Can someone give me small batch concrete 101?

I'm about to make headlight moulds out of concrete, but I've never used the stuff before.

I've been told by someone who has done this before to use concrete because the mould (plug actually?) will last a lot longer.

I'm making clear lenses for a car that you can't buy clear headlights for. It's for a bi-xenon HID projector retrofit.

ANYWAY, the plan is to coat the inside of the glass lens with lithium grease, pour concrete in, drop in some rope. When it hardens, use rope to pull it out, clean off the lithium grease... then cover in bondo and sand to smooth out the indents left in the concrete by the fluted lens. Then compression form new lenses out of acrylic by putting it in the oven until it sags, throw it on the concrete and use a MDF ring to compression form a new lens.

SO yeah. That was more info that you guys probably needed, but I have no idea what to use for concrete. I don't need that much. When I go look at concrete they have like sand, concrete, mixers, and a bunch of crap. It should be pretty easy to figure it out, but I look at it all and go... whaaaaaaaaat???

I can't help you with the concrete questions, but I would love to see pictures of the process when you get it figured out.

rustyvw wrote: I can't help you with the concrete questions, but I would love to see pictures of the process when you get it figured out.

Sure, I'll post up pics when I'm done now that I know at least one person is interested.

Depending on the size of the mold I would recommend using sand mix over the regular sackrete or quickcrete. Most of them will be around 5000 p.s.i some around 3500 p.s.i when fully cured which is 27 day's. It will be plenty hard enough to work with in a day or so, 27 day's is just the industry standard for testing compressive strength. The only difference between sand mix and the regular is the size of the aggregate, regular mix has rocks around a half inch and sand mix uses sand for it's aggregate and is perfect if the mold is going to be less than around two and a half inches thick. Mix it thoroughly don't use to much water and vibrate really well to eliminate voids and let the cream rise up to meet the part. Good luck.

Go to Home Depot and buy some ready mix. 1 bag will probably do.

It will be called "ready mixed concrete mix". Usually Sakrete brand. NOT mortar mix, sand mix, type N, or other variety. If it is a hardware store, just say "WITH the rocks in it..."

There are 2 sized bags. I'd buy the bigger one. More than you need, but they are cheap, and the small one might come up short if you goof.

Just add water. Vibrate it a lot while wet to get the voids out. Sets in a day. Full cure in a few weeks.

I've never heard of making molds out of concrete. I've only done them out of fiberglass.

I am curious to see how this turns out as well. Anything involving concrete we usually let Margie answer.

SVreX wrote: I've never heard of making molds out of concrete. I've only done them out of fiberglass.

Thanks for the rundown.

When I think of fiberglass, I think of fiberglass cloth to make things like motorcycle tails. What is Fiberglass Molds 101?

Similar description to your concrete description.

Mold release (lithium grease) the part, mix up some fiberglass resin, lay in matting, pop loose when hard. It's kind of like those paper mache masks you made as a kid.

Bondo any imperfections, sand.

Will you be making lots of parts?

I'm having trouble with the "mold will last longer" part. Concrete can etch, wear, scratch and break. I'm thinking a concrete mold (plug) would NOT last as long as a fiberglass one (though they DO wear out).

Plus, the finish is a coarser one to begin with. Fiberglass resin can be glass smooth.

Just re-read your post.

The concrete might stand up to the heated acrylic better.

Yeah my understanding is the heat is the problem. The guy who is giving me the crash course in making your own headlight lenses said he tried plaster and it just fell apart on him after one pull. I guess when he made concrete molds he was able to do a run of about 50 sets and the mold hadn't started to degrade at all.

I'm really excited to try this.

Sounds like an awesome project, but what are you going to do with the lenses you make?

I'm doing a bi-xenon HID projector retrofit, so my thick glass lenses distort the sharp light coming out of the projector which is undesirable. My plan is to convert the lenses to thin acrylic in order to have a sharper cutoff.

Here is an example of what it looks like now, and then an example of what it will look like with the clear lenses.

Not much info on concrete here. I'm no expert, but I've mixed a bag of Sakrete or two.

