WO1992018307A1

WO1992018307A1 - A machine for casting hollow objects, in particular concrete pipes, and comprising two mutually axially movable mould parts

Info

Publication number: WO1992018307A1
Application number: PCT/DK1992/000126
Authority: WO
Inventors: Bent Herrmann
Original assignee: Pedershaab A/S
Priority date: 1991-04-19
Filing date: 1992-04-15
Publication date: 1992-10-29
Also published as: DK71191D0; AU1689092A

Abstract

A machine serves to cast concrete pipes substantially vertically by means of successively progressing zonewise vibration compression. The machine comprises inner and outer mould parts (11, 12) which are displaced axially with respect to each other during the casting process. The inner mould part (11) is divided into at least two sections (13, 14) which are interconnected via one or more elastic spacers (15), and which are adapted to rotate with respect to each other. Further, the lowermost section (14) is firmly or rotatably fixed in a stationary or axially slidable part of the machine. The conical portion (19) is moreover provided with a multi-thread screw (20) for compressing the vibrated concrete. By means of the above-mentioned structure it is now possible to cast e.g. concrete pipes having a well-defined higher and more uniform quality throughout the longitudinal direction with a reduced consumption of energy and reduced noise nuisances, than it has been possible in the past to produce with conventional machines of this type.

Description

A machine for casting hollow objects, in particular concrete pipes, and comprising two mutually axially movable mould parts

The invention concerns a machine for casting hollow objects, in particular pipes of concrete or a similar material, substantially vertically by means of successively progressive zonewise vibration compression, comprising inner and outer mould parts, respectively, which are displaced axially with respect to each other during the casting process.

Traditionally, casting of e.g. concrete pipes takes place by filling the ring gap between an inner and an outer stationary mould part from above with fresh concrete which is continuously vibrated. The vibrations are generated by one or more vibrators, which are usually positioned in the inner mould part, from which the vibrations spread to the outer mould part via the concrete. The vibrator must therefore have a sufficiently great capacity to be able to subject the relatively great vibrating mass of the concrete as well as of the entire mould system to corre¬ spondingly great acceleration forces. Furthermore, the vibrator must be capable of providing the energy which the vibrating system consumes, and which is just partly utilized in the actual vibration compression process, since a considerable part of the added energy is instead converted to heat and very trying noise in the actual mould system. In practice, the overall great mass of the system, which is thus to be accelerated repeatedly in rapidly changing directions, limits the possibility of working with sufficiently high vibration frequencies for optimum achievement of the liquidity in the concrete which is necessary for effective compression of it. Since the mass of the vibration system is not constant, but, on the contrary, constantly changes as the mould is filled with concrete, the vibration state of the system will vary greatly during casting. Nor is the concrete vibrated throughout for the same length of time, it being kept in a vibrating state for a longer time downwardly in the mould than upwardly. These conditions in combination entail that the concrete is subjected to a process which varies in dependence on the level of the concrete in the mould, and that the finished pipe therefore cannot be made with the desired uniform and well-defined quality throughout in the longitudinal direction.

In a further development of the above-mentioned method, with a view to obtaining a better result, the vibrator is positioned right at the top of the inner mould part, and the two mould parts are displaced axially with respect to each other during casting. This displacement can take place e.g. in that the outer mould part is stationary, while the inner mould part continuously rises from below up into the outer mould part, while mould is simul¬ taneously successively filled with so much fresh concrete that a specific layer of concrete is constantly disposed around the area of the top of the mould part where the vibrator is present. This layer, which moves upwardly in the mould together with the rise of the inner mould part, will substantially be in the same state of vibration in all the levels of the mould and will moreover be vibrated for the same period of time, so that the vibration compression process can be controlled better and the finished pipe can have a more uniform quality in the longitudinal direction.

However, the previously mentioned drawbacks associated with casting in stationary mould parts are not eliminated completely. The reason is that the vibrations propagate from the top down through the inner mould part and thereby still bring the concrete into vibration states, which depend on the level of the concrete in the mould, during the course of the overall casting process, just as, in this case too, the concrete is vibrated for a longer time downwardly in the mould than upwardly. Nor is it therefore possible by means of this improved method to cast a concrete pipe which fully satisfies the requirements which are today made with respect to a well-defined high and uniform quality of a concrete pipe throughout its overall length.

