Manufacturing Plastic Gears

Need more specific information:

• Measurements that you have made
• Ideal dimensions
• Visual representation of your application - if we don't have this, it is extremely difficult to envision what you are talking about since English is not your primary language.

Mike

Thank you Mike , the gear is in small motors .

Hi again, niya2000007,

Thank you for the illustration but the annotations were both illegible and/or cut off.

Even with your drawing, I still do not understand the "concentricity" issue.

Is this a geared DC motor mechanism? What is it not doing that you want it to do?

We do want to help, but we need adequate information.

Mike

Hi Mikerho :

I am not sure the below advice could help you understand the "concentricity" issue or not .

I am guessing that the molded gears do not assemble nice and easy like they do on the CAD computer model?

So you will want to have your engineers or designers make some gages so you can sort through the gears.

There are variations in the manufacturing process with molds and plastics in general because of humidity and so on.

You need to have a way to measure what is essentially the "CAM" action of these gears. I mean that if you rotate a gear, and the teeth are not truly concentric, you could measure a wave on the pitch diameter as the gear turns. You would normally mark where the high spot is on the gear, and then make sure it matches the low spot on the mating gear so that they work when you assemble.

The person that made that drawing should be able to tell you the "center distance" for all of the mating gears in the small motor. Once you know this value, you can determine whether your gears are the correct dimensions.

I would suggest making a gauge or fixture to measure gears in pairs. If they have been molded too big for example, the mechanism will "bind" and not operate freely.

tks

I'm not following your question. Are you asking how to measure it or how to specify it on the print? or is this a processing issue where a previously "to print" part stress relieves and goes out of tolerance?

This would be an excellent place to utilize Geometric Dimensioning and Tolerancing. Specifically look at total indicated runout and profile of a surface symbology.

If in serious doubt, and it is of importance to you, contact a university and have them either do the calculations, or show you how do . Most probably they would be pleased to help and you, and you would also develop informative and valuable contacts for the future.

Good advice , tks .

Since you work for , I'm assuming you are having problems producing molded gears. There's an old saying, "you can't inspect quality into parts."

Are you looking for a way to inspect existing parts that currently are failing, so that they will pass inspection?

Are you looking for a way to specify the gears, before molding, so that they will be acceptable?

Are you looking for advice on how to mold round parts that don't warp?

No . Lynlynch , this question raised by our prospect from Mexico ,

We think :

A).-check the steel tool, the teeth shape must have the concentricity with other axis(round hole or core) etc. B).-The gates(3 or four pin-point gates) must be the concentricity with above and balance each other.

Tks

Hi Langyaw :

These are really helpful websites , thank you so much .

Hello Niya,

I find it a little difficult to understand the basic question.

Is it a gaging problem or is it a molding problem.

We can help with both but you will probably not have a concentricity problem if you mold consistently and do not have a mold eccentricity problem.

Here are some basics. 3 gates are three problems and two gates are two problems.

You should go to the extremes to gate through one center gate, period.

If there is a solid hub on the gear there will be no problem doing this.

If you do not have a solid hub you should design one in and 2nd O.P the hub off.

I can suggest some designs that allow very quick 2nd O.P to be carried out.

Material choice is important but if gating is on center then even unfilled nylon gears can be produced consistently even though nylon absorbs a lot of water post molding.

Here is the basic concept for gating into the center of a hollow hubbed gear.

1. Extend the moving (B-side) core so that it telescopes into the hotside ( A-side) approximately 3mm or .12". Gate onto the top of this core by allowing plastic to cascade over the core to fill the part.

You can determine the wall section needed around the telescoping core as this is really your 'flash gate'.

Degating -Second O.P.

You can mount your hollow gear hub by lightly 'press fitting' it onto a pin sticking from a lathe chuck. You can accurately and easily cut the flash gate off the hub as it is only the wall section that you allowed to cascade over the pin that has to be cut off.

This could even be done with a simple blade or snips as the flash gate protruding from the hub will not effect the concentricity of the molded gear.

If you have a three plate mold the same concept can be used.

If you need a 'blueprint' ( I am dating myself) or a file I can send it to you.

Hope this helps.

Hello 2tinker :

Could you please send to me at : niya2000007@gmail.com ？ Thank you in advance .

Ni hao ni zaoshang hao,

Sure , I can send you some prints and probably a photo of some small gears that are made using glass filled nylon. It will take me a day or so as I am busy prospecting for a new job.

Good luck.

It sounds like you really need to verify the dimensions of your tool before you go much farther with your parts. Have it sent to a disinterested third party for inspection. Call out concentricity and run-out to +/- .002 mm. Also call out flatness. It does not matter at this point if it fails or not, you still have information.

If the steel is good, then you have a process problem involving injection, cooling, cycle times, or combinations of all.