Rubber is among the most reliable materials in many industries for its elastic properties. Metals are also useful for their rigidity properties. When the two materials are fused, engineers create hybrid products that are both elastic and rigid. In this article, you will learn about the process of creating these hybrids through a process known as rubber-to-metal bonding.
What is Rubber Metal Bonding?
Essentially, rubber-to-metal bonding is a process whereby rubber materials are mechanically bonded to metals. This might sound simple but the process is somewhat complex. The complexity is to ensure that the bond created is strong enough as desired.
Metal and rubber are not compatible in their natural form. To clarify, this means you would not ‘glue’ metal to rubber without manipulating the surface of the materials. This is why the rubber-to-metal bonding process includes a primer and adhesion substance to activate the bonding process of the two materials.
Technicians involved in this process have a deep understanding of the mechanical and chemical properties of rubber and rubber. This knowledge is essential since they rely on it to determine the correct parameters to use in the bonding process.
At the end of the process, the rubber-to-metal bonded components have several properties that are reliable in a diverse range of applications. The products have both rigidity and elastic properties of metal and rubber, respectively. Exploring the whole process, methods, and techniques will help you understand more about these end products.
The Rubber-to-metal Bonding Process
In this 5-step process, we detail what happens from the moment the rubber and metal materials to be bonded are selected. The process has a balance between mechanical and manual steps to ensure effectiveness and quality results.
Step 1 – Preparing the rubber
One thing to note is that not all rubbers can be used in the rubber-to-metal bonding process. This is because there are specific elastomeric properties that are required for a successful bonding that is not found in some rubber materials. Therefore, natural rubber and a few synthetic elastomers such as neoprene are used.
The preparation of the rubber involves making sure the surface of the rubber is free from residues that can prevent adhesion. These residues include oils and grease. Also, this step assumes that the rubber has already undergone the vulcanization processes to give it the desired properties.
Cleaning is among the most common methods of removing these unwanted deposits. Rubber is subjected to appropriate cleaning to remove these residues. The reason we are using the term appropriate is because some cleaning methods can affect the quality and mechanical properties of rubber. For instance, some rubbers cannot withstand cleaning through steam jets.
In this step, cleaning agents such as degreasers and non-corrosive alkaline solutions are used. When the technician has verified that the surface is clean, they dry the rubber and store it appropriately in readiness for the molding process.
Step 2 – Metal surface preparation
Like rubber, not all metals can be used in the rubber-to-metal bonding process. Metals with certain specific properties are selected and prepared for the bonding process.
The preparation of metal is similar to that of rubber. All unwanted materials on the surface of the metal are removed using the appropriate methods. The reason these residues need to be eliminated is because they can negatively affect the quality of the bond created.
Metals contain rust. This substance can be a challenge when trying to bond metal to rubber, and so it must be removed. Since it cannot be dissolved, rust is removed using methods such as sandblasting or machining. While these methods remove the rust, they also create a rough surface which is helpful in the bonding process.
In the metal surface preparation, sometimes the metal is dipped in a phosphate solution. This dip is primarily meant to remove oils and grease on the surface of the metal. Also, the phosphate prevents rust from further forming on the surface down the road.
Step 3 – Application of Primer and Adhesive
This is among the most crucial steps in the whole process because the primer and adhesive are responsible for facilitating the bonding. Without these two, the molding process would not yield a good enough bond.
A primer is a substance that is used to prime or prepare the surface for adhesion. Primers contain chemicals that treat the surface of the substrates, thereby improving their bonding properties. These compounds also prevent contamination of the clean surface by creating a protective coat on the surface.
On the other hand, an adhesive is a chemical compound with bonding properties. They can also be referred to as glue. Adhesives have cohesive and mechanical qualities required to form a bond between two materials. These qualities can be activated either by vulcanization or by the pressure forces. The common types of adhesives include epoxy, polyurethane, and cyanoacrylate.
The application of the primer and adhesive is done in a controlled environment to avoid contamination. Also, it is done right before the molding process starts.
Step 4 – Molding process
In the rubber-to-metal bonding process, the molding processes used are similar to those used in the regular molding of rubber. However, there can be slight modifications depending on the specifications and requirements of the bonding.
