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Could be because of the change in thickness there. A stress-relief crack. More integrity when the wall thickness is consistent. Maybe re-design with a more shallow groove or with corresponding bump on exterior?

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Im going to go out on a limb and assume that the OP'er is making electrical pole/utility pole insulators; you can kind of see some more in the background. While I doubt that this crack would lead to its structural failure(appear to be 1"+ thick), Im guessing the OP'er doesnt want them at all. Id agree with Rae that based on where the crack is forming, that its due to the shape of the slot that has been cast into it. Id suggest maybe trying to compress this portion of your castings when they are leather hard, otherwise I think you'd have to redesign your mold/prototypes. It could be a firing issue/speed. Could also be a drying issue; if the rest of the insulator is very consistent in its thickness, this sharp angled, and uneven wall thickness area would be one spot that would be prone to shrinkage/drying cracks. Not sure what your climate is where you are, and how you dry work, but you may want to try slowing it down some, or evening the drying out.

Are ALL of your insulators cracked in the same spot, or just here and there?

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On 2/15/2019 at 7:27 PM, Vikash bhagat said:

As found groove crack after firing , 

Attachment  :-Top view of Groove with crack in shell Item

IMG_20190215_192345_402.jpg

Same place crack in all type designed insulators , this type of cracking coming in all designed insulators when fired in suttel kiln , when fired in tunnel kiln its (groove crack) appearance is nexible, the wall thickness issues no matter because different type of insulators different different wall thickness ic:- 36kv /630A thickness is 16 mm,  When 36kv/2000A insulator wall thickness at the place of groove  crack is 26mm 

Attachment :- 36kv/630a top view (3D) of groove crack

36kv/2000a top view of groove crack

IMG-20190220-WA0001.jpg

IMG-20190220-WA0000.jpg

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1 hour ago, Vikash bhagat said:

No these are running products , dark material is glaze and, "glued afterward" I am not getting this

So it's all one piece, part is glazed black, the middle is white?

If they weren't cracking before, then something has changed either in the materials or the firing process. Have you verified that the kiln temp is accurate, and the firing schedule hasn't changed? If all that is the same, then something has changed in your materials. I'd guess it's the feldspar.

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This is a problem that would be best solved by a Root Cause Analysis (RCA) effort.  Some questions to answer before jumping to conclusions:
These are example questions NOT a complete list. 
How often are these cracks occurring?
When did the cracks begin to be noticed?
Are the cracks always in the same location? always the same size?
What has changed in the manufacturing process prior to the point in time when the cracks began?
Are the parts made by slip casting?  Have the molds been examined, especially in area where cracks are being found? 

How often are the parts examined for cracking , especially in the area where the cracks are occurring,  throughout the manufacturing process from beginning to the final examination?   Are you sure the cracks are NOT present prior to the firing?   Examine the product at each step from beginning to end to locate where the cracks are beginning to be observable.  Then work backwards from that point to locate the cause.  

Cracks occur because the stresses exceed the material strength in the area of the crack.  Ask questions regarding the process from beginning to the final examination of the finished product; such as, What could have caused these stresses, or weakness in the final product? 

Be specific in the questions.   The next step is to look for answers to each question.  Get specific details on the differences between conditions when the parts were always OK and the conditions now when the parts are not always OK.  

LT

 

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My guess is when they are hanging they get heated all around evenly so less cracking-you can slow down the firing thru critical periods like quartz inversion.

The other option is make a base so they get heated more evenly. You have not shown a photo of the whole piece so its hard to tell what they look like

Here is an insulator made in the 1940s from my collection(this one is my favorite with an red albany glaze)  ,as you can see from the base it has feet that allow an unglazed area and also this base allows for even heating of all surfaces.Look at the foot carefully and you can see how it it was fired.

You are lucky as I have my photo setup going the week.

 

 

in1.jpg

in2.jpg

in3.jpg

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#MAGNOLIA MUD RESEARCH 

>Regularily occuring these cracks

>2 months ago when noticed

> starting from same location ,went towards  ID ,width same, -length changed ,

>No change in manufacturing process and groove cutting after drier , before drier also cut the Groove ,but no improvement 

> the manufacturing process is  filter cake>pugmill>hand making roller for better compact > shaping with the help of jigeer ,

>100% checked from beginning to drier No occuring crack and sure crack didn't showing prior to the shuttle  kiln  

>I think stress May be not matter because Crack occuring all Range insulators as :- 12kv ,24 kv, 36kv, 52kv.  Etc 

Looking for your reply

Thanks 

            Vikash Bhagat

 

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