What Is Plastic Injection Molding?

(Click Infographic to Enlarge)

Plastic injection molding is a manufacturing process that involves the process of injecting melted plastic resins into a prefabricated mold to create a variety of parts and components. This highly versatile and cost-effective process is used to manufacture a variety of products, from children’s toys to complex engineering components.

For more than 35 years, Lerner Molded Plastics has been a leading provider of plastic injection molding services for customers in a wide range of industries. Our state-of-the-art facility allows us to produce quality injection molded parts from the initial design phase through full-scale production and distribution. Whatever your needs, we have the knowledge and skill to create your product or component with the utmost speed and efficiency, all while staying within your budget.

To help you determine whether plastic injection molding is the right choice for your project, we have created a comprehensive guide to the plastic injection molding process, materials, and benefits.

Plastic Injection Molding Process

Plastic injection molding uses high pressure and heat to force melted plastic into a heated mold, where it takes on the desired shape as it cools. The cooled product is smooth and often requires very little additional processing. Although the details of the process may vary depending on the product and material, the plastic injection molding process typically follows these six steps.

1. Mold Creation

A mold is manufactured from a durable, heat-resistant material such as aluminum, steel, or beryllium-copper alloys. Molds are typically CNC machined and finished to extremely precise dimensions to ensure highly accurate and repeatable molded products. Injection molds can be designed with single or multiple gates, through which the melted plastic is injected, and must be able to open on a hinge or sandwich design to allow for part ejection once it has cooled.

2. Clamping

To ensure that the mold does not expand, contract, or become separated when exposed to the high temperatures and pressure of injection molding, the mold must have a secure clamp. The clamping force must be calibrated to ensure that the melted plastic can be injected with enough force to completely fill every part of the mold without material leakage.

3. Injection

The injection process is initiated when plastic beads, powder, or pellets are placed in a hopper, which feeds the material into a heated barrel, causing the plastic to melt while the turning screw simultaneously propels it toward the injection head. The injection head deposits a specific amount of molten resin into the mold at high pressure to ensure that the mold is completely filled.

4. Cooling

After the resin is injected into the mold, the mold is allowed to cool. The plastic hardens as it cools, solidifying into the desired shape. The cooling process is calibrated to ensure that the material cools at the right pace to produce optimal shape, hardness, and surface finish.

5. Ejection

The cooled part is ejected from the mold using built-in ejection pins. It is important to ensure that the part is thoroughly cooled in order to avoid damage. Ejection timing is based on the required cooling time for a given part’s size, shape, and material.

6. Finishing

Plastic injection molded parts are ejected with a smooth surface that requires very little finishing, though specialty coatings and finishes may be added for specialized applications. Most often, the only finishing necessary is the removal of excess plastic around the channels where the resin was injected.

Common Materials

Plastic injection molding can be used to manufacture parts from a wide range of materials, including:

• High Density Polyethylene (HDPE). A versatile, affordable thermoplastic with high tensile strength and stiffness, HDPE is waterproof, resistant to microbial growth, chemically resistant, and offers good electrical insulation.
• Polypropylene (PP). PP is a rigid, crystalline thermoplastic valued for its high strength-to-weight ratio, stiffness, electrical resistance, and chemical resistance. It also bears stress and has high impact resistance.
• Polystyrene (PS). A low-cost thermoplastic with high strength and exceptional clarity, PS is useful for clear packaging and lab equipment, or can be colored for use in consumer products.
• Nylon. Nylon is an extremely versatile thermoplastic blend that can be tailored to meet the needs of particular products. Nylon blends are extremely tough, with high chemical and thermal resistance and low friction.
• Acrylonitrile butadiene styrene (ABS). ABS is an affordable engineering thermoplastic valued for its structural strength, stiffness, impact resistance, chemical resistance, thermal resistance, and electrical insulation.
• Acetal. Acetal isa strong, tough semi-crystalline thermoplastic with exceptional chemical resistance, high mechanical stability, extreme creep resistance, and a low friction coefficient.
• Polycarbonate (PE). PE is a highly durable amorphous thermoplastic with extreme dimensional stability, good impact strength, exceptional optical clarity, and good electrical insulation.
• Polyester (PET / PETG). Polyester is an extremely stress-resistant semi-crystalline thermoplastic with high dimensional stability and exceptional resistance to chemicals and heat.
• Acrylic. Acrylic is an extremely transparent thermoplastic with high resistance to UV radiation, exceptional impact strength, and high dimensional stability.

Advantages of Plastic Injection Molding

Plastic injection molding is an extremely efficient and cost-effective process that can be used to create extremely complex components with a high degree of accuracy and repeatability. Some of the benefits of plastic injection molding include:

• Efficiency. Plastic injection molding uses the same mold for multiple runs, making it a quick and efficient way to produce multiple components. In addition, products straight out of the mold typically have a smooth finish that requires very little additional processing, thereby reducing time and labor.
• Accuracy. Since the plastic resin is injected into the mold at high speed, it is forced to fill every cavity in the mold, ensuring extreme accuracy on even the most complex components.
• Less Waste. Leftover thermoplastic material from the plastic injection molding process can be added to the next load, thereby eliminating process waste. In addition, many of the thermoplastics used in injection molding are recyclable, reducing overall waste.
• Low Costs. Injection molding uses the same molds for multiple products, making it less expensive than many other manufacturing processes.
• Variety of Material and Color Options. A wide variety of plastic materials and blends can be used in the plastic injection molding process to create products in nearly any color.

Plastic Injection Molding Industries and Applications

The versatility, accuracy, repeatability, and many other benefits of the plastic injection molding process make it a popular manufacturing choice for numerous industries, including:

• Building and construction for pipes, fixtures, hand tools, pulleys, and sprockets.
• Food and beverage for packaging, bottles, line process equipment, and dispensing equipment.
• Medical and pharmaceuticals for medical devices, surgical implements, and packaging for medical equipment and pharmaceuticals.
• Automotive for interior handles, knobs, and fixtures.
• Consumer goods for appliances, garden supplies, tools, furniture, and decor.
• Housewares for disposable and reusable silverware, plates, bowls, cups, and mugs.
• OEM for custom-designed mechanical and engineering components.
• Toys for building blocks and other brightly colored children’s toys and games.

Expert Plastic Injection Molding Services From Lerner Molded Plastics

At Lerner Molded plastics, we pride ourselves on providing our customers with the highest quality injection molding services and materials in the industry. From very small tight tolerance parts to extremely large products requiring high tonnage clamping forces, we have the equipment and experience to handle any size part requirement. As an industry-leading plastic injection molding company, it is our goal to ensure reliable, repeatable products that meet even the most stringent industry standards and specifications. Whether you are in need of large-scale production of a simple toy design or intricate custom OEM components for industrial manufacturing, we have the equipment, experience, and skill necessary to provide you with exceptional quality components every time.

To learn more about our superior plastic injection molding services, contact the experts at Lerner Molded Plastics today, or request a quote for your next project.