Patent Description:
Expansion vessels are known in the field of the art. Expansion vessels are widely used in heating systems for buildings, often in combination with a boiler. Expansion vessels are also known for other purposes, such as for suspension systems for vehicles.

Various embodiments of expansion vessels are known. <CIT> discloses an embodiment of an expansion vessel. The expansion vessel comprises two metal halves and is divided into a gas chamber and a water chamber by a membrane. The outer perimeter of the membrane is mounted into the expansion vessel via a membrane holder extending outward with respect to the outer wall of the expansion vessel.

A disadvantage of such an expansion vessel is that it is quite difficult to manufacture and has a limited lifespan.

A method of manufacturing an expansion vessel comprising a housing and a membrane which divides the housing in two volumes, the method comprising:-providing the housing,- providing a membrane comprising a thermoplastic material,-providing a mould having a non-flat form,- providing a vacuum/blowing device which is constructed to deform the membrane according to the shape of the mould by providing an overpressure or under pressure on one side of the membrane,-deforming the membrane with the vacuum/blowing device according to the shape of the mould, thereby allowing the thermoplastic material to deform plastically,-connecting the membrane directly or indirectly to the housing of the expansion vessel, is known from document <CIT>.

The invention provides a method of manufacturing an expansion vessel comprising a housing and a membrane which divides the housing in two volumes, the method comprising:.

The present invention provides an improvement in terms of a membrane having a greater longevity and ease of assembly in an expansion vessel.

In an embodiment, the membrane is connected to an annular membrane holder prior to being deformed with the vacuum/blowing device, wherein the membrane holder is configured to be connected to the housing of the expansion vessel. This stepwise manufacture provides a reliable quality in connections. Traditionally, the membrane is clenched between the two vessel halves by a clenching ring. Although this works, the clenching ring connects three parts in one connection operation. In the present embodiment, the connection operation is split into two separate, consecutive steps, which increases reliability in the assembly process.

Another advantage is that the expansion vessel with the membrane holder does not have a ridge which extends outwardly from the housing, as expansion vessels with clenching rings generally do.

In an embodiment, the membrane material is flat prior to being connected directly or indirectly to the housing.

In an embodiment, the membrane is connected permanently to the housing, directly or indirectly via the membrane holder, prior to the forming of the membrane with the blow-moulding process.

In an embodiment, the housing part into which the membrane is blow moulded is formed prior to the connecting of the membrane material with the housing part. The housing parts may be of metal and are formed in a separate, earlier process. The housing can also be made of any other material which is suitable, such as a synthetic material, in particular a hard plastic.

In an embodiment, the method comprises the subsequent steps of:.

In this way, the blow-moulding takes place in the completed housing.

In an embodiment, the method comprises the step of directly clenching the membrane between the housing parts. This results in a relatively simple expansion vessel. In an embodiment, the membrane is clenched between the housing parts without a clenching ring.

In an embodiment of the method, a region of overlap is formed between the first and second housing parts, the region comprising:.

wherein the membrane is clenched between the housing parts in the clenching region and wherein the housing parts are connected to one another in the connecting region. The clenching region and connecting region may be separate regions. In an embodiment, the membrane may be clenched between the housing parts without the use of a clenching ring. The clenching region and connecting region may be adjoining, separate regions.

This results in a simple and effective way of making expansion vessels.

In an embodiment, the method comprises pulling membrane material from a roll of membrane material, engaging the membrane material with a first housing part, placing the second housing part over the first housing part and cutting away excess material of the membrane. This process can be used to manufacture large series of expansion vessels effectively.

In an embodiment, a first housing part is smaller than a second housing part, and wherein in the blow moulding step the membrane is blow moulded against the second, larger housing part. This embodiment provides the advantage that once in use, the membrane is never subjected to pure tensile stresses. Even if the membrane is expanded to the maximum state, i.e. against the wall against it was blow moulded, it will not be brought under tensile stress because the membrane is only brought into the same position in which it was blow moulded.

