How does a manufacturer make a food or potable grade resin?

Some food and potable grade resins are totally unique and specially developed for specific applications and these resins, as part of their production process are specially treated at different stages of the production, or post-production, to provide a cleaner product. Where a product is available, both as industrial grade and potable grade resins, then the resin will have the same basic chemistry and work in the same manner, but again, it will have additional cleaning stages within the production process, or special post-treatment or cycling between different ionic forms that clean the resin, reducing leachables species from the resin when new.