Rotational molding of medical articles

Background of the invention

The present invention relates to the production of molded parts of silicone elastomers from solvent-based silicone dispersions using a hollow mold and multi-axis rotation until the material is devolatilized to a state in which it is not capable of Flow and be cured.

Rotational molding of many industrial, consumer and medical related parts of a variety of plastics is a common practice. Plastic materials are typically polyolefins in tablets or powders, but some are fluids capable of flowing, such as plastisols, which have a sufficiently low viscosity, for example, less than 5000 cps.

The rotational casting or molding process of the present invention has utility in the manufacture of breast implants and other medical devices and articles that have a thin-walled shell, generally formed of a silicone elastomer, such as tissue expanders and low pressure elastomeric balls. Low pressure elastomeric balloons are used, for example, in catheter fixation, blood flow occlusion, bracitherapy, and as intra-aortic or intragastric balloons for cardiovascular or ear, nose and throat procedures.

Other items include feeding tubes, enema fasteners, catheters, condoms, shunts, and embolic protection devices. The traditional method of manufacturing these items is by immersing a mandrel in a solvent-based silicone dispersion to mold and form the shell.

A particular process for rotational molding a hand prosthesis is disclosed in the document "Computer Controlled Liquid Rotation Molding for Medical Prosthetics", Teoh SH, Sin KK, Chan LS, Hang CC Rotational Magazine, vol. III -3, 1994, pages 10 -16 (XP234285). Specifically, this article refers to the computer control of the thickness of the prosthesis and the effect of temperature and humidity on the kinetics of the bubbles.

Document DE 2015966 (Giehler) also refers to rotational molding in which a mold is rotated around mutually inclined shafts and the material therein is heated. A negative pressure is provided in the mold during the heating phase and during the subsequent cooling phase there is an increase in pressure by the introduction of an inert gas.

Summary of the invention

A procedure for molding the shell of a medical device or other molded object is disclosed. The system includes a multi-axis rotational molding machine in which a mold is mounted. The mold has a cavity in the shape of the object that must be molded. The mold is preferably sealed to maintain vacuum.

In operation, the silicone or other molding material is inserted inside the mold, vacuum is applied to the mold, the mold is rotated around at least two shafts and a molding material covers the interior walls of the mold to form the mold. Wrapped or other desired item.

A molded medical item is also disclosed

Brief description of the drawings

Figure 1 is a schematic illustration of an embodiment of the rotational molding system.

Figure 2 is a schematic cross-sectional illustration of an embodiment of the lower part of the two-piece box mold.

Figure 3 is a schematic illustration in top view of an embodiment of the lower part of the two-piece box mold.

Figure 4 is a schematic cross-sectional illustration of an embodiment of the upper part of the two-piece box mold.

Figure 5 is a schematic illustration in top view of an embodiment of the upper part of the two-piece box mold.

Detailed description

The present invention relates to developments using a rotational molding process according to claim 1 which have direct application in the improvement of the processes and products in these existing medical devices.

The first development achieved by the rotational molding system and the 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 possible so far in most silicone elastomers, including LSRs (Liquid Silicone Rubbers), because silicone elastomers with sufficient physical properties for use with medical devices are generally of high weight. molecular or require input. These materials typically have too high a viscosity to flow freely as required in a rotational molding process. Such higher molecular weight polymers have to be combined with a solvent to make a dispersion that has a suitable viscosity. This solvent-based dispersion, of reduced viscosity, allows the application of the silicone polymer on a mandrel by spraying or immersion, after which the solvent is allowed to evaporate. This solvent-based dispersion has not been practical for use in a rotational molding process since there are no means available to eliminate the significant volume of solvent vapors that are trapped within the closed molds used so far in the processes of rotational molding. However, by adding a vacuum vent hole in the mold, for example, an inner one in the equipment rotation arm, a means is provided to remove the solvent while the arm is rotating and the dispersion material flows and is deposited on the inner surface of the mold. Alternatively, the mold can be constructed of a porous material, such as a porous metal or ceramic, through which the solvent can be evacuated through the entire surface of the mold. This development of the use of a vacuum vent hole or porous mold in a rotational molding process is applicable, not only to silicones, but to any other materials that are solvent based or emit gaseous byproducts during curing, such as polyurethanes or other polymers and the like.

