How do you create a rubber component? A blog around and about the world of elastomers

Rubber parts, or elastomers, are irreplaceable in many areas of our lives; yet most people know little about them so it’s fair to say the subject remains vague and mysterious. There are everyday exceptions like car tires, erasers or rubber bands, but for the most part the real importance rubber products have in daily life remains hidden.  It is only when you know more about the diversity and complexity of elastomers that the reason why this field is so interesting becomes apparent. This is, in part, because an elastomer is frequently invisible in an appliance, even though it may be a component vital for its correct functioning and durability. 

In this blog our hope is to open up the subject and provide a new insight for as broad an audience as possible. Every six to eight weeks we will unravel technical terms; let you in on some interesting facts as well as explaining the ins and outs of elastomers – we have a few surprises coming up too. Whether you are new to the world of elastomers or work with rubber parts, either way the blog is here as an information platform and tool to enable you to make more effective decisions in relation to your selection of rubbers and elastomer products.

1 Why rubber?

It may seem a banal question, but why do we need elastomers? The reason becomes apparent when you think about rubber applications, and how elastomers are deployed. Basically, they are deployed when there is a requirement for a material that is elastic or flexible. In other words the material can flex, and if distorted, its high recoil forces bounce it back to the original geometry of the application. 

Rubber’s good mechanical properties are not lost under dynamic stress, or in harsh climatic scenarios. This means that elastomers have a wide range of working temperature.  They are used ubiquitously as seals to stop leakage of liquids, gases or other substances, in or out of their designed area of operation. Elastomers are resistant to many chemicals, fats, oils, acids, alkalis and steam; can be used in extreme weather conditions and are prized for their dampening properties.

Engineers welcome the design freedom the elasticity properties allow them when deciding the geometry of components. For example, undercutting on parts is possible when effective use of elastomers is made, whereas if other materials are used, this is not an option.

In other words, the potential uses for rubber are huge. You'll find examples of applications here.

2 What is an elastomer? The difference between an elastomer, thermoplastic and thermoset

Elastomers are made from raw rubbers with various additives. We will be looking at the different types of rubber and the manufacture of finished molded rubber parts later.

Apart from elastomers; thermoplastics and thermosets, often referred to as plastics, also have a wide range of applications. The basic difference between them is the type of the crosslinking of polymers from which elastomers, thermoplastics and thermosets are made. Polymers are composed of a large number of different (and the same) monomers. The process in which monomers become macromolecules is called polymerization. 

The polymers of elastomers, thermoplastics and thermosets vary in the strength of the linking or networking.  Elastomers have an open, crosslinked structure whereas thermosets are tightly-meshed. In molded thermoplastics there is no crosslinking of polymers.

In practice, the differences between the polymer structures are expressed by the different properties of the materials. Elastomers are elastic, and after stretching or squashing (usually) reconfigure to their original shape. The crosslinking itself is irreversible; in other words, the shape of an elastomer part that has been crosslinked cannot be permanently changed. Equally irreversible is the crosslinking in thermosets. In comparison to elastomers, thermosets are set hard and are not elastic. Thermoplastics, like thermosets, are hard, but with sufficient heat can be permanently reconfigured. In addition there are also thermoplastic elastomers that (to a certain extent) are flexible, but unlike standard elastomers can be reconfigured with the addition of heat. Consequently, the heat resistance of articles made of thermoplastics or thermoplastic elastomers is usually much worse than that of thermosets or elastomeric parts. Also, their dynamic stress capacity is lower. One benefit of thermoplastics (and thermoplastic elastomers) is they are cheaper to manufacture due to shorter production cycle times.

3 From raw rubber to finished rubber moldings

3.1 Development of a rubber compound

A rubber molding and its properties depend on the type of raw rubber used as its base. However, most rubbers are now produced synthetically. An exception is gum/India rubber, which is largely extracted from the sap of the tree "Hevea brasiliensis" in Southeast Asia and South America.

Figure 4: Composition of a rubber compound

The choice of raw rubber is key to determining the properties of the final elastomer molding, in particular its chemical durability and resistance to environmental forces.  However, additional substances are also mixed into the raw rubber to produce the final rubber or rubber compound. Hardness or tensile strength of a compound can be increased with fillers like soot or silicic acid.  Conversely, softeners reduce hardness and increase the elasticity of a material. A polymer is crosslinked, or cured, by adding different substances. Curing with sulphur or peroxide is the best-known method. In addition various processing aids and additives can be introduced to the mix to make manufacture easier and, for example, improve the flow qualities of a raw rubber compound in a mold. There are other additives that in certain circumstances are useful, for example, bonding agents or, more frequently, antioxidants.

3.2 The vulcanization process

A raw rubber compound is almost entirely malleable so it can be formed into any shape. A polymer only takes on the property of elasticity when it has been cured or crosslinked. This process is known as vulcanization. Crosslinking is irreversible. If you want to make malleable base rubber into an elastomer, you basically need a defined temperature, a certain pressure and time. Parameters vary depending on the rubber base, the geometry and mold wall thickness of the final product. The temperature of vulcanization is typically between 140° C and 200° C.    

There are more or less three ways of vulcanizing and manufacturing rubber components: compression molding, transfer molding and injection molding. The basic principles of these three processes are explained here. There are, of course, many variations and subtleties regarding the construction of the mold and the processing, but in general the method is easy to understand.

