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lukemclachlan

Making A Mould Via Cnc

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Hello Everyone! My name is Luke, as you can see I'm a newbie here. I'm a coffee lover and I dislike my coffee pots so I'm on a venture to make my ideal pot. A friend of mine has designed the shape of the mould and he said that I should probably get it CNC (machine) cut, ready for slip casting.

 

Could you please clarify a few things for me? I have read that the mould should be made from gypsum (my CNC operator can machine it). I would love for the pot material to be porcelain, should the gypsum mould be prepared/mixed differently considering it will be used for a porcelain slip material?

 

The mould is a 3 part mould, two parts for the outside and the third part for the inner hollow centre. I know that I can use a two part mould and simply pour out the slip after x-minutes, however I want a consistent 10mm wall thickness every time I cast. How long should I wait between casting and removing the mould? Also what percentage shrinkage should I be looking at with porcelain?

 

Thanks all for your help and I'll be posting pictures along the way.

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the only thing i know about pouring into a plaster mold is that you have a place to pour into that is where you check thickness.  time has nothing to do with it.  the clay slip is poured into the mold and topped up to keep the level of slip at the top of the pouring hole.  as it dries, not a time limit, the amount of slip that hardens in the pour hole reflects the amount of thickness the pot will be if the excess is poured out at that point. 

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Guest JBaymore

Injection pressure casting is what you are describing.  It is a great way to approach heavy production work (a place I work in Japan uses it for some forms)... but sounds like a lot to take on for what amounts to one-offs.  To fill the spaces, slip is pumped in via a pump with some pressure to it.  Otherwise, the slip does not tend to fill all the voids. Plus you probably need to design with some "sprues" in certain places.... much like molten metal casting.

 

Working molds for hollow slip casting are typically something like "#1 Pottery Plaster".  It is a balance between hardness and water absorbtion.  For solid casting, something like Hydro-cal is sometimes used......for more hardness and "life" out of the mold... and since water absorbtion is less of an issue.

 

Overall mold to fired shrinkage depends on the porcelain body formulation.... but plan on 15 to 18% as typical.

 

best,

 

.......................john

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Yep, lots of variables at work here. How long it needs to stay in the mold depends on the water content of your slip, the absorption of your plaster, and the dryness of your plaster. If you cast more than one cup per day, the time each needs to stay in the mold will be different.

 

Do you really mean a 10mm wall thickness? Isn't that a centimeter? That's crazy thick, and would be terribly heavy. Plus it's going to dramatically increase the time in the mold, and probably cause cracking problems. I wouldn't go over 3mm, more likely 2mm.

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Do you really mean a 10mm wall thickness? Isn't that a centimeter? That's crazy thick, and would be terribly heavy. Plus it's going to dramatically increase the time in the mold, and probably cause cracking problems. I wouldn't go over 3mm, more likely 2mm.

 

Thank you everyone for your input.

 

John I'll be looking at those mould materials so thanks for the tips!

 

Neil yes I was thinking 1cm thick for the container. I read that porcelain 1/2" thick is the maximum I should go for, my porcelain cups are about 4mm thick on the body and 8mm thick on the handles. My parents have 20mm thick porcelain tiles at home. You've made me nervous now. I wanted porcelain because lots of items in the coffee industry are porcelain, but admittedly they're porcelain to retain heat. I don't want to retain heat here, instead I want a cool environment for the beans. I would like a material that can be vitrified, making it impenetrable to outside odours, moisture and oxygen, all of which (especially latter two) can affect the bean flavour. The inside walls should be glazed to make it washable, chemically inert and food safe.

 

Is there a material that springs to mind that can be thick (10mm), vitrified and suitable for slipcasting? Now that I've mentioned vitrification, I image someone will tell me that I definitely can't go over a certain thickness no matter what material I choose, but let's see...

 

Thanks / Luke

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The reason people use porcelain is for whiteness, and because they can charge more for porcelain because it's more precious than stoneware in people's minds. A vitrified stoneware can be just as insulating and durable.

