WO2012052485A1

WO2012052485A1 - Piston for a pneumatic system

Info

Publication number: WO2012052485A1
Application number: PCT/EP2011/068274
Authority: WO
Inventors: Joachim Moeschel
Original assignee: Seal-Mart Engineered Plastics Kg
Priority date: 2010-10-22
Filing date: 2011-10-19
Publication date: 2012-04-26

Abstract

A piston (100) for a pneumatic system, wherein the piston (100) comprises a base body (102) having a bore (104) for receiving a piston rod, two sealing rings (108, 110) on opposing main surfaces of the base body (102), and a magnetic ring (106). The base body (102) is made of a hard plastic material.

Description

Piston for a pneumatic system

The invention relates to a piston for a pneumatic system .

Moreover, the invention relates to a pneumatic system .

The invention further relates to a method of manufacturing a piston for a pneumatic system . Pneumatics may denote the technology of using a pressurized gas to affect a mechanical motion. Pneumatics is that branch of technology which deals with the study and application of use of pressurized gas to affect mechanical motion. Pneumatic power is used in different field of industry, where factory machines are commonly plumbed for compressed air, other compressed inert gases can also be used. Pneumatics also has applications in construction, mining, and other fields.

In a pneumatic tube, a piston may be arranged which may be coupled with a piston rod. The piston may reciprocate within the pneumatic tube thereby either acting on gas in a surrounding or being actuated by such gas.

EP 0,631,057 discloses a seal for a piston operated by a fluidic medium, containing a supporting body of non-magnetisable material, which at least partly encloses an annular permanent magnet and is connected to at least one sealing lip of elastomeric material, in which arrangement the sealing lip projects on the outside beyond the

supporting body in the radial direction and can be brought into sealing engagement with the inner wall of a housing surrounding the piston. The supporting body is of one-piece design and is connected to the

permanent magnet.

However, conventional pistons for pneumatic systems which provide sufficient technical performance are complex and expensive in manufacture. It is an object of the invention to provide a complete piston for a pneumatic system which is safe, reliable and accurate in operation and which is manufacturable with reasonable effort.

In order to achieve the object defined above, a piston for a pneumatic system, a pneumatic system, and a method of manufacturing a multifunctional piston for a pneumatic system according to the independent claims is provided.

According to an exemplary embodiment of the invention, a multi- functional piston for a pneumatic system, particularly for double-acting pneumatic cylinders, is provided, wherein the piston comprises a base body based on rigid plastic material having a bore for receiving a piston rod, and at least two sealing rings on opposing main surfaces of the base body which have a damping function, i .e. act as an integrated bumper stop (alternatively, it may be possible to provide only one sealing ring on one of the opposing main surfaces of the base body or circumferentially surrounding the main body).

According to another exemplary embodiment of the invention, a pneumatic system (such as a pneumatic cylinder) is provided which comprises a piston having the above-mentioned features.

According to still another exemplary embodiment of the invention, a method of manufacturing a piston for a pneumatic system is provided, wherein the method comprises forming a base body with a bore for receiving a piston rod, and moulding, in a separate process, two sealing rings on opposing main surfaces of the base body.

According to an exemplary embodiment of the invention, a piston is provided which can be used as a complete piston for pneumatic applications. Such a piston shall be adapted for low friction reciprocation within a housing such as a hollow cylinder. It shall at the same time be properly sealed with regard to an environment so as to allow to operate a fluidic medium such as a gas like air to actuate the piston, or vice versa. Simultaneously, it should be possible to manufacture such a complete piston with reasonable effort. According to an exemplary embodiment of the invention, this is made possible by integrally forming a base body from one piece which only needs to have two sealing rings on opposing main surfaces of the for instance hollow cylindrical base body. Such a geometry can be achieved, according to an exemplary embodiment, with low effort by first forming the base body, for example using a moulding procedure, subsequently functionalizing a surface of the base body for promoting adhesion of a sealing ring formed thereafter. In contrast to conventional approaches, such a functionalization may be simply obtained by exposing the base body to plasma which activates the surface and allows for a formation, for instance moulding, of the sealing rings so that they are automatically attached to the base body by form closure.

In the following, further exemplary embodiments of the piston will be explained. However, these embodiments also apply to the pneumatic system and to the method.

In an embodiment, the piston may comprise an annular magnet which may be embedded in the base body so as to be only partially surrounded by the base body and to be partially exposed to an

environment. This configuration allows for a very simple manufacture, since the angular ring needs only be placed on a bottom surface of a moulding chamber, and subsequently the base body may be moulded onto the annular ring, thereby automatically forming a connection.

