The Steps of Liquid Silicone Rubber Injection Molding

Injection molding of Liquid Silicone Rubber (LSR) is a process used to produce pliable, durable parts in high volumes. During the process, several components are necessary: an injector, a metering unit, a supply drum, a mixer, a nozzle, and a mold clamp, among others.

Injection molding of Liquid Silicone Rubber (LSR) is a common technology used for the production of different products for medical and electrical applications, among others. In addition to the innate properties of the material, the parameters of the process are critical too. LSR injection molding is a multistep process that is presented in Figure 1.

The first step is the preparation of the mixture. LSR usually consists of two components, pigment, and additives (fillers for example), depending on the desired properties of the final product. In this step, the ingredients of the mixture are homogenized and can be combined with the temperature stabilization system for a better control of silicone temperature (ambient temperature or silicone preheating).

Figure 1 – Schematic of Liquid Silicone Rubber (LSR)

schematic of lsrThe internal part of the product, called inserts, which are embedded in the silicone coating after the injection process, are placed in the mold. These inserts can be kept in ambient conditions prior to injection or preheating.

Liquid Silicone Rubbers are normally supplied in barrels. Because of their low viscosity, the material can be pumped through pipelines and tubes to vulcanization equipment.

The mold is usually heated by water flowing inside the mold or by electrical heaters. Additionally, a cold runner system can be used to prevent the premature curing of the flowing silicone in the runners or its vicinity.

In the second step, the mold is closed by a clamping machine, as shown in Figure 2, and the injection process starts.

Figure 2 – Clamping machine and mold used in silicone rubber injection molding

LSR clamping machineThe injection can be made either under atmospheric pressure (an efficient ventilation system is required) or in vacuum conditions. The injection time depends on the injection pressure determined by the product to be manufactured and the silicone properties, such as viscosity. The latter depends on both the shear rate, increasing during the flow, and the temperature.

Once the mold is filled with silicone, the mold temperature is increased to accelerate the crosslinking process of the silicone rubber, resulting in the material transformation from a liquid to solid state; this step is called the curing stage. The time of the curing stage is specific for a given silicone grade and, of course, product. This is another critical step in the processing, because the material overheating and significant pressure increase inside the mold can be observed as a result of the exothermic effect of a curing reaction and high thermal expansion of silicone.

The final step of the injection molding process of silicone rubber is the cooling of the product inside the mold, followed by a product demolding and final cooling in ambient conditions.

All parameters mentioned above strongly affect the course of silicone molding.  In consequence, all these aspects can be subjected to optimization to ensure the shortest production cycle time while simultaneously keeping the highest final product quality.

Looking to start an LSR injection molding project? Contact us today!

Viscosity of Silicone Rubber

Silicone fluids have important applications in many modern fields including the automotive, electrical, and medical industries. Their advantages include high temperature and chemical resistance, optical transparency, and good electrical properties. Certain application areas and processing steps require a reliable rheological model.

Compounds based on silicone rubbers are designed to process well in molding equipment, especially liquid silicone. Compounds are available in a variety of flow ranges from very stiff for compression molding to very soft for transfer molding. Viscosity is the most well-known expression of rheology and it is used to monitor formulation stability over time and to optimize the sensory and performance attributes.

The rheological properties of silicone rubbers are of importance because they are major determinants in the handling characteristics and adaptations to soft and hard behaviors during processing.

Their plasticities are normally measured by the Mesa Spiral Flow. This test provides an indication of the relative ability of the compound to flow and fill molds. Torque rheometer measurements are sometimes utilized to characterize both flow and curing properties.

Because viscosity is a property that is easy to observe, it is frequently used to monitor formulation stability over time. Rheology has become increasingly important because an understanding of its parameters has allowed formulators to optimize the sensory and performance attributes of the products.

The viscosity of the polymer depends on the molecular weight of the material which, in turn, is a function of the degree of polymerization.

That means, the greater the degree of polymerization, the higher the molecular weight, and the higher the molecular weight, the longer the polymer.  The longer the polymer is, the higher the viscosity. And the higher the viscosity, the more slowly the polymer will flow.

