MXPA98003123A - Method to eliminate imperfections in articles molded by compres - Google Patents

Abstract

The present invention relates to a method for compression molding, a lid having a base wall and a peripheral rim with internal thread, a thermoplastic resin blended with metal particles wherein the metal particles are added in an elliptical or spherical The metal particles are provided in a resin carrier, which is then mixed with the resin in the lid, and the molten mixture is extruded through a nozzle and cut to produce individual pellets or small pieces. Each pellet is deposited in the cavity of a mold that is then closed to mold a cap by compression

Description

METHOD FOR ELIMINATING IMPERFECTIONS IN ARTICLES MOLDED BY COMPRESSION FIELD OF THE INVENTION This invention relates to compression molded plastic articles, such as caps, wherein the resin includes metal flakes.

BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION The conventional system for the supply of melts, used for compression molding articles such as caps, comprises mixing a resin together with dosed amounts of colorant and additives, and feeding them to the resin hopper, and subsequently extruding the resin. At the outlet of the extruder the melt is dosed by a pump for melt, providing a controlled volume of the extruded product. If the molds in which the resin fillers are to be deposited are in a remote position, the extruded product passes through a hose or REF: 27366 conduit, towards a nozzle. As the extruded product emerges from the nozzle, it is cut by a rotating blade to form individual loads and is deposited in successive mold cavities, as they are transported below the cutting blades. This process has not been previously successful in the molding of resins containing additives consisting of metallic pigments, due to the formation of imperfections in the form of pellets, aesthetically objectionable. For example, when the article is a lid having a base wall and a peripheral ridge with a thread on a surface thereof, those imperfections are easily noticed on the base wall. It was found that the supply system described above causes the metallic flakes found in the pigment to be aligned in the flow direction through the hose and the nozzle, so that they are located parallel to the axis of the flow. the pellet of cylindrical shape, as it leaves the nozzle.

During the cutting of the pellet, this alignment remains largely undistributed, so that the union of the cylindrical sides of the pellet and the circular ends of the pellet, there is an abrupt change in the alignment of the metal leaflets. It was found that this change in alignment is one of the main contributors to the effect of the pellet-shaped imperfections, whereby the pellet is deposited in the cold cavity, in a fairly random manner, and the alignment of the flakes is reached to "freeze inside" as the "skin" solidifies while forming against the cooling cavity. The action of the flow of the material during the compression molding differs from that which occurs in injection molding. In compression molding the pellet is brought into contact with the open mold surface and remains in this initial position for the period of time required to close the mold. Only during the final stages of mold closure does the pellet begin to be deformed and displaced to fill the mold. At this time the material that is inside the pellet is flowing, while the initial contact area or "skin" of the pellet remains essentially in its original position. This differs from injection molding where the material flows from a small-diameter central gate or orifice, and immediately flows radially outward and into a completely closed mold. This flow, in injection molding, results in the alignment of the flakes as the melt is forced through the thin sections of the closed mold, immediately. The alignment of the leaflets, in parallel relation to the walls of the housing, results in a good metallic appearance. In US Patent No. 5,328,650, a method for removing imperfections is described, which comprises adding to the resin a blowing agent, prior to extrusion, controlling the temperature of the melt in such a way that the blowing agent is Acti a just before the extrusion of the melt, in the nozzle, the amount of the blowing agent is sufficient to randomize the particulate additive in such a manner that gas bubbles are formed in the extruded mass, in an amount sufficient to substantially reduce the imperfections that normally occur. they are presented in the compression molding of resins containing particulate pigment additives, and in an amount not so great that the gaseous bubbles are not ejected substantially from the melt, during compression molding, to form the top. It is among the objects of the present invention to provide a method for avoiding imperfections in compression molded caps, without the use of blowing agents or without controlling the temperature of the resin. According to the invention, the particles added for the pearlescence comprise spherical or ellipsoidal particles such as aluminum particles, instead of plane pearlescent particles. The resulting caps, molded by compression, have no discernible imperfections. Preferably the spherical or ellipsoidal particles are provided in a plastic carrier compatible with the resin from which the cap is made, and are converted into pellets which are then mixed with the resin to be extruded. A preferred resin comprises polypropylene for the cap and the carrier. When the molten mixture, of the resin and carrier resin with the ellipsoidal or spherical particles, is extruded, cut to produce pellets and compression molded into a cavity, to form a cap, the resulting cap has no discernible imperfections.

