meets both functional and aesthetic standards. Techniques such as incorporating draft angles and ribs contribute to smooth mold ejection and enhanced structural integrity.

Moreover, post-molding operations necessitate careful planning. Processes like cutting and assembly should be streamlined to minimize waste and labor costs.

In addition, other considerations include addressing undercuts, positioning ejector pins, and determining parting lines. Integrating these elements into the design process ensures that the final product is manufactured efficiently and meets all specifications.

Ultimately, crafting parts for LSR injection molding requires meticulous attention to material, mold, geometry, and post-molding operations to achieve optimal outcomes.

Meticulous Mold Design for LSR
Meticulous Mold Design for LSR

Section 4: Mold Design Considerations for LSR

When it comes to designing parts for LSR injection molding, there are specific factors to keep in mind, which may differ from traditional thermoplastic injection molding methods. Here are some key design considerations to ensure success in liquid silicone rubber (LSR) injection molding:

Design Flexibility: Unlike thermoplastics, designing silicone parts is simpler due to the material’s inherent flexibility. This allows for easier part release from the mold and reduces the need for precise ejector pin placement or strict draft angles.

Material Properties: Liquid silicone rubber is shear thinning, meaning it flows easily throughout the mold, accommodating variations in wall thickness. Consequently, silicone parts can be thicker than those made from thermoplastics.

Undercut Consideration: The soft and malleable nature of LSR enables the molding of parts with protruding undercut areas, often eliminating the need for secondary actions for part removal.

Sealing Importance: However, the viscosity of liquid silicone can result in leakage at the parting line if the mold is not properly sealed. Hence, ensuring effective sealing is crucial.

General Guidelines: Adhering to certain guidelines, such as minimizing flash impact, determining appropriate draft angles, and discreetly positioning gates, can help optimize the injection molding process.

Dosing and Mixing: Unlike thermoplastic molding, LSR injection molding requires the precise dosing and mixing of two materials, which are then combined through a mixer to achieve the desired properties.

Process Differences: While thermoplastics are heated to a liquid state before injection, liquid silicone remains in liquid form throughout the process. The mold is heated to accelerate curing after injection, with cooling channels preventing premature hardening.

Additional Considerations: It’s important to ensure a strong bond between the substrate and LSR, anticipate potential dimensional variations, design parts with suitable draft angles, minimize undercuts, choose appropriate textures, and optimize mold design for quality and efficiency.

Demolding Strategies: Although LSR parts are typically demolded manually, automated demolding options can improve efficiency. Designing parts to remain within one mold half and incorporating protruding elements for easier demolding can streamline the process.

Silicone, unfortunately, cannot be recycled. However, a cold runner system offers a solution. This system directs the material directly into the cavity, bypassing the need for runners in molds. As a result, less heat is absorbed by the material, providing greater control over cavity filling.

Furthermore, cold runner molds are advantageous in high-volume scenarios with key benefits in the reduction of material waste, as only the required amount is used for the molded part.

Silicone injection molding machine
Silicone injection molding machine

Section 5: Processing Techniques for LSR Molding

To ensure successful LSR molding, it’s vital for manufacturers to grasp specific processing techniques. Here, we’ll delve into key parameters that LSR designers and operators need to bear in mind:

Starting with Shelf Life, the mixture (A+B) typically lasts around 72 hours at room temperature (25ºC) and 5 to 10 hours at 50ºC, depending on the type of LSR used. Adequate production planning is necessary to prevent the polymer from becoming unusable.

Next, Injection Pressure plays a critical role. While injection pressures for liquid silicone rubber are relatively low, ranging between 4 and 14 MPa, machines can handle much higher pressures (50 to 240 MPa) to expedite cycle times. Counterpressure is usually regulated between 0.5 and 3 MPa.

When it comes to Closing Pressure, there’s a wide variation, from 2 to 50 MPa, with higher pressures required due to forces from parts with high coefficients of thermal expansion, especially softer rubber types.

Injector Temperature is another key consideration. Injection nozzles can operate at temperatures close to the dripping point, crucial for reducing vulcanization times, especially for thicker pieces.

Moving on to Injection Speed and Time, injection speed ranges from 30 to 400 cm^3/second, with injection time typically varying between 5 and 10 seconds.

Considering Mold Types and Degassing, common mold types include joint, diaphragm, pin nozzle, and submarine injection points, with vacuum assistance for mold degassing. For abrasive LSR types, surface hardening of the mold is recommended.

Mold Temperature is also significant, with vulcanization temperature varying between 140ºC and 230ºC, depending on the type of silicone rubber and vulcanization system used.

Vulcanization Time varies depending on silicone rubber type and thickness, typically ranging from 5 to 12 seconds per millimeter of thickness for peroxide vulcanization and 3 to 6 seconds per millimeter of thickness for platinum-catalyzed systems.

Finally, Shrinkage ranges from 2% to 4%, depending on silicone type, composition, and vulcanization system, with lower shrinkage values associated with additional vulcanization systems. Other factors influencing shrinkage include temperature, molding pressure, dimensions, and post-processing conditions.

Section 6: Post-Processing and Finishing Techniques for LSR Parts

Silicone rubber items vulcanized with peroxides usually undergo post-processing in an oven. This step completes the cross-linking process and removes gaseous substances formed during vulcanization, enhancing properties like residual deformations, heat resistance, and adhesion to other substrates. It also improves dimensional stability. However, certain types of silicone rubber don’t require post-vulcanization.

During post-processing, heated ovens with proper ventilation are essential to prevent explosive conditions due to released gasses. Additionally, maintaining a minimum air renewal rate of 125 liters per minute per kg of vulcanized articles is crucial. Furthermore, heating methods, such as burning gas or electrical resistances, can increase danger if not handled carefully. That is why manufacturers often recommend a 4 to 6-hour post-processing duration at 200ºC, with a gradual temperature rise for thicker products to prevent bubble formation.

Moreover, it’s essential to eliminate released gasses outside the work area to avoid health risks. Still, it is important to note that this post-processing phase induces a new shrinkage phenomenon ranging from 0.5% to 0.8%, which should be factored into mold design considerations.

Surface finishes are usually decided based on customer requirements or product design specifications. However, when there’s a choice available, opting for a non-glossy finish is preferred, typically indicated by an SPI mold finish of B1 or lower. This type of finish, lacking a high gloss, prevents LSR from adhering to the mold surface, thus facilitating the demolding process.

Additionally, post-molding processes such as defogging, inspection, and packaging may be necessary to ensure the final product meets quality standards.


Liquid Silicone Rubber molding LSR, is a method of manufacturing that stands apart for its unique versatility and advantages compared to traditional injection molding techniques. Despite the initial investment cost and limited material options, LSR injection molding is widely embraced across diverse industries due to its flexibility in design, high precision, and capability to produce intricate geometries with tight tolerances. This innovative process utilizes liquid silicone rubber to create parts with remarkable precision and adaptability.

Fortunately, TDL Mold Co. offers customizable solutions to address all your Liquid Silicone Rubber molding needs, including, dimensional tolerances, and desired aesthetic appearance.

Should you require assistance with your LSR project, simply click here to leave a message, and a representative will promptly reach out to you.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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