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The History of Plastic Injection Molding

A quick read on the evolution of plastic injection molding

Vaughn Cook RockWell Window Wells
Jeff Crawshaw
January 12, 2021

World War II is a dark period in history. But did you know that the great conflict that split the world into two opposing sides was also a time of opportunity, development, and progress for an emerging plastic industry? With aluminum, copper, steel, and other metals becoming precious commodity allocated solely for military use, plastics came into their own as a manufacturing material. At the same time, the fast-growing aviation and automotive industries fed the demand for inexpensive, mass-produced plastic parts and products.    

Today, plastic is an essential part of our daily lives. The plastic items you come across just about anywhere are in all likelihood the product of a versatile manufacturing technique called plastic injection molding. This piece is the first of a three-part series on plastic injection molding. Read on to know what plastic injection molding is all about, its origin and the current state of the industry.

What is plastic injection molding?

Similar to die casting, plastic injection molding is the process of injecting molten plastic at high pressure into a custom-made metal mold. When the plastic cools and solidifies, the mold is opened, and the plastic molding is ejected. Plastic injection molding is a fast and inexpensive way of mass-producing common-use plastic products.

What raw materials are used?

Thermoplastics and thermoset plastics are the two types of plastic raw material used in plastic injection molding.

Thermoplastics: Thermoplastics melt when heated and harden when cooled. They can be reheated, reshaped and, therefore, remolded multiple times. Thermoplastics are recyclable. Any waste during the manufacturing process can be reused. This makes them cost-effective. Additionally, they are flexible, have a superior finish and high impact resistance (which means they are not too hard and rigid). On the flip side, thermoplastics have a lower resistance to heat and can be more expensive than thermosets.

Popular thermoplastics and their applications: Acrylonitrile butadiene styrene or ABS (computer keyboard caps), polypropylene (car fenders), polyethylene (cable insulators, skincare product containers), polyvinyl chloride or PVC (raincoats), polystyrene (food packaging) and polycarbonate (phone cases).

Thermoset plastics: Thermoset plastics harden irreversibly when heated. Compared to thermoplastics, thermosets generally have higher heat and chemical resistance, mechanical strength, and dimensional stability. However, they cannot be recycled or recast, have lower impact resistance (are more brittle) and are harder to work with, particularly during the surface finish stage.

Popular thermosets and their applications: Fiberglass (bathtubs), silicone (gaskets and seals), bakelite (buttons) and melamine (tableware).

Plastic Injection Molding- Boxes
Plastic injection molding products can be found just about anywhere

Uses of plastic injection molding

You’ll find products and components made by plastic injection molding just about anywhere. Apart from the applications mentioned above, a few more examples of such common-use products are plastic grocery bags, drinking bottles, food containers, toilet seats, remote control and game console casings, toys such as Lego and medical equipment such as syringes. With plastic injection molding, you can make large objects such as trash cans and car dashboards as well as much smaller ones such as bottle caps and automotive parts.       

History of plastic injection molding

As we mentioned earlier, the Second World War was a turning point in the development of plastic injection molding. But the story started long before that. Here is a short timeline:

1868: American inventor John Wesley Hyatt invents celluloid, the first synthetic plastic material, by mixing nitrocellulose, camphor and alcohol in a heated mold.

1872: Hyatt and his brother Isaiah patent the first plastic injection molding machine – a simple device with a plunger that injects plastic through a heated cylinder into a mold. The fledgling plastic industry, which then produced simple items such as combs and buttons, receives a big boost from this invention.

1930s: A period of growth and innovation for the plastic industry, marked by the invention of thermoplastics such as polyolefins, polystyrene, and polyvinyl chloride.

1940s: The industry expands rapidly with World War II feeding a growing demand for inexpensive, mass-produced goods. Plastic fast becomes an affordable alternative to metals and rubber, which are in short supply due to supply disruptions caused by the great war.

