Fastturbovet,
I couldn't tell if you are getting (or have) an injection press. I'll assume that you are.
Cooling:
Jim is right, you're going to need some type of cooling. And a large volume of water is a cost-effective way to start. However, keep in mind that some materials need special processing conditions. For example, when we run ABS, the mold temperature is kept at ~35°C. You won't need a heater (the melted resin heats the mold) , but some sort of regulation is required to keep the mold temperature steady once it gets there. If your parts aren't very critical, watching the mold temperature and adjusting the coolant flow by hand should be enough. Though, this is a CNC forum, so it shouldn't be too difficult to automate.
An aside: A molder friend runs a shop that specializes in high-end engineered resins (i.e $$$). In his travels, he had to inject a resin that requires a mold temperature of ~125°C. He used oil, because water would have turned to steam. They were prototype reeds for a 2 stroke engine and had to withstand high temperatures, flex at 150 Hz, and retain their shape. Really neat material--the part was half a hollow sphere and we couldn't get them to break when we hammered them with a 5 pound sledge.
Parting lines:
I'm a plastic part/mold designer by day and parting lines are the single most difficult aspect of my job. On a part like a cup, the parting line is easy. But on ergonomic devices (that everyone likes so much these days), the parting line can run every which way. And trying to hide it for cosmetic purposes only compounds the problem. Good CAD software is really the only tool you have. Whatever software you have, get comfortable with the parting line tools. The learning curve is steep, and nothing is more frustrating to find out that your software can't generate a parting line for the job you were just awarded.
Also, complicated parting lines are a handful from the machining standpoint. The parting line is basically two faces (one on either side of the mold), that mate to create a seal-off that stops resin flow. A planar parting line is easy; both sides of the mold need to be flat and flush--not hard to do with a CNC. But a complicated parting line involves 3D milling. Getting both sides to match up perfectly is difficult, especially when you take into account things like machine stability and setup inaccuracies. And, if they aren't flush, the part will have flashing when it comes out of the mold. However, there is some allowance. Most resins have a minimum gap through which they will not flow. Kmed mentioned a 0.5" x 0.005" air vent. So, the material he's injecting will not flow through that 0.005" gap. On the parting line, it'll be the same thing. But, I wouldn't go to 0.005" though, 0.0005" to 0.001" is probably the maximum.
Surface finish:
Injection molded parts tell all. Any tiny nick or gouge in the mold will be magnified on the part (especially when the part is in the hands of the QC guy...). If you need a perfectly smooth surface or the parts are for optical use, you will need to polish the mold to a mirror finish. Just like body work, keep using finer and finer grits. However, no sandpaper will give you a true mirror finish--you'll need diamond paste for that (such as:
https://www.travers.com/pdfshow.asp?p=863). Needless to say, it's really tedious work; we usually outsource it.
More often than not, a textured finish will be acceptable (most computer hardware is). The mold will still need to be polished, but not to a mirror polish. It would go for texturing long before the diamond paste is used.
And be sure that your customer really needs a mirror polish. Two years ago, we built 5 molds for a customer. When asked to choose a surface finish (based on our sample cards), he wanted mirror polish. The first shots were delivered to him and he was not happy. The surface finish reflected too much light (making the part look oily), any scratches from handling/installation looked 10 times bigger than they were, and, the parts were installed into an area where every other part near it had a matte finish. Mirror polish sounds cool, but it isn't always required.
However, the runners should be highly polished. Any surface roughness will slow down the melt and increase cycle time and injection pressure.
My last bit of advice: There are a lot of aspects in moldmaking. Taking them on all at once can be daunting. I suggest you assume what you can, and outsource the rest. i.e. Your CNC (3 axis or 5 axis?) will be good for the majority of the work. However, you will eventually come across parts that require a ram EDM to manufacture (such as thin, deep features). Outsource it for now, and buy a machine once it's affordable/required.
You're entering a field that'll never get boring.
Good luck,
Chris Kirchen