I would think you would want to avoid the bagged concrete mixes, just because of the size of the stone. If you're making a mold, seems you'd want something fairly smooth without big chunks of rock in it. I'd go with a mortar for that reason.

Good news is that a bag of concrete or mortar mix is pretty cheap. Get both and see which one you like better. Be sure to mix it thoroughly. And it never seems like enough water when you first dump it in, but if you work it enough, it is. Too much water makes a weaker concrete.

The stones are important to the bond.

Mortar mix is designed for holding rocks together (or bricks, or pavers, etc.etc.etc) It will be VERY weak if it is poured thick without any aggregate (stones) in it.

Additionally, you will have a soft sandy surface that will not be good to finish.

If you do it out of mortar, your mold will fall apart on the first try.

agreed

SVreX wrote: The stones are important to the bond. Mortar mix is designed for holding rocks together (or bricks, or pavers, etc.etc.etc) It will be VERY weak if it is poured thick without any aggregate (stones) in it. Additionally, you will have a soft sandy surface that will not be good to finish. If you do it out of mortar, your mold will fall apart on the first try.

Agreed on the role of the stones, but I think "VERY weak" is a relative term. I've had leftover chunks of thin-set mortar that I've left to cure, and the result is a solid lump that might not have a high tensile strength, but I doubt it would fall apart if somebody looked at it funny. I'd suggest a type S mortar.

How about using some home made pottery as molds? My coffee mug is very smooth and has stood up quite well to heat. Its easier to work with than concrete.

1988RedT2 wrote:

SVreX wrote: The stones are important to the bond. Mortar mix is designed for holding rocks together (or bricks, or pavers, etc.etc.etc) It will be VERY weak if it is poured thick without any aggregate (stones) in it. Additionally, you will have a soft sandy surface that will not be good to finish. If you do it out of mortar, your mold will fall apart on the first try.

Agreed on the role of the stones, but I think "VERY weak" is a relative term. I've had leftover chunks of thin-set mortar that I've left to cure, and the result is a solid lump that might not have a high tensile strength, but I doubt it would fall apart if somebody looked at it funny. I'd suggest a type S mortar.

Thinset is completely different than Sakrete mortar. So is type S.

SVreX wrote:

1988RedT2 wrote:

SVreX wrote: The stones are important to the bond. Mortar mix is designed for holding rocks together (or bricks, or pavers, etc.etc.etc) It will be VERY weak if it is poured thick without any aggregate (stones) in it. Additionally, you will have a soft sandy surface that will not be good to finish. If you do it out of mortar, your mold will fall apart on the first try.

Agreed on the role of the stones, but I think "VERY weak" is a relative term. I've had leftover chunks of thin-set mortar that I've left to cure, and the result is a solid lump that might not have a high tensile strength, but I doubt it would fall apart if somebody looked at it funny. I'd suggest a type S mortar.

Thinset is completely different than Sakrete mortar. So is type S.

Agreed. I offered my experience simply to illustrate the point that a lump of mortar designed for use in thin layers won't fall apart if you pick up a chunk of it.

Sounds like a job better suited to plaster of paris or something. It is very smooth and can easily be sanded to a polished finish.

Modeling clay? As Mr. Snorklewhacker observed, much smoother finish,heat resistant. Available at hobby shop.

I would recommend NOT using acrylic. Polycarbonate has a much high impact resistance, and you can get clear sheets with a UV resistant coating so they don't yellow.

Can you heat polycarbonate enough in your home oven to vacuum form it?

I would rather use polycarb but my understanding is you cannot form it like acrylic.

Taiden wrote: Can you heat polycarbonate enough in your home oven to vacuum form it? I would rather use polycarb but my understanding is you cannot form it like acrylic.

Yes no problem. It becomes moldable around 350*F. You can vacuum form it, brake it, whatever you want. We can make one piece dome skylights with it now. Best to keep an eye on it because polycarbonate will bubble if you overheat it. You will need to tape both sides of any cuts or it will chip. My company uses a hot knife CNC to cut the panels.