Why resin? Lower melting temperature? What kind of resin?
This is really interersting

Resin cures like glue, so melting temperature is irrelevant!

It's all purely theoretical for me right now, looks like Ian has it down with the Smooth-On silicone mold-making resin. Their website has some good picture instructions.

ronsii is right, zero bubbles will be nigh impossible without vacuum depending on what shapes you're doing. Thin bits, fine edges will be difficult. I'd expect a smoother mould surface (e.g. acetone smoothing or sanding the printed part, making silicone moulds and then using them to produce parts) would give less opportunity for the bubbles to stick.

Here is an example where I did this for soap molds- http://www.thingiverse.com/thing:22441

I think you could make about 10 molds printed at 0.1mm, finish them and pour liquid plastic into the molds and make a lot each day.

Unless the melting temperature of the plastic you'll use will be higher than ABS or PLA in which case this won't work i think.

I think resin-casting would be a low-cost option for this - silicone and polyurethane casting resins are widely available (try art and hobby shops to start with), and if you use the right low-viscosity product and design the mold well you may get some quality looking stuff without needing vacuum equipment.

Unless the melting temperature of the plastic you'll use will be higher than ABS or PLA in which case this won't work i think.

That being said, I paid $2,100 for mine, he paid about $1,000 for essentially the same thing.  He has the ability to print in ABS plastic which is what I would suggest you use if you want to sell cases since a PLA case will melt if left in a car in Arizona in the summer.

Then you have to know how to design unless your plan was to just download STLs from websites and print them to sell which is against all the policies of every site that shares STLs in the first place.

I've printed a couple of phone cases for my phones and I don't like how they come out at all because the plastic is not flexible.  In fact, some designs break when you try to remove the phone from the case.  You can use the flexible filament, but it's a lot more expensive, you have to print much slower and it's harder to use.

Then you have to figure what price point you want to sell at.  In PLA at 100 mm/s extrusion, 150mm/s travel speed a case for a galaxy s3 takes about 2 to 2.5 hours to print.  Flexible filament is difficult to use and takes even longer since you have to slow the print speed down.  You are not printing an otterbox, so you might be able to sell a case for $19.99 if you're lucky and they won't fly off the shelf at all because anyone can go anywhere and get a cheap case.  It could take you upwards of over 5 hours to print the case in the correct material.

So if you're plan is to make money with a 3d printer selling phone cases, I would say no it is not a good idea.  If you are good at design at all and network with people, you could offer a prototyping service to shops so you have a fast turn around time for them, but network before you buy!

If you want to get this for a hobby and make a little money on the side (I.e. you're not buying it with the intention of making sales to pay it off) I think it would be good because then you don't have your expectations set for something that is not likely to happen.

My apologies for trying to knock your idea
but you will see visible layers unless you go with a .1 layer height and do an acetone bath.

if you were to print an iphone at .1 depending on your speed it could take 6 to 9 hours for 4 iphone cases
depending on your infill and speed.

3D printers aren't a production work horse depending on the part.

You would be better off printing a case and making a mold however casting plastics
are limited in strength compared to abs however they offer more flexibility then abs.

Anyways you could do 25 to 30 Iphone cases a day but not
Not with one printer

timmu.toke, I'm planning to make something like your idea and I Agree with techbuilder.
You'll need make mold and cast. Take a look on smooth-on com (there is many kinds of materials for cell case and tuts).
Do you know Form 1 from Formlabs? I think that is you best choice for your purpose. 3d SLA printer with 25 microns. More resolution, smooth surface and with a Pro looks.

I'll wait until march-april 2014 to buy Form 1 or another with same specs.

I can sell it with 15-25$. We are talking about 1000+% profit here. Have I misunderstood something?

Without having a multi color printer, without doing acetone vapor baths, without hand painting at least some details on the case I am not sure how it will be SWEET! enough to get $15 for it. Maybe your market is different than mine here. Though I don't use the oh so stylish and trendy iPhone and I do not use a case for the android jelly bean phone I do have.

if you were to print an iphone at .1 depending on your speed it could take 6 to 9 hours for 4 iphone cases
depending on your infill and speed.

3D printers aren't a production work horse depending on the part.

You would be better off printing a case and making a mold however casting plastics
are limited in strength compared to abs however they offer more flexibility then abs.

Anyways you could do 25 to 30 Iphone cases a day but not
Not with one printer