3.2.1 Compression molding

For compression molding the raw material (blue) is divided between the cavities of the two mold halves (figure 5). The mold halves are shut to make the base rubber compound into the desired shape. The molding is then vulcanized under pressure at a temperature between 140° C and 200° C. After a specified amount of time, the mold is opened and the molding released (figures 6 and 7).

Compression molding is suitable for low to high product quantities and particularly for objects with simple geometries.  It is often used to make rubber bonded to metal parts, that is, when inserts of metal or plastic are sprayed and bonded with an elastomeric material. 

3.2.2 Transfer molding

There are three mold parts needed for the transfer mold: top part, middle part and bottom part. The contours of the article being made are reproduced in the bottom and the middle parts. The top of the middle mold part has a ‘pot’ which is connected by a sprue to the mold cavity. The raw, malleable rubber mix is loaded into the pot (figure 8). The raw material is then pressed into the cavity by a piston in the top mold section. The cavity fills (figure 9). After a certain time the mold opens and the molding can be released (figure 10). The sprue may also be removed.

The transfer molding process is performed mainly for articles with complex surface geometries and for metal-rubber parts. The molds are relatively expensive. Transfer molding is suitable for low to high product quantities.   

3.2.3 Injection molding

Injection molding mainly utilizes a horizontal method - as illustrated (the mold halves open horizontally), but it can also be vertical. The raw material is strip fed to a screw automatically, which then transports the material along a canal to the mold cavity (figure 12). At the end of the process the mold opens and the finished molding can be released (figure 13). One of the advantages of injection molding is that the raw material does not have to be placed in the mold manually. The horizontal production method allows molding to be automated, although suitable means to demold the finished parts may have to be found. Nearly all other elastomers can be used in injection molding. Injection molding is used for making liquid silicone components (LSR, liquid silicone rubber). In this case the material is not transported as a feed strip, but by means of a mixer in the screw (this is because LSR is a two component silicone). Injection molding is ideal for the manufacture of large product quantities. The molds are generally relatively expensive.  

4 Rubber is not just rubber

A variety of raw rubbers, or rubber composites, are used to produce elastomer parts.  Depending on the type of application and requirements, EPDM, NBR, HNBR, butyl, FKM / FPM (Viton ®), FFKM / FFPM (Kalrez ®), solid silicone VMQ, liquid silicone, LSR or fluorosilicone FVMQ may be used. Numerous articles and tables are available on the internet describing the different rubbers and their properties. An excerpt from the spectrum of elastomers is available here, although this list is not exhaustive.

It is tantalizing for users to know that, depending on the constituents of a mixture, quite different properties can be achieved for the end rubber. An EPDM with a hardness of 50° ShA can have quite different properties to a different EPDM also with a hardness of 50° ShA. For example, an EPDM mix can be sulphur or peroxide crosslinked, which affects the mechanical properties such as compression set and heat resistance. The addition of other substances means the rubber compounds can be fine-tuned. In principle, however, the choice of the right base rubber is crucial for the application and adjusting the mix is a refinement process.

It should be remembered that there are many different raw rubbers which, in turn, can create an infinite number of rubber compounds.   

5 Criteria for selecting the best raw rubber

There are so many different raw rubbers the user is faced with that the question of which elastomer is best for their application when selecting an elastomer means a number of criteria have to be considered. 

First the technical and mechanical properties required for the rubber part must be identified and, therefore, demonstrated by the base rubber as being present. For example, is a certain hardness specified? How much elasticity is required before breaking? Or is the compression set critical? It is important to consider whether the rubber part will bear only a static load in the application, or whether it will also be stressed by dynamic forces. You can find more about the mechanical characteristics of rubbers and their testings in the blog entry “Elastomer mechanical properties and how they are measured”. 

In addition, the chemical requirements of the product have to be defined. Raw rubber and ultimately the finished elastomeric articles will differ greatly in their chemical resistance. The more that is known about the media the elastomer will be in contact with and for how long it will be subject to what temperature, the easier it is to select a suitable rubber.

Information as to the function the rubber part is also helpful. When selecting the right material the temperature range the elastomer will experience at the point when the specific characteristics of the component are wanted, is particularly important.  There are materials that can be used at temperatures up to 300 ° C, whereas others fail at 90 ° C. It is useful to know whether the elastomer will be used indoors or outdoors, and whether it will be installed in a device and therefore will be out of sight. This sort of information helps to decide whether the elastomer will be exposed to UV radiation or other environmental influences.

Compliance with regulatory requirements is becoming increasingly important. Depending upon the application, elastomers may need to conform to FDA standards, have USP Class IV approval, or be biocompatible and compliant with the flame protection requirements of the UL Underwriters Laboratories.

If that were not enough, it should also be noted that the properties may be interdependent. An increase of temperature may reduce the compression set or tensile strength value.

As the geometry of the elastomer part is key to its function, it is always recommended that the finished component is tested in the application and subjected to stress testing.

Unfortunately, there is no rubber to fit all criteria and property requirements. Selecting a rubber can be a complex process. In this blog, as time goes on, we will be investigating which elastomers are suitable for which application under which criteria.  For now, you only need to bear in mind that various criteria affect the choice of elastomer and the fact that no two rubbers are the same.

If you have any questions or comments on this blog, or in future would like us to focus on a specific aspect of elastomers, feel free to contact us by email at info@hepako.de    

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