Furthermore, by maintaining the surface of the annular ring exposed to an environment, any undesired shielding by magnetic particles within the base body may be safely prevented. Furthermore, the surface of the base body may be a suitable position for such a magnet which may be used in a magnetically controlled piston for position detection purposes and/or for actuation purposes. Furthermore, a ring magnet having an exposed surface may be easily magnetisable after the manufacturing procedure, since the distance between a magnetizing force such as a coil on the one hand and the magnet to be magnetized can be maintained very small .

In an embodiment, the magnet may comprise a rare earth material . Particularly using a rare earth ceramics material for the magnet allows to manufacture the piston with a simple manufacturing procedure. A rare earth material is not very prone to failure even under harsh conditions. Furthermore, rare earth materials are appropriate for being over-moulded with the base body material. The base body should be made of a non-magnetisable material if a magnetic element is embedded therein, for instance for position detection purposes. As an alternative to a rare earth magnet, it is also possible to use other kind of magnets such as a ferrite magnet. Still other embodiments do not have any magnet at all.

In an embodiment, the base body may comprise or may consist of a plastic material . Using a thermoplastic material may be preferred. For example, Polybutylene terephthalate (PBT) has turned out as a powerful material which is mechanically robust and can also be used as a lateral guiding surface for guiding the piston within a cylindrical tube.

Polybutylene terephthalate is a thermoplastic crystalline polymer, and a type of polyester. PBT is resistant to solvents, shrinks very little during forming, is mechanically strong, and is heat-resistant up to 150°C or more. Moreover, such a material is properly combinable with

thermoplastic polyurethane (TPU) which may be advantageously used for the sealing rings which are to be formed on the base body. Plastic material may be preferred for the base body since this allows to form the base body by a moulding procedure. In an embodiment, the at least one of the two sealing rings may comprise or may consist of a thermoplastic polyurethane. The present inventors have surprisingly recognized that such a material is particularly properly appropriate for combination with Polybutylene terephthalate when forming a piston. Hence, a thermoplastic polyurethane may be used for the sealing ring or rings in contrast to conventional plastics. It is believed that thermoplastic polyurethane is less prone to unintentionally adhering to other surfaces.

This particularly holds when the Shore hardness (A) of such a thermoplastic polyurethane is 82, or softer. Apart from the proper compatibility (particularly adhering property) with a hard thermoplastic material such as PBT for the base body, such a TPU can be formed sufficiently flexible to provide for a reliable sealing function in a

pneumatic cylinder.

The base body may comprise a tubular (or hollow cylindrical) structure enclosing or surrounding the bore, wherein the hollow

cylindrical structure may have indentations on opposing surfaces for receiving the two sealing rings. Such indentations may be

circumferentially aligned annular recesses in which a part of the material for forming the sealing rings can be injected by moulding. Such a geometry has turned out to provide for a proper sealing and is at the same time sufficiently stable.

The above-mentioned magnet may be embedded in one of the indentations partially in contact with the base body and partially in contact with one of the sealing rings. In other words, the magnet may be sandwiched between base body and sealing material so as to be protected via undesired influences of the environment.

In an embodiment, the indentations may have an undercut. An undercut may be denoted as a type of recessed surface. In molding it may refer to a feature that cannot be molded using only a single pull mold. By providing an undercut, a mechanical protection against an undesired removal of a sealing ring may be provided, for instance when high forces are exerted. Thus, the sealing rings may be safely prevented from being disassembled from the base body by such an undercut even during fast or frictional motion . The undercut can also have an annular structure and can be formed on one or both sides of the surface of the base body.

Still referring to the previously described embodiment, the magnet may be embedded in one of the for instance two indentations. Therefore, the indentation may fulfil two purposes at the same time. It may accommodate the sealing ring and it may accommodate a magnetic element used for magnetically controlling or actuating the piston.

According to an exemplary embodiment, the piston may comprise a circumferential guide surface for guiding the piston within a pneumatic tube or housing. In an embodiment, the circumferential guide surface may be integrally formed with the base body. In other words, the guide surface may be formed simultaneously with the formation of the base body, for instance may be formed by moulding. Hence, it may be dispensable to provide a separate guide surface which allows to manufacture the piston with reasonable burden. It has turned out to be possible to provide one and the same material for forming the base body and the guide surface without losing performance. One preferred example for such an appropriate material is Polybutylene terephthalate. By adapting the base body so that it at the same time may fulfil the functions of a support body and may also provide an exterior gliding surface or guiding surface, it is possible to manufacture the piston with reasonable effort, since a separate annular guide ring is dispensable.

