METHOD FOR INJECTION MOLDING WITH IMPROVED MOLD PRIMER

Abstract: An in-mold primer which contains a preferentially adhesive resin also serves simultaneously as a mold release agent, and results in RIM (reaction injection molding) plastic articles whose surfaces virtually mirror the mold surface. Such a primer leaves no appreciable residue whatsoever on the mold surface during a production run of a large number of articles, obviating the previously encountered need to clean the mold each time a conventional mold release agent was introduced, each time an article was molded. The preferred resin in the primer is a hydroxylated vinyl chloride-vinyl acetate copolymer (VAGH resin).

Description: METHOD FOR INJECTION MOLDING WITH IMPROVED MOLD PRIMER Background of the Invention I. Field of the Invention 5 The present invention relates to article forming, and more particularly to methods and compositions useful for molding articles from plastic materials. II. Description of the Prior Art Injection molding is a well known method for the 10 manufacture of multiple objects having precisely identical shapes and sizes, for example, automobile fascia and other automobile parts. Conveniently these objects have been molded out of plastic, most notably polyurethane plastic, and have been formed by injection of plastic or plastic 1.5 components into a metal mold. It has been a problem in the past that the formed injection molded plastic has stuck to the mold walls so as not to freely release from the mold subsequent to forming. . One prior method for preventing the plastic from sticking to 20 the mold walls has been to coat the mold with a release agent, such as polymeric wax, silicone or other types of agents, prior to injection of the plastic. In some instances, depending upon the type of plastic used, the formed plastic is baked to rigidify it 5 after the plastic has been removed from the mold. Subsequent to baking, a power wash is applied to the plastic to remove any residual release agent on the surface of the plastic. This wash is required because the release agent 0 provides an insufficiently smooth or insufficiently conditioned surface for the application of paint to the plastic. Once the release agent has been cleaned from the plastic surface, the plastic surface is then suitable for attachment of primer, and the plastic is primed using conventional spray or other equipment or by any other priming method known in the art. Once the plastic is primed, it is given a bake to cure the primer. After the bake the surface generally is sanded and cleaned again. The 5 part is now in condition for receiving a topcoat, and the part is given another bake cycle to cure the topcoat. There have been several problems with using the x&I&ase agents known in the prior art. After a certain (and generally far too short) time, the mold release agent builds IDC- up inside the mold. Because the uneven buildup of the mold release agent causes imperfections on the surface of the plastic, it has been necessary to take steps to remove this buildup after a certain number of mold cycles. The need for these removal steps resulted in either downtime of the mold 15 " associated with the equipment, or a flawed product with uneven and imperfect surfaces, if the mold release agent buildup was not removed at the appropriate time. Another problem apparent in prior methods is that when a release agent is left on the plastic after the power 20 wash, the unremoved release agent couses a loss of adhesion between a subsequently applied primer and the substrate. Conventional release agents are not suitable surfaces for adhesion of primer or topcoat to the surface of the article. This loss of adhesion often results in separation of the 25 primer from the substrate during the sanding operation, and therefore, results in a rejected article. Of course, once any part of the surface of the plastic itself is exposed to the sanding operation (for example, because of excessive sanding), the part needs to be reprimed, cured and resanded. 30 Thus, a further delay in the production process results when the release agent is left on the plastic surface. Summary of the Present Invention The method of the present invention solves these and other problems by use of a primer that is preferentially adhered to the plastic, as compared to the metal mold, at injection or subinjection temperatures. Injection temperatures for reaction injection molding (RIM) processes, for example, are typically on the order of 250°F. The primer of the present invention has excellent adhesion to the plastic, and prevents the plastic from sticking to the mold. Therefore, no other mold release agent is needed in order to insure the free release of the formed plastic article from the mold. With no other mold release agent, there is no buildup that needs to be cleaned up, nor is there a need for a power wash to remove a release agent that isn't present. The primer is also suitable for reception of a topcoat. Thus, the need for sanding is eliminated, provided the mold walls are smooth and a dust-free environment can be maintained. The part can be topcoated immediately after the removal of the mold or after post-cure. The method permits consolidation of the baking requirements of