SO BASICALLY for molding designing, you have to chose first software. For example I am preferring for simple mold designing , Rhinoceroes for complex and more and if there is more analyses needed I use inventor, inventor hsm and moldflow which simulates and is very useful for injection point finding and so on. pretty same can be done in solidworks and in many other software.
First find out what is your 3d model, how it can be molded. Basically you just use boolean operation, but sometimes you have to understand what material wil be molded and you have to expand your mold scales, cause after cooling material will shrink. After model molds case in which 3d model should be cut out and use boolean thats all you have to do. Other diteils will com to you as you contact to manufacturers. There will be part ejection points, cooling piping . And many many other complex things. which cant be fitted here. Thre are bunch of tutorials which you can use to guide in all of these processes
If it is the main mold design what you are looking for, the best way to do it with any CAD software is:
You start with the final molded part model
You have to scale it up between 2% and 5% to cater for shrinkage when it cools. That depends on thermoplastic characteristics of the molded material.
Then you create a new assembly positioning the molded part in a way that will define the contact surface between the mold halves. You must choose a plane where mold halves surfaces will contact.
Once that plane is chosen, you create in context (as part of the same assembly) two blocks that will be the mold halves, with the contact face in coincidence with the chosen plane
After that, for each mold half in turn, you intersect the molded part with the mold half, and keep the resulting solid with molding cavity
With both mold halves you can then modify them as https://grabcad.com/nika.alavidze-1 says, guided by part manufacturer or mold supplier, for placement of injection and ejection points , cooling piping, etc.
There are other subtleties like slanting perpendicular planes around 1º to make ejection of part easier or even possible, use of unmolding lubricants, etc.etc.
Hey anyone interested in this Topic can hit me up i do this for a living and i am willing to share some tips if i have some spare time i even could organise a livestream with active commentary for you guys. Btw i mainly use Solidworks Greetings Patrick
I posted this in another mold design thread, the Protolabs design rules are very helpful to understand what you can and can't do when designing a part for injection molding https://www.protolabs.co.uk/resources/design-tips/