If in use the membrane is pressed against the wall of the small volume, it is also not under tensile stress, because the membrane can contact the vessel wall of the smaller volume without being stretched. Therefore, in use, the membrane is never stretched beyond the blow-moulding position. The absence of stretching was found to have particular favorable effects on the annular region of the membrane adjacent the membrane holder. In use, this region will flex during varying operating conditions. If the deforming is limited to flexing and does not comprise pure stretching, i.e. pure tensile stresses, the lifespan was found to be greater than in a situation where apart from flexing tensile stresses occur due to stretching.

In the operating state, the membrane may be primarily in a flexed position in the small volume. This reduces the stresses in the annular region of the membrane which is adjacent to the annular membrane holder.

This would be different if the membrane divides the volume in a large volume and a small volume and is subsequently blow moulded against the wall of the small volume. If - in use - the membrane is then pressed against the wall of the large volume, the membrane will need to be stretched.

In an embodiment, the method comprises providing a membrane which comprises at least:.

This embodiment allows the membrane to adopt the shape of the vessel in an easy way. Moreover, the membrane does not have to be detached from the mould after being formed.

In an embodiment, the second layer comprises Ethylene Vinyl Alcohol (EVOH).

In an embodiment, the first and third layers comprise one or more layers of the same or a different thermoplastic polymer, preferably a thermoplastic polymer having a melting point temperature, Tm, of less than <NUM> and a glass transition temperature, Tg, of less than <NUM> or a mixture of such polymers, more preferably a thermoplastic polymer preferably selected from one or more of the polymers comprising polyolefins (more preferably polyethylene, such as high density polyethylene, low density polyethylene, propylene, and butylene; including ethylene copolymers and propylene copolymers and butylene copolymers), polyamines, polyesters, and polyurethanes.

In an embodiment, the first and third layers comprise:.

These materials were found to provide good mechanical protection, a good water impermeability and good characteristics for bonding with the housing.

In an embodiment, the method comprises stacking multiple membranes onto one another and connecting the multiple membranes directy or indirectly to a housing of the expansion vessel. Advantageously, the multiple layers provide better air-tightness for certain gasses, for example for nitrogen.

In an embodiment, the method comprises comprising connecting the membrane to the housing of the expansion vessel or to a membrane holder by bonding an outer perimeter of the membrane to the surface of the housing or membrane holder. The bonding may be chemical bonding. Alternatively, the bonding may be physical bonding.

In this embodiment, the surface area of the membrane which contacts the housing or membrane holder can be chosen quite large, so that a subsequent laser welding operation - in which a small local deterioration of the membrane may take place - does not seriously effect the strength or impermeability of the total connection between the membrane and the housing or membrane holder. The surface area of the membrane which contacts the housing or membrane holder may be at least twice the surface area which is affected disadvantageously by the weld.

The combination of a plastically deformed membrane with a laser welding operation has been found to provide a particularly reliable manufacturing operation.

In an embodiment of the method, the laser beam penetrates through the wall of the housing and at least reaches a portion of the membrane holder to form a weld between the housing and the membrane holder.

In an embodiment of the method, the welding comprises providing a line weld which extends circumferentially around the inside of the wall of the expansion vessel for providing a first fluid tight connection between the membrane holder and the housing. With the line weld, a good fluid-tightness and air-tightness can be achieved. In an embodiment, the method comprises:.

The combination of two mating rings was found to be very reliable. The first and second ring may be an inner and outer ring.

One ring may have a convex mating part and the other ring may have a mating concave part, which parts clench and curve the membrane between them.

The present invention further relates to an expansion vessel comprising a housing and a membrane which divides the housing in two volumes, characterized in that the expansion vessel is manufactured with the method according to claim <NUM>, wherein the membrane comprises a thermoplastic material, and wherein in rest, i.e. with equal pressure on both sides of the membrane, the membrane has a non-flat form.

In an embodiment, the membrane comprises at least:.

Traditionally, membranes of expansion vessels, in particular for heating systems, are manufactured from rubber. The present embodiment is a significant step in this field which leads to a longer lifespan.

