DE202008011001U1 - Assembly made of plastic parts welded together - Google Patents

Abstract

Assembly of at least two longitudinally welded at least one weld thermoplastic plastic parts, of which at least a first plastic part was melted along a weld by means of a hot gas, characterized in that at least the first plastic part is a foam plastic part with gas bubbles, which with respect to the molten plastic of the first plastic part are filled with inert gas.

Description

The The invention relates to an assembly of at least two longitudinal welded together at least one weld thermoplastic plastic parts; In particular, the invention relates to a such as a valve cover for a reciprocating internal combustion engine designed assembly.

in the Below, the invention is usually discussed with reference to a valve cover, although the invention also applies to other assemblies of the initially mentioned type, in particular for other vehicle modules.

The ElringKlinger AG presents assemblies designed as valve covers mentioned in the beginning, in which the together with each other connecting thermoplastic plastic parts from optionally Fiber-reinforced polyamide by friction welding were joined together. With the help of friction welding Although two thermoplastic parts in one Step along a generated by the friction welding and in particular endless, that is closed weld but friction welding also does some problems with it: In the so-called cold friction phase, the means before the melting of the plastic during the friction welding, finely divided abrasion, by the plastic parts are polluted, and soiled assemblies are not accepted by the vehicle manufacturers, especially then, when it comes to assemblies to be mounted on motors. Further must all for the friction welding together not only exactly positioned plastic parts to be joined, but be fixed so firmly in their positions that none of the Plastic parts during the friction welding process can be moved from its position; This may be how it turned out The following will be revealed, then be problematic if three plastic parts connected in one step by friction welding should be, namely, if in the finished assembly one of the three plastic parts is placed between the other two one of them by friction welding with the other as well as with the intermediate plastic part was connected. Finally, friction welding requires one linear course of joining surfaces, longitudinal which the plastic parts by friction welding together to be connected.

comparable Problems also arise with modules of the aforementioned Type in which at least one of two by friction welding interconnected thermoplastic parts for reasons the material and weight saving is a foam plastic part. In this case is added that in the cold friction phase, the walls of gas bubbles of foam plastic passing through the welding joint surfaces to be joined the two plastic parts are very close, destroyed can and then also form a finely divided abrasion.

Of the Invention was the object of an assembly of the above defined type with a weld or welds better quality, as well as a method for their production to develop.

These Task can be for an assembly of the beginning mentioned type according to the invention thereby solve that at least a first of the welded together Plastic parts is designed as a foam plastic part, whose Gas bubbles with one opposite the molten one Plastic this plastic part preferably filled inert gas are, and that at least one of them welded together Plastic parts, preferably the foam plastic part (and in particular both plastic parts), along a weld was melted by means of a hot gas or were, a circumstance which is related to the structure of the weld or the joined together by welding joining surfaces the plastic parts can recognize.

The so-called hot gas welding is known per se for the joining of thermoplastic plastic parts (see, for example, US Pat DE-100 19 300-B4 and the EP-1 415 789-B1 ). In this case, a hot gas, which is inert to the plastic melt, directed from one or more nozzles on against the weld to be produced corresponding areas of the plastic parts to be welded together and thereby melted in these areas of plastic, whereupon the plastic parts pressed against each other to form the weld become.

The success of hot gas welding in an inventively designed assembly is surprising, however, because it was initially to be assumed that the gas contained in the joining surface-near gas bubbles leads to losses in the quality of the welded joint and that the heating of the gas bubbles in the course of friction welding for chipping of wall areas such gas bubbles leads, which are very close to the joining surfaces of the plastic parts to be welded together, and that these chipped wall areas affect the quality of the welded joint. However, this is surprisingly not the case, which is currently explained by the fact that in the course of friction welding already in the cold rubbing phase, but at the latest then occurring heating the joining surface-near gas bubbles are destroyed and the gas then escapes, and that formed in the cold friction phase Wall particles of the destroyed gas bubbles in the course of the melting of the plastic of the plastic parts in the plastic melt rise, so that it is also possible to produce a gas-tight weld.

