O-ring manufacturing: how O-rings are made

Choosing the right manufacturing process

The requirements placed on seals are diverse: media contact, pressure and temperature loading and service life differ from one application to the next. The portfolio of sealing elements is correspondingly broad. Just as the optimal product can be found for every sealing case, the right manufacturing process must also be chosen for its production.

The production method influences not only the technical properties of the seal but also its cost. For industrial users, resolving this trade-off is decisive.

What determines the choice of process

From an economic perspective, quantities and acceptable delivery times are the deciding factors. On the technical side, workpiece dimensions, the permissible tolerance range and the physical properties are added. Based on the type of seal and the material, our experts already make a preselection.

The following focuses on two classic seals: O-rings and flat gaskets. You will find an introduction to the basic terms in O-ring basics.

Seal types and materials

The O-ring

The O-ring is the most widely used seal. It is a closed ring with a circular cross-section whose dimensions are defined by the inner diameter and the cord diameter. O-rings are used for static (resting) and dynamic (moving) seals. How the dimensions are denoted is explained in the article on O-ring designations.

The sealing effect is established when pressure elastically deforms the cross-section of the O-ring. A surface pressure builds up between the seal and the groove base on one side and between the seal and the sealing face on the other. For the system to work, the groove design and the material selection must be right.

O-rings consist predominantly of elastomers, which exhibit rubber-elastic properties. The raw product rubber acquires this property during vulcanization. The process cross-links the macromolecules, and the cross-links that form allow the material to return to its original position after an external change in shape.

If the media resistance of the elastomers is not sufficient for a demanding application, the property can be enhanced by encapsulation with suitable materials. These, in turn, often lack the elasticity of an elastomer. Frequently used elastomers are NBR (acrylonitrile butadiene rubber), FKM (fluoroelastomer) and FFKM (perfluoroelastomer). These materials are also known under the protected trade names of the respective manufacturers.

The flat gasket

Flat gaskets are used above all to seal flanges, that is, connections with two flat sealing faces that are common in pipework construction. Here the contact area of the sealing element is considerably larger than with the O-ring, and the circular profile cross-section is absent. Through the contact pressure achieved, for example, by a bolted joint, the flat gasket compensates for unevenness in the faces and seals them.

Accordingly, flat gaskets serve almost exclusively for static sealing cases. Together with the frequently aggressive environment in the chemical, pharmaceutical and food industries, this makes materials other than elastomers of interest as well. Flat gaskets are therefore also made from fibre materials or PTFE (polytetrafluoroethylene). For high pressures, metal-reinforced materials are added. Flat gaskets for special applications also take on very individual shapes and dimensions.

Manufacturing processes for O-rings

The manufacture of conventional O-rings and flat gaskets differs fundamentally. What both have in common are the high demands on precision, cost-efficiency and short-term availability. Customer-specific seals must be able to be manufactured quickly in order to avoid expensive machine downtime at the customer's site.

Two processes are mainly used for elastomeric O-rings: the compression process (compression moulding) and the injection moulding process. For particularly large O-rings, butt vulcanization rounds out the range.

Compression moulding (the compression process)

The compression process begins with producing a blank by extrusion. This is placed into a mould and pressed under high temperature into a circular O-ring. After a final deburring step, the O-ring is finished. The process is suited above all to smaller quantities of seals with large dimensions.

Advantages of compression moulding:

• Suitable for small quantities
• Large dimensions can be produced seamlessly
• Lower tooling costs than the injection moulding process

Injection moulding process

For O-rings in standard sizes that the market demands in high volumes, the injection moulding process is more suitable. The starting material acquires its final shape through automatic injection into the closed mould. After ejection, the O-ring is ready for further processing or sale. Here too, high temperatures act on the material to achieve the vulcanization required for elasticity.

Advantages of the injection moulding process:

• Automated production
• Large runs can be produced cost-effectively
• High quality standard

Butt vulcanization of large O-rings

Particularly large O-rings with common cord lengths of up to 3,000 mm call for a less automated manufacturing process. This takes account of the usually unfavourable ratio between demand volume and tooling costs. The most widespread method is the butt vulcanization of extruded cords. The availability of the round cord in various diameters is the only limitation on design freedom here.

For butt vulcanization, the specialist places the cord ends into special fixtures and applies a bonding compound. Temperature and pressure then act to vulcanize the joints, that is, to link the molecules together. Thanks to the superior physical values of the vulcanized joint, this process has largely replaced the bonding of O-rings that was previously common.

Nevertheless, the joint represents a weak point compared with the extruded cord. The larger manufacturing tolerances compared with the other processes are also inherent to the process.

Advantages of butt vulcanization:

• No tooling costs
• Short, flexible delivery times
• Any inner diameter can be produced

The O-ring processes compared

Which process is the right one results from the interplay of quantity, dimension and tolerance requirement. The following overview classifies the three processes.

Manufacturing processes for flat gaskets

The starting material for flat gaskets is usually sheet-form semi-finished material. Depending on the application, it consists of elastomers, fibre materials or multi-layer material combinations. Flat gaskets in a wide variety of contours can be cut from the sheets by die-cutting, plotter cutting or water-jet cutting. The choice of process is made from the standpoints of quantity, delivery time and material.

Die-cutting

For die-cutting, the manufacturer needs a specific cutting die that a hydraulic press drives into the sheet material. Because of the manageable tooling costs, medium to large runs can be handled economically. For customer-specific flat gaskets, the tool lead time is a disadvantage. There is also a technical limitation: with materials of a cellular and soft structure, the yielding of the material can produce a concave cut edge.

Water-jet cutting

A particularly precise alternative is water-jet cutting. It is predestined for materials where mechanical and thermal processes reach their limits. The high-pressure water jet creates the smallest geometries and bores within tight tolerances. Even soft materials can be processed without deformation, and even simple three-dimensional seals are possible.

A further advantage makes the process interesting for small-batch and one-off production: a digital model of the flat gasket is sufficient to generate the commands for the CNC-controlled machine. If the appropriate semi-finished material is available, production can start immediately once the data has been provided. With materials over 100 mm in height, the increasing scatter of the water jet has a negative effect on precision. Within its intended field of application, the process produces workpieces with very thin wall thicknesses and clean edges.

Plotter cutting

Plotter cutting is likewise suitable for small to medium quantities. An electronically controlled, oscillating knife cuts the contour of the flat gasket from the semi-finished material. In keeping with the absence of a specific tool, only low set-up costs arise. One advantage over water-jet cutting is the size range: depending on the cutting table, very large seals several metres long can be produced. Depending on the material, the process is suitable for maximum material heights of up to 10 mm.

Flat gasket: processes compared

Quantity, tolerance and material also determine the process for the flat gasket. The overview summarizes the three cutting processes.

Flexible production and consistent quality

Whether a seal reliably performs its task depends not only on the material selection and the correct design of the sealing system, but also on the manufacturing standards. On all these questions, the experts at NH O-RING advise you and ensure that the seal meets your technical and economic requirements.

For seals with specific geometries or for prototype parts, particular emphasis is placed on rapid availability. Whether flat gaskets as sheet cut-outs, profile cords or moulded parts: short response times are required here. We select the optimal manufacturing process so that you receive the seals as quickly as possible.

You can read how consistent quality is ensured across every batch in the article on quality control.

Frequently asked questions