How plastic fasteners can be made: Plastic Injection Molding

If you think about it, plastics are an amazing invention.  Before plastics were used in manufacturing, all man-made things were made out of wood, stone, or metal.  Imagine a children’s toy  – let’s say a LEGO set – made out of iron.  Sure, you’d have the world’s strongest boy in no time, but you’d also have a real problem moving the completed set in and out of the toy room.

plastic injection molded lego bricks

Plastic Injection Molded Goodness

And the cost of creating such a set, even using a lighter material such as wood, would be virtually impossible because of the manufacturing tolerances required to fit the pieces together.  Luckily, today we can use plastics in the development of a billion different products.  From the smallest plastic screws used in your iPhone to the side panel on your car, plastics fill a major part of consumer and industrial manufactured products.  Heck, plastic shielding apparently is better than aluminum in protecting astronauts in space.

The use of plastics in replacing metal and other materials in products has increased significantly over the years.  Because of its positive chemical and physical properties such as heat and strength, low cost, speed of manufacturing, and its ability to be manufactured in any size or configuration within the highest tolerances, there has been more plastic produced in the past ten years than in the previous century.  Yes, plastics are non-degradable and therefore not environmentally friendly.   However, a process we’ll discuss here called plastic injection molding plays a positive role in reducing such waste. Plastics used in the injection molding process can be reused as often as required.

How are plastic products made?

There are several manufacturing processes to produce plastic parts. One such way is via plastic injection molding.

What is Plastic Injection Molding?

Plastic injection molding is the most common manufacturing process used to create plastic parts, by injecting molten (melted) plastic into a specific mold – which is formed in the inverse of the product’s shape.   The molten plastic cools in the mold, hardens and takes on the shape of the mold. The mold then opens and the part is ejected out.   Then, the process is repeated automatically.

Interestingly, plastic injection molding isn’t new – it was invented in 1868 by John Wesley Hyatt. At that time, he produced billiard balls by injecting celluloid into a mold. Later, he built a plunger type injection molding machine to facilitate production. Another inventor, James Hendry, improved on Hyatt’s invention and built a screw injection molding machine in 1946.  Both processes are still used today, although they are now computer automated.

Injection molded parts can be simple or extremely complex and versatile, and might include holes, hinges, springs, threads and more.

To get there, a product design is typically demanded by a customer, or mass produced or resale towards generic industry standards (like standard plastic screws [LINK http://www.fastenercomponents.com/standard-fasteners-c-1.htm ), then typically the mold specifications are developed by an industrial engineer or designer.  At that point, the molds are made by a toolmaker with those specifications.  The molds themselves are precision-made from metal, usually either steel or aluminium depending on the product, cost, and durability requirements.

What plastic materials are used in plastic injection molding?

First, not every type of plastic material is a good candidate for injection molding.  Essentially, there are two types of plastics:

  • Thermosets  – which are formed by a heat process but are then set (like concrete) – and cannot change shape by reheating. Good examples of thermoset -developed plastics include melamine (for kitchen worktops), Bakelite (the black saucepan handles), polyester, and epoxy resins.
  • Thermoplastics  – which can be injection molded.

The most common thermoplastic materials (and characteristics of each) are:

  • Acrylic – a group of resins that create very hard plastics.   Acrylics withstand weather and are stable in sunlight. Plus, almost any color can be produced. Used within automotive parts, medical devices, paints, coatings, adhesives, sealants, and construction.  Transparent acrylics might be used in aircraft windows, aquariums, signboards, automobile taillights, bathtub liners, sinks, and your cell phone display screens.
  • ABS (acrylonitrile butadiene styrene):   A mixture of compounds, and extremely versatile.  Used for everything from musical instruments, golf club heads, electronic assemblies, whitewater kayaks and canoes, small kitchen appliances, and toys like LEGO blocks.
  • Nylon:  Known for chemical and heat resistance, being tough and flexible.  Used significantly in washers, spacers and other fasteners, hosiery (aka “nylons”), ropes, and fishing nets.
  • Polystyrene:  A low cost yet low strength and poor longevity.  owever, you see it everywhere. It looks like stiff foam, and Polystyrene is most commonly used in packaging, plastic forks and spoons, model cars, plastic rulers, and hair combs.
  • Polypropylene:  A tough, cheap plastic, with a slightly waxy feel, and ability to be extremely flexible without breaking.  It can be bent repeatedly without breaking. Used for Medical equipment such as syringes, stacking chairs (chairshell is polypropylene), suitcases with integral hinges,(tough and flexible – used for containers)
  • Polyvinyl chloride (PVC): Stiff, hard,tough lightweight plastic. It is utilized indoors and outdoors, and most commonly used in plastic windows and plastic pipes.

What are some of the advantages of Plastic Injection Molding? 

  • Plastic injection molding is a very effective way to manufacture accurate plastic parts and components.
  • It’s fast – saving valuable production time.
  • It’s accurate – so you waste fewer materials. And it’s consistent  -so every finished product is the same, every time.
  • Almost any plastic part or component can be made efficiently and accurately with injection molding — from parts for medical devices to parts used in manufacturing.

But wait, there’s a few design limitations

  • Size matters. Injection molding equipment can only take on jobs that fit within set parameters if the product is to be one complete piece.
  • Similarly, different designs and materials could limit how thin your product wall is.