Because the gas bubbles of the foamed plastic part are filled with a gas inert to the molten plastic such as CO ₂ or nitrogen, and because an equally inert gas can be used for the hot gas welding, the invention does not lead to deterioration of the quality either the weld by oxidation products or other reaction products of the molten plastic.

Furthermore The invention leads to a module of lower weight and to a lesser need for the production of the Assembly required plastic.

at preferred embodiments of the invention Assembly, the gas bubbles have a cell size (Diameter) of about 150 μm or so, and In particular, the cell size is at least almost all gas bubbles in a range of about 5 microns to about 100 microns (resulting in the Present state of the so-called still to be explained so-called physical Foaming of plastics easily accomplish leaves).

There especially for vehicle modules considerable stiffness requirements and strength of the plastic parts is made for preferred embodiments of the invention Assembly recommended, at least one of the plastic parts as fiber-reinforced Part to shape. So that the reinforcing fibers as possible great homogeneity of the foam plastic part or foam plastic parts and size as well allow the distribution of gas bubbles, but also the hereinafter to be described physical foams, we recommend reinforcing fibers whose length on average, a few tenths of a millimeter.

microcellular Foamed plastics can be obtained by chemical or physical Foaming a polymer melt produce. For however, the present invention becomes physical foaming preferably, in particular in one embodiment, which has become known under the name MuCell (in which Term MuCell is a trademark of Trexel Inc., Woburn, Massachusetts, USA). In the MuCell process is used in a mixing screw extruder the thermoplastic material to be foamed plasticized and in the melt a "gas" in the form of the extruder high pressure supercritical fluid finely dispersed; it releases the "gas", whose temperature is above its critical temperature and that is present as a supercritical fluid in which Plastic melt. If this is then injected into the injection mold, the gas occurs as a result of the pressure drop occurring during injection out of the polymer melt and forms in the plastic melt a largely homogeneous foam structure with very small pores (gas bubbles), their cell size is in the range of approximately 5 to about 150 microns. Due to the power of expansion of the Gases become the plastic melt during the cooling process also pressed under sufficiently high pressure against the wall of the injection mold, so that the finished plastic part reflects the tool wall well - the The expansion force of the gas also acts at least almost evenly in all areas of the mold cavity of the injection molding tool.

The MuCell method is used for example in the WO 92/17533 described, and the WO 02/26482 describes the MuCell process in connection with the production of fiber-reinforced plastic parts.

A particularly advantageous assembly according to the invention is characterized by the fact that at least one of them to be welded plastic parts by injection molding and simultaneous physical foaming of a plastic melt produced injection molded part is. In this connection it should be noted that the physical foaming, for example, at the Homogeneity of the foam structure as well as the type of in the pores Detect the gas contained in the foam plastic.

in the MuCell method produced plastic parts according to the invention Assemblies are also characterized by the fact that they are at least almost have no so-called sink marks, which result from that during cooling of the plastic melt in the injection mold the plastic from the wall of the mold cavity of the injection mold retreats; In addition, these plastic parts no significant delays, which otherwise in injection molded Plastic parts are observed and producing qualitatively at least make it difficult to weld properly. These advantages are not just for the above-mentioned expansion power attributed to the gas, but also to that in the plastic melt as a supercritical fluid dissolved gas the fluidity of the plastic melt is greatly improved.

Preferred embodiments of the assembly according to the invention, which are in particular valve hoods for reciprocating internal combustion engines, have three longitudinally joined surfaces welded together thermoplastic Plastic parts, wherein between a first and a second outer part, an intermediate part is arranged, which rests on the first outer part and of which an edge region, in particular its peripheral edge region, with a second outer part facing the first joining surface adjacent to a second joining surface of the second outer part, and wherein a further provided on the first outer part and the second outer part facing third joining surface adjacent to a fourth joining surface of the second outer part. In a plan view of preferred embodiments of the assembly, the third and fourth joining surfaces enclose the first and second joining surfaces.

were the plastic parts over the entire joining surfaces melted, so are the joining surfaces in the finished assembly, of course, no more than interfaces recognizable.