A second aspect of the rotational molding process of the present invention relates to providing a means for molding articles without joints. Most items manufactured by rotational molding are made using hollow multi-part molds. It is often undesirable that medical products intended for implants have a joint or other irregularities on the surface. Even with precision machining of the mold, the articles produced by conventional rotational molding processes have at least one control dividing line on their outer surface, due to the coupled surfaces of the mold. These mold dividing lines are eliminated in the process disclosed in the present invention, firstly by coating the inside of the multi-part closed mold with a thin layer of molding material, such as polyethylene, polypropylene, polyester resin or the like to create a coating of the mold. After the coating has been cast, then the raw material, for example, silicone, polyurethane, or other polymer for the desired article, is injected into the mold cavity and similarly is cast by rotation inside the coating, which produces a laminated construction. When the mold is opened and the construction is removed from the mold, the coating material and the desired article are physically separated, which produces the desired article having a joint-free configuration.

A third aspect of the rotational molding process of the present invention relates to the ability to extract the desired product from a one-piece, reusable, one-piece mold. A one-piece mold of this type may be made of rotationally molded plastic or metal by means of an electrolytic nickel process, cast aluminum, "lost wax" technique or the like. Using any of these mold manufacturing processes, a one-piece mold can be constructed with a very small opening, for example, approximately 2.54 cm in diameter. In this opening there is a drinking fountain to dispense the raw material used in the molding of the desired article and a vacuum connection. Since most medical devices manufactured using the molding system and the process of the present invention generally have very thin walls, for example, from about 0.127 cm to about 0.524 cm, the desired rotationally molded envelope can collapse after be cured by applying a vacuum inside the article and / or by injecting air between the article and the wall of the mold in one piece. The collapsed thin-walled envelope can then be removed from the mold of a piece through the opening when the drinking fountain is removed. This technique results in the desired molded article having an opening corresponding to the size of the mold opening of a piece, which can then be patched

or used in another way, in the same way as the opening of articles produced using a mandrel.

A fourth development achieved by the rotational molding process of the present invention relates to its ability to produce a molded article or shell without plates. The primary component of any of the medical devices mentioned above is the envelope. The current immersion technology for the manufacture of the envelope used in these devices produces a shell hole because the shell must be cut so that it can be removed from the mandrel in which it was formed. The following steps in the construction of these medical devices include plating the hole of the shell with a separate silicone piece or the like. This plating process would not be necessary if the envelope is formed with selected rotational molding procedures since the envelope would be complete and would not require any cutting.

In addition to the four developments described above, there are many other potential benefits and extensions of the concept of rotational molding that experts familiar with the current manufacture of casings and other medical items will appreciate. The process could be modified to produce multi-layered, multi-material laminated envelopes. Surface texture of the shell or other molded article is another possibility.

One of the benefits of the invention is the reduction in the total use of solvents, which allows the condensation and recycling of the solvent, as well as the reduction and almost elimination of the workers' exposure to the solvents used in the molding process . Another possible application is to fill the envelope or other molded article with a contribution, such as a gel, and cure the combined envelope and contribution while still in the mold for precise implant shaping. Soft solid contributions and foamed structures such as NuSil MED4210 elastomer or Applied Silicone liquid foam # 50003 are examples of these contribution materials. Another interesting aspect is that, due to the use of a coating, the mold surface never needs a finish. Silicone material residues are essentially removed compared to an immersion process. Less clean room space is required. The process is more automated, so it should be more reproducible and easier to validate and support. The product obtained is more uniform in thickness, thus improving the performance and quality of the final product. By reducing the use of solvents, the invention is more compatible with the environment and generates less waste that must be disposed of.

Each embodiment of the present invention requires a multi-axis rotation source in which a mold can be mounted. An example of this type of multi-axis rotational molding machine is the Clamshell Compact marketed by FSP Machinery. This two-axis rotational molding machine allows an inert gas to be injected into the mold and controlled throughout the rotation cycle.

