When it comes to selecting the appropriate tolerances, specifically plastic parts, there are several things you need to consider. If you think you can approach it the same way you do a machined metal part, you're very mistaken. Unfortunately, with the role of "the drafter" slowly being phased out and becoming a subset of the engineering role the quality and accuracy of 2D drawings is suffering significantly. Fortunately, there are a few things that can be done to help improve the situation if you find that you're often having pushback from manufacturers and tool makers. This will be high level, if you are interested in more detail, feel free to DM me and I'd gladly go into more detail on anything.
These are some steps to take before finalizing the tolerances on a print:
1.) Engage you manufacturer as early as possible and at every stage of development. Don't lock the design/print until you talked to your tool maker/manufacturer to avoid any unnecessary costs.
2.) You should take every effort to minimize the amount of dimensions on your print. If you ask me how many dimensions should you put, most likely the answer will be LESS THAN YOU HAVE.
3.) Utilizing an industry standard like ISO 20457, establish a global tolerance for all dimensions. Using tighter than standard tolerances on a print creates a lot of problems. If you find you need to deviate too much from the standard, maybe you need to rethink the design or manufacturing process. Industry experts have created the standard and should be utilized to make a reasonable print.
4.) If there are any key or critical dimensions that require that require tighter than standard tolerancing than the global, mark them as critical (circle them) and add the appropriate tolerance. Keep in mind, if you deviate from one of the standards, the desired results may not be achievable, may cost a great deal more to execute, or can create constant quality issues during production. If you can keep the same tolerance band but shift to a min/max situation for fit and interface with other components, this is more ideal then just tightening the tolerance. You should get buy-in form the manufacturer before moving forward.
5.) Before locking in a print (especially necessary in industries that make it difficult to make revisions) get buy-in on the feasibility of the dimensions and tolerances from your tool maker/manufacturer.
***Be careful implementing GD&T symbols. If you haven't heard of the ISO 20457 or equivalent, you can be pretty confident that you don't have the necessary knowledge needed to correctly implement a GD&T symbol for a plastic part. ***
If you take nothing away from this article, 90% of your drawings should be achievable by your manufacturer, acceptable by your manufacturer, and present the most affordable price if you base your tolerances on a standard. Currently ISO 20457 is what I would reference when redlining and the document itself will go into great detail on the appropriate way to tolerance. Last I checked it was ~$180 and worth any penny if you are responsible in any way for 2D drawings. You can visit either iso.org or ansi.org to find the most recent version. Other standards, some which are expired but still very relevant (and much less expensive) are also usable if they cover your specific resin. DIN 16901, DIN 16742, and AQ-102 ("Standards and Practices of Plastics Molders" from the former SPI) are ones I have used in the past and all will be fine to get you in a position for the best outcome.
If you are finding that your manufacturer doesn't want to engage early on (well before you are placing a PO) than feel free to contact me. I'd be glad to have a quick discussion about your part and point you to a manufacturer more suitable to your needs.