Manufacturing of a high-voltage insulator

TECHNICAL FIELD

The invention belongs to the field of electrical insula- tors, in particular to medium- and high-voltage equipment and concerns mainly the manufacturing process of such electrical insulators, in particular hollow polymeric high-voltage insulators as well as production equipment for producing such electrical insulators according to the preamble of the independent claims.

BACKGROUND OF THE INVENTION

Polymer concrete is well known as electrical insulation material for producing polymeric electrical insulators, e.g. high-voltage insulators. It consists mainly of a polymeric binder such as epoxy resin and different grades of filler particles. However, acrylic resin may be selected for the binder depending to the desired properties instead of epoxy resin, too. The filler content is typically between 85 and 97 weight percent (wt.-%) . A repre- sentative prior art reference of suitable material, e.g. an epoxy based polymer concrete as an outdoor insulation material for high voltage insulators is formed by the patent application PCT/EP2007/060633 bearing the title "polymer concrete electrical insulation system" that has been filed on 8 October 2007 by the same applicant.

Also, the usage of acrylic polymer concrete for electrical insulation is known and described in US-A-4210774 and DE 2949358, for example.

The processing of such a polymer concrete with processing methods used for standard filled epoxy compositions /compounds that have typically 65 wt.-% filler results in electrical insulators having a comparatively high void content. Voids are a known problem with highly-filled thermosetting resins. Since HV-insulator require a larger percentage of filler compared to the whole composition, such compositions tend to promote the undesired formation of voids during the manufacturing process even more. Such voids origin e.g. from gas that is present in the liquid molding, i.e. the casting mass or composition. Said gas comprises air entrapped in the composition prior and/or during the casting process, e.g. due to rolling, and/or originates of evaporating chemicals.

The formation of voids of such high-voltage insulators is countered by agitation of the pourable composition. A known method is formed by an agitation by means of vibrations of the highly filled material in order to improve its flow abilities by decreasing the flow behavior of the cast composition such that the mold is filled out even in remote areas easier than without such agitation. Placing cast molds onto a vibration table during casting polymer concrete is a well established technique, but leads often not to void-free products. JP-6320549, for example, discloses a compact casting unit contributing to the miniaturization of molding equipment. The casting unit features an inner and an outer shell forming the mold walls that are fixedly attached to a vibrating table that is excited by a shaker fixedly arranged centrally on an underside of the vibrating table.

In addition to exposing the composition to vibration the JP-63191608 A reference proposes to expose the composition to a combination of vacuum and vibration for removing voids during casting.

Beside the drawback that the machinery used for producing said articles tends to wear rapidly due to the amount of energy brought into its structure, which energy is originating from the vibrating mold, the requirement of a vac- uum generation means even adds to the complexity of said equipment .

The DD-A-140121 discloses a production of fiber reinforced, tank-like articles. In a first step, a polymeric pre-product is applied on an inner surface of a hollow mold responsible for providing an outer shape to the articles. Second, a pressing container bag is applied on the inner surface of the polymeric pre-product and filled thereafter with an incompressible medium, such as gravel, such that the exterior of the pressing container bag comes in close contact with the polymeric pre-product. A vibrating unit that is arranged at the mouth of said hollow mold induces vibratory forces of controlled amplitude and frequency via the incompressible medium by mechanical or electrical field methods to the polymeric pre-product. Ultrasonic vibrations up to 200 kHz are applied in combination with modified epoxy resins. The vibrations are applied for chemically modifying the epoxy resin binder such that wetting, compaction and deaeration of the impregnated material is caused. As an option, shock pulses of up to 200 Hz are superposed to the above high frequencies in order to influence the rheolocical, isotropic or anisotropic properties of the polymeric pre-product. Eventually, the polymeric pre-product is cured by exposure to electromag- netic radiation.

Such a manufacturing method requiring several process steps is cumbersome and thus detrimental to an economic manufacturability of the articles.

BRIEF SUMMARY OF THE INVENTION It is an object of the invention to provide an improved molding process based on vibration agitation to further reduce the void content in a cast electric insulator. A further object of the invention is to provide an apparatus that is suitable for carrying out said improved molding process .