 

I just think a vessel with walls that thick will be terribly heavy, and like I said it's more likely to have a lot of problems when working that thick. Plus it'll cost more to make in materials, production time will be much slower, it'll take more energy to fire that much mass, etc...You can go as thick as you want, you just open yourself up to a lot of headache. There are clay sculptures that are 1-2" thick, but a lot of care has to be taken in dealing with that.

 

Ultimately, if you want it sealed off from the environment, then ceramic is not a good way to go unless you plan on making a lid with a latch and rubber gasket. Functionally I would think a glass jar with a screw top lid would be best, or a heavy walled plastic container.

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Guest JBaymore

How long it needs to stay in the mold depends on the water content of your slip, the absorption of your plaster, and the dryness of your plaster. If you cast more than one cup per day, the time each needs to stay in the mold will be different.

 

Pressure injection casting is a bit different.

 

best,

 

..................john

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Just to add more info:

I once worked in a small pottery factory. The factory had a production order for teacups that were the size and shape of coffee mugs or even a little larger, in porcelain. The molds were filled literally by a hose running from the tank that churned the slip, it had some pressure but not a whole lot, it still took a few seconds to fill each mold. It was mainly a convenience as about six dozen molds were all filled and dumped as close as possible together. The key was shaking the two-part molds gently for a few seconds to get the bubbles out after they were filled. After 10 minutes -I remember that part-, the molds were dumped out. In the humid 50-60's (Fahrenheit) coastal climate they would be the right thickness at that point to let them dry and finish the process. The buyer was Bloomingdales in NY, so if they were the right thickness for that market, maybe a little thicker/longer in the mold for a coffee pitcher?   

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Luke,
 
I am confused regarding the functional purpose of the 'coffee pot' you are designing.
 
Are you wanting a container for:
hot brewed coffee?  or,
brewing coffee?  or,
storing whole coffee beans?  or,
storing ground coffee beans? or,
a drinking vessel for hot brewed coffee? or,

something else?
 
An optimal design will be different for each of the above containers. 
after the purpose is selected you will need to focus on the storage volume.
 
Each of the requirements will have separate influences on the final shape, materials, and mould attributes.
 
Without knowing the purpose it is any advice is biased by the advisor's assumptions and personal preferences.
 
LT

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Hi All and thanks for your input, very valuable indeed!

 

Sorry if it's vague as to the purpose. I have a coffee storage pot for the roasted beans, both whole beans and ground. The capacity when shrunk should be 800cm3, so enough for a 250gm bag. The chap who's making the design of the mould has accounted for volume, also the inner shape of the pot exactly resembles a silo for (I'm hoping) mass flow to a special gravity fed dosing mechanism that I'm having made for the pot. The lid of the pot is air tight and will remain closed, i.e. it won't need to be opened thanks to the said doser.

 

It's interesting that Neil mentioned stoneware, I didn't consider this material for slip casting so it's on my radar now. I was at a garden centre yesterday, research purposes, and saw that some of the flower pots were 15mm thick and it got me thinking, does the material influence the maximum possible thickness, are some more prone to cracking above a certain thickness? I'm not terribly concerned about the weight, though it is at the back of my mind. According to scientific research glass and vitreous ceramics are best for coffee bean storage, I've gone for the latter. I'm not a fan of plastic. The question now is what slip material I should choose?

 

I've now got stoneware slip and porcelain slip on my list of possibilities. Are there any other slip materials that you could suggest, maybe even some uncommon composite materials? The keyword is vitreous ;-)

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Guest JBaymore

The molds were filled literally by a hose running from the tank that churned the slip, it had some pressure but not a whole lot, it still took a few seconds to fill each mold. It was mainly a convenience as about six dozen molds were all filled and dumped as close as possible together. The key was shaking the two-part molds gently for a few seconds to get the bubbles out after they were filled. After 10 minutes -I remember that part-, the molds were dumped out.