The object of the invention is to provide a machine of the type mentioned in the opening paragraph, which, with a minimum energy consumption for the vibration compression process, is capable of casting hollow objects, in parti¬ cular concrete pipes, with a well-defined higher and more uniform quality throughout the longitudinal direction than known before.

This is achieved by the machine of the invention which is novel and unique in that the inner mould part is divided into at least two sections which are interconnected via one ore more elastic spacers, and which are adapted to rotate with respect to each other, the lowermost section being firmly or rotatably fixed in a stationary or axially displaceable part of the machine, the uppermost section containing at least one vibrator.

This structure provides the advantage that the lowermost section of the inner mould part does not participate in the vibration of the concrete, but instead serves to quietly support the not yet set, deposited concrete. Since it is moreover now just the relatively small mass of an axially narrow zone around the position of the vibrator in the uppermost section which is now affected by the vibration forces, the energy supply to the system can be reduced drastically with respect to the conventional technique, whereby the amount of energy lost in the form of heat and noise in the outer mould part is reduced correspondingly. When the overall system consisting of the casting mould and the concrete is arranged as an absorber system with the casting mould as the main system and the concrete as the subsystem in the manner described in the applicant's Danish Patent Application, DK xxxx/91, "An absorber system for casting hollow objects, in particular concrete pipes, by means of vibration compression", which has the same filing date as the present patent application and which is incorporated herein by reference, the supply of energy can essentially be reduced to comprising just the energy which is consumed for vibrating the concrete.

During the mutual axial displacement between the two mould parts, the narrow vibration zone successively travels upwardly with respect to the casting mould and is subjected to quite the same vibration effect in all of its levels, while the already deposited and vibration com¬ pressed concrete is kept at rest by the lowermost section of the inner mould part. Since the concrete is therefore subjected to the same process throughout the casting mould, it will consequently now also be possible to make concrete pipes which have a completely uniform and well- defined high quality in the entire longitudinal extent of the pipe.

The vibrations entail that the concrete is brought into a form of liquid compressible state. The volume of the concrete is reduced during the compression, and in the prior art this causes vertical transport of material to take place in the ring gap between the two mould parts, so that filling of the mould and the compression of the concrete are impeded significantly because of the friction between the concrete and the mould parts. To avoid this transport of material and the consequent drawbacks the uppermost section of the inner mould part has an upper conical portion with a conicity which downwardly narrows the cross sectional area of the ring gap between the conical portion and the inner side of the outer mould part in a proportion which is substantially inversely propor¬ tional to the increase in density which the compression simultaneously imparts to the concrete during the relative axial displacement between the two mould parts.

Compression of the concrete requires that the concrete is loaded with a suitable static pressure, which conventio¬ nally just consists of the weight of the overlying column of concrete. However, loading of the narrow vibration zone, which does not have any greater amount of concrete disposed thereover is relatively small, and, according to the invention, the conical portion of the upper section is therefore equipped with a generally multi-thread screw, which serves to transport the concrete downwardly and to simultaneously apply to the concrete a predetermined down¬ wardly directed static pressure during the compression process, which can thus be controlled optimally by means of a suitable selection of the pitch of the convolutions and the speed of rotation of the uppermost section.

When the spigot end on a concrete pipe is to be cast, a profile ring is usually employed, having the same nominal inside diameter as the pipe. For the spigot end to be cast in immediate continuation of the pipe shank, the convolu¬ tions are dimensioned with an outside diameter which is throughout at least slightly smaller than the inside diameter of the profile ring opening, so that the upper¬ most section can pass upwardly through this opening during the final phase of the casting. The uppermost section of the inner mould part moreover has a cylindrical portion which is in continuation of the conical portion, and which, during the rotation of the uppermost section, acts as a trowel that smooths the concrete and imparts a tight and smooth inside surface to the finished pipe.

Since the vibration zone is axially relatively narrow, and since it is intended to restrict the vibrations to this narrow zone, a rotating vibrator is used, imparting rotating horizontal vibrations to the uppermost section. In a particularly advantageous embodiment another type of vibrator is used, imparting horizontal vibrations to the uppermost section with directions of deflection which lie in a predetermined fixed plane through the axis of the section. This additionally reduces the amount of concrete which vibrates, and since the deflection plane follows the rotation of the section it is simultaneously ensured that the concrete is vibration-compressed completely uniformly around the periphery of the pipe.