These objects are achieved according to the invention by the subject-matter as set forth in the independent claims. Further, particular embodiments are claimed by the dependent claims.

In a first aspect, a method is claimed for producing a polymeric electrical insulator, in particular a high- voltage insulator. Said method comprises the following steps: a) providing an outer mold b) providing an inner mold within said outer mold such that a cavity is formed between the outer mold and the inner mold, wherein the inner mold is mechani- cally decoupled from the outer mold c) providing a castable compound d) filling said compound into the cavity e) curing said compound f) suppressing a formation of voids due to gases by ex- posing the compound to vibrations originating from the inner mold, and g) demolding of the insulator

The term castable compound is understood hereinafter as a pourable, preferably liquid mixture of a binder and a filler plus additives, where necessary. Anyhow, the term compound is also interpreted broadly as a composition comprising such filler and binder in an unsolidified state of the composite concrete.

Said compound is used and understood hereinafter to be the material resulting in a duroplastic plastic, also known as thermosetting plastic or thermosetting polymer that is known for producing polymer concrete articles. Said polymer concrete is commonly understood as a resin plastic that is reinforced with filler, e.g. a filler comprising fibers. Hence the compound referred to in the present invention is a thermosetting compound, in particular a highly filled polymeric matrix thermosetting compound, more particularly a thermosetting compound. The compound comprises a thermosetting compound typically comprising epoxy resin or acrylate resin or unsaturated polyester or other resins used for electrical insulations or a combination thereof. All thermosetting plastics, also referred to as thermo- sets, are polymer materials that irreversibly cure form. The step of curing is achieved through heat, e.g. at a temperature above 100 degrees centigrade, through a chemical reaction, i.e. a two-part epoxy, for example, or irra- diation such as electron beam processing. Depending on their chemical compositions, acrylates and epoxies can also cure at room temperature, for example.

The curing process transforms the resin into a plastic or rubber by a cross-linking process. Where applicable, en- ergy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, three dimensional structure. The thermosetting epoxide polymer cures (polymerizes and crosslinks) typically when mixed with a catalyzing agent or hardener.

The gases referred to at step f) gases may be already present in the compound prior to the filling step and/or gases entrapped in the compound during the filling step.

The technical effect of the inventive method results in a decreased negative impact of the vibrations on the manufacturing device compared to prior art apparatuses having a vibrating table. Such a decrease has the positive effect that the maintenance effort can be decreased compared to known vibrated mold casting methods. Hence the present in- ventive manufacturing method contributes essentially to economic manufacturability of insulators without the ne- cessity of abandoning the positive effects of vibrated insulators, i.e. insulators having fewer voids. Fewer voids lead to a better insulating behavior. A reduction of voids on an outer surface of the insulator contributes essen- tially to an improved outdoor behavior in view of weather resistance and thus to a longer lifetime. All these properties are decisive to the customer' s satisfaction and thus advantageous in view of economics and reliability of a power system where such insulators are built in. Further, it is not decisive for the present invention whether the outer and/or the inner mold are made of metal or not as long their function is ensured. Depending on the requirements does the term mold enclose unheated, heated and preheated molds and/or mold portions or mold parts. Unheated molds may be selected for a compound comprising acrylates. Further, the outer mold and the inner mold comprise several mold elements and may form a multipart-core and/or a multipart outer mold depending on the complexity of the geometries of inner &/or outer contour of the insu- lator to be cast, such as a fuse cutout insulator, and other manufacturing demands .

The present method for manufacturing allows for manufacturing several insulators in one charge of the casting machine, e.g. an APG apparatus (automatic pressure gelation apparatus) , since the inventive method causes little stress on the apparatus. In such case the oscillation /agitation of the inner cores of those several outer cores may be caused for each inner mold separately or for at least several inner molds together. This may be done e.g. by connecting each inner core to a vibration generator that is preferably located outside of the clamping zone of the APG machine or by means of an intermediate transmitter frame that connects several inner molds within said clamping zone of the APG machine with an exterior vibration generator. Alternatively or in addition, the vibration generator is mountable in the inner mold, e.g. into a hole drilled into the inner mold or fixed to the inner mold, e.g. by fastening bolts. As to the process features, the temperature of the metallic mold unit, e.g. mold halves, for casting typical high- voltage insulators are in a range of about room temperature to about 160 degrees centigrade.

