The phrase “injection molding” refers to several technologies and processes that use injection molds. Most commonly, the term is used within industry to describe the general production process that uses such molds and machines that inject substances into those molds. While different materials can be used in the injection molding process, it is most common to use polymer plastics. Such materials are used because they can be easily shaped into desired forms and then can quickly cool down into those forms. Often these materials come with trade-offs between cost, time to produce, and quality.
Thermoplastics are the most used materials in the injection molding process. This is because they can be heated and melted down into a liquid form, which is much easier to pour than other types of materials such as metals or ceramics. The main issue with plastics, when compared to other materials that may be used in the injection molding process, is that they can not tolerate high temperatures.
This is because the molecules within the plastic break down at extremely high temperatures, changing its shape and structure. This typically limits plastics (when compared to materials like metals) in how complicated of a design that may be created. An example of thermoplastics that are used in injection molding includes:
- Polyethylene (PE): A low-density polyethylene, typically found in plastic bags and food storage containers.
- Polypropylene (PP): Has a high melting point, more expensive than other thermoplastics. Used for kitchenware and automotive components.
- Low-density Polyethylene (LDPE): Has a high impact strength, used for plastic bottles.
- High-density Polyethylene (HDPE): More expensive than LDPE, has low moisture permeability and is an electrical insulator. Used in milk jugs and trash bags.
- Polyvinyl Chloride (PVC): Commonly found in plumbing systems and flooring, because it withstands moisture.
- Polytetrafluoroethylene or Teflon: A fluoropolymer that is chemically resistant to high temperatures and acids. Often found in chemical containers and non-stick cookware.
- Nylon: Able to withstand a wide range of extreme temperatures, used widely in the automotive industry.
- Polycarbonate: Used in bullet-resistant glass and safety goggles because it is more shatter resistant than other plastics.
- Acrylonitrile Butadiene Styrene (ABS): A petroleum-based plastic with good impact, heat and abrasion resistance. Often used to make car bumpers and Lego bricks.
2. Thermoset plastics
These plastics are made by crosslinking polymer chains through a chemical reaction that typically involves heat, pressure, moisture or catalysts. Because they are already “set”, thermoset plastics cannot be softened by heat or solvents like thermoplastics can.
- Acrylic: A rigid, durable material that transmits light and is often used in airplane windows and lenses.
- Urea: A key component of thermoset plastics. It reacts with formaldehyde to produce a strong resin that is found in wood products such as particleboard, plywood and MDF (medium density fiberboard).
Rubber is often classified as natural rubber, latex-based rubber, or synthetic rubber. It is used in the production of many types of products, including footwear, hoses and gloves.
4. Metal alloys
There are two main categories of metal alloys: wrought alloy and cast alloy. Wrought alloys are made by mixing metals together to achieve desired characteristics. Most common types of wrought alloys are aluminum, copper and stainless steel.
Believe it or not, glass can be formed into many shapes using injection molding. Used specifically for non-functional requirements, it can be found in items such as vials and test tubes.
Silicone is a very flexible material that can be used in a wide variety of ways, from waterproofing to insulation. When heated, it can create molds. Silicone is often used in the medical industry to make molds for prosthetics.
Ceramics are formed in one of two ways: cold isostatic pressing (CIP) and hot isostatic pressing (HIP). HIP involves using heat to create a more porous material, which can be helpful for coatings or surface treatments. Used mostly for decorative purposes, it can be found in items such as platters and decorative statues.
Injection molding is a process that offers many materials for manufacturers to use. The main materials are metals, glasses, rubber and plastics, but each type of material comes in various forms and brands to offer the best product possible. While injection molding does not necessarily make products more durable or last longer, it can help make them function the way they were intended while also improving their appearance.