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Gearing Up For Plastic Molding

January 31, 2018

Steel yourself: plastic is becoming increasingly more popular as a material for machine gears. In fact, in some gearing applications, plastic is even superior to metal!  

Advances in polymers have made this all possible.  Industrial designers have a better grasp of their benefits and limitations. Today, plastic gears can be manufactured by hobbing or stamping or, for large-volume runs, by plastic injection molding.

Intrigued? Here are the hard, cold facts when it comes to plastic gears, which can be used in the automotive, personal care or medical industries:

  • Lower weight due to the lower density of the material
  • Shock absorption capability and reduced vibration due to elasticity
  • Less noise
  • Less friction
  • Self-lubricating, at least in some applications
  • More amenable to wet environments
  • Safer in food preparation areas

However, as with all materials, there are also some disadvantages to using plastic in gears. These include:

  • Lower load-carrying capacity compared to metal gears of roughly the same size and weight
  • Molded gears cannot retain the same high tolerances that metal gears can
  • Temperature and humidity can affect plastic gearing negatively, even change its shape, because plastic is less stable when compared to metal
  • Most plastics are significantly more costly than that of base metals and their prices are much more volatile due to the price of resin
  • Difficulty in affixing plastic gears to metal shafts

The most common plastics used in gearing are acetal and nylon.  In most applications, acetal is used for plastic injection molded gearing and nylon is used for machined gearing.  Acetal is shape-stable, chemically resistant and is moisture-reisistant.  However, it does need constant lubrication when used in a high-volume production environment.  There is also always a danger the raw material may harbor voids.

Nylon is less stable as it does vary in shape in response to both temperature and humidity. The advantage of nylon for use in gearing is that it is typically self-lubricating.  Moreover, voids are not an issue when using cast nylon.

Many engineers are hesitant to use plastics for gears because plastic is inherently weaker in many categories compared to metal. But there are work-arounds.  One of the most popular methods to improve the strength of nylon gears is to mold the material into a bar and add some type of fiber to the mix, thereby increasing its strength.  The other method for increasing strength is designing-in a carbon steel core.  A steel core can be cut for any shaft-mounting application, yet the outer rim has all of the benefits of plastic.

So what are you waiting for?  Gear up for plastic!


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