The step of curing of typical polymer-concrete high- voltage insulators is performed at a curing temperature of about 60 to about 140 degrees centigrade during about 5 to about 250 minutes depending on the wall thickness of the insulator to be cast and the chemical composition of the compound. There are three different main production methods for filling the compound into the cavity available. Said methods are described below for compounds comprising epoxy resin :

1. "Compression Molding" by use of a very high pressure (around 9.8 to 29.4 MPa) that is applied during the molding process.

2. "Vacuum Casting" that is casting of the compound in a vacuumed fixture. This process does not require high pressure . 3. "Automatic Pressure Gelation", a method that is also abbreviated by APG. This casting process is applicable to high quality mass production like insulators. At APG, the epoxy resin is poured into metallic mold mounted on a heated press until it becomes hard (ge- lation) in the mold. The process minimizes the formation of voids and strains in the cast article. Further, the APG method is ideally suited to the high volume manufacture of both simple and complex parts, offering precisely controlled surface quality and fi- nal part dimensions. Depending on the insulation demands and the weather sus- tainability of the insulator to be cast, one of the above techniques may be selected. Tests revealed that if the compound is filled into the cavity such that the compound is pressurized during at least a predefined time span of the whole filling step, the insulators resulting thereof do have good properties in terms of the presence of any voids, that is a low void content, since the pressuriza- tion urges entrapped gas to leave the compound. If the compound is filled into the cavity from a bottom of the cavity such that its surface level rises about vertically with reference to earth gravity both the entrapped air and/or any entrapped protective gas present in the cavity before the step of filling the compound in, as well as gases from the compound itself are allowed to escape the cavity through appropriate bleeding means, e.g. a bleeding valve or the like.

Exposing the compound to vibrations originating from the inner mold on the compound during the step of suppressing the formation of voids at least partially during the filling step assists the flow of the molding mass since it improves the flowing behavior as it reduces the viscosity of the compound and thus contributes to proper filling of the cavity. This in turn leads to less defective products what is positive in terms of economic value. The compound has to be filled in at such velocity selected such that any entrapped gas has sufficient time to leave the compound. Depending on the degree of freedom, the gas is allowed to escape the compound via the raising surface level of the liquid compound mass, for example.

If the step of suppressing the formation of voids by exposing the compound to vibrations originating from the inner mold is performed during the step of curing said compound, a reduced cycle time compared to prior art castings under vacuum conditions is achievable. In a further embodiment of the inventive method the compound is exposed at least temporarily to vibrations both during the filling step and the step of curing of the compound. Such an embodiment enables achieving particularly short cycle times for the production of insulators.

The wave form or wave shape of the vibrations may be virtually of any shape, e.g. a sinus, a step or square, saw tooth-like or triangular. If the vibrations comprise a sinusoidal shape the vibrations are producible and transfer- rable by several common means that are simple, cheap as well as easy to control, e.g. electromagnetically .

Tests revealed that a good deaeration rate is achievable when applying vibrations having a frequency of about 50 to 300 Hertz, in particular a frequency of about 100 to 200 Hertz. Further, good values are achievable by employing amplitudes with a size, i.e. amplitudes being in a range of about 0.5 to 5 millimeters, in particular in a range of about 0.8 to 2 millimeters for a wall thickness of the insulator of about 30 millimeters. The term amplitude is to be understood as average amplitude hereinafter.

Higher vibration frequencies, e.g. frequencies above 100 Hz, are advantageous in that coarser particles of the compound are moved less compared to vibrations at lower frequencies. Thus the risk of an undesired segregation of the compound is decreased.

The selection of appropriate vibration strength shall be done by consideration of the filler content, the size and shape of the filler particles as well as on the shape of the insulator to be cast and on production requirements, if any.

The person skilled in the art will recognize that adjustments to the amplitude may be required depending on their generation means, e.g. vibrations caused by sound/air waves originating of a displacement of a membrane for ex- ample, electromagnetic waves originating of an exciter and or mechanical agitation by an eccentric tapped fixed on a rotating arbor.