In addition to the smoothing which is performed by the cylindrical portion of the uppermost section, as mentioned above, it is frequently necessary to improve the inside surface of the finished pipe additionally. For this pur¬ pose a surface improving material is pumped via a pipe connection out through at least one opening in the upper¬ most section. When this rotates with simultaneous mutual axial displacement of the two mould parts, this opening describes a screw line, whereby the surface improving material is applied to or incorporated in the concrete surface evenly and uniformly. Examples of useful surface improving materials include fine concrete, plastics or water. With a view to measuring the compression quality of the deposited concrete a sensor is arranged in the uppermost section. This also describes a screw line during the rota¬ tion of the section and therefore successively passes all areas of the deposited mass of concrete, which can thus be detected completely. The vibration compression process can be optimized by adjusting one or more of the parameters comprising the relative displacement speed between the mould parts, the rotational speed of the uppermost section and the vibration frequency and/or vibration amplitude.

These parameters are then controlled by means of the sig¬ nal from the sensor such that the desired density of the concrete is kept constant during the entire casting pro¬ cess.

The invention will be explained more fully by the follow¬ ing description of embodiments which just serve as examples, with reference to the drawing, in which

fig. 1 schematically shows how casting of a concrete pipe takes place by means of a traditional arrangement with inner and outer stationary mould parts,

fig. 2 shows a concrete pipe which is being cast, with a displaceable inner mould part successively rising into the outer mould part as the mould is filled with concrete,

fig. 3 shows the same, but with the inner mould part now replaced by an inner mould part which is divided into two separate sections by means of an elastic spacer,

figs. 4-8 are partial axial views of various stages in the casting of a concrete pipe by means of a machine according to the invention,

fig. 9 is an axial section through the top of the inner mould part shown in figs. 4-8,

fig. 10 is a side view of the top of a rotating inner mould part according to the invention with incorporated sensor,

fig. 11 shows the same, but in a partial axial section, where the sensor is clearly visible,

fig- 12 is a side view of the top of an inner mould part according to the invention with the outer wall partly removed so that a pipe connection to a surface improving material is visible, and

fig. 13 shows the same, but in a partial axial section.

Fig. 1 shows the principle of a traditional arrangement for casting concrete pipes with an inner mould part 1 and an outer mould part 2, both of which are stationary during casting. The arrangement moreover comprises a bottom ring 3, which supports the concrete pipe 5 and also the outer mould part 2 in the shown case. Further, a profile ring for shaping the spigot end of the pipe is provided up¬ wardly in the arrangement. As shown, the mould is com- pletely filled with concrete, following which the finished pipe is now ready for demoulding, which can take place in various ways. For example, the inner mould part, which is fixed on an axially displaceable machine part 6, can be pulled out of the pipe by lowering the machine part 6, following which the outer mould part and the bottom ring 3, which is fixed on a table 7, together with the cast pipe are driven out to the curing site where the outer mould part is then removed. In other embodiments of the machinery the outer mould part is removed directly in the machine before the pipe standing on the bottom ring 3 is driven out to the curing site. However, in all cases the mould parts are stationary during the actual casting, and the vibrations take place everywhere in the concrete, as indicated by its signature symbolizing the vibration state, which entails that the overall system of concrete, mould parts and mechanical fixing parts is vibrated as one great overall mass with the consequent drawbacks. As mentioned above, these drawbacks comprise the necessity of using very large vibrators to keep the great overall mass in constant vibrations, consumption of a considerable amount of energy for this, which is lost to a great degree in the actual mould system as e.g. trying noise, as well as process variations in response to the level of the concrete in the mould, so that it is not possible to cast a pipe with a well-defined and uniform quality in the longitudinal direction.

The arrangement shown in fig. 2 corresponds to the one described above and shown in fig. 1, except that there is just one vibrator (not shown) right at the top of the inner mould part 8, and that this now rises successively in the outer mould part 9 as the concrete 10 is filled into the mould from above. As symbolically indicated by the signature of the concrete, the concrete is then vibrated particularly intensively in a layer around the top, such that the concrete goes through the same process in the various levels of the mould as they are passed by the top of the inner mould part. With respect to the traditional method with stationary mould parts mentioned above with reference to fig. 1, the finished pipes there- fore obtain a more uniform quality in the longitudinal direction in this case, but since the vibrations inevitably propagate from the top down through the inner mould part, the process will still vary to a certain degree with the level of the concrete in the mould, and the finished pipes can therefore still not be cast with a satisfactory uniform quality in the longitudinal direction.

