Concrete mold

Looks good!

Now for the finishing:
http://www.cement.org/decorative/Concrete%20Countertop%20Sealers.pdf

Wax
The most basic sealer is wax. Wax is both a penetrating and film-building sealer,
depending on how much is used and how it is applied. Generally, a high-quality floor
wax that contains carnauba and bees-wax, or just pure bees-wax, is used. Synthetic
microcrystalline is also used. Automotive paste wax should not be used because of the
additives it contains.
Wax produces an attractive, low- to high-sheen finish that brings out the best in the
concrete’s color and visual texture. Wax will usually darken bare concrete (similar to the
way water wets-out or darkens the concrete).
Wax is a very forgiving sealer. It is easy to apply and hard to get wrong. Wax cannot be
scratched, although the concrete itself can get scratched or gouged. Wax is easy to
reapply and usually must be applied frequently to remain effective.
Consider wax to be a sacrificial protectant that must be replenished to remain effective and attractive.
However attractive and easy wax is, it makes a relatively poor sealer. Just about
anything will leave some kind of mark or will stain waxed concrete if left on long
enough, and exposure times must be fairly brief to avoid any kind of staining
whatsoever. Hot temperatures (such as hot sauce pans) can cause the wax to soften or
even melt and soak into the concrete. Acids like vinegar or lemon juice tend to strip off
the wax and can etch the underlying concrete.Wax applied over other sealers often adds very little extra protection. It essentially
serves little purpose other than to provide a psychological boost to the homeowner,
unless the underlying sealer is so poor that the wax actually provides more protection
than the sealer itself.

Not sure if this opens up some problems. But you could smooth the surface post molding. Just mix up a thinner version of the concrete mixture. Fill the holes and smooth them… Since its a level surface it should be fairly easy to get it glass smooth.

Thanks for the tips :slight_smile: I will smooth the concrete after the course is done (because I also want to use it afterwards and spill stuff a lot :laughing: ). Just a bit tight onto the deadline, so I need to finish the legs and doors first. The deadline is also one day earlier because we printed the wrong day on the flyers :laughing: Just finished molding the doors:

Foam test:
http://briankhouw.tumblr.com/post/70698743195

Molding:
http://briankhouw.tumblr.com/post/72139638895

It is made from plaster. Does somebody has experience with a matte paint finish on plaster? I want one black and one white and don’t want to lose too much detail (the same texture with a different color)?

You can paint plaster easily enough, but you have to make absolutely sure that it is dry. Build a hot box out of cardboard, or plywood, or whatever. Put a 100-150W light bulb in it and leave your plaster in it for a few days. Just make sure that the light bulb is far enough from the inside of the box not to ignite anything.

Once the plaster is sufficiently dry coat it with multiple layers of lacquer. Thin the lacquer to a watery consistency and build up multiple coats until you start to see some gloss. Not too much, just enough to seal the porous plaster.

Then apply any kind of paint you want.

Process: http://www.briankhouw.com/metropolis.html

So, I finally finished it :slight_smile: Thanks for all the help everyone!

Pretty freakin’ cool Brian. Thanks for taking us along on your project.

Cool. Nicely done prototype.

Looks pretty cool man! I’d like to see it in its natural environment :slight_smile:

The studio shot of the Night version is pretty epic! Might be nice to photoshop just touch more light on the faceted face to revel some of the texture detail.

Thanks for sharing

Thanks everyone! :slight_smile:

The night version might to be too dark for some monitors, so I changed it a bit. Can you see it clearly now choto?

I will make some new photos when I can put it in my room (have to clean up my room first though :stuck_out_tongue:).

Yup looks a lot better now, just enough light to reveal the form.

Deleted.

This is a great topic and hardly anyone ever mentioned concrete casting here. I can’t find much information on the differences of a PU mold vs. a silicone mold so we have to rely on experience I suppose. How many molding cycles will a 2-part mold for concrete in PU vs. silicone last? The part is a conical lamp base 300 mm tall and a 150 mm maximum diameter at the bottom. It has no undercuts but does have some hard edges at low angles (low-poly style).

It seems that Smooth-On MoldStar 30 works great for silicone but I wonders how it would compare to a polyurethane.

Not based on direct experience, but based on experience with silicone molds in PU casting for production which last for about a dozen cycles. The durometers of PU can be higher, abrasion resistance higher, I’d estimate hundreds of casts.

Just what I wanted to hear, I have no experience and was estimating higher numbers. Thanks!

With any lifetime of a mold estimate, the end of life is where the surface and detail quality falls below an acceptable level. That is project feature and finish requirement dependant. The proper mold itself, if it doesn’t have to radically flex upon release will last for hundreds of thousands (estimate) of casts. Abrasion seems to be the only factor. Concrete pouring chutes are made from urethane, it is a great choice for abrasion resistance.

The repeated heating and cooling of the molding material creates wear as well as de-molding. Soft tooling life spans can depend on factors like mold release, the frequency of mold pours and other such smaller considerations.

I was making parts for a defense contractor that pushed the limits of a silicone mold. The parts required a good amount of mold release to pull. The shine the release agent created was not acceptable, so the parts required a secondary operations to produce a matte finish. As molds wear the seams and detail begin to fail and you are left with more flashing and loss of the intended surface finish.

Depending on the parts end use you can balance creating new molds vs. secondary operations producing the finished surface. You basically see what you can get away with. Also, depending on the parts complexity you can create mold masters. Essentially just a mold of the mold, you can use to scale your production and save mold making time.

Hm, hundreds, then hundred thousands. I need to hear this from someone who has extensive experience in concrete molding. I do hear that PU molds lasts for >100 cycles. You might be right with a dozen for silicone. Our mold will need to keep all of its detail because of the sharp edges in the design.

I will definitely look into creating a more reuseable mold master, thanks!

Call technical support at places like Polytek.

Given that a concrete curing cycle might be 24 hours or more, 27 years of continuous molding would be 10,000 cycles. The urethane would likely be getting chemically shot by then. So yes, 270+ years mold lifetime is an overstatement.