At least one of the two sealing rings may comprise at least one damping element adapted for damping a mechanical force acting on a front face of the piston . Such flexible or elastic damping elements may be provided to prevent undesired damage or other deterioration of a front surface of a piston when abutting against a cooperating surface of a pneumatic system or the like. A damping element may be made of a thermoplastic polyurethane material and may be integrally formed with the remainder of the sealing ring.

In an embodiment, the at least one damping element may be shaped as a plurality of circumferentially arranged arcuate protrusions being circumferentially spaced from one another by a gap. For example, three damping elements may arranged each having an angular section of about 110° with a section of 10° between adjacent arcuate protrusions.

In an embodiment, at least one of the two sealing rings may comprise an annular sealing lip for circumferentially sealing a gap between the piston and a surrounding pneumatic tube. Such a sealing lip may be an end portion which, in a lateral direction, i .e. in a radial direction, extends further as compared to the guide surface of the base body. Thus, such a sealing lip may be in sliding frictional contact with the wall of a cylinder or housing in which the piston may reciprocate under the influence of a pneumatic force, thereby providing the sealing function.

In an embodiment, a single side operating sealed piston is provided, wherein another embodiment provides a double-side sealed piston.

The sealing rings may be adapted for an air-tight reciprocation within a housing such as a hollow cylinder.

In an embodiment, an outer diameter of the piston may be in a range between 20 mm and 50 mm . Particularly, dimensions of 25 mm, 32 mm and 40 mm are possible. In the following, an exemplary embodiment of the pneumatic system is explained. However, this embodiment also applies to the piston and to the method.

In an embodiment, the pneumatic system may be a pneumatic cylinder. Such a pneumatic cylinder may comprise a housing in which a piston as described can be arranged for reciprocating. A piston rod may be guided through the central bore of the piston so as to form a piston- piston rod arrangement. Such an arrangement may then be positioned in a cylinder for reciprocating under the influence of air, or any other fluidic medium such as a gas. For this purpose, a fluid inlet and a fluid outlet may be arranged at the cylinder. It is also possible that magnetic sources and/or detectors are arranged at dedicated positions along the cylinder. A magnetic source may exert a force on the magnet for controlling purposes or may detect a position of the piston by a magnetic detection signal .

Based on such a pneumatic cylinder, it is possible to constitute a pneumatic valve which can be operated with compressed air or another fluidic medium . It is possible to implement such pneumatic valves in an automotive production line, a food production line, a packaging

production line, a clean room production line or a semiconductor production line. The pneumatic system according to an embodiment of the invention may be suitable for these and other purposes, since the described piston is compatible with the strict requirements for such environments with regard to hygiene, cleanness, biocompatibility, and reproducibility.

In the following, further exemplary embodiments of the method will be explained. However, these embodiments also apply to the piston, and to the pneumatic system . The annular magnet may be embedded by insert moulding using a suitable thermoplastic material (which provides a sufficient hardness when solidified), followed by insert moulding of another thermoplastic material (providing a softer material when solidified) for forming the sealing rings. Thereby, the base body and the sealing rings may be rigidly connected to each other.

In an embodiment, the method may further comprise exposing the base body to a plasma treatment before moulding the two sealing rings to promote adhesion of the two sealing rings on the base body. Special treatments are gas phase fluorination (such as gas phase oxi

fluorination), or atmospheric plasma. It is also possible to perform Corona and plasma-surface treatments. Taking such measures may improve wettability and adhesion of TPU materials to many hard plastic materials. A plasma may be denoted as a state of matter consisting of partially or fully ionized gas. Hence, a plasma can be seen as a gas in which there are approximately equal numbers of positively charged particles or negatively charged particles. The provision of a plasma is a powerful means of -activating a surface of the base body before the formation or deposition, for instance moulding, of the sealing rings. This may promote surface adhesion and therefore allows to form a base body/sealing ring arrangement which is robust even in the presence of strong acceleration or deceleration forces. Due to a plasma treatment performed prior to the formation of the one or more sealing rings, it is possible to obtain a piston which is robust even in critical media. Such critical media may be a cleaning agent, lubricating grease, particularly in combination with a high humidity, or hot media. Furthermore, such a piston may be chemically durable.