the primer and topcoat, or the primer may require no more curing than that occurring during the molding operation. The method thereby offers substantial savings of time and energy over prior molding methods. It is, therefore, an object of the present invention to provide an improved in-mold primer whereby a molded article may be easily and freely removed from the mold, thereby preventing sticking of the formed plastic article to the mold walls. It is also an object of the present invention to provide a primer for molded articles that is applied to the mold prior to molding of the articles, instead of being applied directly to the molded articles subsequent to their molding. A further object of the present invention is to provide a primer for molded articles that is applied to the articles contemporaneously with the formation of the articles. It is also an object of this invention to provide a method for the free removal of a molded article from a mold, which method does not require an additional mold release ag.ent besides the agent provided for in the invention. A still further object of the present invention is to provide a mold release agent that, during its use, leaves no residue on the mold surface. A still further object of the present invention is to provide an in-mold primer for molded parts whereby the part can be topcoated after removal from the mold with little or no sanding of the part required. Another object of the present invention is to provide a process for painting injection molded parts which consolidates the baking requirements of the primer and topcoat and thereby offers substantial savings of time and energy. A further object of this invention is to provide an in-mold primer useful in a repeated molding process, which virtually if not completely eliminates cleaning of the mold between the molding of individual articles during a production run of upwards of ten thousand articles, yet which simultaneously provides a clean primed surface not requiring sanding prior to reception of a topcoat or finish coat. A last object of this invention is to provide a primed, molded article whose surface mirrors the shape of the mold surface from which the article is molded, without any treatment of the article besides the use of a preferentially adhesive in-mold primer. Detailed Description of the Preferred Embodiment of the Present Invention The invention as applied to injection molding of plastic parts is generally as follows: A coating of a type hereinafter described is applied to the interior walls of a mold by any method of application known in the art. The mold is then closed and a disparate plastic substrate is injected or otherwise inserted into the mold. The plastic substrate is preferably a RIM plastic, which sets upon the reaction of two or more discreet components which are injected into the mold, advantageously admixed just prior to introduction into the mold. Most preferably, the RIM plastic substrate is a polyurethane formed from a polyisocyanate component and a polyol component. Conventionally, as is well known in the art, a three to five per cent stoichiometric excess of polyisocyanate is employed. The plastic substrate can also be a polyester or polyaramid resin. The forming or fabrication process is then continued until completion, i.e., the part is formed. The formed part is then removed from the mold and ready or nearly ready for application of the topcoat. The part is then baked to cure the topcoat. Many methods for applying a coating to the interior walls of a mold are known in the art. Such methods include manual or automatic spray (air or airless), powder coating, roller coating, curtain coating, continuous dry film application, flow coating and brushing. Many methods for the introduction of a plastic substrate onto a mold surface are known in the art as well. Forming or fabricating plastic parts by the mold process may take place at temperatures ranging from room temperature to 300°F. The mold or die can be made from substrates which include wood, plastic and various metals, and can include plated substrates. The coating need not be restricted to a primer and, in fact, can comprise a single-coat topcoat. Preferably, however, the coating comprises a primer. The coating is characterized in that, at or below the temperature of formation of the plastic piece, it is strongly preferentially adherable to the plastic instead of the material constituting the surface of the mold. For common plastics used in injection molding in a metal mold, the resin in the coating can be of the type found in the following list, but is not limited to: halogenated vinyl polymers; organic acids, esters and/or ethers of vinyl polymers; organic and inorganic acids, esters, and/or ethers of cellulose; or any of the above polymers further modified with functional groups such as, ( but not limited to: oxiranes; carboxyls; halogented alkyls, alkenyls, or alkynyls; hydroxyls; styrenyls; acryloyls; and the like. Many resins typically used in the paint industry are usually undesirable for use as an in-mold primer, because they are designed to adhere to metal rather than preferentially adhering to the plastic part. These include polyesters, alkyd resins, epoxies and others. The part may be removed from the mold by any method known in the art. For example, the part may be peeled from the mold, or may be manually removed from one side of a two piece mold; and, of course, many molds employ ejector pins to facilitate removal. The disadvantages of overspraying a painted part are well known in the art. Therefore, care should be taken to minimize overspray, and thereby provide a high quality finish on the plastic part. A primer according to the present invention advantageously comprises the above-described resin and any or all of a pigment, solvent, an acid scavenger, a plasticizer, a heat stabilizer and a wetting agent. 