Abstract:
A plastic part is provided with decorative metallized surfaces as well as overmolded thermoplasticelastomer grip portions. The plastic part is overmolded with the TPE prior to metallizing the remaining exposed surfaces of the part. A TPE compound with saturated midblocks is used which is stable in the acid environment and elevated temperatures of the plating.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention concerns a process for forming a plastic part, such as a case for a tape measure or other item in which there is a low-durometer grip portion and with other, rigid surfaces of the plastic substrate being plated with a shiny metallic coating. In particular, the invention is directed to a process in which the rigid plastic part is first subjected to an overmolding step to add the grip portion(s), and is then subjected to a plating or metallizing process. This process of overmolding first and then metallizing represents a significant improvement over the conventional technique of metallizing first and then overmolding, as it avoids scratching and damage to the shiny metal surfaces that can occur in the overmolding step. 
         [0002]    Metal plating of plastic materials is commonly done in the industry to provide a decorative, shiny metal finish to a molded plastic part. This can be done using an electroless plating technique or a combination of electroless plating and electroplating. A process for metal plating of plastic parts is described in Frankel et al. U.S. Pat. No. 4,803,097. Another process for applying a decorative chrome electroplate onto a rigid plastic substrate is described in Donovan, III et al. U.S. Pat. No. 6,468,672. These processes involve plating baths of very low pH (acidic) and with elevated temperatures. 
         [0003]    A process for forming an overmolded grip on a rigid plastic substrate is described in Ajbani et al. Pat. Appln. Pub. No. US 2004/0151933. A rubbery polymer is applied onto a plastic part of any of various types, e.g., toothbrush handles, razor handles, hair brushes, pens, kitchen appliances and utensils. The polymer can also be applied as a grip portion onto a tool such as a knife or saw handle, or the case or housing of a tape rule. The overmolding process involves placing the previously molded rigid plastic substrate into an injection or compression mold where the rubbery polymer is applied over some portions of the substrate. Where the overmolding process is used for forming grip portions on a metal-plated plastic part, the action of placing the previously plated plastic part into the mold does often result in scratching or damaging to the other surfaces, including surfaces that are left uncovered by the overmolded grip portions. These scratches are deemed unacceptable in the finished product, and so the parts or cases with scratching are scrapped. 
         [0004]    For this reason, the grips are often formed as separate plastic parts, and attached to the plated rigid plastic part. This has been one preferred technique in applying rubber-like grip portions to chrome-plated plastic tape rule cases, and this technique is described in Wertheim U.S. Pat. No. 5,746,004 and in Blackman et al. Pat. Appln. Pub. Nos. US 2003/0233762 and US 2005/0028397. However, because of the additional manufacturing steps involved in attaching the elastomeric grips, this technique increases manufacturing costs. 
         [0005]    It has been desired to provide an effective, efficient, and inexpensive technique for overmolding rubberlike grips onto metal plated plastic parts, such as tape measure cases, and at the same time to avoid the high scrap rates that seem to be inherent in the current technique. However, that objective has thus far eluded the industry. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0006]    Accordingly, it is an object of the present invention to provide a technique for forming metallized rigid plastic parts with overmolded grip portions, and which avoids the drawbacks inherent with the techniques of the prior art. 
         [0007]    It is a more specific object to provide an overmolding technique in which the rigid plastic substrate or shell is first provided with an overmolded grip portion or portions of a suitable TPE polymer, and is thereafter subjected to plating of the remaining exposed surfaces. 
         [0008]    It is a particular objective to provide an overmolding technique which employs materials and techniques that permit the overmolding to occur prior to the metallization. 
         [0009]    In accordance with an aspect of this invention, a molded plastic part is produced that is at least partly metal-plated and which is overmolded with a thermoplastic elastomer that form grip portions. First, a shell or substrate is molded of a rigid polymer. Then the substrate is overmolded, with a thermoplastic elastomer (or TPE) on some portions of the molded substrate, leaving exposed portions of the molded substrate. After this, a decorative metal plating is applied onto the exposed portions, but not onto the TPE overmolded grip portions. 