The strength of the vibration in view of amplitude size, wave shape and frequency is selected in accordance to the selected filler and to flow requirements given by the shape of the insulator to be cast.

The amplitude of vibration during the filling step is not critical and can be varied as desired as long as it is not too violent as to trap air in the admixture. This can be readily ascertained by observation and sufficient amplitude should be applied to give mobility to the mass. It is preferred to conduct the vibration at the same temperature as the mixing of the filler and the binder to a compound but any temperature below the curing point of the binder can be employed if desired. Even temperatures that are higher than the curing point of resin can be applied. Depending on the embodiment of the inventive method, the outer mold and/or inner mold is/are preheated to the curing temperature prior to mold filling. Agitation by vibration is performed during a certain length of time which is a function of the amplitude and the temperature conditions.

It is possible to embed steel, aluminum or other metallic inserts into the polymer concrete insulators during the casting process, where applicable and desired.

Depending on the requirements and whether the compound is exposed to vibrations during the filling step only, during the curing step only or during at least a portion of both steps, the time span for exposure is longer than a mold filling time and shorter than a pot life. The pot life is understood as the time span before the moment in time where the viscosity of the resin increases. The pot life is also known as working life or usable life and can last to about 250 minutes. Depending on the embodiment of the inventive method, the time span for exposure is in a range of about 2 to about 120 minutes, preferably in a range of about 5 to about 30 minutes, and depends on the shape and/or the wall thickness of the insulator to be cast.

The effective direction of the vibration originating from the inner mold or the vibrations is/are selectable accord- ing to the requirements given by the filler of the compound, boundaries given by the apparatus for production and/or other demands. The vibrating means may be a linear type or a circular, ring-type transducer/ball-type transducer, for example. Hence, emitting vibration in at least one of a longitudinal axis (z), a first direction (x) being transversely to said longitudinal axis (z) and a second direction (y) being transversely to said longitudinal axis and transversely to said first direction (x) may be selected. When casting insulators, the longitudinal axis is mostly defined by the outer mold.

For certain effects to be caused in the compound there are embodiments of the present inventive production method addressing to one or several variations of the vibration in- tensity and/or setting times with no vibration at all, only vibration in a certain direction and/or a reduced vibration strength. Such variation is achievable by selectively controlling at least one of a frequency, amplitude and a time span during which the vibration is performable. The term controlling encompasses both an open-loop control and a closed-loop control. Thus, depending on the requirements and demands, also an alternating vibration cycle having a set of low energy vibrations provided between two sets of high energy vibrations is available, for example. As a matter of course, such a measure and is selectable for the step of filling the compound into the mold unit and/or for the step of curing the compound.

The inner mold or inner core of the inventive method has an inherently stable shape during at least an essential part of the whole manufacturing process. Though, it provides for a physically formstable shape, e.g. a circular cylindrical shape, that is not wobbly and thus contributes to an easy demolding process after curing in case that the inner mold does not remain a part of the insulator after the step of curing. The inner mold of the insulator producible by an embodiment of the inventive method remains within the insulator after demolding. In an embodiment of the inventive method, sleeve-like inner core features circumferential cavities and/or protrusions for enabling a firm form fit with the cast insulator portion consisting of cast compound material. Depending on the desired insulation properties of the insulator and its agitation method, its interior, i.e. the hollow center may be parted in the longitudinal direction by an insulating bridge oriented essentially trans- versely to the longitudinal axis. In an embodiment of the inner mold, the material of said inner mold differs to the material of the compound in order to provide the insulator with special electric properties. Such an inner mold may be formed by a glass fiber reinforced plastic tube (GFK tube) for example, that is caused to emit vibrations from its interior. This may be achieved for example by sound waves impacting on the interior surfaces of the inner mold, i.e. the reinforced plastic tube, and thus excite the plastic tube to vibrate. In an alternative embodiment, the inner mold remaining in the insulator after demolding is a conductor rod or a conductor tube. Such an embodiment may be desired for an electric bushing for example.

Moreover, a non-stick-coating formed by a demolding agent, e.g. a silicone-based parting agent, may be attached to the inner and/or outer mold in order to ease a good de- molding/stripping from the insulators from the molds.

