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 John 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:
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
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.