What Is Plastic Injection Molding?

A guide to plastic injection molding from it's history to today's manufacturing.

We developed this guide to educate the next generation of inventors, entrepreneurs, manufacturers and product managers in plastic injection molding technology. We’ve included history, recommendations, and industry best practices to help bring your ideas to life.


What Is Plastic Injection Molding?

Plastic injection molding is the manufacturing process by which plastic parts are designed, engineered, molded, and produced. In all likelihood, the device you’re using to read this article has some amount of injection molded plastic parts.

Plastic parts can be found in almost every industry: automotive, medical, industrial, consumer goods, construction, aviation, and agriculture.

With modern technology and refinements of this process, manufacturers can produce large quantities of plastic parts with precision and speed.

What Are The Most Important Considerations In Plastic Injection Molding?

It can be overwhelming the number of things you need to weigh into a project. And each process will be different based on the specific needs of a particular project. These are the general factors that impact any injection molding project.

  • Part Design – There are so many features to consider for any given part. What wall thickness do you need? Do you need a solid or cored out part?
  • Material – Picking the right material needs to be researched and evaluation. Which polymer has the properties you need for this part? Will you need brittleness or color? Do you need to account for any special additives?
  • Tooling – There will be many decisions to make regarding tooling depending on how many parts you need, volume, and business goals. How much are you willing to spend? Are you looking for short run or long run mold?
  • Machinery – Each of these factors are interrelated, and the decisions you make about part design, material, and tooling will have a direct impact on the kind of machinery you need.

These are not the only factors and you will have many more decisions to make along the way. Asking the right questions from the start of your relationship with a plastic injection molding company will be vital to the success of your venture.

History Of Plastic Injection Molding

Where did plastic injection molding come from? And how has it changed over time? The next few sections explore the trajectory plastic injection molding from its origins to modern use today.

Key Moments In Plastic Injection History

The origin of plastic injection molding begins in an unlikely place: a 19th century billiards parlor.

Let’s imagine it for a moment.

Image Source: Wikimedia Commons

Stained glass windows cast amber and red lights onto a large oak table. A wooden cue strikes an ivory ball with a crack. The room is spacious and airy, and the sound pierces through the chatter of socialites lounging amid horsehair stuffed couches. In this room, there is not a material, fiber, furniture, or clothing, that is not utterly natural.

This is a world before plastic.

And yet, in this room we can see the beginnings of today’s multi-billion-dollar plastic industry.

In the mid 19th century, a young man named Michael Phelan was making a name for himself in billiards parlors around the U.S. But he was more than just an avid player. Phelan had business sense, and he saw a market opportunity in standardizing billiards tables and products for families to use in their homes.

Creating standards in the industry meant designing and building products that could be reproduced and shipped anywhere.

Phelan joined a business venture to manufacture pool tables. At the height of the factory’s success, 700-1,000 tables were made per year.

Not only did he satisfy his competitive need to perfect the art of billiards, he found a way to make money doing it.

At that time, the cue balls in billiards were made only of the finest ivory, an expensive material harvested from elephant tusks. Phelan and his partners saw the need for a new material for cue balls. He needed something that was cheaper, easier to obtain, and faster to produce on a mass scale.

The Billiard Ball Prize

In 1863, Phelan announced a $10,000 prize to anyone who could come up with a better alternative to the ivory billiard ball. Something that looked, felt, bounced, and smacked just like an ivory ball without the high cost.

Enter John Wesley Hyatt. 

Hyatt was an American inventor, printer, and market opportunist. The story goes that he saw the ad and began experimenting with recipes for a synthetic ball. He had a background in mixing chemicals for printing. He landed on a combination called cellulose nitrate. It was almost what Phelan and his partners were looking for, but it didn’t have quite the same bounce.

So Hyatt was never awarded the Billiard Ball Prize (in fact, no one received the award). But he did patent his invention, “Celluloid.” He also went on to establish a successful company manufacturing billiard balls, false teeth, and piano keys using his newly patented ivory substitute. Celluloid was later more famously associated with film and, unfortunately, fire because it was so flammable.