In an embodiment, the second layer of the membrane comprises Ethylene Vinyl Alcohol (EVOH).

In an embodiment of the membrane, the first and third layers comprise:.

The present invention further relates to an expansion vessel, characterized in that it is manufactured according to the method of claim <NUM>, wherein the expansion vessel comprises a membrane comprising a thermoplastic material, wherein in rest, i.e. with equal pressure on both sides of the membrane, the membrane has a non-flat form. The expansion vessel has a long lifetime and can be assembled relatively easy.

In an embodiment of the expansion vessel, the membrane has a substantially hat-shaped form in rest, comprising an outer region which is substantially flat and an inner region which is at least partially curved and defines a volume. In an embodiment of the expansion vessel, the membrane has a form which in rest is similar to the form of a part of a housing of the expansion vessel.

In an embodiment, the expansion vessel comprises a membrane holder for holding the membrane, wherein an outer perimeter of the membrane is connected to the housing via the membrane holder. In an embodiment, the membrane holder comprises an inner ring and an outer ring between which an outer region of the membrane is clenched. This embodiment can be manufactured relatively simple. In another embodiment, the membrane holder comprises a single ring configured to hold the membrane.

In an embodiment, the expansion vessel comprises a housing comprising a first metal part and a second metal part, wherein the first and second metal parts comprise respective first and second overlapping sections which are constructed to partly overlap one another in a region of overlap, the expansion vessel further comprising a membrane holder which is positioned at the region of overlap, and wherein the first metal part, the second metal part and the membrane holder are connected to one another with a circumferential weld.

In an embodiment, the expansion vessel comprises:.

wherein the inner ring and the outer ring are clicked into one another, the outer region of the membrane being clenched between them.

In an embodiment according to the present disclosure, the membrane holder is configured to force the membrane to curve subsequently:.

when travelling along the membrane from a position just inward of the membrane holder to the outer ridge of the membrane.

The four curves can be defines as four curve regions of the membrane. In other words, the membrane comprises the following four curve regions:.

In an embodiment, the expansion vessel comprises multiple membranes which are stacked onto one another. Preferably, the membranes are connected to a membrane holder, but it is also possible to connect the membranes directly to the housing.

The present assembly further relates to an assembly for manufacturing an expansion vessel, the assembly comprising:.

wherein the assembly comprises a support for supporting a formed housing part of the expansion vessel, wherein the vacuum/blowing device is constructed to provide an under pressure in said housing part or provide an over pressure in the other housing part for blow-moulding the membrane while using said housing part as a mould.

The present disclosure further relates to a method of manufacturing an expansion vessel comprising a housing and a membrane which divides the housing in two volumes, the method comprising:.

The laser welding operation obviates the use of a known clenching ring which protrudes from the outside of the housing and which connects three parts.

The method allows a reliable manufacturing process. The method can be performed with a rubber membrane and with a membrane comprising a thermoplastic material.

The first and second ring may be an inner ring and an outer ring.

In another embodiment, the method comprises:.

This embodiment provides a simple and effective connection operation for three different components, while at the same time creating air-tightness and fluid-tightness.

clicking the first ring and the second ring into one another, wherein the outer region of the membrane is clenched between them. The protruding part may be convex, and the mating receding part may be concave.

The present disclosure further relates to an expansion vessel, comprising:.

In an embodiment of the expansion vessel, the membrane holder comprises:.

In order to form the expansion vessel, the membrane itself is connected to the membrane holder in a fluid-tight manner, either before or after inserting the membrane holder in the housing, and either before or after connecting the membrane holder to the housing.

In the following, the aspects, features and advantages of the present invention will be elucidated further by reference to the annexed figures illustrating exemplary embodiments. In the figures, the same parts or parts having the same function have been identified with the same reference numeral.