Figure 1 is a diagram of a rotational molding system. A two-piece box mold 100 is fixed in a multi-axis rotational molding machine 50 by fasteners that secure the upper mold part 10 and the lower mold part 20 to the fixing base 52 in the upper lock slot 15 and in the lower lock slot 25, respectively. The vacuum connection 75 extends through an arm of the molding machine 50 to the vacuum opening 35. In addition, the material connection tube 77, through which silicone or other molding material (s), polyethylene, polypropylene, nylon, fluoropolymers, polyester resin, polyurethane, epoxy or other coating materials, and / or air is injected into the cavity of the mold 70, it can extend through or along the same arm 55a as the vacuum connection 65 or through the other arm 55b. The hub 51 of the two arms rotates around the A axis in the horizontal direction, while the arms 55 rotate around the B axis, which can be perpendicular to the A axis. This allows the coating material and the silicone material to coat evenly. the surface of the mold cavity 70. The two-piece box mold 100 can be made of copper, aluminum or other materials. The upper mold part 10 and the lower mold part 20 are fitted to each other on the mating surfaces, sealed with o-rings 40, and then locked in to the clamping base 52 of the rotational molding machine of multiple shafts 50. The material reservoir 90 is fluidly coupled to the connecting tube 77 to provide silicone or other molding material, coating material and / or air to the cavity 70. The vacuum source 80 and the solvent condenser 70 are fluidly coupled to the vacuum connection 65.

A two-piece box mold 100 is illustrated in Figures 2-5. The interior of the upper mold piece 10 defines a circular cavity that is used to form the upper part of the shell or the molded article. In cross section, the upper dome cavity 11 may be hemispherical in shape. At the edge 14 of the upper dome cavity 11, near the bottom of the upper mold piece 10, there is a surface of the coupling 13 of the upper mold. The coupling surface 13 of the upper mold joins the coupling surface 23 of the lower mold to form the dividing line of the two-piece box mold 100. Along the outer edge of the coupling surface 13 of the upper mold, located radially outward from the center of the upper mold 10, there is a lip 17 that has a rectangular cross section. The lip 17 extends around the circumference of the lower edge of the upper mold piece 10. The mold will be sealed to maintain a vacuum along the dividing line, in which the coupling surfaces 13, 23 are located when the upper mold piece 10 and lower mold piece 20 are applied to form the two-piece box mold 100. The upper locking groove 15, which is rectangular in cross-section, extends around the entire exterior of the upper mold piece 10, which allows the upper mold piece 10 to be fixed in to the clamping base 52 of the 50 multi-axis rotation molding machine during the molding process.

The lower mold part 20 may be composed of the same material as the upper mold part 10 and may be constructed of copper, aluminum or other materials. The lower mold part 20 may be larger than the upper mold part 10. The lower mold part 20 may have the same outer dimensions or circumference as the outer dimensions or circumference of the upper mold part 10. However, the interior dimensions or the circumference of the lower mold piece 20 and the upper mold piece 10 must match. The lower mold piece 20 has a lower locking groove 25, of rectangular cross-section, which extends around the entire exterior of the lower mold piece 20, which allows the lower mold piece 20 to be fixed on the base clamping 52 of the multi-axis rotational molding machine 50 during the molding process. The interior of the lower mold piece 20 defines a circular cavity 21, which is used to form the lower part of the shell or molded article. The lower cavity 21 of the lower mold piece 20 may be hemispherical in cross-section, similar to the cavity of the upper dome 11, but is more preferably defined by a shallow conical wall 30 that slopes toward a circular opening 29. The circular opening 29 is connected to a circular trough 24 having an outside diameter equal to the diameter of the circular opening 29. The circular trough 24 is coaxial with the circular opening 29 of the lower mold piece 20 and is connected to the opening of the drinker 27, which is also circular. The opening 27 of the trough allows the materials to enter the two-piece box mold 100 when the lower mold part 20 and the upper mold part 10 are coupled.