After tinkering with celluloid for a few years, Hyatt and his brother designed the first plastic injection molding machine. This was a step previous inventors had never really perfected. The Hyatts’ machine was simpler than machines used today, but the principle was the same then as it is now: material is heated to melting and plunged, or injected, at a high pressure into a mold.

20th Century Advancements

The first half of the 20th century brought about a shift in thinking about synthetic materials. Instead of looking for alternatives to natural materials, like ivory, scientists and entrepreneurs were beginning to think of totally original synthetic materials.

The electrical industry had also begun to take off. At the time, shellac was the best electrical insulator. Shellac, a natural material produced by beetles in Thailand and India, was available only in limited supply. So a chemist based out of New York began experimenting with compounds, temperature, and pressure to create a novel material that could insulate electricity.  

Bakelite, patented in 1909, was the result of those experiments, and it was the world’s first completely synthetic plastic. It could be molded, shaped into durable products beyond electrical insulation. It was soon found in jewelry, radios, and telephones. Unlike celluloid, it was not so flammable.

Polystyrene, clear like glass, hard, and brittle, was another new product used for things like plastic cutlery. It could also be puffed full of air to make Styrofoam.

In the 1940s there was another breakthrough in the plastic molding industry. World War II intensified demand for mass produced plastic products, from combs to automobile and aircraft components. That demand drove advancements in technology and materials used during this period.

"A continuous-injection molding and extruding machine, the only one of its kind known to this department, and capable of laying an unbroken seamless tube of plastic or rubber from New York to San Francisco, has come out of the work of Walter P. Cousino, 45-year-old engineer of the Chrysler Corporation."

In fact, in 1941 chemists discovered that polystyrene could be filled with air to make Styrofoam, the material used to make travel coffee cups, coolers, and packaging cushions.

In 1946, the invention of screw injection led to a major shift in the industry that is still being used today. John Hendry revolutionized plastic injection molding by using an auger screw design instead of a plunger to inject liquid plastic into molds.

The ingenuity of the screw design meant raw materials could be heated more evenly, both creating a better product and reducing energy use.

“Screw machines now account for approximately 95% of all injection machines.”

Today, the plastics industry is the third largest in U.S. manufacturing and worth more than $200 billion. The industry is expected to keep growing given demand for bioplastics, injection molding, and 3D printing.

Plastics Market Timeline

  • 1850s – manufacturers begin to look for alternatives to ivory to produce billiard balls
  • 1860s – American inventor John Hyatt creates first celluloid billiard balls
  • 1870s – Hyatt patents first plastic injection molding machine in 1872
  • 1940s – Demand for plastics during World War II leads to expansion of industry and new inventions like Styrofoam and auger screw injection
  • 1950s – Plastics manufacturers develop consumer products
  • 1960s – Synthetic fiber Kevlar introduced
  • 1970s, 1980s – Growth of bioplastics amid environmental and economic concern
  • 2000s – Renewed interest in bioplastics and technological innovations like 3D printing


The Mechanics Of Plastic Injection Molding

You want to know how it all works, and even better how you can get started manufacturing, designing, and delivering the perfect product.

First, let’s review the basics. Molded plastics are made through a combination of the right materials at the right temperature, pressure, and timing.

Most plastics in use today are thermoplastics, which means they melt when heated and resume a solid shape when cooled down.

Injection molding is one of many processes used to mass manufacture plastics. It is an easily scalable process where thermoplastic pellets are melted, pushed into a mold, and cooled into a new, designed shape.

One of the advantages of injection molding over other plastics manufacturing processes is that it’s incredibly versatile. Machines vary in size while there are numerous additives and resins you can combine in the molding process to enhance certain features.

Using a reliable manufacturer, the process results in consistently high quality products.

Injection Molding Machines

This diagram illustrates the mechanics of modern plastic injection molding machines.

diagram of plastic injection molding machine
Image source: Wikimedia Commons


  1. Resin pellets, or plastic granules, are fed into the hopper which guides the material at a specific rate.
  2. The screw rotates inside the heating barrel and moves the materials through the chamber.
  3. As the materials move along the chamber, they begin to heat and melt.
  4. The melted plastic is pushed at a high pressure into the mold cavity. The high pressure ensures the mold is 100% filled.
  5. The plastic begins to cool in the mold.
  6. Once the new shape is cooled and cured it’s removed or ejected from the mold.

