That will depend on the size and number of castings to be cleaned. You can clean up to a few hundred pieces by hand, but for thousands of parts an automated process is more suitable. Are you concerned about the appearance, or the adhesion between the plastic and metal? Because they will be "potted" inside a urethane shell, the adhesion is likely not going to be a major issue; and in fact most of the alkylated methyl polysiloxane mold release products are designed to be paintable release agents (or in this case have good adhesion to urethane plastic). If your company does the molding, switch immediately to a paintable mold release if the currently used one is not paintable. Otherwise, get whoever molds them to change release agents.
If the urethane is not clear, then appearance factors are moot. But if you absolutely insist on getting the castings scrupulously clean, several options are available to you. The least costly will be aqueous detergent cleaning if this release can be removed this way. If not, contact the mold release supplier to identify a solvent to use for cleaning, either by solvent spraying and letting parts drain (better), or by solvent spray or dip followed by manual wiping with clean towels (not quite as good). For non-fragile parts that are not too large (weighing approximately two to three pounds or less) a vibratory cleaning can remove tenacious material, but may need to be followed by a solvent-cleaning step.