The above-described assembly according to the invention with two outer parts and an intermediate part leaves to produce particularly cost-saving, because after hanging up the intermediate part on the first outer part, the two Welds, along the first and the second joining surface or the third and the fourth joining surface are connected to each other, simultaneously produced by hot gas welding Leave after melting the plastic in the areas the second to be generated welds the second Outer part with its joining surfaces immediately against the joining surfaces of the intermediate part and the first outer part is pressed.

at preferred embodiments of the invention Assembly are the two outer parts along one lying in the third and fourth joint surface endless weld gas-tight interconnected what can be accomplished by the fact that the two outer parts in the third and fourth joint surface through the Hot gas melted and then pressed against each other immediately. Depending on the construction and intended use of the invention Assembly can also alternatively or additionally the intermediate part and the second outer part along one lying in the first and second joining surface endless weld to be gas-tight interconnected. This is especially recommended if the second outer part forms at least one cavity together with the intermediate part, which should be gas-tight during operation of the module. Enclose the two outer parts together at least one cavity, which should be gas-tight during operation of the module, this is in preferred embodiments in a plan view the assembly within the third and fourth joint surface. Make first outer part and intermediate part together at least one of these two parts enclosed cavity, which in Operation of the assembly is to be gas-tight, embodiments preferred in which the periphery of the intermediate part of this cavity is adjacent and in a plan view of the assembly the third and fourth joining surface between a periphery at least one of the outer part and the first and the second joining surface are arranged.

In front especially when the plastic parts as components of a valve cover are designed for a reciprocating internal combustion engine, is the intermediate part approximately plate-like and therefore special simply designed; Furthermore, it is recommended then, the first outer part as a shell-like lower part and the second outer part as form approximately hood-like upper part of the valve cover.

in the In view of the advantages of use mentioned above of a foamed plastic as well as applying hot gas welding Embodiments of the assembly according to the invention preferred in which two plastic parts welded together Foam plastic parts are. For an assembly with two outer parts and an intermediate part is advantageously in the both outer parts to foam plastic parts, as a result optimize the material usage and the total weight of the assembly to let. Because the intermediate part is generally a smaller volume on the other hand, it is recommended that this be unfoamed To make plastic part, because then it with a smaller Process costs can be produced as a foamed Between part.

As already mentioned, the invention also relates to a method for producing an assembly of at least two longitudinally welded together at least one weld thermoplastic plastic parts, according to the invention at least a first of the plastic parts by injection molding and simultaneous produced physical foaming of a plastic melt is and wherein at least one of the plastic parts, preferably on both plastic parts, in one or in the to be generated Weld seam corresponding area or areas of plastic through melted at least one jet of hot inert gas whereupon the plastic parts to form the weld be pressed against each other.

In hot gas welding, the hot gas jet could be generated by means of a slot die, if the slot shape is adapted to the contour of the weld to be generated. However, preferred embodiments of the method according to the invention, in which the weld to be produced corresponding and to be melted Is heated portion of the plastic part by means of a group of fine hot gas nozzles, which are arranged according to the shape of the weld. This is particularly preferred when the weld to be generated does not extend in a plane, since then the length of the particular hollow needle-like hot gas nozzles the spatial shape of the weld to be produced can be easily adapted.

In In this context, it should be noted that in the MuCell process manufactured plastic parts for hot gas welding Especially good, because these plastic parts, at least almost have no delays, so that the outlet opening the hot gas nozzle or the outlet openings the hot gas nozzles in a series production with high repeat accuracy in a given small distance from the areas of the plastic part to be melted or the Plastic parts can be positioned or what can for the quality of the weld to be produced is of great importance.

One Hot gas nozzles having tool to design oneself also without substantial additional expenditure, that itself with him on both plastic parts to be welded together in the weld to be generated corresponding areas the plastic melts everywhere at the same time; For this it requires only one tool, which several hot gas nozzle groups has, with which the weld to be generated Heat corresponding areas of the two plastic parts at the same time to let.

at an assembly according to the invention with two outer parts and an intermediate part as described above Even simplify the manufacturing process and with high Repeat accuracy perform that intermediate part is mechanically prefixed on the first outer part, in particular by means of a clip connection, before the intermediate part with the second Outer part or the second outer part with the first Outer part and the intermediate part is welded. This way you can easily ensure that the intermediate part both during the melting of the plastic, as well as in the production of the weld or the welds a desired position on the first outer part and so that the two outer parts of the finished Assembly occupies.