Precision And Parameters

Plastic injection molding requires precision, from the design of the mold to the materials used each step of the way.

There are many interconnected parameters that must be controlled during the entire process.

  • Temperature of the material, mold, and oil
  • Injection, holding, and clamp pressure
  • Injection, cooling, and clamp time
  • Distance for injection, mold-open, and ejection

Partnering with a reliable manufacturer with excellent track records in design and engineering is essential to ensure the project adheres to industry standards and stays within budget.

Selecting The Right Material

Selecting the right resin for the product is one of the first critical steps in the process. A material should be identified based on how the final product will be used. The material could be rigid or flexible, strong or brittle depending on its use. The type of material used will have a significant impact on the design of the injection mold.

What factors should you consider when selecting the right resin? 

  • Use of the final plastic part
  • Mechanical properties
  • Moldabilty
  • Temperature resistance
  • Cost

With advancements in polymer science, there are many options available today. A few commonly used materials for plastic injection molding include ABS (Acrylonitrile Butadiene Styrene) polycarbonate, polypropylene, among others.

With so many choices, it’s easy to get overwhelmed. A qualified manufacturer can help you select the resin best suited for the final product. The right manufacturer or plastic molder can provide expertise that will drive innovation. 

Designing The Mold

The perfect plastic part needs the perfect mold. In the design stage you are one step closer to transforming an idea into reality.

Engineers optimize designs for the parameters and materials set for the project. Designers use technologies like CAD, or computer-assisted design for part design, Mastercam for mold design, and 3D printing for prototyping and design verification.

First, an engineer calculates estimated shrinkage since all materials will shrink some amount when heated and cooled.

Once shrinkage is determined, the next step is to consider physical properties of the material, along with size of the mold, number of cavities in the mold, and cycle time.

It’s also important to consider design standards for building molds, such as draft angles, radiused edges, support ribs, and ejector pins. Each of these serves one or more purposes in the mold to enhance the overall performance and durability of the final product.

The mold itself can be made of steel, aluminum, or other combinations of metals. There are numerous enhancements to consider than can increase the life of the mold once it’s moved to production. The mold design will also factor in any product finishes, such as decoration or company logos.

All molds should be designed with careful attention to each of the parameters. Once it has been rigorously tested and prototyped, the injection molder can begin to build the mold for production.

Molding Systems

Molding systems can be cold-runner molds or hot-runner molds. A cost/benefit analysis is the best way to determine which system to use given the specifics of a project. Each has its pros and cons. An experienced manufacturer will be your best resource when deciding which system to use.

Cold-runner Molds

You can use a variety of polymers in cold-runner molds. These molds are generally easier to produce and maintain, however they may not cycle as fast as hot runner systems. More plastic waste is also generated using cold-runner molds.

Hot-runner Molds

Hot-runner molds are less wasteful, can build larger parts, and have faster cycle times than cold-runner molds. These molds are, however, generally more expensive to build and maintain.

Tooling And Mold Building

Building the mold the right way and with a trusted partner reduces costs in project delays or product defects. The mold may be the largest investment in the project. That means it’s even more important to get it right the first time.

Depending on your use, the metal used to build the mold will vary. Steel is the most common material used. It can last years, even decades. Aluminum is another option, so long as the production run will be short.

The number of cavities will also factor into the price. More cavities may mean a higher cost long-term since it could require more maintenance over time. Customization and parts, including finishes and other complexities, will play a role in the cost.

What are the variables that determine cost of building a mold?

  • Metal Material
  • Cavities
  • Customization

One way to save costs may be to partner with an injection company that both builds the molds and manufactures parts. That way you can bundle expenses and have a lower cost per unit.

Quality Control

Being aware of common defects familiarizes you with what to look for as you research and invest in a plastic injection molding. Knowing and understanding defects also helps pinpoint the underlying cause of the problem.