5 The functions of each additional constituent in a primer are, of course, known in the art. A typical example of a formula for an in-mold primer suitable for use in the present invention is as follows, with the functions of the various components being as indicated: in Formula Percent by wt. (Conductive) Carbon Black (pigment) 2.4 Magnesium Silicate (Pigment) 3.6 VAGH (Union Carbide) (Resin) 12.2 5 Urea - formaldehyde resin (Resin) .8 Epoxidixed Oil (Plasticizer and Acid Scavenger) .4 Propylene Oxide (Acid Scavenger) .1 Tin Merσaptan (Heat Stabilizer) .1 Wetting Agent .1 0 2-Heptanone (Solvent) 80.3 100.0% A resin, to be suitable for use in the present invention, must have relatively poor adhesion to metal and relatively good adhesion to organic substrates. 5 The composition given above, for example, has virtually no adhesion to metal mold surfaces at a temperature below 180°F. The resin must also be durable enough to function in a primer. One resin that meets these 0 criteria is VAGH, a hydroxylated vinyl chloride-vinyl acetate copolymer sold by Union Carbide. The VAGH resin fulfills the above-mentioned characteristics and also has some hydroxyl functionality, which provides reactive sites for chemically bonding the primer to the 5 molded article. This hydroxyl functionality also provides a site at which an amino resin can crosslink vinyl copolymer to further enhance the durability of the primer. The hydroxyl groups may also react with the excess polyisocyanate in the polyurethane substrate to enhance preferential adhesion of the resin to the substrate, instead of to the mold. As indicated above, other polymers may be used in place of the VAGH, with varying degrees of success. Essentially any resin with poor adhesion to metal can be used in the present invention, and the only limit on its adhesion to plastic depends upon the requirements of the end to which the primer will be put. Typical requirements are good ability to adhere to the plastic, good suitability for adhesion to a topcoat, and durability both during the painting process and in use of the product by the ultimate consumer. The simplest way to determine the utility of a given resin for this purpose is to examine the technical specifications relating to adhesion (usually available, from the resin manufacturer) , and select and test those having the aforesaid characteristics and being otherwise suitable for the intended use. The above example is, of course, by way of illustration only. Compositions comprising about 10 to 15 percent by weight of resin, about 1 to 10 percent by weight pigment, about 0.1 to 1 percent by weight heat stabilizer, about 0.1 to 1 percent by weight wetting agent and solvent in a quantity sufficient to permit the application of the composition to the mold surface, are useful in the present invention. The formulation of the example above is the most useful embodiment of the invention known to Applicant. The formulation has been successfully privately tested in molding automobile spare tire covers, automobile grille sections, automobile bumper fascia, automobile bumpers, automobile steering wheels. golf cart wheels, and wheel balancing machine cabinets. While the preferred substrate is polyurethane, the formulation was successfully applied in-mold to the following substrates for these articles, respectively: Dow Spectrim MM-363 and Ashland Arimax 1500; Dow Polyester; Mobay Bayflex 30; Mobay Bayflex 110-25; BASF Bayflex 28, Mobay Bayflex 28 IMR and Ottobock (West Germany) ; Dow 181 System; and Mobay Baydur 724, Mobay Bayflex 110-80 and Mobay Bayflex 110-50. The formulation has also been successfully used as an in-mold primer on Mobay Bayflex 110-25 IMR. Again, there is no appreciable primer left on the mold surface between the sequential molding of individual articles during a typical production run of upwards of ten thousand articles; the mold surface is virtually as clean after the ten thousandth article as either after or before the first, with no cleaning of the mold surface during the production run. This allows the molding of certain substrates in metal molds which are not generally considered oldable by a metal mold surface. For example, polyester resins are usually intended to be and are quite compatible with metal surfaces; that is why such resins are conventionally used to coat metals. In surprising contrast, polyester resins are readily molded with no co patibiliza- tion when the in-mold primer of the present invention is employed. The articles formed when the in-mold primer of the present invention is used possess a surface which literally mirrors the mold surface