Further Features, advantages and details of the invention will become apparent the attached drawings and the following description a particularly advantageous embodiment of the invention Assembly and the method and a welding tool for their production; in the drawing show:

1 an isometric view of three main parts designed as a valve cover assembly according to the invention, namely a shell-like base, a plate-like intermediate part and a hood-like upper part, before joining these three parts together;

2 a front view of the intermediate part;

3 a vertical longitudinal section through the according to 1 right area of the valve cover according to the line 3-3 in 1 ;

4A and 4B the in 3 C section on a larger scale, before the assembly of the upper part, the intermediate part and the lower part ( 4A ) and after joining the three parts ( 4B );

5 a vertical section through peripheral edge portions of the assembled three parts to illustrate the clip connection between the lower part and the intermediate part;

6 a perspective view of a tool for hot gas welding in order to connect the upper part with the lower part and the intermediate part, and

7 a side view of this tool according to 6 seen from the left.

The 1 shows a hood-like top 10 , a plate-like intermediate part 12 and a bowl-like base 14 a valve cover for a reciprocating internal combustion engine. All three parts are plastic parts made of a polyamide, which is the upper part 10 and the lower part 14 According to the invention to act in the MuCell process produced injection molded plastic foam parts, the intermediate part 12 However, a non-foamed plastic part. Furthermore, according to the invention, in particular the upper part 10 and the lower part 14 fiber-reinforced plastic parts, wherein the reinforcing fibers, in particular glass fibers, should be on average a few tenths of a millimeter long.

In the valve cap according to the invention, as will be explained in more detail later, a device for separating engine oil droplets from a gas stream integrated, which is returned from the crankcase of the engine in the intake manifold; Therefore, the valve cover has a Ölabscheidebereich 16 on; Furthermore, the valve cover according to the invention forms a vacuum storage area 18 , for the sake of completeness, it should be mentioned that the formation of the oil separation region 16 both the top 10 , as well as the intermediate part 12 and the lower part 14 are involved (together with other, to be mentioned in the following construction divide) during the vacuum accumulation area 18 only from the top 10 and the lower part 14 is formed.

The lower part 14 is with screw holes 20 fitted in order to mount the valve cover on the cylinder head of the engine (with the interposition of a seal).

In the top of the base 14 is a frame-shaped, level and horizontally extending support 22 for the intermediate part 12 molded, whose course (in a plan view of the lower part 14 ) to the contour of the periphery of the intermediate part 12 is adapted and the support of the on the lower part 14 placed intermediate part 12 serves.

In the same way is in the top of the base 14 a frame-shaped and horizontally extending support 24 for the top 10 formed. Regarding the design of the two editions 22 and 24 will be described in the following 4A and 4B directed.

The 3 leaves the inventive design of Ölabscheidebereichs 16 to recognize even more clearly, the 3 also components of the valve cover according to the invention shows which in 1 are not shown. The 3 shows the oil separation area 16 the valve cover in a vertical section.

In the oil separation area 16 are in the overall shell-like lower part 14 two oil sumps 30 and 32 molded into which flows from the above-described gas flow separated engine oil, through oil drain holes 34 and 36 of the intermediate part 12 (see also 1 ). To return the separated engine oil in the engine oil circuit of the engine, have the oil sump 30 and 32 drainage openings in their floors 38 and 40 on.

Before discussing the components of the valve bonnet according to the invention serving to separate oil droplets from said gas stream, reference will first be made to FIGS 3 . 4A and 4B described as the top 10 , the intermediate part 12 and the lower part 14 assembled and welded together.