A study by Texas Plastic Technologies found that plastics molding defects could be attributed to one or more of the following causes:

  • The molding machine
  • The mold
  • Plastic material
  • The molding machine operator

In the 30-year study, the majority of defects, or 60%, originated in the molding machine. For machine related defects, plastic material resided too long in the heating barrel. Plastic, once overheated, degrades.

Common Defects In Plastic Injection Molding

Each of these common defects has a corresponding cause and solution.

  • Black specks and streaks
  • Blistering
  • Brittleness
  • Bubbles
  • Contaminant
  • Cracking or fine cracks
  • Delamination or separation of layers
  • Flash
  • Flow Lines
  • Shrinkage
  • Sink Marks
  • Splay
  • Warping

Repairs And Modification

Well-designed molds are built to last years. To keep precision equipment running smoothly over time and production cost-effective, repair and maintenance will be necessary.

Mold injection repair involves diagnostics, evaluation, repair, refurbishing, sourcing and replacing parts, and maintenance. Some of the processes used can include welding, milling, polishing, and cleaning.

Higher quality molds will require less maintenance over time, and quality manufacturers will be able to handle most repairs in-house.


There are more than 16,000 injection molding companies in the U.S. Your job is to identify the best manufacturer to suit your project’s needs.

Regardless of the location, the manufacturer you choose should embody three core qualities:

  • Effective Communication
  • Fair Pricing
  • High Quality

You can start by asking these questions:

  • Do they understand your project’s needs and goals?
  • Is the company knowledgeable about raw materials and industry best practices?
  • Is the building clean and are the machines well-maintained?

Choosing A Manufacturer


Plastic injection manufacturers generally fall into one of two categories: low volume and high volume. Low volume manufacturers make 10,000 parts or less. High volume manufacturers produce 1 million or greater parts. You can also look for low volume molders when prototyping to test a part.

One way to narrow your options is to look at manufacturers based on capacity with your own production volume in mind. Accurate estimates of needed volume can help prevent problems like excess parts going to waste or a dip in sales due to limited supply.


Injection molding manufacturers can provide a wide array of services.

Primary services are generally plastic injection molding, mold design, mold making, testing, prototyping, and maintenance.

There are also a number of secondary services manufacturers may provide: engineering, supply chain management, tooling, welding, heat-staking, sub-assembly, and packaging.


Manufacturers serve a number industries, but the most common are automotive, medical, dental, electronic, military, consumer products, telecommunication, and commercial.

Some manufacturers may specialize in just one industry or a subset of an industry, for example medical and surgical components.

Lead Time

The amount of time it takes to complete a manufacturing process will vary based on the type of services needed. Manufacturers quote or estimate lead time on a job by job basis.

Manufacturers can also provide emergency and rush services.

For new projects, it’s important to plan for the unexpected and provide a buffer for contingencies.


The location of your plastic injection molding partner will impact the ease of communication, lead time delivery, access, and emergency service options.

Offshore vs. U.S.-based

Offshore sourcing of plastic injection molding can include a few risks and extra costs that may make it inefficient process for you.

  • Shipping container expenses
  • Duties on imports
  • Unexpected travel if engineering or design problem arises
  • Language and cultural barriers
  • Delays in lead time and communication

U.S. manufacturers are typically more cost effective. Because U.S. manufacturers stand by a code of ethics and industry standards, quality remains consistent.


In the U.S. both you and your manufacturing partner benefit from the legal system. With international suppliers, legal protections will vary country by country. It can also take a much longer time to initiate claims.

With a U.S.-based manufacturer you can rest assured that any patents and intellectual property will be safeguarded.


The closer you are to your production facility, the better control over quality you will have throughout the manufacturing process. Whether it’s a design problem or a defect, the sooner you catch it, the faster you can solve it and minimize costs.

Manufacturers in the U.S. follows safety and quality standards you will not see overseas. Also, these safety regulations keep employees safe.


A well-trained and experienced plastic injection molder can keep your investment and process affordable. Manufacturers will invest in new machines and modify of older machines to increase production capacity and efficiency.


