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When are risers needed in glass casting?

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When are risers needed in glass casting?

Postby cemoore » Fri Jun 24, 2016 6:48 am

Hi All,

I'm using the lost wax casting technique. I'm attempting to find a description of the process of evaluating a model for the necessity of risers to escape any trapped air. Of course I don't want to bother with risers when they aren't needed and surely want to incorporate them where needed. I just don't know how to discern the difference. Does anyone know of a resource that might fully describe the evaluation process?

Thanks!
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Re: When are risers needed in glass casting?

Postby marykaynitchie » Wed Jun 29, 2016 9:04 am

Hi,

We use a sand bed to support the mold for lost wax kilncasting. Here is an article with diagrams showing a suggested arrangement of a shelf on supports with a pile of sand on it, and the mold placed in the sand.

http://www.bullseyeglass.com/images/sto ... eet_08.pdf

We also have a video on lost wax kilncasting, available as part of our subscription program, that shows how the kiln is loaded:

https://videos.bullseyeglass.com/videos ... lncasting/

I hope this helps.

Mary Kay
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Re: When are risers needed in glass casting?

Postby cemoore » Sat Jul 02, 2016 4:47 am

Hi,

Perhaps I didn't phrase my question adequately, but what I am referring to are venting mechanisms from the wax model to the outside of the mold, typically string, wire, or organic material. Thanks for any additional help in determining when/where a riser or vent is needed.

Thanks
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Re: When are risers needed in glass casting?

Postby marykaynitchie » Tue Jul 05, 2016 10:48 am

Hi,

Is that the same as a sprue?

Mary Kay
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Re: When are risers needed in glass casting?

Postby cemoore » Tue Jul 12, 2016 9:32 am

Hi Mary Kay,

Sorry for the delay, I just revised my email notifications to know when there's been a response.

No as to your question, the sprue (at least to my understanding) refers only to the channel through which the molten glass will enter the cavity, with or without an integral reservoir above it for holding the glass. See this online image at http://moragreekie.com/about-me/the-process/, specifically the image labeled 'Stage 3, Adding Risers..." for a visual. The example risers look rather large and I'm guessing are producing warts on the glass that need to be coldworked away.

Related to this question on how to determine necessity of risers, I'm also interested to understand what the smallest diameter riser could be to accomplish the goal of venting.

Thanks again, getting to the bottom of these riser questions would be invaluable.

Cathy
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Re: When are risers needed in glass casting?

Postby marykaynitchie » Wed Jul 20, 2016 8:28 am

Hi Cathy,

We have a video (by subscription) and an article (free) demonstrating lost wax kilncasting with Bullseye glass.

https://videos.bullseyeglass.com/videos ... lncasting/

http://www.bullseyeglass.com/methods-id ... sting.html

Maybe some other Forum members have suggestions.

Mary Kay
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Re: When are risers needed in glass casting?

Postby NancyB » Mon Aug 22, 2016 4:27 pm

Cathy, you don't need to use straws to make your air vents. You can use toothpicks or skewer sticks, which will make much smaller "warts." The placement of them will be something of a judgement call, as every piece will be different. You're looking for places that will trap air, such as at the bottom of the ear (in your example). Since the piece would be cast upside down, the bottom of the ear would be above the top (if that makes sense) and an area that would trap air. Any undercut or sort of flat area like that will trap air.

Also, you sometimes need to vent towards the bottom to prevent a bubble being trapped at the bottom which would prevent the glass filling that area. In your example, she vented at the end of the arm because she needed the glass to flow in there, particularly since the arms are oriented upward when it's upside down.

I hope this makes sense!
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Re: When are risers needed in glass casting?

Postby windsir » Sat Jun 09, 2018 9:14 pm

You can probably determine whether you will need “risers” (I call them “vents”) this way:
Place a small cylinder or cup (like a shot glass or battery) on the palm of your hand
Curl your fingers up, around the cup.

1. your palm represents the piece,
2. the cylinder is your glass sprue,
3. your fingers represent any part of the piece that, in the casting position, rises significantly above your palm.

If the fingers represent any part of your piece that can hold an air buble pushed into the mold in front of a mass of incoming glass, especially if those parts are small, you probably need a vent.

Here’s an example of where a vent (or four) might have helped:

Image


Image
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