The most common molding processes used are injection and transfer molding. Sometimes, compression molding is also used.
Molding is the method used to combine the two substrates — rubber and metal — under certain conditions. The two are placed in a molded cavity and additives such as vulcanizing agents and catalyzers are added.
Injection molding uses a liquid form of the rubber material. Under high pressure, this liquid is transferred to a cavity containing the metal part. Then, this cavity is subjected to heat which facilitates the curing process to make the rubber hard, but elastic. The transfer method utilizes solid rubber and it is more effective for complex parts.
Step 5 – Testing and quality control
The last step involves testing the final product. This involves conducting tests to evaluate the bond created, the properties of the materials, and the overall quality of the product.
Testing and quality control are necessary steps in the whole process. Without them, it would be hard to verify that the product meets your expectations. There are test standards that we follow to ensure that we are operating according to international regulations.
One of the most common tests done is called the load deflection test. This test ascertains that the spring rate is as expected. In other words, it confirms that the rubber is not stiff but elastic enough to maintain the expected properties.
These tests are also done to facilitate the certification of the product. This means that the products have to be tested before they can be declared safe, high-quality, and compliant with industry standards. After this, post-production processes such as sorting, packaging, and distribution are carried out according to the factory workflow.
Controlling the Rubber-to-Metal Bonding Process
The techniques used in the above process vary depending on the manufacturer. Other factors such as the type of rubber component are also taken into consideration. As such, these methods will be different across the rubber industry.
Methods like injection and transfer molding are used for their effectiveness. In rubber processing, these techniques are prioritized. This is because they produce some of the best results in terms of quality. Other methods can be used, but the results could be subpar.
Controlling this process involves picking out the compounds, agents, and other useful materials beforehand. Essentially, manufacturers make sure that everything that is needed for the process is vetted and verified. This is done to ensure that everything used meets the required properties or qualities. This is what contributes to success during and after the process.
Adhesion/bonding Agents Used
As we learned in step 3, adhesion agents are used to activate the bond formation properties of the rubber and metal. Therefore, they are vital additives. While there are many materials that can be used as adhesive agents, there are a select few that are used in the rubber-to-metal bonding process. They include;
Cyanoacrylate Instant Adhesive
The common name for cyanoacrylate instant adhesive is super glue. It is a one-part adhesive agent that is activated by moisture. The phrase one-part adhesive means that the compound is used as is without the need to activate it with other agents.
Cyanoacrylate is used for its unmatched adhesion properties. The reason why it is specifically used is because it facilitates bond formation for materials such as EPDM rubber which has a hard time bonding with metals. This adhesive forms a strong bond between the substrates and removing the bond requires machining techniques.
Epoxies
Epoxy is a polymer created by combining a resin and a hardener. When the two are mixed, they create a cross-linked material that can be used as an adhesive after curing. This substance has quite remarkable bonding properties in its cured state.
Epoxies are two-part adhesives. Unlike cyanoacrylate, epoxies have multiple components making up their molecular structure. The good thing about them is that you can add other additives such as dye and their properties will not be affected. Apart from that, epoxies possess excellent adhesion properties, making them ideal for rubber to metal bonding.
Other Adhesives
Some manufacturers can formulate special adhesives and bonding agents for their own use. These special adhesives can be created to possess specific properties as required. The reason this is an option is for the creation of custom rubber-metal components. Also, there are rubber and metals that are not compatible with cyanoacrylate and epoxies.
How Bonding Agents and Primer are Applied
The application of the bonding agents and primer are controlled process. One of the aspects that is controlled is the application methods used. This is because the method used has an influence on the bonding of the substrates.
Generally, there are four methods that can be used to apply these substances on the surface of the rubber and metal. Each method has its own mechanism and standards that are adhered to during the application process. These methods are;
- Spraying
- Dipping
- Brushing
- Rolling
The choice of the method to be used depends on the nature of the compound, the bonding technique used, and in-house standard operating procedures. Luckily, each of the four methods is guaranteed to create a clean surface for the bonding agents.
Factors that Influence the Bonding Agents Used
The choice of the bonding agent used depends on a number of factors. Remember a bonding agent is just another name for the adhesive. Here are some of the factors that influence the choice of the adhesive.
