Deformed walls when pouring out slip of molds

[Seesee]

I’m having trouble after draining the slip from my moulds the wall where the slip trails and exits gets thicker meaning that by time they dry the wall slumps in. I’m wondering does anybody have any advice on how to avoid this? I can’t reach a tool inside to readjust the wall as the neck won’t allow it, I’ve tried blowing air into it also and leaving it in the mould for longer than needed but eventually all pieces end up the same. I was wondering do I need to make my slip even thinner by adding more deflocculant? Any advice would be much appreciated! 

Edited November 7, 2023 by Seesee
Wrong title

[PeterH]

Pictures might help identify your problem.

You seem to be saying that the casting partially detaches itself from the mould at some point. Two ways that this can happen are:

1) A vacuum develops in the mould as the slip is poured out, pulling part of the casting away from the mould.  Which can happen if the neck of the mould is blocked by the slip at some point (preventing the free entry of air). In extreme cases this is accompanied by a glugging sound. Note that this can be caused either by poor pouring technique or unsuitable slip properties.

2) Excessive trimming before the cast is dry either physically dislodges the wall  or removes support during drying.

One thread on the topic:

 

[Jeff Longtin]

Good Morning Seesee,

Welcome to the Forum. At this point I would say you may have added too much deflocculant. Try adding water instead.

The condition you describe is sometimes referred to as "flabby". When there is too much deflocculant in the slip it will stay wet longer than it should. This will lead to deformation when you open the mold. With the right balance the clay will pull away from the mold and continue to stiffen until you open the mold.

When there is too much water in the slip it will stick to the mold and crack before it pulls away from the mold surface. (Similar to images of a dry desert.)

I've come to appreciate slip that is a little too thin with water.  The form will probably be a little thinner than desired but at least its the form you want. When the slip is flabby the form often deforms as you remove it from the mold and even as it sits out drying on the shelf. 

Good luck!

[Hulk]

I'm reading Seesee's problem description as: there's a thickening where the slip drains out of the mold, which causes the deformation.

Any chance the mold can be rotated whilst draining the slip?

[Jeff Longtin]

I see your point Hulk.

I was responding to the "make it thinner" and "add more deflocculant" comments. (In addition to the "slumps in" aspect.)

If the slip is properly balanced, generally speaking, it will flow out of the mold evenly. Maybe a slight ridge where the drain flow occurs but nothing that affects the shape to any degree.

With too much deflocculant the slip stays wet longer and drains out poorly/unevenly. (And then because that spot stays wet longer it tends to deform more easily.)

In theory you could rotate the mold but the slight complication there is that you will spend a lot of time rotating because the slip may take 5 or 10 minutes to set up. (Until it sets up it will still move in ways you want, or, in ways you don't want.)

[Seesee]

Hey Hulk

thanks so much this is definitely the problem I started rotating the moulds so that the walls would have a even thickness and now no slumping! A lot of trial and error with different slip consistencies. Annoying how the consistency in the tubs need adjusting but I got there in the end! 

[PeterH]

This might give you a better idea of some of the issues involved in getting the deflocculation right.

Determining Dispersant Additions for Casting Slips
https://static1.squarespace.com/static/527ac372e4b0d4e47bb0e554/t/527fd7f1e4b0c046bfa9b90d/1384110065234/Dispersant+Addition+Procedures.pdf

[Harold Roberts]

I have found that water chemistry can much affect clay slips jelling and thinning. Best not to increase your deflocculant. I have found that just a teaspoon or two of common household lye in five gallons of slip will thin out your slip and much alleviate jelling.