According to an exemplary embodiment, such a plasma treating may comprise a Plasma-Assisted Chemical Vapour Deposition (PACVD). PACVD is a process in which a chemical surface reactions may occur on the base body after creation of a plasma of reacting gases. Such a plasma may be created by radio frequency or direct current discharge between two electrodes, the space between which is filled with the reacting gases. Such an activation or functionalization can be carried out in a standard PACVD chamber and therefore with reasonable effort. It has been surprisingly recognized by the present inventors that such a PACVD pre-treatment is a proper basis for the subsequent formation of the sealing rings. This particularly holds for a material combination of a (harder) thermoplastic base body and a (softer) thermoplastic

polyurethane sealing ring.

In an embodiment, the method may further comprise forming the base body over an annular magnet by insert-moulding. Therefore, the first component of the piston may be an annular magnet which can then be over-moulded with thermoplastic material forming the base body. This forms a stable connection between magnet and base body and can be done with reasonable effort. Hence, in one embodiment, the base body may be moulded around the annular magnetic element. The magnet may be inserted into the mold and may be overmolded by a hard rigid plastic material .

In an embodiment, it is possible that the base body is formed by moulding. However, other procedures for forming the base body are possible such as drilling and/or milling and/or turning.

According to another exemplary embodiment, the method may comprise, after the forming -, moulding and -postcuring the two sealing rings and the base body. Increasing the temperature of the base body with the sealing rings formed thereon to a sufficiently high temperature, for instance 150°C, may increase the strength of the binding forces between sealing rings and base body. Therefore, such a -postcuring may render the piston appropriate for operation even under harsh conditions with a high mechanical load. In an embodiment, the annular magnet may be magnetized after the -postcuring. By taking this measure, particularly by manufacturing the system in this order, it is possible to combine both, the postcuring requirement and the magnetization requirement. Tempering before magnetizing may prevent undesired thermally-induced de-magnetization.

The sealing rings may be moulded on the base body or may be separately formed and mounted onto the base body. They may at the same time perform the tasks of sealing and damping.

In one embodiment, turning, milling or drilling technologies may be applied for forming the base body, for instance for smaller series of pistons. For larger series of pistons, it is possible to apply moulding techniques for forming the base body.

The base body of the piston may be formed of a matrix material filled with filling particles. For instance, 1 to70 mass % (particularly 5 to 60 mass %, more particularly 10 to 50 mass %) of the base body may be filling material. The filling particles may be spheres, beads, etc. By such a filling of a plastic matrix with filling particles, the material performance of the piston -will be improved.

For pistons made of hard materials, exemplary embodiment may be used polybutylene terephthalate (PBT), polyamide 11 (PA 11), PA 12, PA 610, PA 10-10, PA 612, PA 6, PA.66, aromatic polyamides such as PPA (polyphthalamide), PPS (poly(phenylene sulfide)), POM

(poly(oxymethylene)) or polyolefine (particularly highly filled polyolefine). It may be preferred to use PA 11, PA 12, POM, PBT or alloys of polyamide and polyolefine for reasons of humidity stability, tribology, abrasion resistance, and costs. Such compounds may be filled with glass spheres (for instance in a range of 10 to 70 mass %), mineral filling (for instance in a range of 20 to 70 mass %), or with carbon, carbon fiber, or graphite (each for instance in a range of 3 to 70 mass %), and/or nanotubes such as carbon nanotubes (for instance in a range of 1 to 40 mass %). Furthermore, such materials (or combinations thereof) for the base body are appropriate for surface activation by a plasma process or by gas phase fluorination.

Such a high degree of filling may contribute to obtain a high resistance against high pressure. Without wishing to be bound to a specific theory, it is presently believed that a high filling percentage renders the piston robust against creeping.

In order to enable the piston to properly perform a guiding function within a piston housing, the tribological behaviour should be such that a low-friction movement of the piston is enabled and that the piston is resistant against abrasion, also by impact of high side loadings. Such properties may be obtained by using carbon particles as a filling material, such as carbon nanotubes, carbon fiber, or graphite.

Such a filling may be also advantageous in view of a reduction of the thermal expansion coefficient of the material of the base body since a high degree of filling results in a low thermal expansion and therefore avoids undesired clamping of the piston within a piston housing, particularly at high temperatures.

The at least partial use of an electrically conductive material for the base body allows to conduct away electrostatic charge from the piston which may result from reciprocation along a surrounding wall . Such an electric conductivity may for instance be achieved by using carbon fiber or carbon nanotubes. This may be important for compliance with official requirements such as the ATEX guideline.

