Plastic Molding 101: Understanding Types, Properties, and Applications

Author: May

Mar. 08, 2024


Plastic Injection Molding Explained

In manufacturing, molding refers to the process of shaping a type of pliable material or liquid by using a rigid matrix or mold. In plastic injection molding, raw polymer resin granules are melted into a liquid and injected into a mold, where the plastic takes the desired shape. Varying levels of heat and pressure may be applied to create the final profile for the end product.

The basic plastic injection molding process consists of the following steps:

  1. Plastic resin materials enter a barrel.
  2. Heat melts the plastic, which is mixed together.
  3. Shot sizes are created, determining the volume of material to be injected into each mold.
  4. The mold closes.
  5. The melted plastic is injected into the mold cavity.
  6. While the material cools and solidifies, the injection molding machine prepares steps 1 to 3 for the next cycle.
  7. The mold opens.
  8. The part is ejected.
  9. The process repeats when the mold closes again.

In today’s ever-evolving world of manufacturing, plastics are being used to make everything from automotive body parts to synthetic human body parts. To create critical components and ensure optimal performance, many manufacturers choose to make use of plastic injection molding.

To suit diverse industry needs, there are several other types of plastic molding available, each offering unique features and benefits. An application’s particular specifications and requirements will determine which type of molding process is best-suited for a specific part.

Below are a few of the most common different styles of plastic molding.

Blow Molding 

The blow molding process follows the same basic steps found in the art of glass blowing. To blow mold a part, the manufacturer inflates a parison — a heated plastic mass, usually in the shape of a tube — with air. The parison inflates until it fills the mold and conforms to its shape. In this way, the plastic is blown into its desired form. Once cooled, the newly formed plastic part is ejected from the mold.

Blow molding is especially useful for economically manufacturing one-piece, hollow objects in large volumes, as the process can quickly create uniform, thin-walled containers — perfect for small objects like bottles, as well as larger ones like storage containers and drums.

Depending on the specific application, manufacturers can use a variety of thermoplastics in blow molding to create a more customized product. Commonly worked materials include low-density polyethylene, high-density polyethylene, polyethylene terephthalate, polypropylene, and polyvinyl chloride.

Compression Molding

Compression molding involves placing a heated plastic inside of a heated mold, then closing it to compress the plastic into the desired shape. Once cooled, the part is removed from the mold. The heating process, called curing, helps ensure that the final product will maintain its integrity and shape. Compression molding offers many unique advantages; it’s both cost-effective and highly efficient. The process is also quite versatile, allowing manufacturers to create parts that vary greatly in thickness, length, and intricacy.

Because compression molding often uses advanced composites for the plastic material, the process yields stronger, more durable parts, making it popular across a range of different industries. For example, compression molding often employs high-strength materials, such as thermosetting resins, fiberglass, and reinforced plastics, resulting in products that are sturdier and more resilient than those offered by other molding processes.

Allowing for the creation of high-strength parts, compression molding is used to produce components for a vast range of applications, including automotive parts, household appliances, clothing fasteners, and body armor.

Extrusion Molding

While other forms of plastic molding use extrusion to insert the plastic resins into the mold, extrusion molding extrudes the melted plastic material directly into the die. This process is unique in that the shape of the die, not the mold, determines the shape of the final product.

Extrusion molding is ideal for manufacturing parts with continuous length and uniform cross-sections. Similar to a plastic injection molding machine, the extrusion molding machine has a screw that turns to feed the plastic resin into the feeder. The molten plastic then moves through a die, creating a long, tubular shape. The shape of the die determines the shape of the plastic tube. Once the extrusion is cooled, it is removed from the machine.

Extrusion molding is well-suited for long, hollow-formed applications, such as tubes, pipes, and straws. Plus, manufacturers can create these parts in many different shapes, including T-sections, U-sections, square sections, I-sections, L-sections, and circular sections.

Injection Molding

Of the various molding processes available, plastic injection molding is considered to be the most versatile, as it can be used to create a variety of parts, ranging in both size and shape. Presses also come in different sizes, based on the pressure they exert and their tonnage.

Injection molding basic principles are fairly simple, but the actual process can be quite complex when it comes to maintaining part consistency. The process involves the injection of melted plastic into a mold, which is made of steel. The mold itself has cavities that will form the parts; once injected, the molten plastic fills the cavities and the rest of the mold. Once cooled, the parts are ejected by pins.

Thanks to its excellent versatility, injection molding can be used to create everything from large automotive parts to small, intricate parts used in surgical equipment. Injection molding also allows for a high level of customization, as various plastic resins and additives can be used, allowing designers and engineers to create unique parts to meet highly complex or unusual application needs. And there are some enhancements and techniques available — such as an array of resin and finish options — for manufacturers looking to create even more specialized parts.

Though it can be expensive to initially make the molds themselves, once built, the production costs become quite low. In fact, injection molding is best-suited for the creation of very high volumes of precise parts; once production begins, the cost per part drops significantly, making the process very economical for high-volume runs.

