Shrinkage of Molded Phenolic Composite Material

Presumably the answer is to rough machine 'em, store 'em for a week and then finish machine 'em?

Mind I'm not sure the material will hold that tolerance with time anyhow? Many plastics absorb water and change dimension...

The clever answer may be to design out the poblem if possible, make it less tollerance dependent. More tollerant of tollerances (tee hee)
Del

If you start out close to the tolerance limit on either end, heating or cooling may throw the tube out of tolerance.

Back in my youth, we'd specify an inspection temperature for complex injection molded parts, that were inspected by government inspectors. Some of them had no concept of thermal expansion/contraction of plastics.

Or better yet, glass-reinforced phenolic should have very little water absorption.

The link is to a resource. No connection to me and no recommendation.

http://www.emcoplastics.com/materials/phenolic/phenolics-glass-reinforced/

Good luck

The OP is talking about a bushing, which presumably involves a rotating shaft of some kind. Glass fiber will wear that shaft significantly...

Thanks for all your comments. My comments are as below:

1) Glass reinforced composite is not a suitable option.

2) Since the problem is of shrinkage (reduction in id and od) and warpage, swelling due to humidity may not be the problem. swelling should increase the od in free condition.

3) I think, my machining tolerance is quite liberal for the bush size.

4) Material does not get heated up while machining, at least by touch. I have also tried rough machining the pieces first and finish machining after a dy or two.

Problem is the unpredictability of shrinkage and warpage as it varies in each piece. I think, the best option would be stop or minimize it, which should be possible, as in the case of metals.

Stress relieving before and/or after machining may be a solution. I tried this on rough machined peice by heating at 85-90deg C in a oven, letting it cool to room temp and them machining. But, the problem of variation and unpredictability continues.

Suggest you measure a piece over an extended period of time and graph it. You may find it just keeps moving and you are chasing the impossible.
Del

I did try that by marking about 12 spots on 3 - 4 pieces. But, measuring also becomes a problem due to warping and ovality, which keeps changing over a period. My worry is the change in dimensions after finish machining.

You are missing the point.
Keep testing a finished part* and see if it ever stabilises. If it never stabilises, then there is no solution using that material and that design.
Del

If you had done that every day from when you first noticed the problem, you would know the answer already.

Del, I agree. But, the point is I can not afford any deviation beyond my tolerance limit 0.004" on diameters (assuming I keep some extra material during finish machining for shrinkage/warpage) after finish machining, which can be done max 2-3 days after rough machining. So, I have to do something to stop deviation beyond 0.004" even after a year.

Phenolic suits my requirement, readily available and is cheaper. Would appreciate if somebody could throw some light on stress relieving procedure for composites.

Molding Compounds - Post-Baking
The above is general in nature.

I suggest slowly heating to just below the Tg (glass transition temperature) of your specific resin. This is different, depending on which type you are using. Comsult your data sheet.

Do NOT machine a solid rod. Start with a tube, the machining stresses will be less with a tube.You might try stress relieving the raw tubes, cut to length, then relieving again, after machining.

You should try stress relieving some tubes in mineral oil, this will help stabilize the heating and cooling process.

Finally, you may have to resort to stress relieving the tube in a constrained condition using a steel rod to control the ID and a steel sleeve to control the OD.

Ask your raw material supplier for help. The have more experience with their material than I do.

You may want to consider compression molding your own custom bushings to the finished size. This will help with the stresses, but drive the cost up.

Good luck.

If the parts are used in a stable temperature and humidity environment, they should be machined in that environment. Keep in mind that the humidity time constant will be long. I'm guessing that your problem is related to the time constant of moisture change in the parts. With a cross section of 0.5 inches, the moisture time constant could be months or even years. You may also find better results using mandrill wound fabric instead of flat material.

A mandrill is a kind of monkey; perhaps you mean mandrel.

--Ed. C.