Also connected to the circular tube 24 of the trough is a vacuum chamber 27 perpendicular to the circular tube 24 of the trough. The inner vacuum tube 27 is connected to the outer vacuum tube 28 which has a diameter greater than that of the inner vacuum tube 27. The outer vacuum tube 28 is connected to the vacuum opening 35 which in turn is connected to the vacuum connection 75. The larger outer vacuum tube 28 allows the vacuum connection 75 to be attached to the two-piece box mold and not enter the circular tube 24 of the drinking fountain.

The lower coupling surface 23 of the mold is different from the upper coupling surface 13 of the mold of the upper mold piece 10. As seen in the cross section of Figure 2, the outer edge of the conical wall surface 30 transitions upwardly in a curve to form the arc 31, so that when the upper mold piece 10 and the piece of lower mold 20 are applied to their respective coupling surfaces, the edge of the arc 31 is aligned with the edge 14 of the coupling surface 13 of the upper mold piece 10. The arc 31 allows the mold 100 to have a smooth transition surface from the upper mold part 10 to the lower mold part 20 and also allows the upper mold part 10 and the lower mold part 20 to form a seal tight when a vacuum is applied to the mold. Radially separated from the center of the lower mold piece 20 and outside the perimeter of the arc 31, an O-ring groove 34 extends around the circumference of the upper edge of the contact surface 23 of the lower mold piece 20. A gasket Typical elastomeric O-ring 4, for example, a Viton® O-ring, is inserted into the groove 34 of the O-ring to maintain sealing integrity between the upper mold piece 10 and the lower mold piece 20. A locking strip 32 is formed around the outer edge of the lower mold piece 20 in which the corresponding lip 17 of the upper mold piece 10 is adjusted to maintain the orientation of the two mold pieces 10, 20 and form a sealing when coupled to each other and locked in the clamping base 52 of the multi-axis rotational molding machine 50 using the locking grooves 15, 25. When the upper mold piece 10 and the lower mold piece 20 are coupled together, the two-piece box mold 100 formed in this way defines the inner cavity 70 of the mold.

The first stage in the manufacture of a shell or other article using the multi-axis rotational molding process of the present invention is to make a coating that covers the inner mold surface of the two-piece box mold 100. The coating must cover the inner surfaces of the upper dome cavity 11 and the lower cavity 21. In this way, covering the inner surface of the mold, any interruptions in the surface are masked, such as dividing lines of the mold, machining marks located on the inner surface of the mold, or minor damage to the inner surface of the mold. The coating can be of any suitable material, but must meet several requirements. First, the coating must have a low extraction capacity level, so that it is biocompatible with the implant shell or other molded article. The coating must also be resistant to any solvent or solvents used in silicone or other materials used in the fabrication of the implant shell or other molded article. The coating material must be able to completely and evenly coat the inner surface of the mold during the rotation of the mold of the multi-axis rotation molding machine. If heat is used to cure the silicone during the molding process, the coating must have a high level of heat resistance. The coating must be easily removable or removable from the surface of the mold and from the cured shell or other molded article. Finally, the coating can be used to provide a desired surface finish to the silicone elastomer or other material, for example, glossy, matte, textured, etc. Suitable coating materials include: polyethylene (Equistar ™ # MP758-772), polypropylene (A. Schulman ™ # PD 8020), nylon (apron® # 8280); fluoropolymers (DuPont® Teflon® PFA), polyester resin (Hypol ™ # 320300-10), polyurethane (Smooth-On Smooth Cast # 305) and epoxy (Polytck® Development Corp. Polypoxy ® 1010), all of which can be found in the market. One skilled in the art may recognize that other similar materials may replace these listed coating materials.

A predetermined volume or weight of the chosen coating material is distributed in the mold to produce a coating of the desired thickness. The coating material is in the form of a fine powder or a liquid depending on the selection of the coating material, provided that the selected material can flow freely. The coating material is inserted into the two-piece box mold 100 through the opening 26 of the drinking fountain and the circular tube 24 of the drinking fountain. The circular tube 24 of the trough extends approximately half into the inner cavity 60 of the box mold 100 and remains in this position during the entire process of forming a lining and the shell or other article. The coating material can be inserted into the box mold before the box mold is locked in the rotational arms of the multi-axis rotational molding machine or after the box mold has been locked in the rotational arms . The closed mold 100 is rotated around two or more shafts, allowing the interior of the coating material to form a consistent coating along the interior surface of the cavity 60. The rotation of the mold around the shafts forms a coating of uniform thickness. If the coating material is composed of thermoplastic materials, heat is applied to make the coating material melt and coat the inner surface of the mold as is done in conventional rotational molding techniques. In the case that a set of chemical substances is used for the coating material system, such as a polyester resin, it is not necessary to apply heat. In addition, air pressure, vacuum, inert gas such as nitrogen or other vapors or solid particles can be applied inside the mold to minimize bubbles or affect the finish of the coating surface in a desired manner.

