Plastic gears are widely used in household appliances, toys, autobobiles and robots. As transmission components, plastic gears are precision components that require tighter tolerances than most other plastic parts. At the same time, they are affected by wear, corrosion and high temperatures, so higher requirements are put forward for their design, manufacturing, testing and material selection.
Why plastic?
In general, there are both advantages and disadvantages of plastic gears. They are briefly listed below:
Advantages:
1.Lower production costs (for high volumes);
2. Light weight and low inertia.
3. Self-lubricating or operates with less lubrication;
4. Corrosion resistant (some plastics);
Vibration damping for quieter operation;
5. Can be made into complex shapes, thereby reducing the number of parts, and the size of the overall assembly.
Disadvantages:
1. Higher initial cost of the injection molds;
2. Lower load capacity;
3. Lower precision;
4. Poorer dimensional stability in high temperature or humid environments;
5. Poorer material properties in harsh environments (high temperatures, humidity, chemical corrosion, etc.).
Plastic gears are more used in applications of low speed, light load, and lower transmission accuracy requirement, and thus a larger backlash is allowed.
The machining of the injection mold
We need to use high-precision methods to machine the mold, like slow wire cutting, helical mirror finishing EDM, turning and milling center, and other precision CNC machines.
While during these processes, the workpiece needs to be deliberately aligned and calibrated on each clamp on the machine to ensure concentricity.
Meanwhile, the dimensions need to be checked carefully to ensure the accuracy of the tool compensation. Since the machine itself is quite accurate, much of the inaccuracy comes from the incorrect tool compensation. This means in the machining process, we only need to take measurements of some dimensions but not all of them, if those dimensions are correct, the others are presumably correct too.
Here is a good example of how to machine the injection molds for a gear. This is a 2-level gear, it consists of smaller helical teeth and larger spur teeth.
The mold part for producing the helical teeth is a separate piece mounted on a bearing, this will allow it to rotate when the gear is pushed out from the mold. This part is produced by the helical mirror finish EDM.
While for the spur teeth, it can be made on a separate mold part that is made by slow wire cutting, with the 2 end surface made perfectly flat and parallel, so there will be no gap after mold assembly.
This is how the helical teeth are made in the mold:
Where does the dimensional discrepency come from?
The size discrepancy of plastic gears mainly comes from several factors:
The injection process
For example, for a gear (material POM) with an outer diameter of 90 mm, the high and low injection pressure can lead to a difference of 0.8 mm. This means that even there is a small deviation from the estimated shrinkage, it can be compensated by adjusting the injection parameters.
The inaccuracy of the injection mold
As mentioned above, we need to use high-end machines for the machining of the mold, if lower-level machines are used, like using medium or fast wire cutting to replace slow-speed wire cutting, or using normal EDM to replace mirror finish EDM, there will be a drop of the precision.
Even when using high-end machines, the human factor plays an important role. If the machining is not done with the utmost attention, it can also lead to a loss of accuracy.
Certainly, there are applications where accuracy is less of a concern, then we surely can use fast wire cutting or normal EDM to machine the mold.
In very demanding applications, the injection mold will be made twice or three times until the final plastic gear is made within tolerances.
The uneven shrinkage of the plastic gear
This is somewhat difficult to predict and control. In mold design, it is assumed that all areas shrink the same amount. When in reality, the shrinkage is not uniform.
First, for the type of crystalline materials (like nylon), the shrinkage in the flow direction and transverse direction are not the same. Second, the shrinkage near the gate will be smaller than at the far end. And third, the characteristics of each type of plastic material are different, and the impact of the same deformation factor on them is also different. They cannot be generalized.
The matter of uneven shrinkage is more complicated, but we can try to find something in common, and can give us some guidance in the plastic gear design. Let’s continue to break it down.
Breakdown of common uneven shrinkage
The uneven shrinkage can be divided into radial (along the circumference of the outer diameter) and axial (along the width of the teeth). They will cause inconsistency in the outer diameter while taking multiple measurements along the radial and axial directions.
Ridial inconsistency
The radial inconsistency in shrinkage is usually caused by uneven thickness on the web area.
This gear has 4 protrusions on the web area. On the OD of 95.7mm, the section corresponding to the protrusion is about 0.25 to 0.3mm smaller than the section without the protrusion underneath.
While this gear has an even wall thickness, on the OD of 38mm, the roundness (consistency of OD with the same gear) can be kept within 0.03mm.
Axial inconsistency
The axial inconsistency in shrinkage is a bit more complex.
First, when the web is too thick, it will sink more in the middle section of the teeth, just like a sink mark.
As you can see from the picture above, if the web is too thick, or if it has a chamfered (or radius) design on the inside corners, the middle section will sink more. It is better to have a thinner web, but we also need to consider the load capacity.
The second situation is uneven sinking at the two end surface than in the middle section. We found this situation with some gears made of POM, as you can see there is a slight gap at the 2 ends from the picture below.
It is hard to explain this phenomenon accurately. First, there is a bit of draft angle along the width, so one end is supposed to be slightly larger or smaller. And second, it may be the center section is restricted by the underneath web, so it shrinks less, while the two ends are free for shrinkage, so they may shrink differently.
In order to decrease its impact, we can increase the thickness of the rim to an appropriate value.
Design tips for plastic gears
From what we have discussed above, here are the important design tips for plastic gears
- Avoid uneven protrusions, gussets, and ribs on the web. When it is necessary to design them, try to distribute them evenly along the circular direction.
- Make the web thinner, as long as the load conditions are satisfied. Use strengthening ribs to avoid a too thick web if necessary.
- The appropriate thickness of the Rim is also very important.
- Have a proper gate design in the injection mold. It is common to have more gates for gears than for other plastic parts. It can help to reduce shrinkage and help to make the dimensions uniform in different directions.
(There are 6 gates in the injection mold for this plastic gear, which is 38mm in Outer Diameter).
Conclusion:
It is difficult to predict and control the amount of shrinkage of plastic gears. The dimensional deviation is allowed for many uses.
While in very demanding applications, we may expect and schedule design changes, the injection mold will be made twice or three times until the final plastic gear is made within tolerances.
Dimensional inspection
Nowadays, more and more computerized equipment are used to inspect gears, such as gear profile inspectors and CMMs, and photographic equipment is also being introduced to inspect gears.
These specialized gear inspection devices are more efficient and accurate, but also more expensive. Manufacturers equipped with these types of equipment are usually not interested in small-volume orders.
However, when orders are small or budgets are limited, inspections are usually performed using hand tools such as common normal micrometers, vernier calipers, and runout testers.
Among them, the outer diameter is an easy dimension to check, assuming that the geometry of the mold is done accurately. This means when the OD is correct, the other dimensions will also be within the specs because they shrink in the same proportion.
The length of the common normal line is another important parameter to check, reflecting the overall accuracy of the tooth profile.
Running test
The running test is an effective way to check the precision of gears. Accurate gears will:
- Have small and uniform running noise
- Have less wear in endurance tests.
Since the noise test is immediate, it is more often used in practice.
Available plastics for making gears
The most commonly used:
- 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 commonly used materials are:
- ABS: it is mostly used for low-end applications 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, and 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.