The addition of functional groups on the siloxane backbone can modify the rheology behavior. Varying degrees of substitution can result in wax-like silicones. The viscosity can be increased by increasing the chain length, adding Si-O units or by employing a crosslinker.

In general, silicone rubber materials exhibit Newtonian rheological behaviors when they have low molecular weights and they are independent of the shear rate applied during processing. But when the molecular weight increases, the polymer becomes entangled and exhibits a viscoelastic response and a decrease in viscosity, called shear-thinning. Depending on the different additives present in the silicone compound, they can be thixotropic or dilatant, depending on the final application and process used in them. The viscosity has a relatively small effect on silicone rubbers’ chemical properties, but it affects the flow behavior and solubility.

The rheological properties can be measured using different techniques. For example, a cone and plate viscometer measures flow characteristics.

Since silicone rubbers are also cured, their behavior is studied by chemorheology, which describes the rheological behavior of crosslinked polymers during the chemical reaction of vulcanization or curing. The change in the silicone rubber flow behavior is determined using rheological measurements during the crosslinking reaction. The results can be used in the evaluation of the material, the design of the process, and the simulation of the process conditions. Here, one of the critical points that can be found is the gel point. At this point, the polymer no longer flows so the silicone must be formed before reaching this point. The increase in temperature of those silicones creates two different effects in the viscosity: First, there is a decrease in the viscosity, but when the curing reaction begins the viscosity becomes higher due to the increase in the silicone’s molecular weight, created by the crosslinks. Rotational rheometers have proven devices to monitor this behavior, because they can capture the transition from solid to liquid.

 

Classification of Liquid Silicone Rubber

To achieve specific application properties, it is sometimes necessary to employ specific LSR technologies.

All silicone elastomers consist of crosslinked polydimethyl siloxane (PDMS) molecules, fillers, and additives.  Here, we are going to explain the classifications of Liquid Silicone Rubber (LSR) and the difference between the grades.

First, we must remember that Liquid Silicone Rubbers undergo addition curing at elevated temperatures. The main applications for this material family are silicone moldings, produced by injection molding, and textile coatings.

The curing mechanism is the addition of modified room temperature vulcanization with mixing ratios, typically 1:1.

Standard Liquid Silicone Rubber evolved from market needs over 25 years ago. They are called general purpose Liquid Silicone Rubbers. The existing standard LSR has been modified over time.

The base chemistry of standard silicones has not changed, however the curing process is now faster, and this allows for a decreased production cycle time and a lower cost per unit.  One of the advantages of the fast curing process is lower molding temperatures.  The typical applications for these materials are in the automotive, healthcare, and domestic appliance industries. The hardness range of these materials is between 10 and 80 Shore A.

Specifically Purposed Silicones

From standard silicones, other silicones with specific purposes have been generated. Some examples of these silicones are briefly described below.

  • High tear resistant LSR was developed to provide maximized mechanical strength for baby bottle applications. They provide a good alternative for latex and other organic rubbers.
  • No post cure LSR is a formulation with a good compression set right after molding. Standard LSR needs post cure to achieve the desired compression set.
  • Heat stabilized LSR has the requirements necessary to operate in ignition systems. Normally, they are colored with a metal oxide in the form of pigment paste. The typical color is black and they do not need to be post cured. Typical applications are in automotive industry.
  • Coolant resistant LSRs must have the minutest changes in properties over time as possible. Normally, they are the natural replacements of EPDM rubber, because the coolant resistance has a lower change in the compression set in hot air, and it has other important parameters such as hardness, tensile strength, elongation at break, and weight change. A typical application is gaskets for radiators, wherein these LSRs come into contact with coolants (monoethylene glycol) in water at temperatures above 100°C and the hot air environment on the outer side of the radiator.
  • Self lubricating LSRs are also referred to as oil bleeding. In appearance, they are similar to standard silicones. The difference is at some point they form an oily film on the surface of the parts.  This film is helpful during assembly, and it improves the hydrophobic behavior of the seal. The oil content range is between 2% and 7%, and the speed and amount of oil bleeding corresponds to the contents.
  • Oil resistant LSRs are formulated to increase the mechanical, physical, and chemical behavior of the material when they are in contact with oil, because, by nature, the resistance of silicone and organic rubbers is low.
  • Self adhesive LSRs are tough enough to stick anywhere desired, yet are easy enough to remove when needed. The advantage of these silicones is the simplicity of combining plastic with silicone rubber without using a primer.
  • Electrically conductive LSRs are made possible by adding substantial amounts of carbon black. Additionally, the viscosity of these LSRs increases almost 8 times, in comparison with a standard LSR.
  • Flame retardant LSRs are used in consumer electronics, especially in high voltage areas.
  • Extra low viscosity LSRs are used for manufacturing insulators.  They can be processed at lower pressures, but the curing times are longer.

