Textured molded part surface

Plastic parts can be post-finished in a number of ways. Film and sheet can be post-embossed with textures and letterpress, gravure, or silk screening can print them. Rigid plastic molded parts can be painted or they can be given a metallic surface by such techniques as metallizing, barrel plating, or electroplating. Another popular method is hot... 

For this work, all samples were either injection molded automotive plaques or parts molded in accordance with our recommended processing conditions. Several different textured and smooth surfaces of varying gloss levels were evaluated. CIELAB color difference units were used exclusively. Finally, a 60 degree gloss meter was used to make sure gloss readings were comparable. 

The exact color a plastic resin or part must match to be acceptable. Resin suppliers often submit color chip samples of the matched resin color to be compared to the molded part. The color chip, or standard, is usually 51 X 76 mm with one polished surface and various textured surfaces on the opposite side. Suppliers use similar standards to verily the color of each lot of resin shipped to their customer. 

With wall thicknesses in the range of 0.040 to 0.060 in, it is clearly impossible to adhere to all the ftindamental rules of proper injection-molded part design (rib thicknesses will be greater than 70% of the nominal wall). This means that surface finish problems will result and texture is required to mask these flaws. In any case, wall thickness variations should not exceed 25%, inside radii should not be permitted to go below 0.020 in, and design details that restrict flow should be avoided. Furthermore, the thinner parts are less rigid at the time of ejection from the mold. Thus, much larger ejector pins must be used than would be used for normal injection molding and there would be more of them. 

Various types of surface finishes are available for plastics and composite parts, such as smooth, textured, molded-in color, and in-mold coating. A textured part surface can be obtained through either a textured mold cavity or a postmold paint process. The former method is the most commonly used. A wide variety of texture designs is available. The surface smoothness, and to some degree the texture, of a plastic or composite is as dependent on the materials used in it as on the surface of the cavity. For example, certain textured surfaces can be obtained only if the proper steel or metal is used in the mold cavity. Also, with any mold the proper cavity steel based on the plastic processed will significantly reduce its wear and tear and extend its useful life [11, 12]. 

Many different textured surface finishes can be produced in GRTP parts by the mold cavity. Textures applied to surfaces opposite projecting ribs and bosses can disguise sinks , surface depressions, or discolorations in pigmented parts. Molded-in styling lines or changes of contour are also effective in masking surface depressions. 

Structures that are typically observed in molded parts and extrudates include anisotropic textures. The higher orientation in extrusion can result in highly oriented rods or strands, at high draw ratios and/or small diameters, or in structures with an oriented skin and a less oriented central core in thicker strands. This skin-core texture is due to a combination of temperature variations between the surface and the bulk and... 

The surface quality and the shape of the mold cavity are transferred to the surface of the molded part. The shape of the article, the position of the possible undercut, and the surface quality, possibly with grained texture, will determine both the choice of the correct molding material and the most suitable production process. Also, the different requirements for processing PE, PP, X-PE, PA, PC, and PVC (e.g., corrosion problems that can occur through hydrogen chloride when manufacturing PVC) should be taken into account in the mold design. 

Often, the cavity temperatures in injection molds have to be increased to avoid, for example, molded part defects such as weld lines or to be abie to exactly replicate surface microstructures or fine-textured mold surfaces. Here, the mold wall temperatures cause long cooling times. 

The requirements on the surface characteristics of the galvanized molds are very different. In dolls parts and toy animals, satin matte surfaces are desirable. To this end, the most strongly undercut galvanized molds are sand blasted with corundum or other abrasives. For textured or smooth surfaces a surface treatment by etching or special blasting with different material mix is necessary. 

As higher temperatures are required to process polycarbonate, heat transfer through the mold to the material can be a problem. The best results are achieved using oil-jacketed molds and finer powder than the 35 mesh (500 tm) commonly used for polyethylene. However, bubbles can still occur in parts and it is common to see textured mold surfaces used to disguise their presence. The use of a nitrogen atmosphere can reduce the yellow color caused by degradation. 

Surface texture can be controlled on the outside surface of the part because the molds are female. Rotationally molded parts generally have a good appearance on the outer surface without sink marks. Matte finish and grains can be readily obtained on the exterior part surface. A glossy finish is difficult to obtain and significantly increases the cost of the mold. Com-... 

Microstructures that are typically observed in molded parts and extrudates include anisotropic textures. The higher orientation in extrusion can result in highly oriented rods or strands, at high draw ratios and/or small diameters, or in structures with an oriented skin and a less oriented central core in thicker strands. This skin-core texture is due to a combination of temperature variations between the surface and the bulk and the flow field in both extrusion and molding processes. For instance, the flow fields in a molded part are shown schematically in Fig. 1.7 [46]. Extensional flow along the melt front causes orientation. Solidification of the polymer on the cold mold surface freezes in this orientation. Flow between the solid layers is affected by the temperature gradient in the mold, and the resulting flow effects [57, 58] result in a rapidly cooled and well oriented skin... 

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