A base body of a hard piston may be manufactured

thermoplastically to achieve a reliable adhesion with a subsequently formed thermoplastic polyurethane (TPU) which may be formed by overmoulding the base body. Such advantageous adhesion properties may be obtained with the following treatment of the base body: According to one embodiment, a vacuum plasma treatment (such as PECVD or PACVD) may be performed. In this context, oxygen, nitrogen, and/or argon may be applied during an offline treatment.

According to another embodiment, a treatment with an

atmospheric plasma may be performed. This may be done in the context of an inline manufacturing method. The term "inline" may denote that the surface activation may be done integrated in and during a manufacturing process of the piston so that directly after surface activation, the overmolding will happen.

According to still another embodiment, a treatment with a gas- phase fluorination, particularly a gas-phase(oxy)fluorination (i .e. a gas- phase fluorination utilizing oxygen) may be performed. This may be done according to an inline or an offline method. The term "offline" may denote that the surface activation may be done in a batch process, i .e. with a large number of base bodies separately from a subsequent procedure of forming sealings. The surface activated base bodies may then be stored even for a long time before sealing formation is performed, i.e. sealing lips or the like will be processed. By gas-phase fluorination, a very stable surface activation may be achieved which also allows to perform the described offline process without degradation of the manufactured piston. Such a gas-phase fluorination may result in the generation of a surface polarisation and may form besides hydroxyl groups, CO groups, COH groups, and/or carboxyl groups also partially fluorinated carbon. Such chemical groups may promote a proper bonding between base body and sealing bodies, and it generates lower friction and better chemical resistance to greases, cleaning agents and in general better barrier properties.

Taking one of these measures may allow to substitute a solvent- based adhesion promoter. Embodiments of the invention are based on the cognition that such a treatment allows for an extremely stable connection between thermoplastic polyurethane and hard thermoplastic material.

The piston reciprocating within a cylinder may have an external diameter in a range between -5 mm and 50 mm.

The magnetic ring may be arranged axially centrally on the base body. Hence, a highly symmetric arrangement may be achieved in which a center of gravity of the base body may be the same as a center of gravity of the magnetic ring.

A preferably round (i.e. free of edges) undercut may be foreseen in a recess of the base body after having processed the magnetic ring in the base body. In a subsequent moulding procedure, material of the sealing rings may be deposited in such a recess snugly fitting with and adhering to the base body.

In certain embodiments, particularly for adjust, tension and short stroke cylinders, pistons may be provided without magnets.

In embodiments which make use of magnets, examples for magnetic materials which may be used in such embodiments are rare earth magnets as a sinter magnet or polymer-bound rare earth magnets (e.g. polyamide matrix . In another embodiment, it is also possible to use ferrimagnets, particularly hard ferrimagnets.

The magnetization can be performed after the entire

manufacturing process in axial form by pulse magnetization. It is also possible to use pre-magnetized rings which may be inserted into a recess of the base body.

The aspects defined above and further aspects of the invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to these examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited.

Fig. 1 illustrates a cross-section of a piston according to an exemplary embodiment of the invention.

Fig. 2 illustrates a pneumatic system according to an exemplary embodiment of the invention having a reciprocating piston therein.

Fig. 3 to Fig. 6 illustrate different procedures performed during a method of manufacturing a piston according to an exemplary

embodiment of the invention.

Fig. 7 and Fig. 8 illustrate cross-sections of pistons according to exemplary embodiments of the invention.

The illustration in the drawing is schematically. In different drawings, similar or identical elements are provided with the same reference signs.

Within a cylinder, a pneumatic complete piston transmits forces of pressurized air in a compact way. An integrated magnetic ring which may be made of a rare earth ceramic may trigger a valve switch at the end of a stroke. Such a valve switch may have the consequence that the piston glides smoothly into an end position and this may also trigger the reverse moving direction of the piston. However, such a magnetic ring is optional . It is also possible that the piston is operated without a magnetic actuation.

In an embodiment, a soft thermoplastic polyurethane (hardness typically < 94 Shore A) as a basis for a sealing ring of the piston can be synergetically combined with a hard thermoplastic material in a two- stage manufacturing procedure. In a first phase, the hard component can be produced as the base body. After a defined plasma treatment of the hard component, this functionalized base body may be over-moulded with a soft thermoplastic polyurethane component. This may result in a form-fitting composed member. Such a procedure may render an expensive two-component injection moulding apparatus dispensable. Furthermore, labour intensive and emission intensive adhesion promoter procedures (involving toxic solvents) - will be prevented. Plasma

Activation Chemical Vapour Deposition may be used as an activation technology for cleaning and functionalizing hard plastic based semifinished base bodies. This is a very simple technology which has turned out to be particularly suitable for forming the piston.