1946: American inventor James Watson Hendry improves on the Hyatt brothers’ invention by building a screw injection machine. This device replaces the plunger with an auger – a rotating screw-shaped device. The auger is placed in the cylinder and mixes the plastic material before injecting it into the mold. It gives the molder better control over the speed of the plastic injection. The result: better quality products. Screw injection machines are still widely used today.

1970s: Hendry develops gas-assisted injection molding, which makes it possible to produce long, hollow, and complex parts. The result is a lighter, stronger product with a better finish, made at a lower cost and with less material wasted. This period is also marked by plastic production overtaking steel production.

Made in China: The plastic injection molding market today

Cut to the present and China is the global leader in plastics production and plastic injection molding. These figures say it all:

Why China

When it comes to plastic injection molding, China’s dominance is rooted in its industrial strengths:

  • You can find plastic injection molding suppliers just about anywhere in China. The East and Central South regions especially have a large concentration of plastic factories. You’ll find suppliers who cater to both large-volume and small-volume orders.
  • The sheer scale of China’s manufacturing capacity means it is capable of producing all types, sizes and quantities of custom-made plastic products.
  • In line with China’s growing adoption of robot technology, many of its plastic injection molding companies use advanced automated processes for high product consistency and production efficiency.    
  • Being the world’s largest plastics producer, plastic raw material is easily available in China.
  • It is also the world’s leading producer of injection molding machines.  
  • China’s transport infrastructure – with its superhighways, high-speed rail networks and ports – is among the most modern and efficient in the world and geared towards facilitating exports. As it extends its infrastructure at home, it is also providing financial assistance to neighbors such as Myanmar and Pakistan to build ports and rail terminals to service its own export hubs.     
  • All of this means that China’s supply chain integration is highly superior.

If you’re thinking of sourcing plastic injection moldings from China, here’s why it’s a good idea:

  • Plastic injection molding prices in China are highly competitive. The overall efficiency of the manufacturing process keeps costs low and ensures that you don’t compromise on quality.
  • Plastic injection molding China suppliers have both experience and expertise. You can expect an experienced supplier to not only manufacture your goods but offer engineering support to make it a better product.
  • Sourcing from China means access to its vast resources and technologically advanced equipment.
  • There’s no need to buy your own equipment (including a plastic injection molding machine, which can be a heavy investment). You also save on maintenance and repair costs and workers’ salaries.

Advantages of plastic injection molding

  • High-speed production. The cycle lasts between a few seconds to a few minutes, depending on the mold’s size and complexity.
  • Mass production with consistent measurements. It shouldn’t be a problem to produce 200,000-odd parts before the equipment requires repairs or maintenance (though the material used has a say in this).
  • Cost-efficient. As a raw material, plastic is cheaper than rubber or metals. Manufacturing and labor costs are also low, and more and more manufacturers are using automated processes that in turn further reduce manpower. Low waste rate – especially in the case of thermoplastics, which can be recycled and remolded – also helps keep manufacturing cost low.
  • Design flexibility. Plastic injection molding is perfect for producing detail-heavy and intricately shaped parts.
  • Good surface finish. Usually, very little post-production work is required, though special finishes such as a shiny look, leather-like finish or engravings are also possible.
  • Wide choice of materials and colors. Also, more than one plastic can be used simultaneously. For example, a PVC/ABS blend is intended to have the best of both materials but with few disadvantages of either.

Disadvantages of plastic injection molding

  • High initial tooling cost. Complex molds with multiple cavities can cost thousands of dollars to design and manufacture.
  • High initial lead time due to product conception, design, tooling, and testing. The tooling stage can take between 15 days and three months.  
  • Thermoset plastics cannot be recycled.
  • Performance of recycled material is inferior compared to new material.
  • Not cost-effective for very large moldings.
  • Warpage (shape distortion during cooling) and shrinkage.

Next: The plastic injection molding process

Sourcing Allies is a team of expert China sourcing agents that has helped western customers manufacture and source products from low-cost regions since 2006.

For more on China sourcing visit our website or write to us at info@sourcingallies.com.

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