Product Volume
In most cases, the choice of the bonding agent is influenced by the product volume. This means that the number of products being created using rubber-to-metal bonding. The reason why this factor is considered is because it has implications on the cost and efficiency of the whole process.
For small-volume manufacturing, adhesives that are cheaper and readily available are used to help with the economies of scale. The same can be applied to large-volume production. However, most manufacturers opt for more affordable alternatives in cases where there is large production. This helps in realizing more returns.
Manufacturing Space
Apart from the choice of adhesive, the manufacturing space influences other choices such as the molding technique and type of substrate. If you think about it, smaller spaces tend to have a lot of constraints which can affect productivity. Therefore, choosing the most applicable option helps alleviate these constraints.
Price of Product
Realistically, you would not expect the manufacturer to use premium adhesives for everyday components. It would not make sense for business. However, the fact is that most of the adhesives used create quite strong bonds, premium or not.
Type of Metal
Technically, there are metals that do not react well with certain adhesives. No matter how refined the surface of the metal is, the bonding agent will not bind.
In such cases, rubber manufacturers have to be thorough in selecting and identifying the bonding agent to use. This could involve experimenting with different kinds of adhesives to determine the most suitable. Alternatively, some manufacturers can also formulate new bonding agents or reinforce the existing ones to make them usable.
Type of Rubber
Like metal, some rubbers have a hard time bonding with select adhesives. Some adhesives can affect the properties of rubber while others fail to stick to the material completely. In such situations, the manufacturer experiments with a few choices before deciding. This step is important since it contributes to a positive outcome by the end of the process.
Curing Time
The curing time is the amount of time the adhesive takes to stick onto the rubber or metal substrate. Also, when the materials have been bonded in the molding processes, they are allowed to cure.
These two periods are critical and they influence the choice of the bonding agent. Some adhesives take more time while others take less to cure. Taking these considerations into account, the rubber manufacturer decides the best option based on the turnaround time of the project.
Techniques for Bonding Rubber to Metal Bonding
So far, we have referenced a few of the techniques used in this process of creating rubber-metal hybrids. However, we have not gone into details and explained how they occur. Let us learn some of the common techniques.
Over-molding
This technique involves the application of rubber on top of metals, just like the name implies. Over molding is a special type of injection molding. The resultant component from this process has several desirable properties.
Overmolding happens in one of two methods;
- Two-shot molding – This method uses a single mold
- Pick-n-place molding – Uses two molds
Both two-shot and pick-n-place techniques are automated and highly controlled. However, the specific control techniques used in the methods will vary. Both methods are used to improve vibration dampening, impact resistance, and grip.
Complete encapsulation/insert molding
For this technique, the metal part is completely covered by rubber material. It creates a hybrid where the metallic part is completely unnoticeable. In most cases, complete encapsulation is used in electronics to create wires and cables. This technique is rather straightforward and effective, unlike over-molding.
Complete encapsulation can be used in applications where the rubber is used to protect the metal on the inside. Also, it can be for rubber products that require the strength of the metal. In either case, this technique is quite effective.
Transfer molding
This is among the most common rubber-to-metal bonding methods. As we indicated earlier, it involves solid rubber and a cavity. During the possess, the rubber is heated to a melting temperature to facilitate the bonding.
The transfer molding technique is highly used because it is quite effective. It is also fast which is a desired quality for high-volume production. Lastly, this technique is relatively affordable since it does not consume a lot of resources.
Injection molding
The final technique that you need to know about is injection molding. In this process, liquid rubber is pumped into an open cavity under high pressure. The liquid joins the metal that is preloaded into the cavity and the curing process starts. Injection molding is also quite effective but more expensive than transfer molding.
Metals that can be bonded with rubber
Metallic compounds have quite specific properties that are not found in other compounds. For that reason, the bonding capabilities of metals are limited to specific metals. These metals can bind with rubber and have specific properties.
Some of these properties include;
- Good mechanical properties such as strength, durability, and stiffness.
- Good thermal resistance. Since rubber is used in areas prone to high temperatures, this property is quite essential.
- Machinability. This is the ability of the metal to be cut, shaped, hammered, and manipulated easily. It is an important property.
- Highly available. The availability of these metals is high in most parts of the world. More importantly, they exist freely in nature.