 

In pressure casting the molds are filled under some significant pressure, pressure is increased (or maintained) until the water is gone into the molds, not dumped out (because the is no "excess" to dump out. 

 

What you are describing is pretty much "normal" traditional slip casting.

 

Unless I misunderstood his description, he described a mold that has a center core (that will be the hollow interior part).  That is like lost wax casting in metal (except there is no wax to burn out before the metal goes in.  With that type of mold......... traditional casting would not work.

 

Here is the process for sinks:  https://www.youtube.com/watch?v=Z25IjnsMz1k

 

best,

 

.....................john

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Unless I misunderstood his description, he described a mold that has a center core (that will be the hollow interior part).  That is like lost wax casting in metal (except there is no wax to burn out before the metal goes in.  With that type of mold......... traditional casting would not work.

 

That's exactly what I'm doing John. I've been traditional casting that throws out the slip after x minutes but as I said in my first post I'll be using the inner mould do dictate the thickness, plus some very simple pressure mechanism to force the slip into the mould whilst it's drying. Is my naivety showing through here - should I be aware of some pitfalls and possible modifications? Thanks / Luke

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Guest JBaymore

 Is my naivety showing through here - should I be aware of some pitfalls and possible modifications? Thanks / Luke

 

Luke,

 

I think it might be a small dose of "not enough information" yet at this point.  I am assuming here you are doing your 'due diligence' on starting up a production for a very commercially viable object.  And hence eventually, if the business plan numbers work, heavy production.  For a heavy production setting... and large number of objects sold.... likely this is a viable idea.  Small run......... maybe not.

 

The only experiences I've had with it (in Japan) .... there was some significant investment in equipment and also in the actual master and working mold making.  Not as elaborate equipment as the video I shared above... but still, compared with more handcraft type studios, an expensive setup.  Continuous slip blungers and reservoirs, high pressure slip pumps, casting racks that clamp molds in gangs to resist pressure, air compressors to do the release from the molds, "nice" working gang molds, and so on.

 

The design work for the pieces created is/was done "in-house".  (note....... some work is /was done in Rhinoceros 3-d modeling software and CNC machined for the original mold master objects.)  Then the mold work (master and working gang molds) was done by people that specialized in the production of this type of mold for ceramic use (in Mashiko-machi).  Mashiko is a long way from the actual pottery production center...so that tells you something about the need for someone who knows what they are doing in making the molds for this process.

 

Maybe a field trip to a place like Kohler is in order?

 

Also.... most all of my friends in japan are dealing with this next issue also.........

 

Within about 6 mos. to 1 year after your object hits the market.... there will be low price knock-offs coming out of China and Southeast Asia.  Over the past 20+ years working in Japan..... I've watched the demise of a LOT of the more 'production oriented' ceramics facilities.  Chinese imports are killing that market in Japan also for Japanese made work.  Can't compete on price.

 

The only reason the place I mention above can continue to work on their production lines (they have a VERY diverse production range from one-off hand thrown and handbuilt, to slip cast , jiggered and jollied, and so on) and is that the finishing process for the work cannot be economically reproduced easily (wood-fired, yakishime, charcoal reduction finish). And they also are designing/producing new objects constantly to stay ahead of the imports.

 

So maybe keep in mind that the item you are thinking about will possibly have a short exclusive window in which to make big profits.  Sober reality in the globalized world.

 

best,

 

.......................john

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Hi Jon my sentiments exactly re: China, initially these pots are for my kitchen but if they work and I can drum up interest then I'll have to go down the mass production route and I will, I promise, avoid China like the plague. I have a contact in India, easier to communicate with, I trust them implicitly, slightly more expensive but worth it. You hit the nail on the head there as to why the Japanese company is still (I take it) profitable...by staying ahead of the curve and if I ever do come to market, which is my plan after this prototype (if I have any money left over), then I'll have to constantly evolve the product.

 

My (Indian) engineer will have the mould drawing ready next week, I can share some stuff along the way. I'm off to see a slip casting supplier tomorrow for porcelain and stoneware slip.

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