Introduction
There are numerous methods for silicone parts production, liquid silicone rubber (LSR) and room temperature vulcanization (RTV) silicone rubber are two main types of silicone rubber for injection molding. Injection molding is a highly automated manufacturing process for complex silicone parts with medium to high production. However, this process will be more complex due to additional step with specific material grades.
Room Temperature Vulcanization (RTV) Silicone Rubber
RTV Silicone Rubber Types
RTV silicone rubber has a wide range of properties. It is easy to process with low viscosity, low shrinkage and excellent mechanical properties. Depends on different catalytic curing system, RTV silicone rubber are divided into three grades:
Two-part RTV
This grade has the fastest curing process, but this material is difficult to manufacture, for the reason of less time to form final products. The curing reaction in silicone materials can occur at room temperature or high temperature, this depends on the catalyst types in production process.
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One-part RTV
This material has a slow curing rate once only at room temperature. Although metering this RTV silicone is sufficient to start the curing reaction, environmental moisture is required to activate.
Special One-part RTV
This grade silicone rubber has high curing rate at high temperature. This material has similar process to the conventional one-part RTV.
Molding Methods for RTV Silicone Rubber
Normally, the geometry structure and properties requirement of final silicone parts will determine the final grade of RTV silicone and its molding process. We should consider various factors to select the most fitting molding process:
Injection Molding
Description: Injection molding is suitable for complex RTV products, RTV materials are forced into hot mold cavities by a ram or plunger. The silicone parts are formed from materials once two halves of the mold close.
Materials: Low viscosity and short curing time
Variables: Temperature, pressure, shot-size, viscosity
Advantages: Automated, high productivity, short cycle time, tight dimensional tolerance, low cost per part.
Disadvantages: Material waste in Runner, defect in injection molding like warping, bubbles, sinking.
Application: small-size parts, complex geometry, thin walls.
Gravity Casting
Description: Gravity is applied to fill the cavities in the mold.
Materials: Medium viscosity, long curing time
Variables: Different viscosity
Advantages: Suitable for small details and this walls
Disadvantages: High implementation cost
Application: Housings, cases
Reaction Injection Molding (RIM)
Description: Reaction injection molding is similar to injection molding. However, a vacuum mixer replace the normal mixer to mix RTV silicone rubber before injection process.
Material: Low viscosity, short curing time
Variables: Temperature, pressure, viscosity
Advantages: Fast production time, high tolerances, suitable for large silicone parts
Disadvantages: High cost equipment, only economical for large-volume production
Application: Bumpers, cases
Liquid Silicone Rubber
Liquid Silicone Rubber Types
Liquid silicone rubber material has excellent mechanical and chemical stability, it has low compression set and can withstand extreme temperatures. LSR is normally two-compounds with the platinum catalyst. The curing reaction starts once two compounds come in contact or at high temperature. Because of the two-compound composition and liquid state, LSR materials require uniform mixing with ratio of 1:1 before injection molding.
Molding Methods for Liquid Silicone Rubber
As different additives in LSR compounds, its viscosity will vary. This will affect the performance in different molding process. Therefore, we need to consider different factors to select the most suitable molding methods:
Liquid Injection Molding
Description: Liquid injection molding is similar to normal injection molding. The only different is uniform mixing of two compounds before the processing.
Material: short curing time, very low viscosity.
Variables: Temperature, pressure, shot size, viscosity.
Advantages: Shorter cycle time than conventional injection molding, intricate design, automated process, high dimensional tolerances, low materials waste.
Disadvantages: High cavity wear, easy to generate flash, premature scorching problems.
Application: Seals, O-rings, gaskets, keypads, shielding, cable boots, electronic contacts.
Compression Molding
Description: The preform silicone is placed in mold cavities, then hot molds close by high pressure. Silicone materials flow and fill the cavities completely, in order to get required structure. The preform is in various types, such as powders, pellets, granules, and specific liquid.
Material: Medium viscosity and curing time.
Variables: Temperature, pressure.
Advantages: Low cost, small batches, low material waste, short set-up time.
Disadvantages: Preform preparation step, require special surface finish to improve flowability.
Application: Seals, Gaskets, cushion pads, O-rings, hoods, fenders, automotive parts.
Transfer Molding
Description: This method is similar to injection molding. The preform or material is in a reservoir tank, then forced out by plungers to fill the hot cavities through a sprue.
Material: Low viscosity and medium curing time.
Variables: Temperature, pressure.
Advantages: Short cycle time, high tolerance.
Disadvantages: Secondary operation for flash removal (trimming, cutting, grinding), perfect control of temperature and pressure.
Application: small-size part with detail features.
Blow Molding
Description: This method normally produces hollow silicone parts. The silicone rubber will be extruded in tube-shape form, then stretch the tube inside the mold with air pressure, finally get required silicone form.
Material: Medium viscosity and curing time.
Variables: Mold temperature, viscosity, wall thickness, curing time.
Advantages: Low cost, small to large batches production.
Disadvantages: Material waste, low precision.
Application: Technical parts, fuel tanks, air ducts, surfboards.
Rotational Molding
Description: This is another method for hollow parts production. Place a certain amount of LSR material inside the mold, then rotate and heat the mold. The material adheres to mold wall and begin to cure.
Material: Medium viscosity, long curing time.
Variables: Temperature, viscosity, rotational velocity.
Advantages: Small batches, stress-free parts.
Disadvantages: Material waste, low temperature control, expensive equipment.
Application: Covers, containers, boxes, cases.
Select the Best Manufacturing Method
Although different manufacturers prefer different molding methods, there are guidelines to assist us selecting the most perfect method. In the following graph, it shows the general categories of molding methods and relation to parts complexity, curing time, and viscosity. In silicone rubber, the selection of silicone compounds depends on the final parts, such as primary function, temperature requirements, whether exposed to chemicals.
Important Factors in Molding Selection
There are many parameters to determine which is the best molding method. Normally, the final part cost is the most critical factor, but it isn’t the easiest and the most suitable method. For this reason, we should consider the economical balance between the material, product design, mold tooling, equipment, and workspace. Furthermore, this process also need to ensure the final products satisfy all design, density, behavior, quality and reliability requirements.
The most critical factors in molding process selection are as following:
Material
Except above-mentioned properties, we also need to consider other conditions, such as material storage, handing, waste management. Most silicone rubber have specific handing condition, this will affect the final properties of molded materials.
Processing
The simpler the final products, the simpler and easier the molding process. We should avoid unnecessary operations once the curing reaction starts. It is necessary to analyze products design and operation sequences carefully, combine or eliminate steps.
Tooling
In most case, the molds have the highest percentage of the initial cost. The mold is critical to the parts quality like texture, tolerance, total production volume, production efficiency.
Conclusion
All in all, we should consider different molding processes for silicone parts production. This selection of the best process requires the knowledge of final parts application, silicone rubber or LSR materials behaviors, the equipment and techniques.
Miwo is the most professional liquid silicone molding company in China, we have the knowledge, experience and advanced technology to optimize our production process, ensure high quality of LSR injection molding, LSR over-molding, LSR 2-shot molding products.
Contact us now for more information about different silicone rubber materials and molding methods.