In an embodiment, such a material combination and manufacturing technology may be combined for manufacturing a double operating complete piston for use in the field of pneumatics. Such a double operating pneumatic piston may fulfil the tasks of sealing, guiding, damping end sections as well as providing a magnet function for triggering a valve switch. In an embodiment, the parts piston, guiding, ring, sealing and magnetic ring can be integrally combined in a single piece.

In an embodiment, the following manufacturing procedures may be carried out for producing a piston :

- forming a base body from a hard plastic material by injection moulding around a magnetic ring

- plasma treatment or gas phase oxi-fluorination

- over moulding the pretreated piston with a soft thermoplastic polyurethane - - -post curing the over-moulded piston

- magnetizing the postcured piston

Fig. 1 shows a perspective cross-sectional view of a half of a piston for pneumatic system according to an exemplary embodiment of the invention. A cut has been made through a symmetry axis of the piston 100 so as to also expose also components of the piston 100 which are embedded in an interior thereof.

The piston 100 comprises a base body 102 made of a hard plastic material such as Polybutylene terephthalate. The base body 102 has a central bore 104 for receiving a piston rod (not shown in Fig. 1). In other words, the base body has a basically hollow cylindrical geometry or a disk-shaped geometry. Furthermore, an annular magnet 106 made of a rare earth ceramics is embedded in the base body 102 in a way that the surface of the magnet 106 is entirely surrounded by the base body 102. The annular magnet 106 is arranged at a central position in an axial direction.

As can be taken from Fig. 1, a first sealing ring 108 is arranged with a form-fit connection on a first main surface 112 of the base body 102. The second sealing ring 110 is arranged with a form-fit connection on an opposing second main surface 114 of the base body 102.

The base body 102 consists of Polybutylene terephthalate, whereas the two sealing rings consist of a thermoplastic polyurethane with a Shore hardness (A) of 82.

The hollow cylindrical structure of the base body 102 has

indentations on both sides. One indentation is adapted for receiving the magnetic ring 106, the sealing ring 110. The other indentation is adapted for receiving the sealing ring 108. Both indentations have undercuts 116, 118 shaped as annular recesses for an improved securing of the sealing rings 108, 110.

Moreover, the base body 102 has a circumferential guide surface

122. When being arranged within a housing or pneumatic tube (see Fig. 2), the circumferential guide surface 122 being an integral part of the base body 102 and being also made of Polybutylene terephthalate, guides the piston 100 within such a pneumatic tube. As can be further taken from Fig. 1, each of the sealing rings 108, 110 comprises arcuate protrusions 124 on either side of the base body 102, adjacent arcuate protrusions 124 being spaced by gaps 126. The arcuate protrusions 124 function as damping elements and provide for a damping of a mechanical force acting on a front face of the piston 100 when the piston 100 abuts on an end face when being arranged within a housing. Integrally formed with such damping elements, each of the sealing elements 108, 110 has a circumferentially arranged annular sealing lip 128 with slanted pivotable end portions. This provides for an air sealed reciprocation of the piston 100 within a cylindrical housing.

Fig. 2 shows a pneumatic cylinder 200 as an example for pneumatic system according to an embodiment of the invention.

Piston 100 is arranged for reciprocating (see double arrow 202) within a hollow cylindrical housing 204. A piston rod 206 is guided through the central bore 104 of the base body 102. Two pressurized air connections 208 and 210 are provided as well to supply air to a desired side of the piston 100 within the housing 204. Furthermore, two magnetic transducers 212, 214 are arranged at opposing end sections of a reciprocation chamber 216 in which the piston 100 reciprocates.

In the following, referring to Fig. 3 to Fig. 6, different procedures or states during performing a method of manufacturing a piston for a pneumatic system according to an exemplary embodiment of the invention will be explained. Although Fig. 3 to Fig. 6 show the different procedural steps within different apparatuses, it is also possible to integrate a part or all of these components in a single manufacturing device, for instance in a production line or within a common housing.

Fig. 3 shows a moulding -cavity 300 which comprises two opposing moulding parts 302, 304 which can be pivoted relative to one another by a hinge 306. In a closed position, as shown in Fig. 3, the moulding chamber 300 encloses a hollow space 308 which has the shape of a base body to be formed. A magnetic ring 106 is placed in the hollow chamber 308. By opening a valve 310, liquid plastic material may be supplied from a container 312 into the hollow chamber 308 so as to form the base body 102 by moulding. By subsequently opening the moulding parts 302, 304, the base body 102 can be taken out of the chamber 300 with the magnet 106 embedded therein.

