DEVICE AND PROCESS FOR THE PRODUCTION OF SLABS OF THERMOPLASTIC MATERIAL

Abstract: A device for the production of slabs of wax or other thermoplastic material is provided with two endless conveyor belts (1, 2) placed opposite each other and one above the other between which the wax is fed, with side guide (17) running along with and between the parts of the conveyor belts facing each other and with cooling means (9, 10) for cooling the wax. The device contains side guide belts (17) with two flat parallel surfaces, each of which sealingly engages with the opposite surface of the corresponding conveyor belt part. The device is suitable for processing wax with a sticky character due to the horizontal position of the belts and the relatively large sealing areas between the belts and the side guides.

Description: Device and process for the production of slabs of thermoplastic material. The invention relates to a device for the production of slabs of wax or other thermoplastic material, provided with two endless conveyor belts placed opposite each other and one above the other between which the wax is fed, with side guides running along with and between the parts of the conveyor belts facing each other and with means for cooling the wax. Such a device is known from NL-A- 69.19602. In this known device, the side guides are designed in the form of flexible hollow tubes with a round cross-section. These tubes are introduced at one side between the parts of the conveyor belts facing each other, and are discharged at the other side. According to the invention, provision is made for the side guides in this case to be designed as endless belts. This means that the side guide can always be reused for lateral sealing of the conveyor belts, which means that from the economic point of view it is sensible to select a belt of high-grade material of a suitable shape for this, in order to ensure a good seal. In the known side guides the tubes are compressed slightly between the facing parts of the conveyor belts, which means that a sealing power is produced, and also a flattening of the tubes, which produces a sealing surface of a specific width on the conveyor belts. Given the hollow shape of the tubes, this sealing power and the sealing surfaces must remain limited, so that only a relatively weak seal can be ensured. This is a disadvantage particularly with sticky materials such as wax, where the problem occurs that after some time sticky material residues can be left between the side guides and the conveyor belt surface, thereby causing the seal to be lost in the end. According to the invention, a better seal can now be obtained by each side guide belt having two flat, parallel surfaces facing away from each other, each of which can mate in sealing fashion with the opposite surface of the corresponding conveyor belt part. Through a suitable choice of the dimensions of the side guide belts, the mating surfaces of the side guide belt, on the one hand. and of the conveyor belt parts, on the other, can produce a good seal, even as regards sticky products such as wax. According to a preferred embodiment of the invention, provision is also made for each of the side guide belts to be guided around over two pulleys whose centre line is at right angles to the surfaces of the conveyor belt parts facing each other. An advantage of this design is that the side guide belts can be introduced between the conveyor belt parts at any point in the lengthwise direction of the conveyor belts, and can also be guided out of them again at any point in the lengthwise direction. This makes it possible to make the side guide walls shorter than the conveyor belts, and to provide them only at those points between the conveyor belts where the material to be processed, such as wax, is still liquid. According to a practical embodiment, provision can be made in this case for the pulleys to be located next to and outside the conveyor belt parts facing each other, and for each side guide belt in the plane of the pulleys to project relative to the pulleys, in such a way that the projecting section of the part of the side guide belts which faces the conveyor belts is between the conveyor belt parts facing each other. Provision is preferably made for the side guide belts to have larger dimensions in the plane of the pulleys than they are at right angles to said plane. In this embodiment a relatively large contact surface is obtained between each side guide belt and the conveyor belts, which benefits the seal. According to the invention, provision can also be made at the guided part of the side guide belts outside the conveyor belts for stabilization means above and below the side guide belts for holding the guided part flat. In particular, if the dimensions of the side guide belts are considerably greater in the plane of the pulleys than at right angles thereto, there is in fact a risk of the side guide belts "flipping over" in said guided parts. This flipping over can be reliably avoided by means of the above-mentioned stabilization means, which can be, for example, rollers whose centre line runs along the radius of the pulleys. The side guide belts can be designed in various ways, but provision is preferably made for each of them to comprise a V-belt which on the largest trapezium side is provided with a rectangular strip of bendable material. This rectangular strip of bendable material forms the part of the side guide belt which can be inserted between the facing parts of the conveyor belts. The V-belt and the strip are preferably made of polyurethane and are attached to each other by means of adhesive. In order to increase the seal between the side guide belts and the conveyor belts, provision can be made for the conveyor belts to be supported and/or held by pressure at the level of the region over which the side guide belts can extend. As already mentioned above, slabs can be made of materials such as wax with the device according to the invention, as described above. Thanks to the good sealing of the side guides, the slabs are a very regular shape such as can be seen, for example, in the straight side edges. For this, it is also important that before solidification the still liquid wax is sprayed uniformly over the entire width of the conveyor belts between the side guides, so that air inclusions or other irregularities can be reliably avoided. For this