A quick read on the evolution of plastic injection molding
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.
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.
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).
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.
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:
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.
Cut to the present and China is the global leader in plastics production and plastic injection molding. These figures say it all:
When it comes to plastic injection molding, China’s dominance is rooted in its industrial strengths:
If you’re thinking of sourcing plastic injection moldings from China, here’s why it’s a good idea:
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.
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