The in 3 denoted by C and in the 4A and 4B on a larger scale cutout leaves the edition 22 of the lower part 14 for a peripheral edge area 42 of the intermediate part 12 detect; on this edition 22 becomes the intermediate part 12 placed in the correct position and by means to be described later on the lower part 14 mechanically fixed. Outside (toward the periphery of the valve cover) of the peripheral edge portion 42 of the intermediate part 12 have the lower part 14 and the top 10 before the welding of the two parts with each other in the circumferential direction of the valve cover extending and aligned ribs 44 and 46 on, where the two outer parts are welded together. Furthermore, the peripheral edge region 42 of the intermediate part 12 at its top, also along the periphery of the intermediate part 12 extending rib 48 in whose area the upper part 10 and the intermediate part 12 be welded together; at the top 10 but also one of the ribs 48 be provided corresponding to this and aligned, not shown in the drawings rib, so that the outer part 10 and the intermediate part 12 can be welded together at these two ribs.

In the 4A and 4B are the four joining surfaces in which the top part 10 with the lower part 14 and the intermediate part 12 with the shell 10 be welded or are, with 50 . 52 . 54 and 56 designated. A first joining surface 50 gets from the top of the rib 48 formed, it lies a second joint surface 52 at the bottom of the shell 10 across from. A third joining surface is from the top of the rib 44 educated; it is a fourth joining surface 56 opposite, which from the bottom of the rib 46 is formed. In the then possibly no longer existing border or joining surfaces 50 and 52 then lies after the welding of the upper part 10 with the intermediate part 12 a first weld, and in the joining surfaces 54 and 56 lies after the welding of the upper part 10 with the lower part 14 a second weld.

Before welding the three parts 10 . 12 and 14 the ribs point to each other 44 . 46 and 48 and optionally on the upper part 10 existing, the rib 48 opposite and described above rib in accordance with 4A vertical direction according to the invention a certain excess on (compared to in 4B shown state after welding the three parts together), so that during hot gas welding after the melting of the plastic in the areas of the four joining surfaces, the three parts 10 . 12 and 14 can be pressed against each other so that they are in the in 4B shown positions are connected relative to each other.

The main advantage of the in the 4A and 4B illustrated arrangement of the four joining surfaces can be seen in that after placing and prefixing of the intermediate part 12 on this and on the lower part 14 the plastic of the three parts in the areas of the four joining surfaces can be melted simultaneously and in one step, and then the upper part 10 against the intermediate part 12 and the lower part 14 pressed and so in a single step both with the intermediate part 12 as well with the lower part 14 is connected via welded joints.

The gas flow described above, in which fine and very fine oil droplets are dispersed and from which this oil mist is to be deposited in the valve cover according to the invention, passes through a in the 1 and 2 illustrated inlet opening 60 in the bottom of the base 14 from below into the valve cover and then flows through a passage opening 62 (see also the 1 ) from below into the oil separation area 16 of the top 10 , what in 3 was indicated by the arrow F. So that the gas stream to be treated not with already deposited and in the oil pan 30 oil in contact is the lower part 14 with a partition 63 Mistake. In the further course, the gas stream flows through nozzle openings 64 a wall 66 that of the oil separation area 16 of the top 10 and the intermediate part 12 formed cavity of the valve cover in two chambers A and B (see 3 ). The from the nozzle openings 64 generated gas jets then hit one as a whole 68 designated baffle, which from the top 10 extends down into the chamber A and at which the gas flow is deflected downward, so that they have a gap between the baffle wall 68 and the intermediate part 12 flows through, as in 3 was indicated by the arrow F '. Subsequently, the gas flow hits a baffle plate 70 which are from the intermediate part 12 hineinerstreckt upward into the chamber A and through which the gas flow is deflected upward so that they over the baffle plate 70 flows away, as in 3 was indicated by the arrow F ''. The gas stream then leaves the chamber A through an opening, not shown in the drawings, in the oil separation area 16 of the top 10 ,

On the upstream side of the wall 66 in the chamber B separated oil droplets flow on the upstream side of the wall 66 down, and this separated oil flows over the drain hole 34 in the oil sump 30 , On the upstream side of the baffle 68 and optionally on the upstream side of the baffle plate 70 drop in the chamber A separated oil droplets or flow also down and get to the intermediate part 12 ; There resulting oil then flows through the drain opening 36 in the oil sump 32 from. The flapper 70 but mainly serves the purpose through the baffle wall 68 isolated and on the intermediate part 12 formed bottom of the chamber to prevent oil from being entrained by the gas stream, but it through the drain opening 36 to expire.