The History of Liquid Silicone Rubber and Injection Molding

History of LSR Injection Molding

Liquid silicone rubber and the liquid injection molding (LIM) process play a pivotal role in the manufacturing process of a broad range of industries. This is changing the very nature of the manufacturing world. It makes possible the fabrication of a wide range of parts for just about every industry imaginable. The fabrication process allows for a high productivity level, due to quick cycle time.

After the design and production of the molds, the operator can change the materials and colors with minimal effort. The typical injection molding machine has a self-gating automated device that moves parts through the manufacturing process effectively and efficiently, with little or no manual labor required. This lowers per unit labor costs.

The process has its disadvantages. For example, there is a high cost of entry for tooling and the injection molding machine. Part design depends on the creativity of the designer. Parts cannot be hollow; they must be completely solid.

The process requires the separate fabrication of each piece. In addition, the precision required for tooling and the fabrication process requires skills and expertise in design and plastic injection molding manufacturing. The advantages of the injection molding process easily offset the disadvantages.

The injection molding fabrication process, which has evolved along with the plastics and synthetic rubber industries, has its origins in the late 19th century. During this period, American and European chemists began experimenting with different rubbers and residues from chemical mixtures, which moved liquid injection molding to its current position as a dominant fabrication process in a variety of industries.

Here are some key moments in injection molding history and an injection molding timeline.

From Natural Polymers to the First Injection Molding Machine

Modern plastics as we know them today have their origin in the late 19th century. During this period, numerous European and American chemists experimented with various types of rubber and residues from chemical mixtures. All plastics fall under the scientific term polymers.

A polymer is a large molecule comprised of many smaller units, or monomers, bound together. The monomers are linked together from end-to-end by chemical bonds, which form a long chain. Polymer materials flow and fabricators mold them into parts.

Natural polymers, such as horn, shellac, tortoiseshell, and a variety of resinous tree saps have existed since the beginning of time. Humans have fabricated natural polymers for centuries. They formed them into an array of articles such as combs and jewelry, employing a process that required the application of heat and pressure.

Alexander Parkes first introduced synthetic polymers (plastic) to the world in Birmingham, England around the mid-1850s. Originally named “Parkesine” after its inventor, the material consisted of mixed pyroxylin, which is a form of partially nitrated cellulose. Pyroxlin is also a major component of plant walls, along with camphor and alcohol.

Parke’s mixture resulted in a hard, flexible, and transparent material. He demonstrated his invention at the International Exhibition of London in 1862. The plastic had several desirable qualities which hold true to this day. It melts and forms easily, and it retains its shape after it cools.

However, Parkesine discovered that other qualities offset such attractive characteristics. It had a high production cost, and it was very fragile, and highly flammable.

In 1886, John Wesley Hyatt invented a new material called celluloid, which received a U.S. patent in 1869. Celluloid had its genesis when the production of ivory in the mid-1800s could not keep up with demand for the material. Phelan & Collander, a United States billiard ball manufacturer, offered a $10,000 prize for anyone who could develop a suitable alternative, according the Plastic Historical Society.

Hyatt, building on the research into cellulose nitrate conducted by innovators like Parkes, discovered the benefit of adding camphor into the mix. Hyatt incorporated the use of heat and pressure. The heat melted the camphor, which made it into a solvent for the cellulose nitrate material. The use of camphor diminished the necessity for additional volatile solvents. Hyatt eventually received patents for several ideas involving the making of celluloid.

