The purpose of this section is to list and discuss some of the factors that have an effect on the finish and appearance of molded products and to present standards covering four classes of finish.


Machined Finish of Mold

The machined finish of the mold has considerable effect on the surface finish or appearance of a rubber product.

The best finish can be obtained from a highly polished steel mold, free from all tool marks or other imperfections. Naturally, this type of mold is quite expensive to construct and maintain and is not generally required unless surface finish is of paramount importance from either an appearance or functional standpoint. In addition, it may be desirable in some cases to chrome plate the mold in order to maintain the required surface finish under production conditions.

The commercial type mold is a machined steel mold made to conform to good machine shop practice. Machine tool marks will not ordinarily be polished out of this type of mold. It should be noted that regardless of how highly the mold itself is polished, the appearance of the rubber surface will depend to a large extent upon the factors outlined in the following paragraphs.

Type of Rubber Material Used

The type of rubber material used can greatly affect the appearance of the rubber product. Some compounds lend themselves to a bright glossy surface while others may be dull as molded or become dulled very easily during handling or storage. Also, there are some rubber compounds to which antiozonants are added to impede attack from ozone. As these compounds age, the antiozonants "bleed out", giving the product a colored or waxy surface. This is a common practice and the product should not be considered imperfect or defective in any way. This or other specification requirements may make it impossible to produce a product with a glossy surface.

Mold Release Used

There are certain rubber compounds that can be removed from the mold with the use of little or no mold release lubricant, while others require the use of considerable quantity of mold release lubricant. The latter may have the appearance of being oily.

If the surface of the rubber product is to be bonded to other materials in its application or is to be painted, the designer should designate this on the drawing so that the manufacturer may use a mold release lubricant that will not impair adhesion quality.

Flash Removal Method

Some of the many methods used to remove flash from rubber parts may affect the appearance of the finished product. As an example, hand trimming will ordinarily have no effect, while tumbling may result in a dull surface.

Method of Designation of Finish

The symbol "F" followed by an appropriate number selected from Table 6 shall be used to designate the type of finish required.

An arc enclosing the actual area included by this designation and a leader to the finish number designates the type of finish desired. The use of a finish symbol on the surface does not preclude the possibility that other surfaces may require different finishes. However, the use of the standard notation is desirable wherever possible to eliminate the repetition of finish symbols and maintain simplicity. See Fig 10.

Always permit "Commercial Finish" (F3) whenever possible.

Table 6
Drawing Designation Description
F1 A smooth, polished and uniform finish completely free of tool marks, dents, nicks and scratches as produced from a highly polished steel mold. In areas where F1 is specified, the mold will be polished to a surface finish of 10 micro-inches (250nm) or better.
F2 A uniform finish as produced from a polished steel mold. In areas where F2 is specified, the mold will be polished to a surface finish of 32 micro-inches (800nm) or better but with very small tool marks not polished out.
F3 Surfaced of the mold will conform to good machine shop practice and no micro-inch finish will be specified. This is "Commercial Finish".
F4 Satin Finish.
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