Turning to <FIG>, a membrane <NUM> comprising a thermoplastic material is provided which is connected along its outer ridge <NUM> to a membrane holder <NUM>. The membrane holder can be a metal ring. The membrane holder <NUM> is manufactured from a corrosion resistant alloy. The thermoplastic material of the membrane is elastic, which allows an elastic deformation of the membrane when a pressure difference exists between the two opposing sides of the membrane.

A mould <NUM> is provided and a vacuum device or blowing device <NUM> is provided for creating an underpressure or overpressure on one side of the membrane, in order to deform the membrane. The mould <NUM> may be one half <NUM> of the housing of the expansion vessel, or at least a part of a housing of the expansion vessel.

The combination of membrane holder <NUM> and membrane <NUM> is placed in the vessel halve <NUM> or in the special forming mould. The mould is a part of the housing, and the membrane holder is welded to the housing part via a laser weld. The housing part into which the membrane is blown moulded is formed prior to the connecting of the membrane material with the housing part.

The welding operation may take place prior to the deforming process and is further discussed herein below. In this way a permanent connection is created between the membrane and the vessel halve prior to the forming of the membrane with the blow-moulding process.

In an embodiment falling outside the scope of the claims, the mould is a separate mould. The membrane holder may be connected to the mould in a variety of ways.

A heating device <NUM> is positioned at the bottom of the vessel halve or mould in order to provide heat to the membrane <NUM>. A vacuum is created in the space <NUM> defined between the membrane <NUM> and the vessel halve <NUM>, by sucking out the air through an opening <NUM> with the vacuum device <NUM> which is in fluid connection with the space between the membrane and the mould <NUM> via a conduit <NUM>. Simultaneously, the membrane is heated with the heating device <NUM>. The thermoplastic material in the membrane becomes soft and mouldable and deforms in a plastic way. The plasticity results from the thermoplastic nature of the material. The membrane substantially adopts the form of the vessel halve <NUM> or, in the embodiment falling outside the scope of the claims, special forming mould <NUM>.

When the membrane has deformed, the heating device is turned off and the membrane cools back to the original temperature. The membrane adopts its new, non-flat form, while remaining deformable due to its elasticity, as the membrane should be in order to let the expansion vessel function properly.

Different materials can be used for the membrane <NUM>. In an embodiment, the membrane is composed of multiple layers. This aspect is further discussed in relation to <FIG>.

It is also possible that multiple membranes of the above mentioned kind are stacked, and together act as a compound membrane. The different membranes are connected at the circumference of the outer region via the membrane holder.

The layers of the membrane are all elastic, in order to allow the membrane to deform in its normal use temperature, when in use the pressures inside the expansion vessel varies.

Other kinds of membranes which can be plastically deformed with pressure and heat may be used.

If a special forming mould is used, the membrane holder and membrane <NUM> are first deformed in the special forming mould, are subsequently disconnected and removed from the mould, are placed in the housing and welded to the housing.

If the housing is used as a mould <NUM>, the membrane <NUM> is left in place. After the deforming process, the heating device and the vacuum/blowing device are removed from the housing part. Next, the remaining parts of the expansion vessel are fitted.

Turning to <FIG>, in an embodiment, the membrane holder <NUM> comprises an inner ring <NUM> and an outer ring <NUM>, which fit into one another and form a click-connection. An outer region <NUM> of the membrane <NUM> is positioned between the two rings <NUM>, <NUM>. In this embodiment, the membrane <NUM> may not comprise a thickened outer ridge.

The inner ring comprises a receding part <NUM> which extends along the outer circumference of the inner ring. The outer ring comprises a protruding part <NUM> which extends along the inner circumference of the outer ring. However, this may also be vice versa, with a protruding part on the inner ring and receding part on the outer ring. The receding part and the protruding part mate with one another to form the click-connection when the inner ring is moved into the outer ring in the direction of arrow <NUM>. The receding part <NUM> may be concave, while the protruding part <NUM> may be convex. The outer region <NUM> extends upward between the inner and outer ring.

The outer ring may comprise a seat portion <NUM> for a seal. A rubber sealing ring <NUM> may be provided at the seat portion <NUM> of the outer ring, to provide air-tightness and fluid tightness between the inner ring and outer ring. This function of air-tightness and fluid tightness may also be provided by the membrane <NUM> itself.