from which the article is formed, while cleaning of the mold surface during sizable production runs is eliminated. It is the use of a resin in a top position, which functions both as a primer and as a mold release agent, that characterizes this invention. Those skilled in the art will understand that the other components of the composition given in the example provide the opacity, color, conductivity, heat resistance, stability, wettability, and similar characteristics required by the environment or circumstances under which the primer would be employed as a primer. Having described my invention, however, many modifications thereto will become apparent to those skilled in the art to which it pertains, without deviation from the spirit of the invention, as defined by the scope of the appended claims. I claim:

Claims: - I I- C LA IMS 1 1. A RIM process for forming a surface 2 finished, molded plastic article from a mold, 3 comprising the steps of: 4 (a) dispersing a resin-containing composition 5 on a mold surface, the resin in said composition having 6 by itself substantially no adhesion to the material of 7 said surface; 8 (b) introducing a different RIM plastic ) substrat onto said mold surface over said composition; IX) and " : 11 (c) incorporating said resin onto the surface 12 of said substrate by curing said substrate in said mold 13 and subsequently demolding said substrate; 14 wherein said resin is a hydroxylated vinyl 15 chloride-vinyl acetate copolymer possessing sufficient 16 hydroxyl functionality to strongly adhere to said 7 substrate by chemically bonding to it; and 8 wherein said process is characterized in that 9 no residue is left on the mold surface after the 0 forming of a great plurality of articles despite the 1 absence of any cleaning of the mold surface between 2 such formings. 1 2. The process according to claim 1, wherein 2 said process is carried out employing a metal mold. 1 3. The process according to claim 1, wherein 2 said process is carried out employing a composition 3 additionally comprising at least one member selected 4 from the group consisting of: pigments, solvents, acid 5 scavengers, plasticizers, heat stabilizers and wetting 6 agents. 1 4. The process according to claim 1, wherein 2 said process is carried out employing a composition 3 comprising: 4 about 10 to 15 percent by weight of said 5 resin; 5 about 1 to 10 percent by weight pigment; 7 about 0.1 to 1 percent by weight of g plasticizers; g about 0.1 to 1 percent by weight heat 0 stabilizers; 1 about 0.1 to 1 percent by weight wetting 2 agent; and 3 solvent in a quantity sufficient to permit the 4 application of said composition to the surface of said 5 mold. 1 5. The process according to claim 1, wherein said process is carried out employing a composition 3 comprising: 4 about 2.4 percent by weight conductive carbon black; about 3.6 percent by weight magnesium silicate; 8 about 12. 2 percent by weight hydroxylated v inyl chloride-v inyl acetate copolmer; about 0.8 percent by weight urea-formaldehyde resin; about 0.4 percent by weight epoxidized oil; about 0.1 percent by weight propylene oxide; about 0.1 percent by weight tin mercaptan; about 0.1 percent by weight wetting agent; and about 80.3 percent by weight 1-Heptanone. 6. The process according to claim 1, wherein said different RIM plastic substrate is selected from the group consisting of polyurethane, polyester and polya amid resins. 7. The product of the process according to claim 1. 8. The product of the process according to claim 4. 9. The product of the process according to claim 5. 10. The product of the process according to claim 6. 11. An in-mold primer for a product formed by a mold, said primer comprising a resin, and said resin having a substantially greater adhesive preference for the substrate forming said product than for the material constituting said mold. 12. The invention according to claim 11, wherein said primer additionally comprises at least one member from the group consisting of: pigment, solvent, acid scavanger, plasticizer, heat stabilizer and wetting agent. 13. The invention according to claim 11, wherein said resin comprises a polymer. 14. The invention according to claim 11, wherein said primer further comprises: about 10 to 15 percent by weight of said resin; about 1 to 10 percent by weight pigment; about 0.1 to 1 percent by weight of plasticizer; abo ut ø . l to 1 pe rcent by we ight we t t i ng agent ; and solvent in a quantity sufficient to permit the application of said primer to said surface of said mold. 15. The invention accord ing to cla im 11 , wherein said resin is a hydroxylated vinyl chloride-vinyl acetate copolymer . 16. The invention acco rd ing to cla im 11 , wherein sa id res in compr ises a member of the class consisting of halogenated vinyl polymers ; organic acids of vinyl polymers ; esters of vinyl polymers ; ethers of vinyl polymers ; organic acids of cellulose ; inorganic acids of cellulose ; esters of cellulose; and ethers of cellulose . 17. The invention according to claim 16, wherein said member of said class is modified by the addition of at least one functional group from the group consisting of oxiranes, carboxyls, halogenated alkyls, halogenated alkenyls, halogenated alkynyls, hydroxyls, styrenyls and acrylonyls. 