         [0010]    In a preferred embodiment, the thermoplastic elastomer is a compound having saturated elastomer midblocks and which is adapted to withstand exposure to the acid environment and elevated temperature of a plating bath. The decorative metal plating is carried out by preparing the exposed portions of the molded and overmolded substrate in a bath of chromic acid to etch the surface of said exposed portions. Then the substrate is subjected to electroless palladiumizing in a bath of stannous chloride and palladium chloride to produce a palladium tin layer on the surface of the exposed portions. A subsequent bath removes any plastic residue from the palladium tin layer on the exposed portions. Then the part is placed in successive plating baths of copper, nickel, and chrome for electroplating the palladium tin on the exposed portions, and produces a durable, shiny metal surface. 
         [0011]    Favorably, the thermoplastic elastomer or TPE is a material having a shore hardness between about 40 and 90, although this is not critical. The material may have a tensile strength of substantially 60 Kg/cm 2 . One example of an acceptable type of thermoplastic elastomer is Empilon (a trademark of Ho Tai Industrial Company, Ltd, of Taipei, Taiwan) of grade PHA 61B. Other types of TPE could be employed, depending on the product. 
         [0012]    The step of palladiumizing should be preceded by neutralizing in a bath to remove from the surface of the exposed portions of the substrate hexavalent chrome that results from the etching step. The steps of palladiumizing and removing plastic residue should be followed by dipping the substrate in a bath containing nickel sulphate, ammonium chloride, sodium hypophosphite and sodium citrate. 
         [0013]    The above and many other objects, features, and advantages of this invention will be more fully appreciated from the ensuing description of selected preferred embodiments, with the description to be read in conjunction with the accompanying Drawing. 
     
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0014]    The sole Drawing FIGURE is a perspective views of the case or housing for a tape measure rule or tape measure, formed by a process according to a preferred embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    The FIGURE shows an example of a plastic molded item, namely a case  10  for a tape rule, which is formed of a rigid plastic material, e.g., ABS or polystyrene or polycarbonate, with overmolded grip portions  12  formed of a suitable TPE, which in this embodiment is Empilon PHA 61B. Here the case is fabricated as left and right halves, i.e., a cover half and a hub half, and each of these is subjected to metallization of exposed surfaces  14 , i.e., those not occupied by the grip portions  12 . 
         [0016]    As discussed before, the tape rule case is but one example, and the technique can be used for many other tools and implements or other products where a rubbery grip may be desired. 
         [0017]    The rigid plastic right and left halves of the case are first formed by a traditional plastic molding technique. In one example, the case substrate is molded of ABS. Then each housing half is provided with the overmolded grip portions, applying the Empilon TPE material. Favorably, one possible TPE has the following characteristics, namely, a specific gravity of 0.91, a Shore (A) durometer hardness of about 40 to 90, a tensile strength of about 60 Kg/cm2, an elongation (at break) of 500%, a melt index of 2 grams per 10 minutes at 200° C. for a 5 Kg test sample, and an adhesive strength with ABS of 6 Kgf/2.5 cm. In this example, the TPE is black, but if desired could be of another color. This TPE can favorably be an SEBS polymer, with styrene end blocks and polybutadiene and butylene mid blocks or center blocks. The Shore hardness can be higher or lower than the range mentioned above, depending on the article. This particular compound has high weather and oxidation resistance, high processing stability, and normal processing stability at temperatures up to at least 190° C. and up to 260° C. 
         [0018]    After the overmolded grip portions  12  have been added, the overmolded ABS substrate is placed onto a hanging rack, and is prepared for processing in a number of successive chemical baths, where the metallization is accomplished on the exposed surfaces  14 . 
