Understanding the Different Types of Plastic Molding

Published on January 3, 2024

Plastic is one of the most used materials globally for fabricating products and components across various industries and applications. Though plastic injection molding is considered the most advanced technology for making plastic parts, several types of plastic molding methods can be used instead, which, in some cases, work better for an application. Though injection molding usually offers the most efficient and economical processes, other methods have their place. Understanding these different plastic molding processes can help manufacturers better understand which will best suit their final product.

The Various Types of Plastic Molding

When fabricating parts made from different types of plastic, molding involves shaping material either in a flowable form that goes into a mold or malleable enough to be shaped within a rigid matrix. Depending on the method used, molding plastics requires distinct ranges of pressure and heat to be applied to produce the final product from various types of plastic. Molding processes most often utilize polypropylene, polyethylene, or other polymers in powdered or liquid form.

Though injection molding is one of the most popular ways to mold different types of plastic, molding techniques vary to suit a diverse range of needs. Industries use other plastic molding methods to meet certain specifications and requirements for their components and products. The type of plastic molding a manufacturer uses depends upon the process that best suits the application. The main types of plastic molding are blow molding, compression molding, extrusion molding, injection molding, and rotational molding.

Blow Molding 

The basic concept behind blow molding is similar to that of blowing glass. It’s an especially economical technique for quickly creating large numbers of hollow objects, particularly containers with thin walls and uniform shapes. It’s an excellent method for making smaller bottles, large containers, and drums of 13 gallons (50 liters) or more. While it works well for making simpler products, it’s not plastic molding useful for fabricating complex parts.

The blow molding process follows these basic steps:

  • A preform needs to be made to ensure consistency and accuracy of the blow molding process.
  • A mold is injected with molten plastic to form its external shape, which is clamped around a core rod to form its internal shape to form the neck of the container.
  • The mold is then opened, with the core rod clamped into a chilled blow mold.
  • This core rod opens to allow compressed air to inflate the piece into its finished form.  
  • Once cool, the mold is opened, with the finished piece removed from the core rod.

Blow molding allows manufacturers to utilize various thermoplastics, including high-density polyethylene, low-density polyethylene, polyethylene terephthalate, polypropylene, and polyvinyl chloride.

Compression Molding

Both economical and very efficient, compression molding helps manufacturers make components that vary in complexity, length, and breadth. Compression molding’s curing process uses heat, enabling final products to keep their structural integrity and shape. This technique makes incredibly strong components and is used for making a range of products, including body armor, fasteners for clothing, household appliances, and vehicle components. The process of compression molding is a relatively simple one.

Steps for compression molding involve:

  • Heated plastic is placed within a heated mold.
  • Plastic compresses within the mold into the desired form in curing.
  • After cooling, the component is removed.   

Compression molding produces more durable and stronger parts and uses more advanced types of plastic. Molding with materials like fiberglass, reinforced plastics, and thermosetting resins, this method offers more robust construction and greater resilience than other types of plastic molding, though it’s less useful for making more intricate parts.

Extrusion Molding

Unlike other types of plastic molding that utilize molds to shape the final product, extrusion methods involve inserting plastic resins directly into a die, determining its final form. This technique uses an extrusion molding machine that’s used in injection molding and is used for piping, straws, tubes, and other lengthy, hollow shapes.

As an example, the extrusion molding process for silicone involves:

  • Silicone is mixed then rolled to make it easier to work.
  • Trapped air is removed from the material via air compression.
  • At this point, any specialty ingredients – like a vulcanizing agent – is added to create specific properties.
  • Machines are warmed up.
  • Rolled silicone is put inside the machine, choosing a specific nozzle to create a certain shape.
  • Silicone is heated so that it keeps its form as it moves through the die to create a tubular shape.
  • Finished product is removed from the machine, cleaned, measured and packaged.