The outer ring also defines a curve <NUM>, in this case at the seat portion <NUM> The inner ring defines a mating curve <NUM>. The curve has the function of curving the outer region <NUM> of the membrane, toward the mating parts <NUM>, <NUM>.

The outer ring <NUM> further comprises a membrane guide 46A which is curved and prevents damage to the membrane when the membrane is deformed. The inner ring <NUM> comprises a similar membrane guide 46B which is curved and which prevents damage to the membrane <NUM> when the membrane is deformed in an opposite direction.

When travelling from a middle portion <NUM> of the membrane to an outer ridge <NUM> of the membrane, the membrane first starts to curve at a meeting location <NUM> where the membrane meets the inner ring and the outer ring. The curvature is in a first direction of curvature. Subsequently, the membrane curves in an opposite direction of curvature at the curve <NUM>, <NUM> toward the mating parts <NUM>, <NUM>. The membrane subsequently curves through the curved form defined by the mating parts <NUM>, <NUM>. When travelling towards the ridge <NUM>, the membrane first curves inwardly toward the center of the membrane holder and subsequently curves outwardly away from the center of the annular membrane holder. Above the mating parts <NUM>, <NUM>, the membrane extends over a small distance and ends at the ridge <NUM> of the membrane.

When the membrane holder and membrane are connected, the membrane may be deformed, as is discussed in relation to <FIG>.

Turning to <FIG>, it is shown how the membrane holder <NUM> is connected to the housing while in a same operation the two parts of the housing are connected to one another. The housing <NUM> comprises a first housing part <NUM> (also referred to as a first vessel halve) and a second housing part <NUM> (also referred to as a second vessel halve), which have overlapping sections <NUM>, <NUM>. The housing parts <NUM>, <NUM> may be halves but need not be of the same size. The housing parts <NUM>, <NUM> are preferably made of steel. The housing has a circular, cylindrical shape. The wall of the housing parts <NUM>, <NUM> has an inner side and an outer side. The overlapping sections <NUM>, <NUM> overlap in a region of overlap <NUM>.

It will be understood that the words "metal halve" does not mean that the two metal parts need to be exactly equal in size. Rather, the two metal halves should form the housing together, and can have different sizes. Alternatively, the housing can comprise a one-piece unit, which is easy to manufacture. The housing can be of any shape, preferably, of a right circular cylinder shape. The housing comprises a wall having an inner side and an outer side. The housing can be made of metal or any other suitable material, such a synthetic material, in particular a hard plastic.

The method comprises inserting the membrane holder <NUM> into the housing of the expansion vessel in such a way that an outer perimeter <NUM> of the membrane holder <NUM> is circumferentially abutted to an inner wall of the housing part <NUM>.

A laser welding device <NUM> is provided from the outside and a laser beam <NUM> is directed toward the housing parts <NUM>, <NUM> and the membrane holder <NUM>. The housing parts <NUM>, <NUM> and the membrane holder <NUM> are welded to one another. The three parts may be connected to one another in a single circumferential weld. The circumferential weld creates and air-tight and fluid tight connection between the membrane holder <NUM> and the housing which is formed by housing parts <NUM>, <NUM>.

The laser beam provides a high energy deeply into the contact area between the housing and the membrane holder, and generates a narrow and deep weld. The depth of the weld can be up to <NUM>.

When the expansion vessel is completed, the membrane <NUM> divides the volume formed by the housing into two parts, with a first part acting as a first chamber for storing water, and with a second part acting as a second chamber for storing gas.

As the membrane holder <NUM> is welded to the inside of the wall of the housing, the housing per se forms a protection layer with respect to the membrane holder for protecting the membrane holder from the corrosion or damage by, e.g. a collision. This prolongs the usage period of the expansion vessel.

