My friends at Kemstertek, a consulting organization dedicated to Elastomers and its related areas, shared the contents from their e-book ‘Core Rubber Concepts‘.

Rubber in its different forms demonstrates the ability to bond to all kinds of metals. And bonded rubber is stronger. Rubber to metal bonding is a means by which rubber is mechanically bonded to a metal insert during the moulding process. Typical applications for rubber to metal bonding include, any part requiring the combination of the flexibility of rubber and the stability of a metal.

You may have observed that manufacturers across many industries rely on rubber to metal bonding for their components. This is because rubber not only produces a very strong bond on metal, but it also can be used to combine several components into a single assembly. Automotive and industrial manufacturers increasingly are turning to rubber to metal technology to reduce their raw number of components, eliminate vibration and improve the performance of individual components and sub-assemblies used in harsh environment applications.

Examples of Rubber-Metal bonded include small mounts for motors to large locomotive suspension parts, parts used for the isolation of noise and vibration in automotive and engineering applications, larger components designed to decouple translational movement for bridges and buildings.

Rubber-Metal Bonding Process

Briefly, the Rubber-Metal bonding process begins with the inserts that are first prepared for production using a grease removing system to rid your parts of any contaminants before the adhesive is applied. Subsequently, the heated adhesive is applied on to the inserts using a technique similar to your spray painting. Once the metals are prepared, the inserts are then physically placed, one at a time, into each cavity of your mold. Next, the rubber moulding process is started using the appropriate rubber moulding machinery. After the mold is closed, and the moulding begins, the adhesive on the metals is activated, allowing the inserts to bond to the rubber.

Four Steps to Effective Rubber-Metal Bonding

This is a topic from Kemstertek’s book ‘Core Rubber Concepts‘. Read on.

The scope of the following discussion limits only to bonds formed with metal parts while the rubber undergoes vulcanization. This does not cover bonding of vulcanizates with metals at the raw stage or after curing is completed.

1. Part Design and Manufacturability: The geometry of the metal insert should be such that proper rubber filling should happen while molding and the areas adjoining the insert should get enough molding pressure. The mould should be designed for a positive shut-off on the metal insert to cut the amount of flash. Provision should be made to vent off entrapped air or otherwise it will lead to blisters or weak spots at the interface of rubber and metal that will lead to a lack of bonding and premature failure.
2. Metal Preparation: Generally a freshly sanded or grit blasted surface should be sufficient to give a strong bond. If more strength is required, chemical adhesive treatments are to be done. Commercial adhesives are available in two pack systems, a primer, and an adhesive. Proper application as prescribed by the manufacturer in terms of grammage, curing, and ageing are to be strictly followed. Adhesive application or molding should be done immediately as the surface of sand blasted metal is prone to oxidation which will affect the bond quality. For high-performance bonds, the brass coating can be made on the metal surface by electro deposition of copper and zinc followed by diffusion and maturation process to form a durable brass coating. Conventional metal coatings processes like phosphating, galvanizing, and polyisocyanate treatments were also reported to be useful.
3. Adhesion Promoters: Adhesion promoters can play a vital role in the adhesion and sustenance of a bond due to ageing in hot and humid conditions. Cobalt salts like cobalt naphthenate are proven adhesion promoters in tyre skim compounds that come in contact with steel cords. Other additives like silica, organic resin formers, organic sulphides, zinc oxide, and organofunctional silanes are also found effective.
4. Base Polymer and Fillers: Generally polar rubbers are found to be easier to bond than non-polar rubbers like butyl rubber. Sulphur cured rubbers are easier to bond as sulphur is believed to interact with some of the added bond promoters in the recipe. Fillers are to be carefully selected so as to reduce the differential stress at the interface. Excessive loading of fillers reduce the rubber hydrocarbon percentage at the interface and is not advisable. Epoxy modified natural rubber as an adhesion promoter is found to improve the adhesion properties.

Reach out Kemstertek on team@kemstertek.com

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