Fig. 3 shows an arrangement according to the invention with an inner mould part 11, which, like in fig. 2, rises in the outer mould part 12. However, the inner mould part 11 is now divided into an upper section 13 and a lower section 14 via an elastic spacer 15, which may e.g. be of rubber or a similar material. In this case too the vibrator (not shown) is positioned at the top of the inner mould part, viz. the upper section 13, but elastically isolated with respect to the lower section 14, which therefore does not participate in the vibration of the concrete 16, but, on the contrary, serves as a support for the deposited and not yet set concrete which is kept at rest in this manner against outer impacts. As indicated by the signature of the concrete, it is now just a relatively narrow zone of concrete around the upper section which is in a vibrating state at a given point of time. The under¬ lying concrete 18, which usually constitutes the greater part of the overall mass of concrete, is kept at rest by the lower section 14, as mentioned. Owing to the highly reduced mass of the narrow vibration zone it is now possible to use a vibrator which has a much smaller capacity than the vibrators used in conventional machines of this type. The resulting considerable reduction in the vibration forces and the vibration energy entails that the outer mould part is now just vibrated to a minor degree, so that the energy loss and noise nuisances are limited significantly.

When the overall system, which consists of casting mould, mechanical fixing parts and concrete, is adapted as an absorber system with the casting mould as the main system and the concrete as the subsystem in the manner described in the applicant's Danish Patent Application No. xxxx/91, "An absorber system for casting hollow objects, in parti- cular concrete pipes, by means of vibration compression", which has the same filing date as the present application and is incorporated herein by reference, the supply of energy to the system can essentially be restricted to just comprising the energy necessary for vibrating the narrow zone of concrete around the upper section 13.

The small mass of this zone with respect to the mass of the overall system enables the use of much higher vibration frequencies than before, whereby the intended liquidity in the concrete is obtained optimally. When the concrete is in this state, it can easily be compressed, thereby reducing its volume and correspondingly increasing its density. To prevent this reduction in volume from causing vertical transport of material in the relatively narrow ring gap between the two mould parts, which would lead to an energy-consuming friction which would also impede the free course of the casting process in an adverse manner, the upper section 13 is provided with an upper conical portion 19 whose conicity is adapted such that the cross sectional area of the ring gap is narrowed downwardly in a proportion which is inversely proportional to the increase in density which the compression simul¬ taneously imparts to the concrete during the relative axial displacement between the two mould parts.

For the compression to take place, the concrete, as ex¬ plained above, must be brought into a liquid state by the vibrations, and the concrete must be affected by a verti- cal static pressure which is usually provided by the weight of the overlying column 18 of concrete. In the arrangement of the invention shown in fig. 3, the concrete column positioned above the narrow vibration zone is rela¬ tively low, and the static pressure on the concrete is therefore small and varies greatly with just small changes in the filling rate. The upper section 13 is therefore adapted such that it is caused to rotate during casting by means of a drive assembly (not shown), and the conical portion 19 of the section is moreover provided with preferably several convolutions which, at a specific speed of rotation, apply to the concrete a predetermined down¬ wardly directed pressure which is considerably greater than the pressure from the overlying concrete column. This pressure can be regulated by changing the speed of rotation and by means of a suitable selection of the pitch of the convolutions. It has been found that the optimum pressure impact on the concrete can be obtained best when this pitch is relatively small and when there are several convolutions which together form a multi- hread screw.

The conical portion 19 of the upper section 13 downwardly merges into a cylindrical portion 21, which serves as a trowel smoothing the surface of the concrete during the rotation of the section 13. The cylindrical portion 21 also serves to calibrate the inside diameter of the cast concrete pipe, after the compression process at the conical portion 19 has been completed. If the height of the cylindrical portion is too low, the calibration will be incomplete, since it can then partly take place in the transition zone to the not yet finish-compressed concrete, and if the cylindrical portion is too high, too great friction forces can easily occur between the portion and the concrete, which is thereby affected by a torque, which can entail deformation in the pipe. The best solution to these conflicting conditions is that the height of the cylindrical portion is between 0.1-1.0, preferably between 0.3-0.7 and in particular between 0.4-0.6 times the diameter of the portion.

