How to find reliable injection mold and molded parts manufacturers?
Here are some ways that probably work best when used together.
Word of mouth-Talk to colleagues and others who have prior experience with procuring injection molds and parts. Perhaps your customer has experience with a particular molder. Ideally you'll come up with a relatively short list to choose from. Certainly you will hear who it may be best to avoid, har har!
Internet presence-You can often get a feel for a company based on the quality of their website. The more sought-after companies will generally be more professional in this regard, they will list industries and applications that they specialize in and may even list some of their customers which you can contact and ask questions of. Molders often have different specialties so it's important to make sure their skill set is aligned well enough with the parts you wish to make.
Certifications-Quality companies often have particular certifications 'proving' they have audited quality systems in place, and are familiar with things like first article reports, SPC, PPAP submittals, etc. If they are unfamiliar with the industry 'lingo' it's best to proceed with caution.
Boots on the ground-Nothing beats an 'in-house' visit where you send a knowledgeable person or a small team to meet with, ask questions and observe the tooling and molding operations first hand.
It certainly bears a due amount of diligence and work to make a good choice, after all, you'll be spending tens to hundreds of thousands of dollars on a tooling project.