Hyatt’s work with celluloid, or cellulose nitrate plastics, led to the development of the stuffing machine, which was the predecessor to injection molding.

The original injection molding machine, which Hyatt and his brother Isaiah Hyatt patented in 1872, used crude technology when compared to the injection molding machines of today. The apparatus employed a plunger, which looked like a large hypodermic needle. It injected molten plastic through a heated cylinder and into a two-part mold. The Hyatt brothers introduced the first multi-cavity mold in 1878.

Some of the early objects produced through this fabrication process include:

common objects made of silicone

The highly flammable nature of cellulose proved a serious hazard for manufacturers. Although it continues in use as the primary material used in the production of table tennis balls, other synthetic plastic began to replace cellulose.

In 1879, employing techniques from a combination of polymer science and rubber technology, Bouchardt created synthetic rubber from an isoprene polymer. The synthetic rubber could contain any unnatural polymer material with the mechanical properties of an elastomer. Such materials allowed extensive elastic deformation under stress. Upon relieving the stress, the material returned to its previous size without lasting deformation.

Synthetic Plastics: 1900s Through Post World War I

In 1903, three Englishmen – Charles Cross, Clayton Beadle, and Edward Bevan – received a patent for artificial silk, also known as art silk. It provided a much safer alternative to the cellulose-based fabric called chardonnay silk. Mass production of artificial silk began in 1905. The feedstock for artificial silk consisted of cellulose, developed from wood pulp.

With the technical name of cellulose acetate, art silk gained recognition under the trade name rayon. Manufacturers produced rayon in massive quantities well into the 1930s, when it was replaced by better synthetic fibers. Despite being inexpensive to produce, art silk’s popularity declined because the material becomes weak when wet and creases easily.

A by-product of cellulose acetate called cellophane hit the market in 1913. This transparent sheet-form material had multiple consumer and commercial applications. Cellulose acetate also replaced cellulose as the stock material used to manufacture movie and camera film. The limitations of cellulose fueled the next significant advance in synthetic plastics: phenol or phenol-formaldehyde plastics.

Leo Hendrik Baekeland, a Belgian-born American chemist living in New York State, discovered phenol while attempting to develop an insulating shellac to coat electrical wires for electric generators and motors. The combination of phenol and formaldehyde formed a sticky mass when heated, which hardened when allowed to cool. Building on this process, Baekeland found he could mix the material with wood asbestos, flour, or slate dust to create composite materials with different characteristics.

Presented to the public in 1912, Bakelite became the first authentic plastic and the first thermosetting plastic. Thermoplastics refers to plastics that become moldable when heat is applied, solidify when cooled, and can then be reheated and remolded.

From 1910-1912, German and British scientists built on the research performed by Bouchardt, as they developed other processes for creating synthetic rubber.

1930s Through Post World War II

nylon introduced in 1927

Beginning in 1927, at a cost of $27 million, the DuPont Corporation commissioned a top secret project. The effort resulted in the creation of polyamide (PA), or nylon, one of the most important plastics ever created. This 100 percent pure synthetic fiber, introduced at 1939 World’s Fair held in New York City, was used in the manufacturing of women’s nylon.

During the Second World War, all nylon output went toward the war effort and the fabricating of parachutes for pilots and paratroopers. The bulk form of nylon has exceptional wear resistance qualities when infused with oil, which makes it a popular choice in the production of bushings, bearings, and gears. The heat-resistant quality of nylon makes it a sound material choice for automotive under-the-hood applications and other mechanical components.

Advances in modern technology, including chemical processes, led to a proliferation of new materials used for liquid injection molding. German manufacturer, IG Farben, developed polystyrene (PS) and polyvinyl chloride (PVC).

PS plastic was used to make plastic model kits and similar items. Polystyrene was invented by Dow in 1938. The plastic is still widely used today. Polystyrene was also used in the manufacturing of products under the names styrene foam and Styrofoam. This high-impact version of the material, introduced in the late 1950s, continues in use today in the manufacturing of novelty items and toy figurines.

