The Advantages of Liquid Silicone Rubber for Injection Molding

With Contributing Expertise From: Luis Marrero

The Advantages of Liquid Silicone Rubber for Injection Molding

Injection molding is a fabrication process used for the production of parts using thermoplastic and thermosetting materials. The method consists of feeding the material into a heated barrel, mixing and forcing the material into a mold cavity by employing a reciprocating screw or ram injector, and finishing with the injection molded part being cured to the configuration of the mold cavity.

The injection molding process has been around since the late 1860s when billiard ball maker Phelan & Collander offered a reward to anyone who could come up with a material that could be used in lieu of ivory in the production of billiard balls. A man by the name of John Wesley Hyatt rose to the challenge and invented a process to fabricate billiard balls by injecting celluloid into a mold. John and his brother Isaiah patented the first injection-molding machine in 1872, with its key feature being a basic plunger mechanism that injected the plastic into a mold through a heated cylinder.

Over the next five decades, the industry advanced at a snail’s pace, with manufacturing primarily limited to the production of plastic buttons, collar stays and hair combs. However, after World War II there was an increase in demand for inexpensive, mass-produced consumer goods which lead to a sharp increase in popularity towards the injection-molding manufacturing sector.

In 1946, there would be another injection molding revolution when a James Hendry would build the first screw injection molding machine. Hendry’s invention, which ended up superseding the Hyatt plunger device, had an added an auger fixed inside the cylinder which facilitated the mixing of the injection material before moving it forward into the mold. This innovation allowed manufacturers to add colored or recycled plastic to fresh material and thoroughly mix the components before they were to be injected into the mold. Of the injection molding machines in use today, most are screw injection molding machines.

Although there have been numerous injection molding process variations, the compression and transfer molding process and the use of plastic and high consistency rubber (HCR) have been dominating manufacturing procedures in the medical, aerospace, electrical, construction and industrial applications for years until recently.

Indeed, the tide has turned and manufacturers across a wide variety of industries have discovered the advantages of Liquid Silicone Rubber (LSR) for injection molding and for the fabrication of molded parts. One such advantage is that LSR produces medium- to high-volume rubber parts more efficiently and economically than thermoplastic and other molding processes.

LSR Properties, Grades and Applications

In its basic form, silicone rubber consists of sand or silicon dioxide, with material manufacturers processing the sand into silicon metal and reacting the material with methyl chloride. It is employed a variety of additional fabrication methods to create many types and grades of silicone, including fluids, polymers, compounds and finished rubber bases. Compared to most plastics, silicones have a structural backbone of alternating atoms of silicon and oxygen, and these organic side chains engender curability and other beneficial characteristics.

LSR consists of a high-purity platinum core silicone, developed by Dow-Corning in the 1970s, and sets it apart from high-consistency, gum-based silicone materials, which at one time dominated the market. Indeed, LSRs consist of compounds made from a silicone oxide base and LSR material can also come in a variety of types and grades, providing companies with a selection of textures, from grainy to smooth, for their end-products. By modifying specific side groups of the material, manufacturers can alter LSR to meet certain defined functions or specifications.

Although some features will remain the same for all LSRs, each material can have different properties. LSR has some processing attributes that are similar to thermoplastics. Simultaneously, LSR also possesses some prominent characteristics that parallel a thermoset.

Other LSR properties include:

  • Chemical inertness
  • Resistance to bacteria, UV light, Ozone, and harmful radiation
  • Superior light transmittance and illumination
  • Clarity
  • Gas permeability
  • High elongation
  • Superior dielectric strength
  • Excellent tensile strength, tear strength
  • Ability to maintain flexibility and operate in temperatures ranging from -55°C to +300°C
  • Low compression set at extreme temperature ranges
  • Odorless and tasteless
  • FDA biocompatibility
  • Odorless and tasteless
  • Unlimited pigment options
  • Hydrophobic
  • Hypoallergenic

Manufacturing LSR comes with many options, risk-free. One such option is to choose from different levels of hardness, which is measured with a durometer, depending on its intended usage. Another, is that silicone can be sterilized through a variety of methods such as dry heat, gamma radiation, e-beam, steam autoclaving and ethylene oxide (ETO). Also, manufacturers can include additives and other fillers to formulate LSR materials to withstand higher temperatures, oil and other conditions. Adding phenyl units to the material mix can extend the low temperature capabilities of LSR.