As can be taken from Fig. 4, the base body 102 made of

Polybutylene terephthalate is then inserted into a vacuum chamber 402 of a Plasma-Assisted Chemical Vapour Deposition apparatus 400. A control unit 404 controls supply of precursors in containers 406, 408 by correspondingly controlling the state of valves 410 and 412, respectively. The base part 102 with the embedded magnetic ring 106 is placed on a heating plate 414 for heating the semi-finished part of the piston. A plasma source 416 for generating a plasma 418 and a plasma chamber 402 can also be controlled via the control unit 404. Furthermore, an outlet valve 420, also being controllable by the control unit 404, is arranged to allow draining of material from the plasma chamber 402.

With the apparatus shown in Fig. 4, it is possible to subject the base plate 102 with the embedded annular ring 106 to plasma 418 for functionalizing a surface thereof in order to prepare it for a subsequent formation of sealing rings.

Such a formation of sealing rings can be performed in a moulding apparatus 500 as shown in Fig. 5. The moulding apparatus 500 also comprises two cooperating moulding parts 502 and 504 having a shape which allows the base body 102 with the embedded annular magnet 106 to be placed within such a chamber. Additionally, hollow spaces 506 are foreseen which are shaped so as to allow to form sealing elements on dedicated parts of the base body 102. By opening valve 310, material may be supplied from a container 312 into the moulding -cavity of the injection moulding-machine 500 which allows to form the sealing elements 108, 110 from a thermoplastic polyurethane material.

As can be taken from Fig. 6, the almost finished piston 100 may then be inserted into a post-treatment chamber 602 of a post-treatment apparatus 600. As a next step, the semi-finished part may be heated, for instance using a heating coil 604 allowing to increase the temperature within the post-treatment chamber 602 to a desired value. This may anneal the piston 100 to improve its durability and robustness. Operation of the ohmic heating element 604 can be performed via a switch 606 operable by a control unit 608 and allowing to couple an electric current provided by a current source 610 to the heating wire 604.

After this heating, the control unit 608 may activate a switch 612 allowing a current source 614 to activate a magnetizing coil 616 for magnetizing the annular ring 106.

The piston 100 may then be removed from the post-treatment apparatus 600 and may be assembled within a pneumatic system such as the one shown in Fig. 2.

Fig. 7 illustrates a cross-section of a piston 700 according to another exemplary embodiment of the invention.

As in the embodiment in Fig. 1, the magnetic ring 106 is arranged axially centrally on the base body 102 in Fig. 7. The axial direction corresponds to symmetry axis 704 of the hollow cylindrical base body 102 and equals to a reciprocation direction of the piston 700 within a cylindrical housing. In other words, a distance D between a symmetry plane 706 of the annular magnet 106 and the upper main surface 708 of base body 102 equals to a distance D between the symmetry plane 706 of the annular magnet 106 and the lower main surface 710 of base body 102.

With such a symmetric arrangement of the annular magnet 106 in the axial direction, one and the same annular magnet 106 may be conveniently used for cooperating with two reed contacts arranged on opposing reverse positions of the piston within a cylindrical housing.

Advantageously, the annular magnet 106 is located at an outer circumference of the piston 700, i.e. very close to the circumferential surface of the hollow cylindrical base body 102. Hence, the distance between the annular magnet 106 and the cooperating reed contacts (not shown) is very small so that a precise magnetic control is enabled.

However, for preventing undesired abrasion of the material of the annular magnet 106 when moving in direct contract with the surrounding wall of a cylinder, at least the exposed surface of the annular magnet 106 is coated with a protection layer 702. Protection layer 702 is made of a sufficiently robust and tight material. -- Protection layer 702 may be made of a material which provides a corrosion protection for the annular magnet 106.

Fig. 8 illustrates a cross-section of a piston 800 according to another exemplary embodiment of the invention.

In contrast to Fig. 7, the annular magnet 106 is arranged to be completely surrounded by material of the base body 102. However, a radial distance d between the circumferential surface of the base body 102 and the radially outer end of the annular magnet 106 is very small (for instance less than 10% of the radius of the base body 102) so that the distance of the annular magnet 106 to the -reed contacts is

sufficiently small to allow for a strong magnetic interaction. As in Fig. 7, the annular magnet 106 is arranged axially centrally on the base body 102.

