Doubt about plastic gears 3

[Galvano]

I have noticed that many plastic gears have an internal cutout on both sides of the sprocket.

In the company there is a need to find some plastic gears and I have seen that they are in a manufacturer's catalog, but since I don't know if they can supply them to me and they will have to manufacture them; That interior emptying has some function or is it just to lighten weight.

Attached photo where you can see that emptying on both sides. It exists in many plastics.

 

[Compositepro]

These are injection molded parts. The shape is optimized to use the least amount of material possible for reduced cost. Also, thick sections cause problems due to resin shrinkage and extended cooling time during molding.

 

[Pud]

It does make the gears lighter, but it also does more:

1. Reduces wall thickness and hence time to mould.

2. Thick walled plastics are likely to have shrinkage voids inside. These act as stress raisers.

3. Helps stop warpage when moulding as shrinkage due to the polymer crystallising varying with cooling time.

I would guess the only way to get a 'solid' gear is to have one machined from bar or sheet stock.


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[Galvano]

the teeth are mechanized right? The inner diameter comes with a H9 tolerance. Is this diameter machined or does it come directly out of the mold?

 

[Compositepro]

The gear is molded as one piece. The teeth and bore are not machined after molding. Machined plastics generally do not have as good a surface finish as molded surfaces.

 

[mfgenggear]

I don't have first hand experience with plastic molded gears. What is the surface finish and dimensional size held. But I have worked with machined plastics. It is tough

 

[Pud]

Surface finish is determined by the finish on the tool. Usually to all intents and purposes it's a mirror finish.

We used to make a 120mm od part (not a gear, circular but no teeth). In acetal (POM) could comfortably hold 0.1mm on diameter)



Politicians like to panic, they need activity. It is their substitute for achievement.

 

[mfgenggear]

Pud thanks for sharing that experience.
.1 MM is generally not precision enough for gears with very close backlash while in mesh , but that said like I said, not experience with molded plastic gears.
any one here designed molded plastic gears?

 

[Galvano]

The one with the teeth also seems to me to be a very good finish, but I have seen a video on YouTube where they inject plastic into a mold and the gear comes out with its teeth.

 

[mfgenggear]

Galvano
fundamentals on gear trains are the same metal vs plastic. what makes a difference is what fit, form or function is required.
and what are acceptable cycles. a post machining may be required in the bore to be more precise.
it is better to have more back lash, then to be to tight and bind. plastic is more economical for very high volume.
but may not be durable. with high wear.

 

[Compositepro]

Plastics have over five time the CTE of metals and will not be used in high load or high precision applications. They are relatively very low cost to produce in large quantity, and perfect for many applications like printers and copiers.