         [0019]    The substrate or part is first degreased to clean it of grease, dirt, fingerprints and the like, using a standard degreasing compound in aqueous solution at about 50-60° C. for several minutes. This is followed by a water rinse. Then the part is placed into a sulfuric acid solution with a wetting solvent, to enhance the hydrophilic behavior of the exposed ABS surfaces  14 . After this the part is dipped into an etching bath of chromic acid and sulfuric acid at about 65 to 75° C. for a sufficient time (in one example, 8-15 minutes) to etch the surface lightly. After a brief water rinse, the part is neutralized in a bath of CP hydrochloric acid for about a minute to a few minutes. This removes any hexavalent chrome from the etched surface to prepare for the subsequent steps. This is followed by a water rinse. 
         [0020]    The part is next dipped into a bath containing hydrochloric acid, stannous chloride, and palladium chloride in aqueous solution, at ambient (room temperature) for about 90 to 180 seconds (this time is not critical, and can be varied). This is followed by a water rinse. This produces a layer of stannous palladium, i.e., palladium tin, which is chemisorbed onto the exposed ABS surfaces  14 . Then the part is peptized by dipping it into hydrochloric acid at 45 to 60° C. for 90 to 180 seconds (again, this time range is not critical, and depends on the product being plated). This removes plastic residue from the palladium tin. A water rinse follows this step. 
         [0021]    After this, the part is subjected to a number of plating steps for plating successively in copper, nickel and chrome. 
         [0022]    The part is dipped into a nickel plating bath of nickel sulphate, ammonium chloride, sodium hypophosphite and sodium citrate in aqueous solution at 30 to 40° C. for about several minutes (e.g., 5 to 10 minutes) to form a base layer of electroplated nickel. After a water rinse, the part is placed in a copper pre-dip bath of copper pyrophosphate and phosphoric tetrasodium at about 45 to 55° C. for several minutes to enhance the conductivity of the nickel base layer. Then there is another water rinse. 
         [0023]    Copper plating is carried out in a bath of copper sulphate with sulfuric acid for about a sufficient time to create a shiny surface for the part, and in one possible example this is for about 8 to 15 minutes, with current applied at about 2 to 4 amps/dm 2 . After water rinsing, the part is placed in an activation bath of sulfuric acid for a short time, e.g., 30 to 90 seconds, to remove oxidation from the metal surface. Then there is a water rinse, and the part is placed into a nickel semi-dipping bath, of nickel sulphate, ammonium chloride, and boric acid, at a temperature of about 50 to 60° C., a pH of about 4.0 to 4.6, for a sufficient time, e.g., 4 to 6 minutes at a current density of 2 to 8 amps/dm 2 . This enhances the anti-corrosive properties of the metallization. Then the part undergoes a second nickel dipping under the same conditions and this produces a shiny surface on the part. After a water rinse, the part is dipped into a chrome plating bath of chromic acid and sulfuric acid at about 35 to 43° C. for a few minutes, e.g., 3 to 4 minutes at 10 to 30 amps/dm 2  to give the metallization an anti-wear and anti-corrosive property. 
         [0024]    In one example, the solutions used may have concentrations as follows: Degreasing, 7%±1%; Etching, 46%±2%; Neutralization, 5%±1%; Palladiumizing, 9±2 g/L; Peptizing, 2 to 6 g/L; Initial Nickel dipping, 4±0.5 g/L; Copper pre-dipping, 21±2 g/L. In the remaining metal plating, for Copper dipping, 22±2 g/L; Nickel semi-dipping, 20±2 g/L; Nickel dipping 25 to 30 g/L; and Chrome dipping, 20±2 g/L. The skilled process engineer can adjust these as need be to achieve optimal results in any given application. 
         [0025]    The post-overmolding plating as described above does not adversely affect the overmolded TPE grip portions. The incidence of scratching or damage to the plated or metallized surfaces is greatly reduced as compared with the more traditional approach of first plating and then overmolding. 
         [0026]    While the invention has been described with reference to a specific preferred embodiment, the invention is certainly not limited to that precise embodiment. Rather, many modifications and variations will become apparent to persons of skill in the art without departure from the scope and spirit of this invention, as defined in the appended claims.