Once the lining has formed, the next stage is the formation of the shell or other desired article. The circular tube 24 of the trough is kept in the opening 26 of the trough during the entire curing process of the coating and the molding material. To keep the tube 24 of the drinking fountain clean and to keep the vacuum during the casting stage, the outer end of the drinking fountain has a removable lid. Silicone or other molding material is injected into the mold. A predetermined amount of molding material is inserted depending on the size and desired thickness of the envelope or final object. For breast implants, the mater desired materials are usually silicones dispersed in a solvent. NuSil MED 10-6605 is a good selection for a dispersion of silicone acetoxy cured at room temperature (RTV). The NuSil MED 10-6400 can be used as a platinum catalyzed heat-cured silicone (HTV) dispersion. Tin catalyzed silicones or polyurethanes can also be used, as well as other silicone elastomers or solvent systems.

After the silicone or other molding material has been dispensed into the cavity 70 of the mold with the coating through the tube 24 of the drinking fountain and the opening 26 of the drinking fountain, the mold is rotated around at least two axes at the same time that a vacuum is applied inside. The vacuum can be applied in different ways. The vacuum can be applied to the trough of a closed mold through the vacuum opening 35. The vacuum can also be applied to the inner cavity or chamber in which an open cast mold is rotating. Alternatively, the mold may be constructed of a porous material and a vacuum may be applied to the outside of the porous mold. In addition, the positive pressure using either air, nitrogen or other gases or in combination, can be applied intermittently to facilitate the removal of bubbles in the silicone elastomer or other molding material. The bubbles must be removed to allow a smooth and uniform surface of the coating, and ultimately, of the shell or other molded article. In the case of RTV silicones, which require the presence of some water molecules in the mold cavity to carry out the condensation reaction, the positive pressure gas applied could include water vapor.

The silicone or other molding material is rotated and allowed to cure when the arms of the rotational molding machine rotate around its axes, thus forming the desired shape. Rotating the mold at a higher speed can compensate for a lower viscosity level of the inserted materials. Heat is applied, if necessary, to accelerate the curing process. The silicone or other material "fixes" and stops flowing when it rotates and heals in position along with the coating material. If a laminated piece is desired, the above steps can be repeated. If an additional wall thickness is desired in the shell or other molded article, the steps can also be repeated.

After the curing cycle has been completed and the silicone or other molding material has cured to the desired thickness, the mold is opened in the dividing line, that is, where the contact surfaces of the upper mold piece and of the lower mold piece converge. The envelope or the formed article surrounded by the liner is removed from the mold. The shell or other molded article is separated from the coating by one of the appropriate procedures that follow for the coating system: dissolving the coating in a suitable solvent, melting or burning the coating away from the shell or molded article more resistant to the coating. temperature; tear or breakage of the liner separating it from the shell; or detachment of the flexible liner formed from the shell and removal thereof through the opening in the liner produced by the trough opening. The coating can be discarded, or if the coating has not been damaged or dissolves depending on the process of separating the shell from the shell or molded article, the coating can be reused again in the process.

The mold is cleaned, if necessary, of any particles that may have remained from the previous production of the coating and the shell or other article. After cleaning, the mold is ready for the next cycle. If the coating used above is in a satisfactory condition, the coating can be reused in the next molding process, with or without a two-piece box mold.

If the shell is used for breast implants, the shell formed is smooth for assembly or transformation consistent with the usual way in creating a final breast implant product. For example, the implant shell may be filled with a silicone gel, saline or other biocompatible contribution material well known to those skilled in the art.