Figs. 4-8 show various stages in the casting of a concrete Pips by means of a machine according to the invention. The machine, which is only shown in part, corresponds to the machine shown in fig. 3, and the same reference numerals are used. Further, the machine may be of any type, pro¬ vided that it can retain and guide the mould parts as shown and described in the present patent application. In the shown embodiment the uppermost end part of the outer mould part 12 is shaped as a filling hopper 22, which downwardly merges into the profile ring 4, which is thus firmly connected with the outer mould part. Above the filling hopper 22 there is moreover provided a conveyer belt 23 for filling the mould with fresh concrete in a manner known per se at the desired rate. The bottom ring 3, which carries the weight of the concrete, rests on a support ring or support leg 24, which in turn rests on a number of rubber buffers 25 for preventing the vibrations in the concrete from propagating to the table 7.

In fig. 4, the casting process is in the initial phase, where the inner mould part 11 is raised precisely so much that the cylindrical portion 21 of the upper section 13 closes the central opening in the bottom ring 3.

Simultaneously, fresh concrete is filled into the mould by means of the conveyer belt 23, and, as will be seen, the socket of the pipe is almost already finished. During the continued rise of the inner mould part 11, the shank of the pipe is cast, as shown in fig. 5, and in fig. 6 the shank is finished, and the casting of the spigot end almost completed. The supply of fresh concrete is now stopped.

In conventional methods, the spigot end of concrete pipes is normally formed by means of a profile ring which is independent of the outer mould part, and which is finally pressed down against the spigot end during casting to provide the necessary static pressure for compressing the concrete at the spigot end. This involves considerable problems in compressing the concrete at the spigot end correctely and uniformly, while obtaining a sufficiently accurate tolerance of the length of the pipe. This, how¬ ever, is possible by means of the arrangement shown in figs. 4-8, since the profile ring 4 is now firmly connected with the outer mould part 12, and the distance between the end faces of the pipe therefore constitutes a fixed quantity. Filling and compression of the spigot end are simultaneously ensured by means of the compressing action of the multi-thread screw 20 of the conical portion 19 on the concrete at the spigot end, and, for the screw 20 to perform this function also in the final phase, the supply of fresh concrete is continued so that the upper section 13 is kept covered by a layer of concrete until the spigot end is substantially finish-compressed. For this finish-compression to take place it is, as will appear, necessary in the final phase to feed an excess of concrete to the mould, which, as shown in fig. 7, is compressed in the lowermost conical part of the filling hopper 22 during the continued rise of the inner mould part.

Until the inner mould part 11 has reached the position shown in fig. 7, where also the excess of concrete in the filling hopper 22 has been compressed, the vibrator (not shown) is kept in continuous operation and thereby emits the vibrations shown by the symbol 26. Then the vibrator is stopped and the demoulding operation begins, the inner mould part 11, as shown in fig. 8, being again lowered to be withdrawn completely from the finished concrete pipe, which now has such a stable shape because of the compression that it can carry itself without support from the mould parts. The same applies to the excess of concrete which is disposed in the filling hopper 22, and which therefore does not drop out; even though the inner mould part is lowered and no longer supports the excess of concrete. However, when the next pipe is cast, the concrete residue will be entrained by the fresh concrete and used together with it.

For the spigot end of the pipe to be cast and compressed in the manner described above, the upper section 13 of the inner mould part 11 must be capable of passing the central opening of the profile ring 4, and the convolutions 20 of the conical portion 19 therefore have an outside diameter which is sligthly smaller than the diameter of this opening.

The outer mould part may be removed in a variety of ways known per se, which e.g. comprise driving the mould part with the finished pipe to the curing site and removing it here, or lifting the filling hopper and the outer mould part clear of the cast pipe in the actual machine, but separately. In the particularly advantageous embodiment shown in figs. 4-8 the filling hopper and the outer mould part are intregal, as mentioned before, and, after the conveyer belt 23 has been pulled out to the side, they are lifted as a body clear of the finished pipe, which now has such a stable shape that it can be driven to the curing site without support. However, during this transport and until the pipe has cured, it remains standing on the bottom ring 3.

The inner mould part 11 may be firmly or rotatably fixed on the axially upwardly and downwardly displaceable machine part 6. It has been found that the inner mould part can more easily be moved up and down in the deposited concrete when its lower section 14 simultaneously rotates quite slowly. In that case the upper and lower section 13, 14 are caused to rotate in separate directions for the resulting torsional moment, which affects the concrete, to be as small as possible. In an advantageous embodiment of the machine the inner and outer mould parts 11, 12 are moreover displaced in counter phase during demoulding, so that the tensile forces, to which the cast, but not yet set pipe are subjected, substantially neutralize each other.