DESCRIPTION OF THE PREFERRED MODALITIES This invention relates to the compression molding of plastic caps, wherein a resin, together with dosed amounts of colorants and additives, is fed into the resin hopper, and subsequently extruded. At the outlet of the extruder the melt is dosed by a pump for melt, providing a controlled volume of extruded product. If the molds on which the resin fillers are to be deposited are in a remote position, the extruded product passes through a hose or conduit, towards a nozzle. As the extruded product emerges from the nozzle, it is cut by a rotating blade to form individual charges and is deposited in successive mold cavities, as they are transported below the cutting blades. Each load is compression molded to produce an article such as a lid having a base wall and a peripheral rim having an internal thread. According to the invention, the particles added for the pearlescence comprise spherical or ellipsoidal particles, such as aluminum particles, instead of plane pearlescent particles.The resultant compression molded caps have no discernible imperfections.Preferably, the spherical particles or ellipsoidal are provided in a plastic carrier, compatible with the resin from which the cap is made, and are converted into pellets which are then mixed with the resin to be extruded.A preferred resin comprises polypropylene, for the cap and for the carrier When the molten mixture of the resin and carrier resin with the ellipsoidal or spherical particles is extruded, cut into pellets and compression molded into a cavity to form a cap, the resulting cap has no discernible imperfections. preferred comprises metal flakes and a plasticizer coating on the particles Typical plasticizers comprise dioctyl phthalate, diisodecyl phthalate or mineral oil. In one example, the particles have the following properties: Maximum retained in mesh 325 1.0% Diameter 19 microns Plasticizer based on dioctyl phthalate 20% These particles comprise SILVEXMR 221-20-A manufactured by Silberline Manufacturing Co., Inc., Tamaqua, Penns i 1 vania. The amount of particles used comprises approximately 0.5% to approximately 3% by weight of the molded cap. Satisfactory results have been obtained in which the particles comprise approximately 0.75% by weight of the molded cover. In this way it can be seen that a method has been provided to avoid imperfections in compression molded caps, which contain metallic particles, without the use of blowing agents or without controlling the temperature of the resin. It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (18)

1. In a method for compression molding a lid having a base wall and a peripheral rim with internal thread, a thermoplastic resin mixed with metal particles, where the molten mixture is extruded through a nozzle, is cut into pellets or small individual pieces, each pellet is deposited in the cavity of a mold which is then closed to compression mold a lid, the improvement characterized in that it comprises adding, "in the step of adding particles, elliptical or spherical shaped particles.

2. The method according to claim 1, characterized in that the particles are provided in. a plastic carrier that mixes with the thermoplastic resin.

3. The method according to claim 2, characterized in that the particles comprise aluminum particles.

4. The method according to claim 2, characterized in that the particles are coated with a plasticizer before being mixed with the carrier.

5. The method according to claim 4, characterized in that the plasticizer is selected from the group comprising dioctyl phthalate, diisodecyl fatalate and mineral oil.

6. The method according to claim 4, characterized in that a plasticizer comprises dioctyl phthalate.

7. The method according to claim 1, characterized in that the amount of particles comprises approximately 0.75% by weight of the plastic lid.

8. The method according to claim 1, characterized in that the thermoplastic resin of the lid and the carrier comprises polypropylene.

9. The method according to any of claims 1 to 8, characterized in that the amount of particles comprises from about 0.5% to about 3% by weight of the plastic lid.

10. A plastic cover, molded by compression, characterized in that it comprises a base wall and a peripheral rim with internal thread, the lid has distributed therein a plurality of elliptical or spherical particles.

11. The compression molded cover according to claim 10, characterized in that the particles are provided in a plastic carrier that is mixed with the thermoplastic resin.

12. The compression molded cover according to claim 11, characterized in that the particles comprise aluminum particles.

13. The compression molded lid according to claim 12, characterized in that the particles are coated with a plasticizer before being mixed with the carrier.

14. The compression molded lid according to claim 13, characterized in that a plasticizer is selected from the group consisting of phthalate-dioctyl, diisodecyl phthalate and mineral oil.

15. The compression molded lid according to claim 13, characterized in that a plasticizer comprises dioctyl phthalate.

16. The compression molded cover according to claim 15, characterized in that the amount of particles comprises approximately 0.75% by weight of the plastic cap.

17. The compression molded cover according to claim 16, characterized in that the thermoplastic resin of the lid and the carrier comprises polypropylene.

18. The compression molded cap according to any of claims 10 to 17, characterized in that the amount of particles comprises from about 0.5% to about 3% by weight of the plastic cap.