PVC creates a rigid, durable material with excellent heat and weather-resistant qualities. Manufacturers use the material for a variety of applications, including:

  • Plumbing
  • Gutters
  • Exterior siding
  • Computer casings
  • Electronic gear

A chemical process can soften PVC for the fabrication of rain gear, shrink-wrap, and food packaging.

Although research had been ongoing since 1910, it was not until 1931 that Neoprene, the first synthetic rubber, became publicly available. The large-scale production of synthetic rubber began in Germany. Neoprene has excellent resistance to chemicals, including oil and gasoline, and withstands extreme heat. This made it appealing for the fabrication of fuel hoses and insulating material for machinery.

In response to the Japanese assuming control of most of the natural rubber-producing regions during the Second World War, the United States government embarked on a secret project to develop and refine synthetic rubber processes.

Also in the 1930s, the Corning Glass Company began research into creating a plastic material with the qualities of both glass and plastic or silicone. Glass-based silicone would have the following properties:

  • Temperature resistance
  • Moisture resistance
  • Chemical inertness
  • Dielectric

In 1942, Corning formed a joint venture with Dow, agreeing to develop and manufacture silicone. From 1943-1960, the partnership of Dow Corning manufactured silicone for a number of industrial uses, including:

  • Aircraft engines
  • Industrial lubricants
  • Textile treatments
  • Personal care
  • Biomedical devices

In 1946, an American inventor named James Watson Hendry manufactured the first extrusion screw injection machine. The machine included a rotating screw, which gave the operator better control over the injection speed. This innovation resulted in higher quality products. The mixing action of the machine allowed the addition of additives, such as color and recycled materials, to the plastic material.

The friction generated by the action of the screw also enhanced the heating process, which resulted in reduced energy consumption. Today, screw injection mold machines account for approximately 95% of all injection molding machines.

From the mid-1950s through 1965, a series of innovative thermoplastics came on the market, particularly high-density polyethylenes, typically used to fabricate mold cups, containers, and lids.

The next innovation for injection molding technology occurred in 1956, when W.H. Willert received a patent for a reciprocating screw plasticator. This machine incorporates a reciprocating screw that moves back and forth during the cycling process. After the mixing, the screw stops turning. Then, the screw moves forward and, much like a plunger, injects material into the mold. During the plastication process, the screw moves in a backwards motion against the hydraulic back force.

Liquid Silicone Rubber – Advances in Liquid Injection Molding

Product designers, engineers, and managers have the challenge of choosing a high-quality elastomer for a variety of critical applications, including medical devices, aerospace, and automotive components. These professionals must make a critical assessment of an array of material properties and processing capabilities to best determine which options meet their demanding performance specifications and budget requirements.

Decision-makers must have clearly defined performance criteria, such as durometer, elongation, and modulus tear. The availability of numerous material and fabrication processes makes it a challenge to determine the optimal material based on desired performance requirements.

The repercussions of choosing the wrong material can cost time, market share, and money. To make the best decision, management must gather as much information as possible about each material, including whether the application requires a solid or liquid silicone rubber.

LSR contains polymers with lower molecular weight, which means shorter chains and better flow. It has become the go-to material for a variety of applications since the 1960s, including:

  • Medical devices
  • Medical tubing
  • Food service products
  • Other industrial, construction, and consumer applications

Although users may test and consider a variety of biomaterials, silicone has a unique combination of biocompatibility and performance characteristics necessary for a variety of uses.

characteristics of silicone

  • It’s tasteless, odorless, stainless, and chemically inert.
  • It’s bacterial resistant and easy to clean and sterilize.
  • Silicone is biocompatible and short/long-term implantable.
  • It’s lubricious for less invasive biological applications.
  • It can withstand temperature extremes (180°F to 600°F).
  • Silicone is resistant to fatigue and compression set.

By the early 1960s, synthetic rubber production exceeded the production of natural rubber. By the 1970s, manufacturers began producing LSR on a massive scale.

