Injection molding for plastic gears with undercut

[israelkk]

We could not release the 0.3 module injected plastic gears from the mold without destroying the gears. It seems that the gear teeth in the undercut area (quite large undercut) shrinked and the force needed to release the gear is too high.

Does anyone familiar with releasing difficulties of injection molding of plastic gears due to the shrinkage of the gear teeth in the undercut area of the gear? Any documented reference will be appreciated.

 

[dimjim]

Can you redesign the pinion so it
does not have undercutting by using
one less teeth or using a modified
pinion? If you have plenty of contact
ratio, you can redesign the undercut
to have a tangent point at a strait
verticle with the tooth centerline
but you would be removing even more
of the involute profile.

 

[israelkk]

dimjim

The problem is that the gears were designed by an outsourcing company and we would like to have documented references to back our injection problems to face them with the hard facts that their design is flaw.

 

[eromlignod]

Have you looked into using a Fellows Stub Tooth dual-pitch type gear?

Don
Kansas City

 

[israelkk]

eromlignod

Thanks, I know how to fix the problem using profile shift. However, the work was done by an outsourcing company and we need documented collaboration to our claim that their design was flaw and doesn't allow proper injection for the gears.

 

[dimjim]

Interesting problem. I wonder if you
know the mating gears and you can
revise the fillet to ensure no interference
in the fillet area and still provide
a good part that will not cause such
severe shrinkage. Is the tooth form
also shrinking on the blank? I realize
this is a very small gear. What is the
width and bore of the pinion?

 

[gearcutter]

You probably already no them, but try to get in touch with these people

http://www.akgears.com/index.htm

 

[Spurs]

There is a solution to your molding problem:

1) increase the pack pressure and hold pressure and times to make sure that as much plastic as possible can be squeezed into the mold cavity - this will reduce the amount of in mold shrinkage

2) To increase even more packing, increase the size of your gates - dont be afraid to go real real real big - the resin supplier recommend a gate diameter of 50% the thickes cross section of the part. - If your molding unfilled acetal - dont be afraid to go .1" diameter or more

3) After the gate is frozen, keep your cooling time to a bare minimum - just enough to refill the barrel - get the part of of the mold as fast as possible so it doesnt sit there shrinking in the mold cavity

4) Add some undercuts into the opposite half of the tool to help pull the part out

5) See if you are allowed to add fillers into the material (with customer approval) to reduce shrinkage

6) Polish the gear ring profile to ensure that it is not sticking because of surface finish

Undercuts are molded by the millions in plastic gears every day of the year. They are a fact of life. Maybe this isnt what you wanted to hear, but its reality.

http://www.webgearservices.com

 

[MikeHalloran]

Did you get a second opinion on the design of the mold?

This would be a good time.



Mike Halloran
Pembroke Pines, FL, USA

 

[K2gears]

The problem you have is corrected with:

1.) Proper ejection. ie EJ pin/sleeve size, placement, speed, timing.

2.) DRAW polishing of the tool.

3.) Proper material compounding. ie PTFE + silicone + structural fillers.

4.) Pack pressures resulting in proper part density.

Rarely do we need additional undercuts if the tool and part design is correct. ID's and profiles have no draft and do not require draft to mold properly. Your problem can easily be solved with an experienced gear molder.

http://www.K2plasticsinc.com

Klaus Kremmin
Bergen, NY, USA