In an additional step of the inventive production method, a polymer-concrete insulator is coated with a silica coat- ing provided e.g. by Sierracin Corporation, Sylmar,

Calif., which is a proprietary abrasion resistant coating type Sierracote 311 by dipping the insulator into the coating liquid after solvent cleaning and rotating under a heat lamp at 90 degrees centigrade for about 45 minutes in order to confer an enhanced outdoor-properties to the in- sulator.

The acceleration of the release of entrapped gas may be supported in addition to the vibration by adding an additive which causes a surface tension of the compound to drop . Removing the inner mold of the insulator or vice versa, e.g. by stripping off at the step of demolding enables a reutilization of the inner mold for further castings. This is particularly valuable in case that the inner mold has a complex shape. Such a method is suitable for producing dome-shaped hollow surge arrestors, for example.

Certain properties of the exterior surfaces or portions thereof of the insulator are achievable by applying a layer causing or having said desired properties in the outer mold prior to filling said compound into the cavity. By doing so, enhanced weather shield and/or electrical properties are conferrable to insulators.

Conducting the step of filling of the compound in the cavity and/or the step of curing said compound at least temporarily in a vacuum or nearly vacuum or low-pressure en- vironment contributes positively to the degassing quality in that the void-rate is further decreased such that particularly good results in terms of the insulator quality are achievable. Moreover, exposing the compound to vacuum accelerates the degassing process what is desired in view of a short cycle time.

It is an essential feature of all the embodiments of the inventive methods referred to above that the compound, in particular the thermosetting compound, remains chemically unaffected by the vibrations. This ensures keeping all properties of the compound at anytime under control during the whole manufacturing process of the electrical insula- tors and thus contributes to the quality of the insulators to be cast.

An electrical insulator, in particular a medium voltage or high-voltage insulator being produced by any one of the inventive production methods referred to above will feature advantageous properties in terms of a good surface quality due to few voids, if any, and few voids within its insulator body what is desired in view of its electric and dielectric properties. The advantages referred to above in view of the method generally apply analogously for the apparatus described below. Likewise the advantages referred to in regard of the apparatuses discussed below apply for the inventive manufacturing method. In a second aspect, an apparatus for manufacturing an insulator, in particular a high-voltage insulator is claimed. Said apparatus is particularly suited for performing at least the step of filling a castable compound into a cavity of a mold unit. The compound is preferably a thermosetting compound, in particular a polymeric thermosetting compound. Said mold unit in term comprises an outer mold and an inner mold that may be multipart molds each. Prior to the casting process, that is the filling of the compound in the mold unit, the inner mold is arranged within said outer mold such that said cavity is formed between the outer mold and the inner mold. Said inner mold is functionally connected to a vibration generator and arranged in the apparatus such that it is mechanically decoupled from the outer mold. The mechanical decoupling contributes essentially to the quality of the apparatus in term of its lifetime as it experiences almost no stress due to vibrations. This allows the apparatus, e.g. an APG machine, to be built less massive what is in turn very advantageous in terms of demold- ing where typically a movable die, also known as platen or mold, needs to be opened and shut as quick as possible during a working cycle of an insulator since the amount of energy required for moving the movable die is lessened.

Further, the frequency of maintenance of the inventive apparatus is lowered compared to apparatuses having a vi- brating table since the inventive apparatus wears less. Both the low maintenance rate and the resulting increased availability of the machinery together with the good properties of the molded/cast insulator are essential factors in terms of economics and customer satisfaction. Although heated metallic molds are common in the APG method other mold materials may be selected as long as the functionality is ensured.

The term functional connection has been selected to encompass bodily transmitting means between the vibration gen- erator and the inner mold, e.g. a hose, pipe or cable wire, such as a wire for mechanical excitation or a hollow body for acoustic excitation, but also to non-physical functional connections such as a ferromagnetic inner molds that is caused to vibrate by means of a electromagnetic vibration generator being located in or proximate to the mold unit or to mold halves, for example.