Designers can also create products with complex geometries and undercuts that would be impossible with thermoplastic materials. LSR is suitable for the custom manufacturing of medical device components ranging from parts for blood analysis machines to surgical instruments, and compared to plastics, LSR materials do not stick to the patient’s skin, nor are they harmful to body hair.

In its final stages, the LSR material is cured through a method that contains non-hazardous substances. Therefore, even if a portion of the substance is under-cured, there is no threat of hazardous chemicals being released.

Other grades of LSR materials include:

The Advantages of Liquid Silicone Rubber for Injection Molding

How is LSR Valuable to Different Industries?

Although the medical industry has used silicone since the 1950s, more medical device manufacturers are finding LSR highly appealing because of its purity and other distinctive properties, including biocompatibility with human tissue and fluids, stability, hypoallergenic features and high level of performance. LSR is also chemical, mold and bacteria resistant.

In the lighting industry, LSR has also begun to replace glass and plastics for numerous optical applications because of its flexibility and transparency, and its ability to maintain those characteristics over time. Not only does LSR resist cracks, scratches and discoloration, but the material also allows manufactures to fabricate two parts into one — thus reducing assembly and overall manufacturing costs.

Food-grade LSR has the purity, strength and stress resistance needed to fabricate kitchen products, from the gaskets and seals required for industrial cooking appliances to the bottle dispensers and baking trays used in residential kitchens.

For general-purpose applications that require only basic physical properties, LSR material that consists mainly of silica is suitable. This material typically has mid-properties in terms of temperature resistance, tear strength, percent elongation, solvent resistance and other characteristics.

Liquid Silicone Rubber (LSR) Injection Molding Advantages

Here are several problems you might face with other materials, which you would not with LSR:

By-Products:  LSR does not generate by-products. Its molecular structure makes it appropriate for a wide range of applications, however, the same structure means it can’t be broken apart or recycled. The superior bonding strength between the silicon and oxygen atoms keeps the polymers from degrading under temperatures that most thermoplastics cannot withstand. LSR’s characteristics of purity, chemical inertness and its ability to endure sterilization processes makes it the optimal choice for medical devices, food uses and baby care products.

Restrictive design: Generally, LSR has design guidelines that are comparable to thermoplastics, but it offers more freedom in certain areas. LSR gives designers more flexibility in the design process because the material is more forgiving when compared to thermoplastics.

Planes: Planes that run the same direction of the mold opening usually require some draft, which allow milling of the mold. The mold design requires about one degree per inch of the mold depth in order to prevent the scraping of the component against the mold. Since shear-thinning LSR flows easily into the mold, it can navigate thin walls that would result in fill issues for thermoplastic materials. This characteristic allows for the design of parts with varying thickness.

Sink: Sink is also not an issue with LSR because the heat solidifies the material, which mostly cures before it cools. Because sink is not a problem, designers can make parts with features that are thicker than the average thermoplastic material permits.

Undercuts: When it comes to the handling of undercuts, this may be the biggest difference between LSR and thermoplastic materials. Fabricators can mold rigid thermoplastics materials with undercuts, but it requires complicated methods used by LSR, such as pickouts or the use of side-actions that produce the undercut feature, and then withdrawing the material prior to the mold opening and ejection of the part.

Compressibility: Compressibility is an innate characteristic of LSR. The property creates some capabilities in the design of parts that would be unattainable with plastic materials. Another benefit to the use of LSR is the ability to combine it with different thermoplastics without the need for a priming step. Part designers can add the part only on the surface area as functional purposes. This eliminates the need to fabricate the substrate with additional thickness (material) in order to withstand the heat of the vulcanization process. These advantages result in smaller parts that weigh less and facilitate a more economical silicone manufacturing process.

