The process of designing Liquid Silicone Rubber injection molds requires a comprehensive understanding of both the molds and the unique molding process of Liquid Silicone Rubber.While these molds share similarities with thermoplastic molds, the distinct properties of Liquid Silicone Rubber (LSR) create notable differences. Key aspects include LSR’s lower viscosity, leading to shorter mold filling times even under low injection pressures, and the need for efficient venting systems to prevent air entrapment. Additionally, unlike thermoplastics, LSR tends to expand when heated and contract slightly when cooled, affecting how it adheres to the mold surfaces during the molding process.
Shrinkage Rate in LSR Molding
Understanding and managing the shrinkage rate is crucial in Liquid Silicone Rubber injection molding. While LSR does not shrink significantly inside the mold, it typically experiences a shrinkage of 2.5% to 3% upon demolding and cooling. The exact shrinkage level depends partly on the rubber’s formula. From a mold design perspective, several factors influence the shrinkage rate. These include the mold temperature of the rubber at the time of demolding, the pressure inside the mold cavity, and the subsequent rubber compression.
The position of the injection point is also a critical consideration. Shrinkage along the direction of rubber flow is usually more significant than in the perpendicular direction. The product’s dimensions also play a role; thicker products generally exhibit lower shrinkage rates compared to thinner ones. Additionally, if a secondary vulcanization process is required, the rubber may shrink by an additional 0.5% to 0.7%.
Parting Line Considerations
Establishing the parting line’s location is one of the initial steps in designing silicone rubber injection molds. Grooves strategically located along the parting line primarily achieve venting, which is crucial for preventing air entrapment and reducing strength loss at the seams. The positioning of these grooves at the last area reached by the injected rubber enhances the venting process.
Given the low viscosity of LSR, the precision of the parting line is paramount to prevent flash (excess material spilling out of the mold). Despite this precision, the parting line is often visible on the finished product. The ease of demolding is influenced by the product’s geometric dimensions and the position of the parting surfaces. Designing the product with slight draft angles ensures consistent adhesion to the required half of the mold cavity.
These considerations are essential in achieving optimal results in LSR molding. By carefully managing shrinkage rates and parting lines, manufacturers can produce high-quality silicone products with minimal defects, ensuring efficiency and consistency in production.
Effective Venting in LSR Molding
Venting plays a crucial role in the injection molding of Liquid Silicone Rubber (LSR). As LSR fills the mold, it compresses the air trapped inside, which must then be expelled through vent grooves to avoid defects. These grooves facilitate this process, typically ranging from 1mm to 3mm in width and 0.004mm to 0.005mm in depth. If venting is incomplete, it can trap air within the rubber, often resulting in the finished product having white edges or air pockets.
Creating a vacuum inside the mold serves as an optimal solution for venting. This involves designing a gasket along the parting line and using a vacuum pump to evacuate air from all cavities quickly. Once the vacuum reaches the predetermined level, the mold closes completely, and the injection process begins.
Some injection molding machines allow for variable clamping force. This feature enables molders to initially close the mold under low pressure until 90%-95% of the cavity is filled with LSR, facilitating easier air expulsion. The clamping force is then increased to prevent overflow as the silicone rubber expands.
Injection Points
In the design of Liquid Silicone Rubber injection molds, particularly in the section on injection points, the implementation of a cold runner system is a crucial aspect for efficient molding. This system maximizes the benefits of LSR, significantly enhancing production efficiency. The main advantage of using a cold runner system is the elimination of the need to remove sprues, thus reducing labor intensity and material waste. This can also shorten operation times in many cases.
A needle valve system controls the injection of LSR. Many manufacturers equip their injection nozzles with pneumatic switches as standard, enabling versatile placement within the mold. Additionally, some mold makers have developed a specialized open cold runner system. This space-efficient system permits multiple injection points within a confined mold area. It facilitates mass production of high-quality silicone rubber products without needing to separate the sprue.
When using cold runner systems, it’s important to maintain a significant temperature differential between the hot mold cavity and the cold runner. If the runner is too hot, the rubber may begin vulcanizing before injection. Conversely, if it cools too quickly, it can absorb excessive heat from the mold gate area, leading to incomplete vulcanization.