Fig. 9 is a fragmentary view of the uppermost part of the inner mould part 11 shown in figs. 4-8 with the upper section 13 and the lower section 14. In this case the mould part is built as a plate structure, and the spacer 15 is a ring consisting of rubber or another suitable elastic material. The ring, which is positioned in recesses in the sections 13, 14 and keep these mutually spaced, can rotate freely with respect to at least one of the sections by means of a slip means, and the outer side of the ring is flush with the outer side of both sections. The ring ensures that no noticeable vibrations are trans¬ mitted from the upper to the lower section 13, 14.

A rotating vibrator 27 with swing bodies 28 is incorporated in the upper section 13. This vibrator 27 may be of any structure and will therefore not be described more fully here. The vibrator is caused to rotate by means of a drive assembly (not shown) via a vertical shaft 29 and an elastic coupling 30, preventing the vibrations from the vibrator 27 from propagating to the shaft 29 and from there further on to the rest of the mould system. The vibrator 27 is positioned in a sleeve 31, which is in turn connected with the outer wall in the upper section 13 of the mould part by means of two ring-shaped plates 32a, 32b. Further, the sleeve 31 is downwardly fixed on a ring- shaped plate 33 with bolts 34. This ring-shaped plate 33 is in turn fixed to an underlying flange 36 with bolts 37, the ring-shaped plate 33 and the flange 36 being kept mutually spaced by a plurality of rubber buffers 35 serving to prevent the vibrations from the vibrator 27 from being transmitted to the flange 36 and from there further on to the rest of the mould system. This is thus completely vibration-insulated from the upper section 13 by means of the rubber ring 15, the elastic coupling 30 and the rubber buffers 35.

The flange 36 is rotatably journalled with a ball bearing 39 in a vertical stationary pipe 40, which is fixed on the machine part (not shown) which serves to raise and lower the entire inner mould part during the casting process. The flange is moreover welded to a vertically downwardly extending pipe 38, which can be caused to rotate by means of a drive assembly (not shown) arranged downwardly in the inner mould part or directly below it.

As previously mentioned, the vibrator 27 rotates continuously during the actual casting process, and the upper section 13 simultaneously rotates in the same or the opposite direction. The vibrations take place with a frequency of between 50 and 250 Hz, and the upper section 13 rotates at a rate of up to 200 rotations per minute.

Instead of the shown rotating vibrator, it is possible to use a vibrator which just brings the uppermost section into horizontal vibrations with directions of deflection which lie in a fixed plane through the axis of the section 13, and which therefore rotate with the same speed of rotation as the section. When this vibrator type is used, the mass of the concrete amount subjected to vibrations at a given moment is additionally reduced to a significant degree with the consequent advantages in the form of e.g. reduced energy consumption and reduction in trying noise from the mould system.

The optimum vibration compression conditions which are capable of keeping the density of the concrete at the desired level during the entire casting process, can be provided by adjusting one or more of the parameters, the relative speed of displacement between the mould parts, the speed of rotation of the uppermost section, as well as the frequency and/or amplitude of the vibrations. These parameters are controlled by preprogrammed computer in response to the signals from a sensor 41, which is positioned in the upper section 13, as shown in figs. 10 and 11. The sensor may e.g. be adapted to measure the rate of an ultrasonic pulse during the passage to and fro through the wall of the cast pipe to the inner side of the outer mould part 12 (fig. 11), and the sensor system is then to be constructed as a transceiver system measuring the time delay of the reflected signal from the outer mould part. It will be seen from fig. 10 how the sensor 41 describes a screw line because of the rotation of the upper section 13 and the rise of the inner mould part 11. The wall of the cast concrete pipe is hereby inspected completely in its entire extent, and the mentioned ultrasound rate can be used as a measure of the quality of the compression by comparison with reference values known in advance, and the quality of the compression can thus not only be registered passively, but also be controlled actively if desired. For this purpose not only ultrasound sensors may be use, but also other suitable forms of sensors, e.g. X-ray sensors.

