Processing Polymers

There are numerous methods for producing plastic parts and many will be discussed in this post. Of the various processes, molding and extrusion are the most predominant. The best method will depend on the polymer being used, the part geometry, and the part’s function. The definition of plastic molding is it is a group of manufacturing processes used with a closed tool called a mold, to produce parts. The inside of the mold is in the shape of the part, when the high temperature mold is filled with the molten polymer and closes, the part is formed during the curing. Extrusion is another polymer processing method that can be used to produce plastic parts that are typically linear in shape. The extrusion process also uses hot molten plastic and a heated tool, however the extrusion tool is an open tool, called a die.

Material Consideration

Generally speaking, injection molding is a highly automated process, and for complex parts with medium to high production requirements, is a well-suited process. Silicone Rubber and thermoplastic polymers are both processed using injection molding, however processing silicone rubbers can be more complex due to additional steps, such as mixing the material that may be required depending on the specific grade. The following are processes used for processing the two main types of silicone rubbers: room temperature vulcanization silicone rubber and Liquid Silicone Rubber (LSR):

Types of Room Temperature Vulcanization (RTV) Silicone Rubber

This type of silicone rubber is available with a wide range of properties. In general, it is easy to process and has low viscosity, low shrinkage, and good mechanical properties. Depending on the type of molding employed, different curing systems are used:

1. Two-part grades have the fastest cure, but this can make the material difficult to process, not allowing much time to form the product. The curing reaction could occur at room temperature or at high temperatures, depending on the catalyst used.
2. The one-part material has a slow cure rate that occurs only at room temperature. Metering the silicone rubber is enough to start the curing reaction, but environmental moisture is necessary to activate.
3. Special one-part grades cure at high temperatures, and the cure rate is fast. This material processes similar to a conventional one-part silicone rubber.

Molding Processes for RTV Silicone Rubber

The type of part and the grade and type of curing chosen will determine which of the following molding processing methods is most fitting:

Injection Molding

Description: It is well-suited for complex products. The material is forced into the mold’s hot cavity, using a ram or plunger, and forms the part when the two halves of the mold close and the material [1].

Material: Low viscosity and short cure time

Variables: Temperature, pressure, viscosity, shot volume (the amount of material injected into the cavity)

Advantages: High productivity, automated, low cost per part, short cycle times, little to no flash, tight dimensional tolerances

Disadvantages: Material used in the runners (waste); defects in part due to injection (warpage, bubbles, and sinks) are common therefore best for parts with thin walls.

Applications: Small parts, thin-walls (automotive body panels)

Gravity Casting

Description: Gravity is used to fill the cavity

Material: Medium viscosity, long cure time

Variables: Viscosity

Advantages: Low-cost process, low-cost equipment

Disadvantages: Long cycle times, process not for thin wall products, small batches

Applications: Scale models, dental plates.

Vacuum Casting

Description: The mold has a jacket which is used to create the vacuum. The air is pulled out and the silicone rubber is then able to fill, even in small spaces.

Material: Low viscosity, medium cure time

Variables: Viscosity, vacuum

Advantages: Thin walls, small details

Disadvantages: High implementation costs

Applications: Cases, housings

Thin Wall Reaction Injection Molding (RIM)

Description: Similar to injection molding. A vacuum is used to mix the silicone rubber before it is injected into the cavity.

Material: Low viscosity, short cure time

Variables: Viscosity, temperature, pressure

Advantages: Close tolerances, fast production times, large parts.

Disadvantages: High cost equipment, economically viable only for large production runs.

Applications: Bumpers, cases.

Types of Liquid Silicone Rubber (LSR)

Liquid Silicone Rubber materials have low compression set, good mechanical and chemical stability, and can withstand extreme temperatures. Most LSRs used are 2-Part LSRs in which platinum is used as the catalyst, and the reaction begins when the two parts come in contact or at high temperatures. Due to its two-part composition and liquid physical state, LSRs always require a good mixing before processing.

Molding Processes for Liquid Silicone Rubber (LSR)

Depending on the additives in the compound, the viscosity will vary and will affect the selected molding process. The following are the molding processes for LSRs and considerations for each:

Liquid Injection Molding (LIM)

Description: Liquid injection molding is similar to conventional injection molding, with the difference being the required mixing of the raw material before the processing [1].

Material: Very low viscosity and short cure time

Variables: Temperature, pressure, viscosity, shot volume

Advantages: Short cycle times (even in comparison to conventional injection molding), intricate design, very close dimensional tolerances, automated process, low material waste.

