Rotational molding of medical articles

#This invention relates to producing castings of silicone elastomers from solvent-based silicone dispersions using a hollow mold and multi-axis rotation until the material devolatilizes to a non-flowable state and is cured.

#Rotational molding of many industrial, consumer, and medical related parts from a variety of plastics is commonplace. The plastic materials are typically polyolefins in pellet or powder form, but some are flowable liquids, such as plastisols, with sufficiently low viscosity, i.e. less than 5000 cps.

#The rotational molding or casting method and system of the present invention has utility in the manufacture of breast implants and other medical devices and articles having a thin-walled shell, typically formed from silicone elastomer, such as tissue expanders and low-pressure elastomeric balloons. Low-pressure elastomeric balloons are used, for example, in catheter fixation, occlusion of blood flow, bracytherapy, and as intraaortic or intragastric balloons for cardiovascular or ear-nose-and-throat procedures. Other articles include feeding tubes, enema cuffs, catheters, condoms, shunts, and embolic protection devices. The traditional method of manufacturing these articles is by dipping a mandrel in a solvent-based silicone dispersion to cast and form the shell.

#FIG. 1 is a schematic illustration of an embodiment of the rotational molding system of the present invention.

#FIG. 2 is a cross-sectional schematic illustration of an embodiment of the bottom piece of the two-piece case mold.

#FIG. 3 is a top view schematic illustration of an embodiment of the bottom piece of the two-piece case mold.

#The present invention relates to four novel developments using a rotational casting process that have direct application for improving the process and products for these existing medical devices.

#The first development achieved by the rotational molding system and process of the present invention relates to its use in the rotational molding of silicones and other solvent-based or gas-emitting materials. This has not been feasible heretofore with most silicone elastomers, even LSR's (Liquid Silicone Rubbers), because silicone elastomers with sufficient physical properties for use with medical devices are usually high in molecular weight or require fillers. These materials typically have too high of a viscosity to be free-flowing as is required in a rotational molding process. Such higher molecular weight polymers need to be combined with a solvent to make a dispersion having a suitable viscosity. This solvent-based, reduced viscosity dispersion allows application of the silicone polymer onto a mandrel by spraying or dipping after which the solvent is allowed to evaporate. Such a solvent-based dispersion has not been practical for use in a rotational molding process since there is no ready means to remove the significant volume of solvent vapors that are trapped within the closed molds heretofore used in rotational molding processes. However, by adding a vacuum vent to the mold, e.g., one internal to the rotating arm of the equipment, a means is provided to remove the solvent while the arm is rotating and the dispersion material is flowing and being deposited on the inner surface of the mold. Alternatively the mold may be constructed of porous material, such as a porous metal or ceramic through which the solvent can be evacuated over the whole surface of the mold. This development of utilizing a vacuum vent or porous mold in a rotational molding process is applicable not only to silicones, but to any other materials which are solvent-based or emit gaseous by-products during curing such as polyurethanes or other polymers and the like.

#A second aspect of the rotational molding system and process of the present invention relates to providing a means for molding seamless articles. Most articles made by rotational molding are made using multi-part hollow molds. It is often undesirable for medical products intended for implantation to have a seam or other surface irregularities. Even with precision machining of the mold, the articles produced by conventional rotational molding processes have, at a minimum, a witness parting line in their outer surface due to the mating surfaces of the mold. These mold parting lines are eliminated in the process disclosed in the present invention by first coating the inside of the assembled, multi-part mold with a thin layer of molding material such as polyethylene, polypropylene, polyester resin or the like to create a mold liner. After the liner is cast, then the raw material, e.g. silicone, polyurethane, or other polymer, for the desired article is injected into the mold cavity and similarly rotationally cast inside the liner resulting in a laminated construct. When the mold is disassembled and the construct is removed from the mold, the liner material and the desired article are physically separated resulting in the desired article having a seamless configuration.