18. The invention according to claim 11, wheein said primer comprises a composition simultaneously suitable (a) as a mold release agent in the molding of a plastic article, and (b) as a surface finishing agent for said article; said composition characterized by its adherence to the surface of said article, and its nonadherence to the surface of said mold. 19. A process for providing a coated surface on an article formed by a mold, comprising the steps of: applying a primer to a surface of said mold; forming said article by said mold; and removing said article from said hold; wherein said primer comprises a curable resin which, when cured, has a substantially greater adhesive preference for the substrate forming said article than for the material constituting said mold, such that said primer forms a coated, surface on said article which mirrors the surface of said mold and which leaves the surface of said mold substantially free of said primer when said article is removed from said mold. 20. A process for forming a surface finished, molded plastic article wherein a composition used as a mold release agent becomes chemically bound to the substrate forming said article and incorporated on the surface of said article, said composition thereby serving as a surface finishing agent. 21. A RIM process for forming a surface finished, molded plastic article from a metal mold, comprising the steps of: (a) dispersing a resin-containing composition on a metal mold surface, the resin in said composition having by itself substantially no adhesion to the metal of said surface; (b) introducing a RIM plastic substrate onto said mold surface; and (c) incorporating said resin onto the surface of said substrate by curing said substrate in said mold and subsequently demolding said substrate; wherein said resin is selected from the class consisting of those resins possessing sufficient hydroxyl functionality to strongly adhere to said substrate by chemically bonding to it. 22. The process according to claim 21, wherein said process is carried out employing a composition comprising a hydroxylated vinyl chloride-vinyl acetate resin. 23. The process according to claim 22, wherein said process is carried out employing a composition comprising: about 10 to 15 percent by weight of said resin; about 1 to 10 percent by weight pigment; about 0.1 to 1 percent by weight of plasticizers; about 0.1 to 1 percent by weight heat stabilizers; about 0.1 to 1 percent by weight wetting agent; and solvent in a quantity sufficient to permit the application of said composition to the surface of said mold. 24. The process according to claim 22, wherein said process is carried out employing a composition comprising: about 2.4 percent by weight conductive carbon black; about 3.6 percent by weight magnesium silicate; about 12. 2 percent by weight hydroxylated v inyl chloride-vinyl acetate copolymer ; about 0.8 percent by weight urea- formaldehyde resin; about 0.4 percent by weight epoxidized oil; about 0.1 percent by weight propylene oxide; about 0.1 percent by weight tin mercaptan; about 0.1 percent by weight wetting agent; and about 80.3 percent by weight 2-Heptanone. 1 25. A process for forming a substrate into a 2. surface finished, molded article by introducing said 3' . substrate onto a mold surface, comprising the steps of: 4 applying an in-mold primer to said mold 5 surface; 5 introducing said substrate onto said primer on 7 said mold surface; and g forming said substrate into said article and 9 bonding said primer to said substrate; 10 wherein said primer comprises a resin having a 11 substantially greater adhesive preference for said 12 substrate than for the material constituting said mold 13.. surface, such that the surface finished, molded article 14 so formed is removable from said mold surface without 15 leaving any appreciable residue on said mold surface, 16 while said surface finished, molded article mirrors the 17 configuration of said mold surface. 1 26. The product of the process according to 2 claim 25. 1 27. The process according to claim 25, 2 wherein said substrte comprises a RIM plastic. 1 28. The process according to claim 27, 2 wherein said plastic comprises polyurethane. 1. 29. The process according to claim 25, 2: wherein said primer comprises a resin possessing 3 sufficient functionality to strongly preferentially 4 adhere to said substrate by chemically bonding to it. 1 30. The process according to claim 25, 2 wherein said primer comprises a hydroxylated vinyl 3 chloride-vinyl acetate copolymer. 31. The process according to claim 30, wherein said primer comprises: about 2.4 percent by weight conductive carbon black; about 3.6 percent by weight magnesium silicate; about 12.2 percent by weight VAGH resin; about 0.8 percnet by weight urea-formaldehyde resin; about 0.4 percent by weight epoxidized oil; about 0.1 percent by weight propylene oxide; about 0.1 percent by weight tin mercaptan; about 0.1 percent by weight wetting agent; and about 80.3 percent by weight 2-Heptanone.

Inventors: KELLY BRIAN M, KELLY, BRIAN, M.
Applicants: KELLY BRIAN M, KELLY, BRIAN, M.
Application year: 1986
Main classification: B28B7
IPC codes: B29C33, B29C37, B29C39, B29C45, B29C67