In the field of medical device manufacturing, liquid silicone rubber injection molding (LIM) is a process that has several advantages over other methods. The technology can reduce product strength and weight, while being more cost-effective than some alternatives, and still producing the final product within the parameters set by equipment and regulators.
The Molding Process of Medical Device Manufacturing
There are many kinds of molding processes for medical device manufacturing, including thermoplastic, micro molding, compression molding, transfer molding, etc. Choosing the right molding process is very important in creating a successful and high-quality final product. Here are some common options:
• Transfer Molding
By transfer molding, hydraulic plungers push rubber through the runner and gate into specific cavities. The benefit of transfer molding is that it requires very little setup time, and it is very useful for both small and high-volume production. Contract manufacturing organizations (CMOs) can also use the process for the secondary molding of metal and plastic inserts.
• Compression molding
Compression molding involves the preparation of organosilicon outside of the molding process. Position the shot in each mold cavity, seal with the top, then compress to ensure access to all areas of the mold. The compression molding process is suitable for large geometric parts and encapsulation molding applications.
• Micro-molding
This is a form of LIM and is suitable for components requiring injections of less than 6 grams. This applies to products that require strict tolerances and strict flash requirements. It is commonly used in parts of minimally invasive surgical tools, valves, and seals. Because of its precise tools, micro-molding is also used in micro-sensory and microfluid products.
• Silicone and Metal Bonding/Insert Molding
Silicone rubber is bonded to the primer substrate by insert forming and cladding forming. The challenge here is to ensure that the inserts remain in the correct position throughout the molding cycle. Inserts are pretreated to produce maximum bonding strength and durability.
• Thermoplastic Injection Molding
Thermoplastic injection molding is a reversible molding process in which plastic particles are melted, forced into a mold to take on their final shape, and then quickly cooled and hardened. This method is ideal for creating parts with excellent surfaces and finishes. Precise, fully functional components and parts can be manufactured for prototype and full-scale production runs.
• Insert/Wrap Molding
A thermoplastic material molded around extruded, metal inserts, or other functional parts that have been placed in plastic injection molds. The advantage of using this technique is that it can create durable integrated components at a lower assembly cost while maintaining quality.
• Liquid Injection Molding (LIM)
Liquid injection molding (LIM), uses silicone rubber or liquid silicone rubber (LSR) to make highly durable silicone parts that can withstand extreme conditions. Liquid injection molding involves injecting liquid silicone rubber or rubber into a prefabricated mold to form a device or part, which is then heated to cure and solidify the shape.
The Benefits of Silicone Liquid Injection Molding
Liquid injection molding using liquid silicone rubber offers numerous advantages to medical device manufacturers, making it one of the most popular solutions for manufacturing rubber parts. Here are some benefits:
Durability: Liquid injection molding lasts a long time, even in hot and cold temperatures, so you’ll often find LIM parts used in cars and aircraft, as well as medical fields. Liquid injection molding is less likely to warp or melt, as plastic injection molding parts are, due to silicone’s flame-retardant and heat-resistant properties.
Strength and Flexibility: Silicone LIM components are both pliable and strong, which means they can stretch up to six times their length before snapping or breaking. This increases the durability aspect, but also means that it expands the number of potential uses across multiple industries.
Excellent Mechanical Properties: Liquid silicone injection molding is suitable for equipment with moving parts. It increases the durability of complex parts that require cushioning and materials to keep the parts working together without excessive wear and tear.
Bio-compatibility: Silicone rubber is bio-compatible, making it a safer and less allergenic alternative to other materials used in medical devices.
Accuracy: The liquid silicone injection molding process is very accurate, which means the parts do what they are supposed to do.
Automation and 24/7 production: Once set up, liquid injection molding can be automated. This means that the process can run almost 24/7 and requires little human intervention and monitoring.
What Are The Materials Used in Liquid Injection Molding?
Silicone is commonly used in liquid injection molding (LIM). It is a synthetic liquid rubber. Different silicone manufacturers and durometers can be used in LIM, each with advantages and use for specific applications.
• Liquid Silicone Rubber (LSR), usually platinum-cured silicone, is probably the most common type of silicone used in liquid injection molding. It is bio-compatible and very durable, making it the silicone of choice for medical devices and instruments.
• Fluorinated Liquid Silicone Rubber (F-LSR), as the name implies, uses fluorosilicone, using the element fluorine as a way of improving the chemical resistance of the final product. Fluorosilicone is a popular choice for automotive parts as well as precision parts and smaller parts or spares.
What is The Liquid Silicone Rubber Injection Molding Manufacturing Process?
As mentioned above, two types of silicone are commonly used in liquid injection molding, namely liquid silicone rubber (LSR) and fluoro liquid silicone rubber (F-LSR). In medical device manufacturing, LSRS is commonly used to produce biocompatible components.
To make parts using liquid silicone rubber (LSR), injection molding machines typically use A metering device that provides two components and then mixes parts A and B (one containing a platinum-based catalyst) into a uniform suspension that passes through a static mixer before entering the injection molding machine. Silicone rubber does not cure unless the two parts are mixed.
The silicone liquid injection molding machine comprises a syringe, a measuring unit, a supply drum, a mixer, a nozzle, and a mold fixture. In short, the injection device will move the silicone into the mold by pressure, and the material needs to solidify in the mold. The machine will be cooled to keep the silicone cool and uncured until it reaches the mold, the heat of which will allow the part to reach its final geometry. Molds need to be properly vented or use an in-mold vacuum to protect components from bubbles or leaks. Sometimes a clean room is necessary, especially in the production of medical equipment. This is a simplification of a very complex process that involves various moving parts to ensure a good final product.
Certain considerations should be kept in mind when using liquid silicone injection molding to design parts and products. In general, design constraints are not very different from thermoplastic injection molding. Here are some potential issues to look out for:
• Shrinkage: Liquid silicone rubber will not shrink like thermoplastics when inside the mold. But once the silicone is removed from the mold, it shrinks by as much as 3 percent when it cures.
• Exhaust: Trapped air can damage the silicone liquid injection molding, so exhaust must be carefully set up to avoid air buildup in the mold. Complex part designs may require the use of in-die vacuums.
• Thermal tearing: The point at which silicone components are removed from the mold is critical as thermal tearing may occur. This is a simplification of a very complex process that involves various moving parts to ensure a good final product.