It should be noted that the term "comprising" does not exclude other elements or features and the "a" or "an" does not exclude a plurality. Also elements described in association with different

embodiments may be combined. It should also be noted that reference signs in the claims shall notstrued as limiting the scope of the claims.

Claims

C l a i m s

1. A piston for a pneumatic system, the piston comprising :

a base body having a bore for receiving a piston rod, wherein the base body comprises or consists of thermoplastic material;

two sealing rings on opposing main surfaces of the base body; and

an annular magnet embedded in the base body.

2. The piston of claim 1, wherein the annular magnet is only partially surrounded by the base body and is partially exposed to an

environment.

3. The piston of claim 2, wherein the annular magnet comprises a rare earth material, particularly rare earth ceramics, more particularly polymer-bonded rare earth ceramics.

4. The piston of any one of the above claims, wherein the base body comprises or consists of Polybutylene terephthalate, polyoximethylene, and polyamide, more particularly one of PA 11, PA 12, PA 610, PA 1010, PA 612.

5. The piston of any one of the above claims, wherein at least one of the two sealing rings comprises or consists of a thermoplastic polyurethane.

6. The piston of any one of the above claims, wherein at least one of the two sealing rings is made of a material having a Shore hardness of 80 or more.

7. The piston of any one of the above claims, wherein the base body comprises a hollow cylindrical structure enclosing the bore, the hollow cylindrical structure having indentations on the opposing surfaces each for receiving a respective one of the two sealing rings.

8. The piston of claim 7, wherein at least one of the indentations has an undercut.

9. The piston of claims 2 and 7, wherein the magnet is embedded in one of the indentations.

10. The piston of claim 9, wherein the magnet is embedded in one of the indentations partially in contact with the base body and partially in contact with one of the sealing rings.

11. The piston of any one of the above claims, comprising a

circumferential guide surface for guiding the piston within a pneumatic cylinder, the circumferential guide surface being integrally formed with, particularly being made of the same material as, the base body.

12. The piston of any one of the above claims, wherein at least one of the two sealing rings comprises at least one damping element arranged and adapted for damping a mechanical force acting on a front face of the piston.

13. The piston of claim 12, wherein the at least one damping element is shaped as a plurality of circumferentially arranged arcuate protrusions being circumferentially spaced from one another.

14. The piston of any one of the above claims, wherein at least one of the two sealing rings comprises an annular sealing lip arranged and adapted for circumferentially sealing a gap between the piston and a surrounding pneumatic tube.

15. The piston of any one of the above claims, wherein the base body is formed of a matrix material filled with filling particles.

16. The piston of claim 15, wherein the filling particles comprises at least one of the group consisting of glass beads for low creeping, and carbon-based fillers for low friction behaviour.

17. The piston of claim 15 or 16, wherein 1 to 70 mass % of the base body are made of the filling material.

18. A pneumatic system, the pneumatic system comprising a piston of any one of the above claims.

19. The pneumatic system of claim 18, adapted as at least one of the group consisting of a pneumatic cylinder, applied in an automotive production line, a food production line, a packaging production line, a clean room production line, and a semiconductor production line.

20. A method of manufacturing a piston for a pneumatic system, the method comprising :

forming a base body with a bore for receiving a piston rod, wherein the base body comprises or consists of thermoplastic material; embedding an annular magnet in the base body; and

moulding, in a separate process, two sealing rings on opposing main surfaces of the base body.

21. The method of claim 20, further comprising exposing the base body to a plasma treatment or a gas phase treatment before moulding the two sealing rings to promote adhesion of the two sealing rings on the base body.

22. The method of claim 21, wherein the plasma treatment comprises a Plasma-Assisted Chemical Vapour Deposition.

23. The method of claim 21, wherein the plasma treatment comprises a vacuum plasma treatment.

24. The method of claim 21, wherein the plasma treatment comprises a treatment with an atmospheric plasma.

25. The method of claim 20, further comprising exposing the base body to a gas-phase fluorination, particularly a gas-phase(oxy)fluorination, before moulding the two sealing rings to promote adhesion of the two sealing rings on the base body.

26. The method of any one of the above claims, wherein the base body is formed by moulding.

27. The method of claim 26, wherein the base body is formed over an annular magnet by insert-moulding.

28. The method of any one of the above claims, comprising, after the forming and moulding, -postcuring the two sealing rings and the base body.

29. The method of claims 27 and 28, further comprising, after the postcuring - magnetizing the annular magnet.