The vibration generator referred to above comprises at least one of a mechanical excitation means, an electrical excitation means and an acoustical excitation means. A me- chanical excitation means may be formed by an eccentric tappet, whereas an electrical excitation means may be formed by a magnetizer and an acoustical excitation means may be formed by acoustic wave generator, for example. Depending on the requirements and on feasibility boundaries also a combination of the above excitation means is applicable. However, other agitation means are conceivable as long as the function is assured.

In case of acoustical excitation, the inner mold may feature an interior cavity acting as the excitation means. Shock waves of the kind of air wave pulses that are generated e.g. by an air pressurizing pump and a suitable con- trol device are transmittable through a transmitting means, e.g. suitable hose, to said interior cavity. There, the shock waves or sound waves impact on the interior surfaces of the inner mold, i.e. a reinforced plastic tube, and thus excite the inner mold to vibrate. It is also conceivable that said interior cavity does extend only to a limited portion of the inner mold to achieve the required effect .

If the effective directions of the vibrations need to be adjusted, one or several excitation means are employable.

In order to disconnect the vibration generator from the vibrating inner core, it is particularly advantageous to arrange the vibration generator outside of the mold unit and to connect it merely functionally with the excitation means that is formed by the inner mold or the inner core respectively.

An embodiment of the inventive apparatus features a vacuum generation means in addition to exposing the compound to vibrations for accelerating the degassing process. Depend- ing on the limitations said vacuum or low- pressure/negative-pressure may be present only in a limited space located in or next to the mold unit or the casting machine itself is arranged within such an environment. In the latter case, the casting equipment is ar- ranged in a vacuum chamber or a vacuum-sealed chamber. The duration of the vacuum is subject to variations depending on the desired effects. Thus the vacuum is applied on the compound mass during at least a predefined time span of the whole filling step only, for example. If an insulator for a surge arrestor is to be produced, the typically cylindrical inner mold, that is responsible for the inner shape of the hollow insulator to be cast, extends along a longitudinal axis defined by the typically also cylindrical outer mold. However, also hollow polymer-concrete insulators having a more complex geometry are producible by the inventive ap- paratus and the inventive method. As an example, the inner mold of such an apparatus has at least one lateral branch- off with regard to said longitudinal axis in order to form the intermediate contact of a fuse cutout insulator that maybe required for mounting the fuse cutout insulator onto a pole.

As indicated before, the inner mold and/or outer mold is a multipart-core in embodiments for producing insulators requiring complex geometries of an inner and/or outer con- tour of the insulator. Such an embodiment will most likely be the preferred choice for producing the insulator for a fuse cutout insulator for example.

In order to achieve certain effects like optimal setting and optimal energy assignment, the inventive apparatus is furnished with at least one control means for controlling at least one of a frequency; amplitude and a time span (duration) during which the vibration is performable.

Further embodiments, advantages and applications of the invention will become apparent from claims or claim combi- nations and when consideration is given to the following detailed description and the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Such description makes reference to the annexed drawings, which are schematically showing in Fig. 1 an apparatus for conducting an improved molding process according to the present invention; and Fig. 2 a detailed view of a mold as shown in figure 1 which mold is employed for casting an electric insulator . In the drawings identical and/or functionally identical elements parts and designated by identical reference numerals, unless indicated otherwise. DETAILED DESCRIPTION OF THE INVENTION

An apparatus 1 for conducting an improved molding process according to the present invention is described with reference to the enclosed figures 1 and 2. The molding, i.e. a casting process is employed for producing an electrical insulator, in particular a high-voltage insulator that is made of a compound comprising a resin (e.g. epoxy or acry- late) as a binder and a filler (e.g. silica) for forming the polymeric concrete after curing. The apparatus 1 comprises an automatic pressure gelation device. Figure 1 shows merely a stationary, first mold half 2 and a movable second mold half 3 schematically whereas most of the other APG machine is not shown in figure 1 in order to enhance readability of the figure. The second mold half 3 is movable back and fro along a moving axis 4 of the APG machine for demolding purpose. This is indicated in figure 1 by a double-headed arrow.

A tank 5 comprising pourable, preferably liquid compound 6 forms the material source for the insulator 7 to be cast (see figure 2 for reference) . Pressure 8 is applied on the surface of the compound 6 in the tank 5 in order to squeeze the compound 6 from the tank 5 through a duct 9 to a mold unit 10 that is seated partly within each of the mold halves 2, 3 and to suppress shrinkage of the casting during the curing process.