Turning to <FIG>, another embodiment of the expansion vessel <NUM> is shown. The expansion vessel comprises a housing <NUM>. The membrane holder <NUM> is first connected to a housing part <NUM> (or vessel halve). To this end, the assembly of the inner ring, outer ring and membrane is positioned at the inner side of the housing part <NUM>. The membrane holder is connected to said housing part <NUM>. Next, the housing part <NUM> is connected at its ridge <NUM> to another housing part <NUM> shown in dashed lines. Both the connection between the membrane holder and the first housing part and between the first housing part and the second housing part are performed by laser welding from the outside.

The membrane divides the total volume in a first volume <NUM> and a second volume <NUM>.

The membrane holder <NUM> is welded to the inner wall of the housing by the laser welding device by directing the laser beam at a contact area between the membrane holder <NUM> and the inner wall of the housing. The laser beam provided by the laser welding device penetrates through the inner wall of the housing and a tip of the laser beam at least reaches a portion of the membrane holder <NUM> to form a weld between the housing part <NUM> and the membrane holder <NUM>. The weld comprises a line weld which extends circumferentially around the expansion vessel. The line weld is provided at the contact area between the membrane holder <NUM> and the inner side of the wall of the housing part <NUM>. The line weld provides a first fluid tight connection between the membrane holder <NUM> and the housing <NUM>.

During the welding operation, the laser beam <NUM> is moved along the entire outer circumference of the housing, i.e. perpendicular to the plane of drawing, and performs an annular weld which connects the membrane holder to the housing.

Turning to <FIG>, a membrane <NUM> is shown. The membrane comprises a first, protective layer <NUM>, a second, gas-impermeable layer <NUM> and a third, protective layer <NUM>. The three layers are interconnected via first and second adhesive layers <NUM>.

The second, gas-impermeable layer may be composed of Ethylene Vinyl Alcohol (EVOH). EVOH is known for its good impermeability.

Turning to <FIG> a method of bonding a membrane <NUM> onto a membrane holder <NUM> is shown. <FIG> shows how the membrane <NUM> is directly bonded to the membrane holder <NUM>. The bonding may be chemical bonding. A bonding agent may be used. Bonding agents are known in the field of the art. The bonding may also be physical bonding. The surface area of the bonded connection is rather large, i.e. the membrane can be bonded over a considerable width <NUM> onto the membrane holder <NUM>, see <FIG>. The width <NUM> extends parallel to the housing. The connection extends circumferentially around the inner side of the membrane holder <NUM>.

<FIG> shows how the membrane holder is welded to the housing parts <NUM>, <NUM> while at the same time connecting the housing parts <NUM>, <NUM> to one another. This connection is performed by laser welding from the outside. The parts <NUM>, <NUM> and <NUM> form a region of overlap <NUM>. If the laser weld deteriorates the quality of the cured connection between the membrane <NUM> and the membrane holder <NUM>, this will only locally affect the cured connection and not deteriorate the overall quality of the connection between the membrane and the membrane holder, because a width of the weld is substantially smaller than the width <NUM> of the cured connection. In another embodiment, a first laser weld is performed to weld the membrane holder <NUM> to housing part <NUM> and in a second, separate laser weld, the housing part <NUM> is welded to housing part <NUM>.

Turning to <FIG>, in a next step, overpressure is applied by blowing air or another gas via the opening <NUM> into the expansion vessel. The membrane <NUM> is deformed against the housing part <NUM>, and heated with a heating device <NUM>. The membrane <NUM> becomes plastic and deforms plastically. The housing part is used as a mould <NUM>. Next, the heating device <NUM> is removed or switched off; the membrane <NUM> cools and becomes elastic again.

Turning to <FIG>, a similar process as in <FIG> is shown, except that the membrane <NUM> is connected directly onto a housing part <NUM> by bonding. The bonding may be chemical or physical bonding. A bonding agent may be used. Subsequently, the housing part <NUM> including the membrane <NUM> is connected to another housing part <NUM>. Next, the membrane is plastically deformed by a combination of pressure and heating, wherein the housing part <NUM> is used as a mould.