In the 1970s, James Watson Hendry developed the first gas-powered injection molding process. Hendry’s invention allowed for the mass production of complex hollow components, which cooled quickly. The invention also permitted designers more flexibility. It improved part strength and finish, and reduced production time, weight, costs, and waste. Beginning in 1972, fabricators began employing robotic technology to remove the parts from the molds.

In 1979, plastic output exceeded steel production. Several years later, a Japanese firm introduced the first all-electric injection molding machine. In 1990, many manufacturers began using aluminum molds in plastics injection molding.

Injection Molding Process: Reliable and Long-Lasting

The history of LSR, as well as the history of injection molding fabrication, will continue to evolve as new trends shape material and machinery technology. Innovations in production processes and material manufacturing will continue to play a steady role in improving material properties and enhancing injection molding machinery, as well as fabrication processes.

Silicone comes in a range of product types that fabricators can tailor for a variety of applications. The choice of materials depends on the processing criteria and the properties necessary for the final product.

Consult with an LSR project expert at SIMTEC to learn more about the process and your options.

Silicone Rubber Platinum-Based Catalysts

Description of the Platinum Chemical Reaction that Forms Silicone Rubber

The silicone industry extensively uses the platinum-catalysed hydrosilation reaction in which a silicon-hydrogen (Si-H) bond is added across an unsaturated carbon-carbon double bond (C=C) of an olefin, resulting in the formation of a silicon-carbon (Si-C) bond.

Hydrosiliation can be used for the synthesis of monomers; for example, Figure 1 shows the addition of a methyl dichlorosilyl-substituted compound, which, upon hydrolysis, gives a polymer, polysiloxane, with hydrocarbon, functional groups.

hydrosilation of silicone

Figure 1 – Hydrosilation

Hydrosilation is a reaction widely used in the silicone rubber industry for the preparation of monomers containing silicon-carbon bonds and for crosslinking polymers, resulting in a variety of products.

Hydrosilation is used to a much greater extent in industry to produce crosslinking reactions.  The crosslinked network shown in Figure 2 is created by the addition of platinum to a mixture composed of a difunctional vinyl-containing polydimethyl-siloxane and the multifunctional Si-H-containing copolymer of polydimethyl-siloxane and me-thylhydrogen siloxane.

Figure 2 shows the crosslinking in the polymer chain which has been catalysed by platinum, the M, D, T, and Q are used where they mean: mono, di, tri and quaternary oxygen substitution.

crosslinking network silicone

 

 

 

 

 

 

Figure 2 – Crosslinking network

Crosslinked silicones obtained by platinum catalyst are used in many applications, such as: automotive gaskets, paper release coatings, pressure sensitive adhesives, and baby bottle teats, to name a few.

The oxygen substitutes a silicon atom and the superscript is used to describe substitution at silicon. Methyl as in Mvi refers to vinyldimethylsiloxane, D is dimethylsiloxane and DH is methylhydrogen siloxane.

The formulation of platinum cure silicone rubber must address the following issues:

Strength – Unfilled silicone rubber has extremely poor mechanical properties and will crumble under the pressure of even a fingernail. The most effective reinforcing filler is hexamethyldisilazane treated, fumed silica. If the clarity is important in the application, the reinforcing resin must be different.

Hardness – The higher the density of the crosslink, the higher the hardness of the silicone rubber.

Consistency – This depends on the viscosity of the base rubber and the variety of low surface area fillers, for example, calcium carbonate and precipitated silica.

Time of cure – The selection of platinum catalyst generally controls the preferred temperature of cure. Platinum in vinyldisiloxanes is usually used in room temperatures curing.  Platinum in cyclic vinyl-siloxanes are used in high temperature curing.

Work time – Is related the speed of cure. Apart from temperature, moderators (or retarders) and inhibitors are used to control the work time.  Moderators slow, but do not stop, the platinum catalyst. Inhibitors stop or “shut-down” platinum catalyst.

Shelf life – In a fully compounded silicone, the shelf life can be affected by moisture, differential adsorption of reactive components by fillers, and inhibitory effects of trace impurities.

Compounding – All but the lowest consistency silicone rubbers are typically compounded in sigma blade mixers, two-roll mills, or, for large scale production, twin-screw extruders.