The mold unit 9 also comprises a vacuum chamber 11 for contributing to an effective degassing process of the compound such that the insulator 7 resulting thereof features a very low void content such that its electrical, dielec- trie and weather resistance properties are good.

The mold unit 10 shown in figure 1 is shown in a simplified manner in order to improve the readability and under- standability of said figure. In figure 1 the step of filling of the compound 6 into preheated mold halves 2, 3 is not finished yet. Hence a surface level 12 of the liquid compound 6 in the mold unit 10 is displayed. More details of the mold unit 10 as shown in figure 1 are explained with reference to figure 2. The portions of the steely mold halves 2, 3 forming an outer mold are shown to be enclosed in the vacuum chamber 11. Figure 2 reveals further that an interior 13 of the outer mold defines a longitudinal axis 14 along which a core-like cylindrical inner mold 15 is arranged in a concentric manner such that a cavity 16 is formed between the outer mold 2, 3 and the inner mold 15. Said cavity 16 is limited in the longitudinal direction by lids 17 each and by a plurality of weather sheds in a circumferential direction relative to the longitudinal axis 14. Since the inner mold 15 is shorter than a distance between the lids 2 the differing distance is distributed at each end of the inner mold 15 by disk-like decoupling elements 18 having damping proper- ties in view of vibrations that are emittable from the inner mold 15 such that the inner mold 15 is mechanically decoupled in terms of vibrations at least to a large extent to the AGP machinery, in particular the mold halves 2, 3. The inner mold 15 is a metallic, in particular a steel tube featuring an empty interior that has a circular cylindrical cavity 22 that acts as the excitation means 22, here as a ring-type transducer means. Said inner mold 15 may be coated with a special layer of material in order to simplify the demolding of the inner mold 15, where necessary.

The inner mold 15 is caused to vibrate by acoustic excitation. Hence, a vibration generator 19 in form of an air pressurizing pump and a suitable control device 23 is functionally connected via a transmitting means 20 in form of a hose 20, with the inner mold 15 in the mold unit 10. Said hose 20 is arranged in a vibration damping and sealing manner in an area of a joint 21 formed between the molds halves 2, 3. Said vibration generator 19 is located outside of the APG machine in order to avoid negative in- fluence on the latter.

Alternatively, the vibration generator can be compact enough to be mounted inside the inner mold, with just a pressurized air hose or electric line in the case of an electric generator coming into the mold. Shock waves of the kind of air wave pulses are generated by the vibration generator 19 are transmittable through the hose 20 into said interior cavity 22. There, the shock waves or sound waves impact on the interior surfaces of the inner mold 15, i.e. a reinforced plastic tube, and thus excite the inner mold 15 from its inside to vibrate.

With such an embodiment of the excitation means 22 in the inner mold 15 vibrations with an effective direction parallel to the longitudinal axis 14 (z) and transversal thereto in any direction radial to said longitudinal axis 14 (x, y) are producible.

The curing of the compound 6 is performed by heating means, e.g. by heating elements in the mold halves 2, 3.

Demolding is achieved by moving the movable second mold half 3 off the stationary first mold half 2 in a direction x as indicated by the double-headed arrow. The circumferential surface of the inner mold 15 is such that it contributes to an easy removing of the inner mold 15 during the demolding process. In case of re-used inner molds 15 it is advantageous if the transmitting means 20 remains at least partially attached to the inner mold 15 and the vibration generator 19 in order to reduce the cycle times and handling effort.

In an alternative embodiment, the inner mold 15 is made of a glass fiber reinforced plastic tube. In this case the vibrations suffer far more damping between the excitation means 22 and the surface being in contact with the compound compared to a metallic inner mold.

List of Reference Characters

1 apparatus

2 first mold half

3 second mold half

4 moving axis

5 tank

6 compound

7 insulator

8 pressure

9 duct

10 mold unit

11 vacuum chamber

12 surface level

13 interior

14 longitudinal axis

15 inner mold

16 cavity

17 lid

18 decoupling element

19 vibration generator

20 transmitting means

21 j oint

22 vibrating means

23 control means