RDI Reference

Introduction

The process of adhering rubber to metal components are widespread techniques in the rubber industry. Generally the same considerations and procedures are applicable for rubber to rigid non-metallic components, but the adhesion values may be lower. Only the broad aspects of rubber-to-metal molding are covered here, and more precise information can be provided by the rubber manufacturer involved.

GENERAL INFORMATION

Application

Various adhesion levels can be obtained. For instance, to obtain adhesion on critical products, such as motor mountings, very close controls are usually required, both on metal and rubber preparation. With less critical products, such as some pedal pads and closures, which require only enough adhesion for assembly, close controls are not necessary.

The adhesion level is directly affected by types of metal, metal surface preparation, non-metallic inserts, compound variation, compound tensile strength, and type of elastomer.

Drawings should clearly state adhesion requirements and any other factors which can explain the degree of adhesion required and the method of testing. A clear understanding between customer and rubber manufacturer is essential.

Methods of Obtaining Adhesion

The method most commonly used to obtain adhesion between rubber and metallic or non-metallic components is the use of adhesive cements. Prior to the use of these special adhesives, the surface of the insert must be clean and free of contamination.

The inserts may be prepared by suitable methods such as degreasing, blasting, and/or a suitable chemical treatment. When any one of these preparatory processes is objectionable, it should be noted on the drawing. The rubber compound is then vulcanized to the prepared inserts to obtain the desired adhesion.

Design Factors and Limitations

(A) Avoid localized stress raising irregularities

(B) Minimize edge effects. Break, coin, or otherwise eliminate sharp edges of all metallic members covered by the rubber. Provide fillets in the rubber at junction line with inserts where possible. Where fillets are not possible, extend the rubber beyond the edges of the inserts which would otherwise terminate line to line with the rubber.

(C) Minimize surface roughness of metallic members in area of adhered rubber.

(D) Avoid welding a molded rubber component to a machine or structure to prevent unnecessary heat deterioration. When welding is mandatory, design metallic member as a heat sink and provide for assembly techniques which will keep the adhered rubber area of the metallic member below 302 deg. F.

Test Methods for Determining Adhesion Values

Adhesion testing is done in several ways, depending upon the application and the product design. The methods recognized for this testing are treated in full detail in ASTM Test Method D 429. These methods are:

Method A: Rubber adhered between two parallel metal plates.

Method B: 90 deg. stripping test, rubber adhered to one metal plate.

The above methods are used primarily for laboratory development and testing production parts. These methods may be modified and applied as described under RMA Production Test Methods section as follows.

RMA PRODUCTION TEST METHODS

Method A: Used where two metal surfaces, not necessarily parallel, can be separated until the specified adhesion value is obtained using the projected adhered area. The area to be considered should be the projected active adhered working area of the smallest metallic member, excluding fillets, overedge, and radii. Very irregular areas are to be given special consideration.

Method B: Used where the rubber can be stripped from the entire width of the part to obtain a specified adhesion value or where the rubber can be cut in 1 in. wide strips. Specimen rubber thickness shall not exceed 3/8 in. In rubber sections over 3/8 in., values should be negotiated between customer and supplier.

Acceptance Criteria

Looseness contiguous to the adhered areas at corners, fillets, mold parting lines, and back-rinding will ordinarily be acceptable.

The adhesion strength is usually considered to be satisfactory if the failure causes permanent distortion of a metallic member.

If the deformation of the rubber section under test far exceeds the functional service requirements, this factor should be taken into consideration when establishing a reasonable adhesion value.

It is recognized that conditions for adhesion will exist where a quantitative value cannot be obtained. In these instances, it is customary to pull the rubber from the metallic member and examine the nature of the failure. The acceptable degree of adhesion must be agreed upon between the customer's test methods and fixtures should be identical with those of the rubber manufacturer and correlation procedures established.

Methods of Designating Adhesion Values

The design engineer when writing specifications, should use a designation to obtain suitable adhesion for the purpose intended.

Methods of testing, such as tension pull or shear pull or 90 degree stripping and the minimum destruction values, as well as the design of special testing fixtures should be specified on the drawings. ASTM D2000 - SAE J200 has two types of adhesion designations for adhesion of vulcanized rubber to metal.

(A) Adhesion by vulcanization, designated by K11 or K21.

(B) Adhesion by the use of cements or adhesives after vulcanization, designated by K31.