Disadvantages: High tendency to generate flash, high cavity wear, problems due to premature scorching.

Applications: Gaskets, seals, O-rings, key pads, cable boots, shielding, electronic contacts.

Compression Molding

Description: A preform silicone rubber is placed in the cavity. The preform can be granules, powders, pellets or a specific amount of liquid. The hot mold is closed using high pressure and the material flows, filling the cavity completely [1].

Material: Medium viscosity and medium curing time.

Variables: Pressure, temperature

Advantages: Low cost process, small batches, short set-up times, low material waste.

Disadvantages: Additional step to generate a preform, special surface finish to improve the flow

Applications: Gaskets, seals, O-rings, cushion pads, automotive parts, hoods, fenders.

Transfer Molding

Description: Similar to injection molding. The material (preform) is in a reservoir and forced out using a plunger to fill a hot cavity through a sprue.

Material: Low viscosity and medium curing time.

Variables: Pressure, temperature

Advantages: Short cycle time, close tolerances.

Disadvantages: So much flash is generated that secondary steps are necessary to finish the product (trimming, cutting, grinding). A perfect control of pressure and temperature is necessary.

Applications: Smaller and more detailed parts.

Blow Molding

Description: This method produces hollow parts. The silicone rubber is extruded in a tube-shaped form, and the tube is placed inside a mold where air pressure is applied. The part is formed by stretching [2].

Material: Medium viscosity and medium cure times.

Variables: Viscosity, curing time, mold temperature, wall thickness.

Advantages: Low cost process, can be produced from small to large batches.

Disadvantages: Waste is formed, low material precision.

Applications: Fuel tanks, air ducts, technical parts, surfboards.

Rotational Molding

Description: This method produces hollow parts. A certain amount of Liquid Silicone Rubber is placed inside the mold, then the mold rotates and heats. The material adheres to the wall due to the centripetal force, and begins to cure [3].

Material: Medium viscosity and long cure times.

Variables: Rotational velocity, viscosity, temperature.

Advantages: Stress free parts, small batches.

Disadvantages: High material waste, expensive equipment, low temperature control.

Applications: Containers, cases, covers, boxes.

Selecting the Best Manufacturing Process

While a company may have preferences about the type of molding they want to use, there are guidelines that can assist when selecting a new method. Figure 1 shows the general categories of molding and their relation to the part complexity, curing time, and viscosity. With silicone rubbers, a compound is developed based on the type of manufacturing process being used, however there are general purpose compounds that can be used in multiple manufacturing processes. Therefore, selection of the compound will depend on the part – its primary function, temperature requirements, if it will be exposed to chemicals, and of course, the manufacturing process [4].

Figure 1. Molding method related to curing time, viscosity, and complexity of the final part [3].

There are many parameters evaluated to determine which molding method is best. Often the final part cost may be the most important factor, but it should be cautioned that although a method may be cheaper, it may not be the easiest or the most suitable for the part. For this reason, it is necessary to consider an economical balance between choice of material, the product design, tooling, equipment, and floor space. The process must also assure that the final product meets all design, quality, behavior and reliability requirements.

The following are the most important factors in selection of molding process:

• Material: Additional to above-mentioned properties, other conditions should be taken into account, such as material storage, handling, and waste management. Most silicone rubbers have specific handling conditions that can affect the final properties of the molded material.
• Processing: The general rule-of-thumb is: “the simpler the final product, the simpler and easier the mold processing”. Unnecessary operations, especially when the curing reaction has started, should be avoided. It is a good idea to carefully analyze the product design, the sequence of operations, and, if possible, combine or eliminate steps. For example, it is always desirable to avoid processes where flash is produced. If available, process simulation tools are helpful in selecting the best process.
• Tooling: In some processes, the molds are the highest percentage of the initial cost. With this said, the mold is very important and critical to the quality of the part (texture, tolerances, etc), the total volume to be produced, and the required rate of production should be considered.

In conclusion, there are a multitude of processing options to be considered for producing silicone rubber and Liquid Silicone Rubber (LSR) products. Selection of the best process requires knowledge of the final application of the product, the behaviors of the available LSR or silicone rubber material options, and the equipment and techniques. It is advisable to consult a reputable, molder. SIMTEC Silicone Parts for example is one of North America’s leading injection molders of Liquid Silicone Rubber, regarded as LSR experts they have the knowledge, experience and advanced technology to optimize their process to produce high quality LSR, two-shot, and multi-shot LSR products.