Figs. 12 and 13 show a pipe connection 43 which terminates in an outlet opening 42 in the upper section 13. The pipe connection 43 is connected with a reservoir (not shown) of a surface improving material, which can thereby be con- veyed to the outlet opening 42 by means of e.g. a pump (not shown). This describes a screw line in the same manner as shown in fig. 10 for the sensor 41, and the sur¬ face improving material will therefore be applied com¬ pletely uniformly over the internal surface of the con- crete pipe, where it is additionally smoothed by means of the cylindrical portion 21 of the upper section 13. The surface improving material may be fine concrete or plastics which settles as a thin layer on top of the concrete, or e.g. just water which brings the outermost surface of the concrete into a liquid state which can easily be smoothed to an even and tight layer.

As appears from the foregoing, the machine of the invention now makes it possible to cast a concrete pipe with a well-defined high and uniform quality throughout the longitudinal direction and with a reduced energy con¬ sumption and transmission of less trying noise than known before. Because of the described control system the pro¬ cess is moreover self-adjusting, such that a uniform quality is ensured from pipe to pipe in series of a desired size. To this should be added that the process can easily and quickly be switched from one quality to another by changing the production parameters via the control system.

Although embodiments of the machine according to the invention for production of cylindrical pipes have been described above and shown in the drawing, other embodi¬ ments of the machine are readily conceivable within the scope of the invention, and such other embodiments may e.g. be adapted to cast pipes which are four-sided or six- sided exteriorly. Correspondingly, for the casting there may conceivably be used other materials, which are suit¬ able for casting by means of vibration compression, than concrete e.g. the material described in the Danish Patent Application 1175/89, "A method of making acid-proof sulphur concrete pipes".

Claims

P a t e n t C l a i m s:

1. A machine for casting hollow objects, in particular pipes of concrete or a similar material, substantially vertically be means of successively progressing zonewise vibration compression, comprising inner and outer mould parts, respectively, which are displaced axially with respect to each other during the casting process, c h a r a c t e r i z e d in that the inner mould part is divided into at least two sections which are inter¬ connected via one or more elastic spacers and which are adapted to rotate with respect to each other, the lower¬ most section being firmly or rotatably fixed in a statio- nary or axially displaceable part of the machine, the uppermost section containing at least one vibrator.

2. A machine according to claim 1, c h a r a c t e r ¬ i z d in that the uppermost section has a lower cylindrical portion and an upper conical portion with a conicity which downwardly narrows the cross sectional area of the ring gap between the conical portion and the inner side of the outer mould part in a proportion which is substantially inversely proportional to the increase in density which the compression simultaneously imparts to the concrete during the relative axial displacement between the two mould parts.

3. A machine according to claim 2, c h a r a c t e r - i z e d in that the diameter of the cylindrical portion corresponds to the desired inside diameter of the cast pipe.

4. A machine according to claim 2 or 3, c h a r a c t e r i z e d in that the height of the cylindrical portion is between 0.1-1.0, preferably between 0.3-0.7 and in particular between 0.4-0.6 times the diameter of the portion.

5. A machine according to claim 2, c h a r a c t e r i z e d in that the conical portion is equipped with least one convolution.

6. A machine according to claim 5, c h a r a c t e r i z e d in that the outside diameter of the convolut is equal to or slightly smaller than the diameter of cylindrical portion.

7. A machine according to one or more of claims 1-6, c h a r a c t e r i z e d in that the vibrator is ad to bring the uppermost section into horizontal vibrat with directions of deflection which rotate about the of the section.

8. A machine according to one or more of claims 1-6, c h a r a c t e r i z e d in that the vibrator is ad to bring the uppermost section into horizontal vibrat with directions of deflection which lie in at least o predetermined firm plane through the axis of the sect and which rotate together with the section.

9. A machine according to one or more of claims 1-8, c h a r a c t e r i z e d in that the uppermost sect of the inner mould part is formed with at least one o opening, which communicates via a pipe connection or like with a pressure source for feeding a surface improving material, such as fine concrete, plastics o water, to the outlet opening.

10. A machine according to one or more of claims 1-9, c h a r a c t e r i z e d in that a sensor for measu the density of the concrete is provided in the upperm section of the inner mould part.

11. A machine according to claim 10, c h a r a c t e r ¬ i z e d in that one or more of the parameters, the relative displacement speed between the mould parts, the speed of rotation of the uppermost section, as well as the vibration frequency and/or vibration amplitude are controlled by means of the signal from the sensor such that the desired density of the concrete is kept constant during the entire casting process.