Avoiding bubbles in Impression-Casting Material

gmackugler · Jul 22, 2019

I am using flexbar reprorubber casting material (2-part compound injected into part through nozzle) to create impressions of an internal chamfer on a plastic part, for measurement on an optical comparator. I've been experimenting with different flexbar products (thin-pour and medium body), but they are both producing bubbles in the casts and the bubbles are throwing off my measurements.

Does anyone have any tips for avoiding bubbles in the casts?

Thanks

gregormarwick · Jul 22, 2019

Just the other day while I was not really paying attention to it, Youtube decided to play me a video about artisan soap making.

They added rubbing alcohol to the mix to stop bubbles forming as the mix hardened.

I have no idea why I've retained this information, or if it would have any positive effect on reprorubber.

Scottl · Jul 22, 2019

Usual practice is to de-air it in a vacuum vessel immediately after mixing.

You don't need high vacuum or a fancy container. A pressure cooker with the vacuum tubing connected where the weight sits will work. One place I worked at used a large dia. pipe spool ( a short piece of pipe welded to 2 flanges) sitting on a rubber sheet on the bench with a chunk of thick Lexan as a lid with a rubber gasket.

For vacuum even a cheap compressed air siphon will get you down to about 25 inches of mercury, plenty enough for this.

gmackugler · Jul 22, 2019

Scottl said:
Usual practice is to de-air it in a vacuum vessel immediately after mixing.

You don't need high vacuum or a fancy container. A pressure cooker with the vacuum tubing connected where the weight sits will work. One place I worked at used a large dia. pipe spool ( a short piece of pipe welded to 2 flanges) sitting on a rubber sheet on the bench with a chunk of thick Lexan as a lid with a rubber gasket.

For vacuum even a cheap compressed air siphon will get you down to about 25 inches of mercury, plenty enough for this.

Thanks for your response! I am hoping to avoid adding any semi-sophisticated equipment to this process though, as this process would be for high-volume QC inspections and the added complexity would be difficult to implement.

gregormarwick · Jul 22, 2019

gmackugler said:
Thanks for your response! I am hoping to avoid adding any semi-sophisticated equipment to this process though, as this process would be for high-volume QC inspections and the added complexity would be difficult to implement.

I take it there's no way to get into the feature with the putty?

I've only ever used the putty, never had problems with bubbles forming. Maybe I've been lucky, or is the putty perhaps less prone to this?

gmackugler · Jul 22, 2019

gregormarwick said:
I take it there's no way to get into the feature with the putty?

I've only ever used the putty, never had problems with bubbles forming. Maybe I've been lucky, or is the putty perhaps less prone to this?

Can't get into it with putty. We are measuring the angle of the chamfer relative to the central axis of the threads, and are getting bubbles along the threads (which is throwing off the thread-detection app on the optical comparator). The putty definitely wouldn't fill out the threads.

Dan from Oakland · Jul 23, 2019

pull a vacuum on it after you cast it. make a box the part will fit in. Degassing before pouring also makes a big difference.

Scottl · Jul 23, 2019

gmackugler said:
Thanks for your response! I am hoping to avoid adding any semi-sophisticated equipment to this process though, as this process would be for high-volume QC inspections and the added complexity would be difficult to implement.

If you don't want to use vacuum your only other option is to find a material that does not need to be de-aired, and they DO exist, although perhaps not in silicone.

This article lists different techniques for molding without bubbles.

How to make Silicone Moulds without air bubbles

Conrad Hoffman · Jul 23, 2019

We degas most of our epoxies in a small centrifuge, rather than vacuum. Just be careful with fillers, as they can end up at the bottom of the mix!

Delw · Jul 24, 2019

I tried that stuff once, cause I ran out of the good stuff and had the same problem with bubbles so I tossed it.
we use the same stuff they use at dentist for impressions (forget the name but the company has been around since the 90's).
white tubes blue white and yellow label if I recall.
I have a case of it sitting at the shop if I remember I will get the name of it sometime tomorrow. the color of the stuff is white and yellow

Delw · Jul 24, 2019

heres the stuff we use. we love it and use alot of it.

cb750chris · Jul 24, 2019

We use Panasil (I think that's how it's spelled). Dental stuff also, in the same tubes as the picture above. No air bubbles unless you dont mix it well or don't get it into the nooks and crannies well. Mix it right on the desk and it dries in a few min. Only special process is to make sure the part is cleaned with alcohol or acetone first, or coolant oil will keep the surface of the mold from hardening correctly. The boss says its pricey, but it's the best of the ones we have tried.

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gmackugler · Jul 26, 2019

cb750chris said:
We use Panasil (I think that's how it's spelled). Dental stuff also, in the same tubes as the picture above. No air bubbles unless you dont mix it well or don't get it into the nooks and crannies well.

Do we know if these dental products have comparable dimensional accuracy and stability as the products advertised as "metrology-grade" like Flexbar and Plastiform? I wouldn't want to make a cast of my part only to have the cast shrink over the next hour.

cb750chris · Jul 26, 2019

I can't say for sure over long time frames (days/weeks), as I usually take measurements within 10 min to an hour. I can say that I've compared the measurements from a mold to other measurements of the same feature using CMM, micrometers, etc and they're usually within tenths of each other. That's all assuming you're a little OCD (like me) about making a good mold and how you set it up and measure it with a comparator.

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