- Low weight. Weight is a consideration that is considered since it controls the final weight of the component.
The most common metals used in the rubber-to-metal bonding process are
- Copper
- Aluminum
- Brass
- Beryllium
- steel
While brass is on this list for other properties, it has some complexity when dealing with it. Brass contains lead in its molecular composition. Lead can affect the bonding capabilities of materials negatively, making brass harder to machine. To counter this problem, low-lead content is used in the rubber to metal bonding process.
Factors that influence the type of metal substrate used
The choice of metal can be influenced by a few factors. They include;
Ability to withstand the rubber molding process
The rubber molding process involves several additives and conditions that can destroy certain materials, including metals. As such, metals that have inferior properties such as low boiling points cannot be bonded with rubber. The metal needs to be able to withstand the high vulcanizing temperatures in the mold.
Strength and durability requirements
The reason why metals like steel and aluminum are used in the rubber to metal bonding is that they are strong. In particular, these metals are not easily degraded by pressure and other mechanical stressors.
Durability on the other hand refers to the amount of time the metal can last without becoming degraded. Generally, this involves the ability to resist rust or not get affected by it. Rust is among the most notorious compounds that degrade metals. However, the phosphate applied in the second step of this process prevents further formation of rust.
Weight considerations
Heavy metals are generally not suitable for rubber-to-metal bonding. This is because they increase the weight of the final product. Also, heavy metals are challenging to handle in the manufacturing process. Therefore, metals that have manageable weight are mostly prioritized, as long as they meet other requirements.
Cost-effectiveness
In any business, cost is a major factor that affects decision-making. Different metals cost differently depending on various factors. That is just a fact. To ensure the best value for money, manufacturers choose metals that are more affordable for mass production. However, premium metals can also be used in custom productions.
Customer specifications
If the customer specifically states that they want aluminum or steel, that is what the manufacturer uses. This is because the customer has their reasons for specifying the metal. If you are unsure of what metal you should use, the manufacturer can also help you narrow down during the consultation phase of the project.
Benefits of Rubber to Metal Bonding
Working with rubber is quite rewarding. The material has several properties that we rely on as humans every day. Adding the properties of metals to the existing rubber properties means creating a powerful and highly reliable product. This is one of the major benefits of rubber-to-metal bonding. The other major benefits of rubber-to-metal bonding include;
- Improve vibration dampening. This means that the end product of this process has the ability to absorb mechanical shock. Vibration dampening is required in many applications where structural integrity and performance are crucial.
- Better sealing. Rubber is used to make several types of sealants, including o-rings. When these components are reinforced with metal, they become better sealants.
- Corrosion prevention. In this case, the rubber is used as a shield for the metal. Rubber is known to have a high resistance to corrosion. Therefore, it effectively protects the encapsulated metal against corrosion.
- Electrical insulation. Rubber is also known to have insulation properties. Therefore, it can be used as an insulator for the metal. This is beneficial in applications such as electronics.
Application of Rubber to Metal Bonding
If you think about it, there are many industries that can benefit from rubber-metal hybrid products. This process creates such capable products that any regular products can emulate. In other words, most of the existing products can be molded with metals for various reasons.
However, the reason that would not be practical is the cost implications and manufacturing complexities. Therefore, the rubber to metal bonding is mostly reserved for niche applications and use cases.
Common metal-bonded rubber components
Currently, the majority of the metal-bonded rubber components in production fall into a few categories or industries. These are mostly areas where rubber is currently being used in high volume. These industries include medical, automotive, aerospace, construction, and agriculture.
All these industries need components that can exhibit the strength and elastic properties of the rubber and metal hybrids. In that case, some of the common products include;
- Ball joints
- Suspension bushings
- O-rings and seals
- Engine mounts
- Conveyor belts
- Medical equipment isolators
- Seismic dampers
There are more products or components that can be molded with metals but these are the most common of all. Moreover, they vary depending on the manufacturer.
Conclusion
Rubber-to-metal bonding is a complex and integral process in the rubber industry. Its success in creating rubber-metal hybrid components has improved the reliability of rubber in several industries. We hope that this comprehensive guide has outlined the process, control measures, and all underlying factors.