Based on 2 and 5 will now be explained how the intermediate part 12 at the bottom 14 is prefixed, so it when welding the upper part 10 with the intermediate part 12 and the lower part 14 in the correct position on the lower part 14 is positioned. In this context it should be mentioned that the 5 a slightly different periphery of the intermediate part 12 shows as the 3 . 4A and 4B ,

Near the peripheral edge area 42 ' of the intermediate part 12 are at the bottom tongue-like first locking elements 74 molded, and indeed has the intermediate part 12 along its periphery several such spaced apart locking elements 74 on whose lower free ends are made of the in 5 shown position elastically to the left (according to 5 ). To the inside of the wall of the base 14 are like locking lugs designed second locking elements 76 formed, in positions which the positions of the locking elements 74 match if the intermediate part 12 on the lower part 14 is up. Because the locking elements 74 and 76 are provided with oblique sliding surfaces, snap the locking elements 74 under the locking elements 76 one, if one the intermediate part 12 from above on the lower part 14 hangs up and pushes down, so that the above-described locking or clip connection between the intermediate part 12 and the lower part 14 results.

Based on 6 and 7 the basic structure according to the invention of a tool for hot gas welding is explained below, with which in the areas to be welded together (joining surfaces) of the upper part 10 , the intermediate part 12 and the lower part 14 the valve cap according to the invention, the plastic can melt in one step and everywhere at the same time, whereupon the upper part 10 immediately against the intermediate part 12 and the lower part 14 pressed and so welded to these two parts.

That in the 6 and 7 shown and as a whole with 100 designated tool has several compressed gas connections 102 for supplying a gas which is inert and pressurized with respect to the plastic melt, which is in particular nitrogen. The compressed gas flows in a manner not shown in a heat exchanger 104 , which contains at least one not shown electrical heating element, with which the compressed gas when flowing through the heat exchanger 104 is heated. At the top and bottom of the heat exchanger 104 there is a nozzle plate in each case 106 respectively. 108 ; the nozzle plate 106 is with a first group 110 of fine, hollow needle-like hot gas nozzles 112 provided, which plate from the nozzle 106 extend vertically upward and have at their upper ends fine outlet openings for the hot gas. In the same way is the nozzle plate 108 with a second group 114 from fine, hollow needle-like hot gas nozzles 116 provided, extending from the nozzle plate 108 extend vertically downwards and have at their lower ends fine outlet openings for the hot gas. The hot gas nozzles are removed from the heat exchanger 104 fed with hot gas.

What simplicity in the 6 and 7 was not or only partially shown, are the following features of the tool according to the invention:
After the first and second joining surfaces 50 . 52 lie in a different horizontal plane than the third and fourth joining surfaces 54 . 56 , indicates both the first nozzle group 110 , as well as the second nozzle group 114 two subgroups of hot gas nozzles each 112 respectively. 116 in which the hot gas nozzles of one subgroup are longer than those of the other subgroup or the nozzle openings of one subgroup in another, according to 7 horizontal plane lying as the nozzle openings of the other subgroup. Furthermore, according to the invention, the nozzle openings of a subgroup of the group 110 arranged along a first curved path, the nozzle openings of the other subgroup of the group 110 along a second curved path, the nozzle openings of a subgroup of the group 114 along a third cam track and the nozzle openings of the other subgroup of the group 114 along a fourth curved path, wherein the course of the first curved path the course of the second joining surface 52 , the course of the second curved path the course of the fourth joint surface 56 , the course of the third curved track the course of the first joint surface 50 and the course of the fourth curved path the course of the third joint surface 54 equivalent. Furthermore, the nozzle openings of the hot gas nozzles of the four subgroups lie in such planes that the tool 100 from the lower part 14 and the intermediate part 12 lifted top 10 between the latter and the lower part 14 including intermediate part 12 insert and position so that the nozzle openings of the different hot gas nozzle subgroups of the joining surfaces associated with the subgroups all have the same small distance, which is preferably of the order of 2 mm. In this context, it has a positive effect that according to the invention produced by the MuCell method upper and lower parts 10 respectively. 14 at least have no significant delays, so that the second, third and fourth joining surface 52 respectively. 54 respectively. 56 are and are in accordance with the 4B horizontal levels.

