How to work with your plastic manufacturer on plastic gears

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custom plastic gears injection molding

Finding a plastic gear manufacturer is a more demanding job

As transmission components, plastic gears are precision parts, they call for tighter tolerances than most other structural components. At the same time, for each of the custom plastic gear projects, we need to deliberately choose the right plastic material with the required knowledge. This puts higher requirements on the plastic gear manufacturers than other plastic injection molders. This post is to help you understand how to effectively locate a manufacturer to produce your plastic gears with fewer costs and risks.

Before we get started, let’s get some basic knowledge about plastic gears first.

Why plastic, and what the downside is?

With the development of plastic material and molding processes, plastic gears are finding more applications to replace metal gears. They are becoming larger, stronger, and more complex in shape.

Some of the plastic gears are manufactured by CNC machining, but most of them are injection molded without secondly machining.

machining of plastic gears
injection mold plastic helical gear close view

Generally speaking, plastic gears have both upsides and downsides. They are briefly listed below:

Upside:

  1. Lower production cost (for large volume).
  2. Lightweight, less inertia.
  3. Self lubricant or operation with less lubricating.
  4. Corrosion resistant.
  5. Shock absorption, running more quietly.
  6. Can be made to complex shapes, thus reducing the number of parts, and the size of the whole assembly.

Downside:

  1. Higher initial cost in injection molds
  2. Lower load capacity
  3. Lower accuracy
  4. Less dimensional stability under high temperature or moist environment.
  5. Poorer material property under harsh environment (high temperature, different chemical corrosions, etc.)

It is more difficult to conduct strength calculation

If you are new to gear designing, you can read this post to get some preliminary knowledge quickly.

If you have the full knowledge of gears, the challenges will be the uncertainty of plastic material properties. Engineers are usually accustomed to metal gear designing because they can easily find data on handbooks to conduct strength calculations, while for custom plastic gear design, it is always harder to find enough data to do similar calculations. Meanwhile, many plastics weakened at high temperatures, while their metal gear counterparts can still perform quite steadily at that same temperature range.

However, plastic gears can be securely used in some situations:

  1. Lightly loaded application where the temperature will not rise too high due to a lower speed and lighter loads;
  2. In the applications where plastic gears have been proven to perform a satisfactory job. You can just copy the gears or make some small changes.

What plastic materials are appropriate for making gears?

These materials are used for most custom plastic gear applications:

  • POM (acetal): It is easy to be injection molded with good dimensional stability, at the same time, it has great strength, ductility, and anti-wear, anti-corrosion, and humidity resistant properties. This is the primary material for plastic gears.
  • PA6/PA66/PA46 (nylon): It has great strength and wear resistance, but it absorbs moisture which leads to instability in dimensions. In other words, it swells while absorbing water. They are mostly used in transmission with heavier loads.
  • TPEE: this is an elastic material, which means it absorbs shocks and reduces noise in transmission.

Other less used materials are:

  • ABS: it is mostly used for low-end application with lower costs, like toys.
  • PC: as an amorphous polymer PC has great dimensional stability in the injection molding process, in other words, it can be molded to the shape of the mold cavity with less contraction. The disadvantages are it is a poorer self lubricant, it also has poorer fatigue resistance.
  • PPS: this is a quite expensive material, but it has great dimensional stability and yet offers mechanical strength and durability. This is often used for harsh applications like pumps and robots.
  • LCP: it is another expensive material with excellent dimensional stability and can be made to high precision, it also tolerates a high temperature of 220℃ and chemical corrosion, but it offers less strength. It can be used in watches.

Keep in mind that there are different forms to each kind of material: unfilled, reinforced with glass fibers, and filled with lubricant material (mostly PTFE or silicone), so it is rather a narrowing down process to make a choice based on each application.

Criteria for selecting a qualified plastic gear manufacturer

Now that we have the basic knowledge about plastic gears and the manufacturing of them, it is time for us to come to the main topic of this post–what kind of plastic injection molders should we look for?

I have written a post about how to find a qualified plastic injection molding manufacturer, but since gears require tighter tolerances, they place more requirements on the manufacturing condition and operators:

  1. The molding machine needs to be in good (new) condition with the capability of precise control of molding temperature, pressure (injection pressure and holding pressure), injection speed.
  2. The mold needs to be properly designed to allow proper material flowing and cooling, accurate temperature control, and need to consistent from cavity to cavity. For plastic gears, a flow mold analysis is always recommended before entering mold production.
  3. The manufacturer needs to know about gear parameters and understand proper measuring methods.
  4. The operators need to pay close attention in the manufacturing processes and keep measuring the gears, he needs to know how to adjust the molding parameters, so as to make sure the gears are made within tolerances.

In simpler words, you need to find the manufacturers equipped with high-end molding machines, responsible and detail-minded operators, and intelligent, experienced mold designers.

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