Jump to content


Photo
- - - - -

Cracking In Mold


  • Please log in to reply
8 replies to this topic

#1 Strelnikov

Strelnikov

    Advanced Member

  • Members
  • PipPipPip
  • 72 posts

Posted 19 August 2013 - 09:00 PM

I have a mold in which as the slip dries it cracks.  What causes this and what can be done to prevent it?  I'm using slip straight from the shop, no used stuff added.



#2 BeckyH

BeckyH

    Advanced Member

  • Members
  • PipPipPip
  • 85 posts

Posted 19 August 2013 - 10:47 PM

When does it crack? What shape is the object you are casting? Long, narrow stems will crack where they attach to the bowl of a goblet. Plates and bowls tend to crack across the bottom.
A few possible reasons:
The differences in thickness of different parts of the object are great enough that they dry at very different rates.
You are dumping the extra slip before the casting is thick enough.
You are letting your casting get too dry in the mold. You should pull it out when it is leather hard.
Do you use a defloculant? If your slip is sludgy instead of smooth and creamy the differences in viscosity can cause problems.

#3 Strelnikov

Strelnikov

    Advanced Member

  • Members
  • PipPipPip
  • 72 posts

Posted 20 August 2013 - 08:58 PM

It cracks as it dries before it is pulled out of the mold.  It is a bowl that is raised in the center.  I think the problem is that the corners around the center part are not smooth enough.  Is it possible to sand a mold to change its shape a bit?



#4 stephsteph

stephsteph

    Advanced Member

  • Members
  • PipPipPip
  • 36 posts

Posted 21 August 2013 - 01:08 AM

i usually do not let my pieces dry all the way in the mold, and release them as soon as possible. it may be that as the piece is shrinking it is getting caught, either in  a change in the relief or profile in the piece , or in a gouge or something.


Stephani Stephenson

Revival Arts Studio

http://www.revivaltileworks.com

 


#5 Mart

Mart

    Advanced Member

  • Members
  • PipPipPip
  • 283 posts

Posted 22 August 2013 - 01:33 AM

It cracks as it dries before it is pulled out of the mold.  It is a bowl that is raised in the center.  I think the problem is that the corners around the center part are not smooth enough.  Is it possible to sand a mold to change its shape a bit?

 

Yes it is, it's plaster. Be really careful and visualize the effect the change has, before you start with alterations.



#6 PeterH

PeterH

    Advanced Member

  • Members
  • PipPipPip
  • 194 posts

Posted 22 August 2013 - 05:11 AM

Streinkov

It cracks as it dries before it is pulled out of the mold.  It is a bowl that is raised in the center.

 

Assuming that you are using a 1-part drop-out mold and your de-molding problem is only  marginal: Try to

drop the cast from the mold repeatedly as it dries, so you de-mold as soon as you can. Alternatively support

the upside-down mold just above a surface, so the bowl has a chance of dropping out itself.(Maybe on a board,

and dropping the board an inch or so from time to time to encourage de-molding.)

 

Otherwise, it sounds like the slip is shrinking onto the mold as it dries (and shrinks slightly). Do you have any

reason to doubt this? Do you really have to pull the bowl from the mold, if so how strong and localised is the

adhesion.

 

There are three ways that you can address a shrink-on problem: remove the offending area (or reduce its severity

"sufficiently"); de-cast very early (but the released form may not be self-supporting); use a parting agent[*] (and risk

a slight surface blemish). You may need to use them in combination.

 

When I've had the problem it was localised to one "face" of a multi-part mold:

 

- I tried to de-mould this face as soon as the casting was strong enough to allow this without tearing, but without

  success. By the time the casting was strong enough to pull/lever off it was firmly stuck.

 

- Using a parting agent made de-molding possible, but the freed face was very floppy  (distorting ~1cm to de-mold).

 

- I had to firmly support this face to restore its shape, and support it while it dried to leather-hard. Luckily

   it was a shape that permitted this (a flat face with an egg-shaped depression) and the other parts of

   the mold didn't get in the way.

 

It rather sounds like you are using a 1-part drop-out mold. Which makes supporting a floppy de-molded bowl

difficult.

 

Regards, Peter

 

* IIRC talc for lo-fire and nepheline syenite for hi-fire. 'ware dust hazards, and maybe make a dusting-bag from 

old tights (or even a tea-bag).



#7 Strelnikov

Strelnikov

    Advanced Member

  • Members
  • PipPipPip
  • 72 posts

Posted 23 August 2013 - 09:39 AM

Peter,

 

Yes it's a 1-piece drop-out mold that I made myself.  The cracking doesn't start appearing until the piece is almost ready to release.

 

I think I'm going to try to modify the mold a bit to see if the cracking problem goes away.  I made two other 1-piece drop-out molds that are working fine.

 

Thanks very much for your help.

 

Evan



#8 Strelnikov

Strelnikov

    Advanced Member

  • Members
  • PipPipPip
  • 72 posts

Posted 23 August 2013 - 09:41 AM

i usually do not let my pieces dry all the way in the mold, and release them as soon as possible. it may be that as the piece is shrinking it is getting caught, either in  a change in the relief or profile in the piece , or in a gouge or something.

 

It doesn't seem to be getting caught on anything.  It is shrinking away from the mold, it just starts cracking as it shrinks.



#9 Strelnikov

Strelnikov

    Advanced Member

  • Members
  • PipPipPip
  • 72 posts

Posted 23 August 2013 - 09:41 AM

 

It cracks as it dries before it is pulled out of the mold.  It is a bowl that is raised in the center.  I think the problem is that the corners around the center part are not smooth enough.  Is it possible to sand a mold to change its shape a bit?

 

Yes it is, it's plaster. Be really careful and visualize the effect the change has, before you start with alterations.

 

 

Okay thanks very much.






0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users