Extrusion molding can also make these forms into circular sections, I-sections, L-sections, T-sections, and U-sections. Types of plastic molding materials used in the extrusion process include acrylic, acrylonitrile butadiene styrene (ABS), nylon (polyamides), polyacetal, polycarbonate, polyethylene (PE), polypropylene, polystyrene, polyvinyl chloride (PVC) and silicone.

Injection Molding

This method is considered the most adaptable molding method for many different types of plastic. Molding via this technique can create various shapes and sizes of parts and other products, with the machines available in assorted sizes, depending on how much pressure is needed for the application. Plastic injection molding uses fairly basic processes, though it can fabricate complex parts with greater consistency than other types of plastic molding. This versatility makes it useful for various products, including car body parts, intricate internal vehicle components, prosthetic devices, and surgical equipment.

Design engineers can make highly complex and unusually shaped parts with a wide array of plastics available. Though the molds are quite expensive, the cost per piece becomes lower the higher the volume produced. Plastic injection molding is very well-suited for mass production. Able to produce precision parts consistently in large quantities, it’s considered the most efficient and cost-effective solution for molding many types of plastic. Molding processes can even be altered to include resins with special additives, specific properties, various finishing options, and other enhancements.

The basic process for plastic injection molding entails the following phases:

  • Plastic resin is put into a barrel.
  • Resin is heated, mixing different ingredients together.
  • Volume of material is determined through the creation of shot sizes that are then injected into the mold.
  • Mold is shut.
  • Mold cavity receives injection of melted plastic.
  • Material cools and sets, with the equipment then prepared for the first three steps in the next cycle.
  • Mold is opened once sufficiently cooled.
  • Component is then ejected from the mold.

Injection molding can be used with hundreds of different types of plastic. Molding processes using this method most commonly entail use of thermosetting and thermoplastic polymers, though certain injection molding machines can be used for alloys and metals, glass, elastomers, and even candy.

Rotational Molding

Also sometimes called rotomolding, rotational molding uses high heat and rotating movements to coat the inside of a mold to form a component. The technique is most suitable for single-piece containers that are large and hollow, producing components that have better flexibility in their design, greater strength, and uniform wall thicknesses. Molds are typically made from fabricated steel sheet or cast aluminum. Rotational molding is also very efficient and environmentally friendly, as the process wastes very little material, while excess is often reused. Though it’s an inexpensive method, it’s also a much slower process than other types of plastic molding.

Rotational molding follows these basic steps:

  • Resin is pulverized from pellets into powders, the fineness dependent on the application and type of machine used.
  • Additives or colors are added to ensure the piece has the necessary properties.
  • Powdered resin polymer is measured and placed into a hollow mold firmly locked in place, with the amount used determining wall thicknesses.
  • Inner walls of mold coated with a release agent to prevent component from sticking to mold.
  • Rotations per minute are determined to ensure adequate distribution of material to the walls for proper part thickness.
  • Heating time is calculated to prevent thermal degradation from excessive heating and incomplete melting if not heated long enough.
  • Molten polymer is allowed to cool, hardening and solidifying it as the machine continues to rotate.
  • Moving air cools the outside of the mold, either naturally or via fans.
  • Internally air is sometimes used to maintain part’s dimensional stability while cooling.
  • To hurry cooling times, water sprays are sometimes used, though this may affect properties and shape.
  • Once cooled, a hollow mold tool is used to remove the component.
  • In some machines, air injection systems are used to remove the part.
  • Component can then be coated, painted, welded, assembled or otherwise finished.
  • Part is then inspected and packaged.

The vast majority of products that utilize rotational molding techniques use polyethylene. The five types most often used are cross-link polyethylene (XLPE), high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE), and medium-density polyethylene (MDPE).

To learn more about the types of plastic molding available, the injection molding experts at Spaulding Composites are here to help. We also have mold-making capabilities and can work with molds made by third parties. To determine which method is right for your project and how Spaulding can assist, contact us today.