LSR injection molding advantages manufacturers can realize include:

  • Shorter LSR molding cycle time that can be measured in seconds as opposed to minutes for plastics.
  • Premixing LSR in a closed system prevents contamination from the environment.
  • Flow properties of LSR makes it the ideal material for molding smaller, complex and tight tolerance parts.
  • Results in less scrap from excess rubber molding material than other methods.
  • Ability to over-mold LSR onto thermoplastic, metal, magnesium and other materials.

Manufacturers should work with an expert to ensure selection of the appropriate LSR material to match the application. The material’s superior thermal, electrical and chemical resistance characteristics make it an excellent choice compared to thermoplastics elastomers.

Liquid Injection Molding Advantages

Another important consideration, and why the selection of LSR makes it the right choice for your project, has to do with the liquid injection molding (LIM) manufacturing process.

LIM methods begin with a two-part liquid silicone compound formula, parts A & B — a catalyst and a base. Typically, the machine automatically delivers the material from supply containers or drums by a pneumatic or a hydraulic metered pumping system to a static mixer at a constant 1:1 ratio.

The static mixer blends the two materials into a homogenous concoction, which triggers the cure system. From the static mixer, the LSR flows into the injection unit, which feeds the material into the mold cavity by way of a runner and gate system. The LSR is held in the mold under extreme pressure and high temperature at an established cycle time to ensure an optimal level of vulcanization of the rubber — where the material is cured. When the cycle ends, the parts are ejected or removed from the cavities, and the next cycle begins.

LIM allows manufacturers to fabricate a broad range of precise and near flashless molded silicone parts and silicone over-molded assemblies at high volume and tight tolerances.

Some other advantages of LIM over thermoplastics include:

The Advantages of Liquid Silicone Rubber for Injection Molding

Compared to other rubber molding methods, LIM has higher startup and shutdown costs, which makes it better suited for high-volume production runs. In addition, the runner systems can lead to increased gross material weight unless countered with cold runner systems or other low-waste options.

Why Choose LSR for Your Project?

In the last decade, silicone material providers have made substantial progress in LSR formulations which have resulted in performance gains, processing stability and competitive pricing. These advances in LSR technology have leveled the playing field, and for many applications they have also provided a better option than plastics.

Here are just a few of the industries/applications suitable for LSR materials:

The Advantages of Liquid Silicone Rubber for Injection Molding

  • Medical/Healthcare: catheters, medical tubing, endoscopy,
  • Automotive: gaskets, O-rings, seals, LED lenses for optical coupling
  • Optics: exterior lighting, professional lighting
  • Electronics: LED lenses, protective sleeves, lighting guides
  • Consumer goods: headsets, ear buds, nose pads, goggles

Companies and individuals must perform a careful evaluation of a wide range of material properties and processing methods for critical applications, especially for medical devices, in order to meet rigid and demanding performance specifications and budget requirements.

It is important to have clearly definitive performance standards in relation to certain properties such as elongation, durometer and thermal resistance, in order to select the correct material. Selecting the wrong material or process can be detrimental to your business. Not only can it create large setbacks, it may also result in loss of market share, revenue and time.

At first glance, it may be difficult to select LSR as your material of choice because the cost is often higher than other materials. However, LSR offers manufacturers, especially fabricators of high-volume parts, many benefits compared to plastics and HCR. As a raw material, the benefits provided by LSR can easily justify its use despite the initial higher cost as manufacturers will have the ability to perform long production runs with minimal labor. Indeed, it is possible to have near complete automatic operation of the process — a single operator can attend to and maintain multiple liquid injection molding machines. This cost saving aspect is extremely important to manufacturers in terms of increased process efficiency who are also faced with the rise in labor costs, making LSR the optimal choice of material for your production needs.

Despite any perceived shortcomings, LSR provides manufacturers consistency and cost-effectiveness for a wide range of applications, such as automotive, medical, infant care, electronics and consumer products.

SIMTEC is a top Global injection molding company that works exclusively with Liquid Silicone Rubber.Consult with one of our LSR project experts to learn more about the benefits of liquid silicone rubber for injection molding.

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