For products injected through conventional gating systems like submerged gates or cone-shaped gates, it’s advisable to use small-diameter feed ports (typically 0.2 mm-0.5 mm). As with thermoplastic materials, a balanced runner system is crucial for the uniform filling of all mold cavities. The use of flow simulation software in designing runner systems can greatly simplify mold development and validate the effectiveness of the molding process through trials.
Demolding LSR Products
Demolding can be challenging with vulcanized liquid silicone rubber due to its flexibility and tendency to adhere to metal surfaces. However, LSR’s high-temperature tear strength allows for effective demolding under normal conditions without damaging even larger products. Common demolding techniques include using stripper plates, ejector pins, and air blasts. Other techniques like roller stripping, slide plate demolding, and automated demolding systems are also widely used.
Precision in the demolding system is vital. Excessive gaps or wear in ejector pins can cause flash. Conical or mushroom-shaped pins improve contact pressure, seal better, and aid demolding.
Mold Materials
In choosing mold materials for Liquid Silicone Rubber injection molds, it’s essential to select specialized steels and treatments that ensure optimal performance, especially under conditions of high temperature and wear. These material choices are pivotal in defining the durability and efficacy of Liquid Silicone Rubber injection molds.
For the mold base, manufacturers generally use non-alloy tool steel (No. 1.1730, DIN code C45W), a standard due to its balance of durability and machinability. When molds need to withstand temperatures ranging from 170°C to 210°C, pre-hardened steel (No. 1.2312, DIN code 40 CrMnMoS 8 6) becomes the preferred choice, offering enhanced impact resistance crucial for high-temperature applications.
For mold plates housing the cavities, it’s essential to use tool steel treated with nitriding or tempering processes. This treatment ensures that the mold can endure the high temperatures involved in LSR processing while maintaining its structural integrity.
In situations where LSR has a high filler content, such as oil-resistant grades, molds require materials with higher hardness. Options like bright chrome-plated steel or specially formulated powder metals (No. 1.2379, DIN code X 155 CrVMo121) are ideal. These materials offer superior wear resistance, a critical factor in maintaining the quality and precision of the mold over time.
Designing molds with high-wear components as replaceable parts is a strategic approach. This mold design lets specific sections be replaced, not the entire mold. It simplifies maintenance and extends mold life, saving costs over time.
The inner surface of the mold cavity plays a pivotal role in determining the quality of the finished product. Polished steel materials are the go-to for molds intended for transparent products, ensuring a flawless finish. Steel surfaces treated with chrome/nickel or PTFE/nickel coatings are highly effective for enhanced wear resistance and ease of demolding. These coatings not only prolong the mold’s life but also aid in the smooth release of the final product, minimizing defects and ensuring consistency.
Temperature Control
In LSR molding, electric heaters like bands, cartridges, or plates are key for effective temperature control. It’s critical to ensure a uniformly distributed temperature field across the entire mold, promoting even curing of the LSR. For large molds, oil temperature-controlled heating systems are both economical and efficient.
Covering the molds with insulating plates is a beneficial practice to minimize heat loss. Inappropriate heating in parts of the mold can lead to significant temperature fluctuations, causing issues like gas entrapment. A surface temperature that’s too low slows the rubber’s curing, causing demolding problems and lower product quality. It’s essential to maintain a gap between the heaters and parting lines to prevent mold plate warping and avoid flash or burrs on the final product.
It’s vital to ensure complete thermal separation between the hot and cold ends in molds designed with cold runner systems. Special alloys, such as 3.7165 (TiA16V4), are ideal for molds requiring thermal separation due to their low thermal conductivity. Placing insulating plates between the mold and its base minimizes heat loss to enhance the heating system’s efficiency. This energy-conserving setup ensures consistent temperatures, enhancing the molding process and product quality. Such proactive temperature management is key to the precision and excellence required in LSR molding.
Conclusion
Designing Liquid Silicone Rubber injection molds necessitates deep knowledge of Liquid Silicone Rubber characteristics. The mold design should promote efficient filling, rapid curing, and superior quality in Liquid Silicone Rubber injection molding. This approach ensures not only high-quality silicone products but also maximizes economic benefits for processors in the silicone rubber industry.
