If your plastic housing requires a high gloss surface, you need to plan for that in your assessment of the mold designer/builder. Polishing the inside of a mold to achieve a high gloss finished part is a process that has not changed in quite some time. It is a manual process and requires a great deal of technique, dare we suggest “art”, to do it well and consistently. Achieving a highly polished lustrous metal finish requires three major processes. You start with grinding, then move to hand stoning, and complete the effort with diamond polishing.
After machining, grinding is the first step and while the crudest step, it is like most other tools. Using a grinder is easy; knowing how to use it well is a function of experience. Used correctly, it will set up the mold for the next step in the timeliest manner. How well the mold has been ground determines what grit of stone the hand polishing will start with.
Hand polishing is essentially, a process of producing scratches that get finer and finer as the work progresses. Skill and experience with go a long way in this step. The goal is to get to the desired stage of finish, but the byproducts are timeliness and efficiency, which feed back into cost.
Lastly comes the diamond polishing and how much will be required is a function of how lustrous the final product must be. Essentially a series of diamond compounds, literally diamonds suspended in a delivery product like oil. Sequentially, descending grits of compound are applied with a brush and low rpm rotary tool. The mold surface must be cleaned thoroughly between each compound change, with each step producing less and less detectible swirl marks. Eventually, the mold maker is manually using felt and then tissue, cleaning after each iteration, to get to their final desired surface.
If you want the shine on your product, the mold maker has to incorporate that into your mold. Achieving that shine is somewhere between a skill and an art form. Knowing how well that will be achieved and how it will translate into cost is a question worth asking up front.
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