According to the invention, the nozzle openings of the hot gas nozzles 112 . 116 Diameter in the order of 1 mm, and for parts to be welded together, preferably a polyamide is used whose melting temperature is about 270 ° C; then the tool used for hot gas welding is operated so that the temperature of the gas leaving the hot gas nozzles is in a range from about 350 ° C to about 400 ° C.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 10019300 B4 [0007]
• EP 1415789 B1 [0007]
• WO 92/17533 [0014]
• WO 02/26482 [0014]

Claims (17)

Assembly of at least two longitudinally welded at least one weld thermoplastic plastic parts, of which at least a first plastic part was melted along a weld by means of a hot gas, characterized in that at least the first plastic part is a foam plastic part with gas bubbles, which with respect to the molten plastic of the first plastic part are filled with inert gas. Assembly according to claim 1, characterized in that that the gas bubbles have a cell size of have a maximum of about 150 microns. Assembly according to claim 2, characterized in that that the gas bubbles are a cell size in the range of about 5 to about 100 microns exhibit. Assembly according to one of claims 1 to 3, characterized in that at least one of the plastic parts a fiber reinforced part is. Assembly according to claim 4, characterized in that that the reinforcing fibers on average a few tenths of a millimeter are long. Assembly according to one of claims 1 to 5, characterized in that at least one of the plastic parts the thermoplastic is a polyamide. Assembly according to one of claims 1 to 6, characterized in that at least the first plastic part one by injection molding and simultaneous physical Foaming a plastic melt produced injection molded part is. Assembly according to one of claims 1 to 7, which three longitudinal joining surfaces with each other has welded thermoplastic plastic parts, characterized characterized in that between a first and a second outer part an intermediate part is arranged, which on the first outer part rests and its peripheral edge portion with a second outer part facing first joining surface to a second joining surface of the second outer part adjoins that one on the first outer part provided and the second outer part facing third Joining surface to a fourth joining surface the second outer part adjacent, and that in a plan view on the assembly the third and the fourth joint surface the enclose first and second joining surfaces. Assembly according to claim 8, characterized in that that the first as well as the second joining surface in a first level and the second and the third joint surface lie in a second plane, which is opposite to the first Plane is offset in the direction perpendicular to this plane. An assembly according to claim 8 or 9, characterized that the two outer parts along one in the third and fourth joint surface lying endless Welded gas-tight interconnected. An assembly according to any one of claims 8 to 10, characterized in that the periphery of the intermediate part at least one of the first outer part together with the intermediate part formed cavity is adjacent and that in a plan view the assembly the third and the fourth joint surface between a periphery of one of the outer parts and the arranged first and second joining surface are. Assembly according to one of claims 8 to 11, characterized in that the second outer part together with the intermediate part and / or the first outer part at least forms a cavity, which in a plan view of the assembly within the third and the fourth joining surface is located. An assembly according to any one of claims 1 to 12, characterized in that the plastic parts as constituents a valve cover for a reciprocating internal combustion engine are designed. Assembly according to claims 8 and 13, characterized in that the intermediate part is approximately plate-like is designed and that the first outer part is a shell-like Lower part and the second outer part about Hood-like upper part of the valve cover form. Assembly according to one of claims 1 to 14, characterized in that two welded together Plastic parts are foam plastic parts. Assembly according to one of claims 8 to 14, characterized in that both outer parts foam plastic parts are. An assembly according to any one of claims 8 to 16, characterized in that the intermediate part is a non-foamed Plastic part is.