Injection molding is an investment in your product and your company. The initial design and mold build are the largest upfront costs in manufacturing, however that cost is shared across the volume of parts produced.

For short run, low volume molding projects that only require a handful of parts, the mold cost may be lower while the price per part is higher. Meanwhile, for long run, high volume projects, the initial cost of the mold will be higher, but the price per part will be lower.

There are a number strategies to consider to help reduce the cost of plastic injection molding, like eliminating undercuts, unnecessary features, and cosmetics.

An experienced engineer can also reduce costs with efficient design.


Plastic injection molding use a number of safety measures to protect workers. Proper training and routine equipment inspections are the first step to preventing injury. Emergency stop controls, guards, mats, and procedures are also used to keep the workspace safe.


As the industry as a whole continues to improve efficiency, total energy costs are reduced. That means even the smallest improvement can lead to lower energy usage and less environmental impact. The choice of machine as well as the type of polymer determine how much energy is used during the manufacturing process. Innovation and research in bioplastics are additional ways the industry seeks to reduce our carbon footprint.


The plastic injection molding industry has almost a million employed workers in the U.S. It employs engineers, technicians, designers, production managers, mechanics, not to mention project managers and numerous specialists.

Advantages Of US-based Plastic Injection Molding

When you choose an American company as your plastic injection molding partner, you invest in your community, the domestic economy, and your business simultaneously.

The plastic industry provides jobs to 900,000 Americans in mold making, packaging, machine operation, and more. Not only that, U.S. manufacturers pay taxes that benefit you, your employees, and your community.


With higher quality processes, standards, and trained workers, production will be more efficient. There will be fewer delays. And when problems arise, they can be quickly identified and resolved. These efficiencies are what make partnering with U.S. manufacturers affordable.


The “Made in U.S.A.” stamp is more than just savvy marketing. It conveys trust that the part was produced with sound practices, safety standards, and high quality materials. In other words, it’s a reliable part.

Lead Time

When you work with domestic manufacturers, you benefit from shorter lead time. It’s also a way to reduce costs of delivery and shipping.


Labor used to be one of the biggest expenses in manufacturing. But these days, thanks to automation and computerized machinery fewer workers may be required on the premises. With a highly trained tech savvy workforce, labor in the U.S. is well positioned to support increased demand for plastics manufacturers.

For your project, proximity, reliability, and trust in your plastics manufacturer can accelerate your business success.

Risks With Overseas Outsourcing

To cut costs some production managers may consider outsourcing plastic injection molding to overseas companies. But there are a number of risks associated with outsourcing, which can lead to more expensive production.

Quality control

Low quality materials, inadequate training, and poorly constructed plastic parts can damage your business reputation, not to mention cause safety concerns.

Copyright infringement

One of the biggest risks beside quality of product is copyright infringement or plagiarism. In some countries there are few or no standards in place that protect you and your intellectual property.

Language barrier

If there is no common language spoken between you and the manufacturer, you will likely need to hire a bilingual translator. Even with a translator, communication will not be ideal.

Selecting the wrong company can put your business and reputation on the line.

Remember, when selecting a manufacturer there are three key factors to consider: communication, fair pricing, and quality.


Since its introduction in the 19th century, plastic injection molding has fascinated entrepreneurs, inventors, manufacturers, and scientists alike. As technological advancements and material improvements have been made, the industry has revolutionized the way we experience the world and do business. Today’s U.S. based plastic injection molders are more precise, efficient, and environmentally sound than ever before.

Call To Action

If you’re ready to take the next step on your project, contact Murray Plastics today. Murray Plastics will fit the process to match your needs, whether it’s small plastic parts to large components.

About Murray Plastics

We can take your sketch on a napkin to a two- or three-dimensional design, build, prototype, and produce the product from start to finish.

With 20 years in business, Murray Plastics represents the combined expertise of industry professionals with proven results. Here are a few benefits you’ll get when partnering with us.

  • 24-hour production cycle
  • 51,000 square-foot warehouse
  • Local delivery
  • 20 years in business
  • Quality customer service


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