Plastic injection molding is a highly reliable solution for producing large numbers of precise, consistent components. It’s also more efficient and cost-effective than other molding styles, in that it produces much less waste. As a result, injection molding is most often used for the manufacture of high-quality parts in high volumes.

Rotational Molding

Rotational molding, also known as rotomolding, uses high temperatures and rotational movement to coat the inside of a mold to form a part’s desired shape. First, the mold is filled with a polymer powder. The heated mold then rotates on two perpendicular axes so that the powder adheres to the entire interior of the mold. As it continues to rotate, the mold eventually cools and is removed, ultimately forming an even-walled component.

Rotational molding is best suited for the creation of large, hollow, one-piece containers, such as tanks. Though cost-effective, it is not a fast-moving process. However, rotomolding wastes little material, and what excess material is produced can often be reused, making it an economical and environmentally friendly manufacturing process.

Other key advantages include the ability to produce parts with consistent wall thicknesses, enhanced design flexibility, and great strength

Intro to Plastic Injection Molding eBook

Explore the World of Plastic Injection Molding with Confidence

Download our eBook, “An Introduction to Plastic Injection Molding.” Inside we cover:

  • Types of plastic molding, their benefits & applications
  • Types of presses and their capabilities & advantages
  • How to determine plastic injection molding costs
  • Common mistakes to avoid
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Each of these plastic molding processes offers unique features and advantages, making each style well-suited for different types of applications.

At The Rodon Group, our designers and engineers are well-versed in custom, high volume plastic injection molding and work closely with all of our clients to ensure their unique needs are met. In most situations, there are several ways to approach a manufacturing challenge, and we can provide expert insight into the various techniques and materials best suited to a specific project.

To learn more about plastic injection molding and how it can help with your next project, download our free eBook, “Plastic Injection Molding 101.” Contact us or request a quote to start your solution. 


The manufacturing of polymer products involves several distinct processes, each with specific benefits and applications. Despite the increasing popularity of additive manufacturing (commonly referred to as 3D printing), most rubber and plastic parts are manufactured using traditional molding processes.

This blog post is designed to help you understand how leveraging the different types of molding processes produces durable, high-quality rubber and plastic components for the unique demands of your next project. After briefly defining the molding process, we’ll break down the advantages and applications of four different types.

What Is Molding?

Molding is one of the five core processes involved in rubber and plastic manufacturing, alongside casting, forming, machining, and additive manufacturing. Though they share some similarities, the processes are quite different.

A molding process uses pressure and heat to fill the cavity of a mold, a rigid frame also known as a matrix. The raw material, typically a pliable one like plastic, glass, or ceramic, is kept under pressure until it solidifies or vulcanizes (i.e., begins to harden). Advantages of molding include greater accuracy, versatility, and efficiency.

Molding is different from casting, which is when a liquid or melted polymer is poured into a mold and solidifies to form the part. No pressure other than gravity is used to fill the mold and form the part. The advantages of casting include durability and a greater complexity of design – however, it may not be possible to cast large parts.

4 Types of Molding Processes: Applications and Advantages

Molding serves a wide range of industries and offers exceptional quality, sustainability, and adaptability of parts. Understanding the types of molding processes can be pivotal in ensuring the success of your next project. Here are some of their main applications and advantages.

1) Compression Molding

The compression molding process is used to make rubber and plastic parts. In the rubber compression molding process, a preformed section of rubber is placed in a heated mold, then the mold is closed and held under pressure until the rubber takes the shape of the cavity and vulcanizes. Vacuum-assisted compression molding is similar but placed under vacuum to remove any gases and ensure the best possible mold filling and shape adherence.

Compression molding offers several benefits over other molding processes, including low tooling costs and fast lead times. However, the process requires manual handling of the mold and products, resulting in slower cycle times.

2) Melt Molding

When applied to thermoplastic materials, compression molding is referred to as melt molding. For this fabrication technique, the polymers are heated above their melting point, so they can assume a liquid form.

The resulting polymer is then allowed to cool and solidify in the form of the mold. When temperature conditions are properly controlled, melt molding can deliver better mechanical properties than compression molding.

3) Transfer Molding

In the process of transfer molding, the material is loaded into a chamber prior to being forced into the mold. The chamber may be in the molding machine or in the mold itself.

Transfer molding may be used with rubber or plastics, although the process for each material is slightly different. Tooling geometries are a bit more complex than compression molding but still exhibit lower cost than injection molding.

4) Injection Molding

The injection molding process can be used with both thermoplastic materials and thermosets. Injection molding machines inject molten polymer materials under high pressures and speeds into closed molds. Though creating these molds can be more expensive, the process can ultimately provide finished parts at a high production rate.

To learn more about the different characteristics and benefits of injection molding for complex polymer components, be sure to check out our recent blog post on this topic.


Whether you’re looking to ramp up high-volume production or need short-run custom products, look for a manufacturing partner with a variety of molding processes under one roof. With robust capabilities across the four types of molding, the right partner can help you custom-design the right solution using the best molding process to meet your specifications, efficiency objectives, timeline, and budget.

Plastic Molding 101: Understanding Types, Properties, and Applications

4 Types of Molding Processes in Rubber and Plastics Manufacturing




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