Turning to <FIG>, another embodiment of the expansion vessel is shown in connection with another method of manufacturing the expansion vessel. A roll <NUM> of membrane material is provided from which the membrane material <NUM> is spooled. A first housing part <NUM> is provided, also called a first vessel halve <NUM>. The membrane material which comes from the roll is flat. The membrane material, in particular a region which is intended to form the outer region of the membrane, is engaged with the first housing part <NUM>, in particular on the ridge <NUM> of the first housing part <NUM>. A second housing part <NUM>, also referred to as a second vessel halve <NUM>, is engaged with the first vessel halve <NUM>. Excess material of the membrane can be cut away.

This embodiment does not have a separate membrane holder, i.e. the membrane <NUM> is connected to the housing parts <NUM>, <NUM> directly. However, a membrane holder may be used.

Turning to <FIG> and <FIG>, the second housing part <NUM> is placed over the first housing part <NUM>. To this end a region <NUM> of overlap is formed. The region comprises:.

wherein the membrane is clenched between the vessel halves in the clenching region and wherein the vessel halves are connected to one another in the connecting region. The clenching region and connecting region may be separate regions. The clenching region and connecting region may be adjoining, separate regions.

The first housing part <NUM> has an overlapping section <NUM> comprising an inwardly receding section <NUM>. The inwardly receding section <NUM> forms the clenching region <NUM> with a part <NUM> of the overlapping section <NUM> of the second vessel halve <NUM>. The membrane is clenched between the vessel halves. The part <NUM> may be an inwardly protruding section of the second vessel halve, in particular an inwardly protruding rim <NUM>. The part <NUM> presses against the inwardly receding section 206n and clenches the outer region <NUM> of the membrane there between.

The outer region <NUM> of the membrane extends downward relative to the middle portion <NUM> of the membrane <NUM> and is clenched between an upwardly directed receding section <NUM> and the part <NUM> of the second housing part <NUM>.

The connecting region <NUM> is formed by a wall section <NUM> of the first housing part <NUM> and a wall part <NUM> of the second housing part <NUM>.

<FIG> shows that the first housing part <NUM> is slightly smaller than the second housing part <NUM>. <FIG> shows that the first volume <NUM> is slightly smaller than the second volume <NUM>.

<FIG> shows that after the first and second housing parts are engaged, they are connected with laser welding.

<FIG> shows that the membrane <NUM> is blow-moulded against the housing, and in particular against the larger housing part <NUM>. This has the positive effect that in use, the membrane will never undergo pure extension, or stretch, with accompanying tensile stresses.

<FIG> shows that in the use situation the smaller volume <NUM> is filled with the liquid or gas that needs to be able to expand, and the larger volume <NUM> is filled with the gas that provides the back pressure on the membrane. Opening <NUM> is an opening for a medium which needs to be able to expand and which is typically configured to be connected to a conduit system. The opening <NUM> is an opening for a gas which provides the back pressure.

Claim 1:
Method of manufacturing an expansion vessel (<NUM>) comprising a housing (<NUM>) and a membrane (<NUM>) which divides the housing in two volumes, the method comprising:
- providing the housing (<NUM>),
- providing a membrane (<NUM>) comprising a thermoplastic material,
- providing a mould (<NUM>) having a non-flat form,
- providing a vacuum/blowing device (<NUM>) which is constructed to deform the membrane according to the shape of the mould by providing an overpressure or underpressure on one side of the membrane,
- providing a heating device (<NUM>) which is configured for providing heat to the membrane during the vacuum/blow moulding process,
the method comprising the further steps of:
- deforming the membrane with the vacuum/blowing device according to the shape of the mould, while providing heat to the membrane with the heating device, thereby allowing the thermoplastic material to deform plastically,
- cooling the deformed membrane to below the temperature wherein the thermoplastic material deforms, thereby forming the thermoplastic material in a non-flat form according to the shape of the mould, and
- connecting the membrane directly or indirectly to the housing of the expansion vessel,
wherein the method comprises using the housing or a part of the housing of the expansion vessel as the mould.