purpose, it is known to use a container with a nozzle for feeding the starting material such as molten wax between the side guides and the conveyor belts. In order to obtain a uniform infeed of the starting material between the side guides and the conveyor belts, the device according to the invention is designed in such a way that the nozzle is provided with a slit-type aperture which extends over the entire width or a considerable part of the nozzle. In that respect a further improvement an be obtained by having a baffle extending in the nozzle downstream of the aperture in the direction of flow of the wax, in such a way that there is a slit or are slits between said baffle and the top wall and/or bottom wall of the interior space of the nozzle for uniform distribution of the wax over the press aperture. The uniform distribution of the starting material can also be favourably influenced by the container being approximately the width of the aperture and being provided with holes disposed over said width for the infeed of the wax to the side of the baffle facing away from the aperture. It can also be arranged that the container is provided on the inside at regular intervals with baffles placed at right angles to the container wall in which the channels are located, while the container spaces formed by the baffles are connected to each other by recesses in the baffles. The diameter of the holes is preferably in the range 2 - 20 mm. Furthermore, the invention relates to a process for the production of slabs of wax, or other thermoplastic material with a deyice as described before in which wax is fed between two endless conveyor belts of a conveyor device placed opposite each other and one above the other, and is cooled while passing through this conveyor device and discharged in slab form from said device. Such a process is, apart from NL-A-69.19602, known from an advertisement of the Sandvik company in "Chemical Engineering", 18 December 1978, p. 46. Cooling takes place through spraying the top conveyor belt on the top side and the bottom conveyor belt on the bottom side with water. The pressure of the two conveyor belts on the material fed through ensures that good contact between the product and the two cooling belts is ensured. It is stated that various waxes are placed on the belt from an apparatus for forming strips (a "strip former"). In this case a strip former is understood to be an apparatus which makes molten strips approximately 20 mm wide flow out at approximately equal distances from each other in the lengthwise direction of the cooling belt. At the end of the belt these strips are cooled and fed into a granulator which makes toffee-like shapes of them. Hot- melt adhesives are often put in this form. The strip former as such comprises a pipe with holes, from which the viscous, molten mass flows onto the cooling belt. This method cannot be used for plastic wax types, which are the subject of the process according to the invention, because it is impossible with a low-viscosity material to obtain an adequate thickness on the belt without the strips running together, and because the strips cannot be granulated should they be produced in a reasonable form. The granulator connected downstream of the conveyor/cooling device instantaneously becomes clogged. Applicants carried out production experiments with becomes clogged. Applicants carried out production experiments with the device used in this process, and this problem was found, and material in slab form with the desired dimensions was not obtained. Dutch Patent Application 79-06467 discloses a process and device for the production of slabs of thermoplastic material, such as wax or a thermoplastic plastic, in which the starting material is fed unsolidified into the space between two parallel, spaced apart vertical parts of two endless conveyor belts, the material being placed on the returning parts. A major disadvantage of this known process is that, in order to obtain a reasonable production output, the device has to be of a height which is very considerable and is virtually impossible in practice. Besides, with the known device there is a great risk of soiling of the outflow aperture and of other elements which are essential for the device, such as the guide wheels for the conveyor belts. No seal is used along the edges, so that thermoplastic material will come out at the sides. The use of a solvent is not mentioned anywhere, so that it is likely that the method described is not suitable for material which has a tendency to stick to the conveyor belts. In general, waxes are refined and mixed with other waxes or with plastics, synthetic resins, pigments and the like, by melting them together at a temperature between approximately 80 and 150°C. If the mixture is mixed homogeneously, it must be put in a form in which the wax can be transported and stored. According to a usual process, the wax or wax compound is placed in liquid form in a mould and subsequently solidified, cooled, removed from the mould, and packed. In the most suitable process of the state of the art, so-called slabbers or cooling presses are used. The cooling is carried out by means of cooling water. The slabbers can in general be filled and emptied approximately five times per normal working day. This method has, however, major disadvantages. For example, it has been found that the solidified slabs often have sizeable shrinkage cavities. With this method most waxes have a shrinkage of 10 - 15X between 100°C and 20°C. An additional problem is shrinking away of the wall. For, the heat transfer is not ideal because the filled chamber is at rest and a solidified layer of wax arises from the cooled side. In the case of sticky products it is difficult to unload the slabbers. The removal of the slabs often involves the use of some force, which leads to damage of the slab surface. In a subsequent operation the adhesion to the slab is then even stronger. This is a cumulative process which in the long run necessitates a complete overhaul of the cooling plate. Other disadvantages of slabbers are also the frequent occurrence of blockages of the cooling plates, water leaks and, in particular, the hard work involved. According to another known method, a large number of wax moulds are suspended between two endless chains. The wax moulds are filled automatically and conveyed by the slow-moving chain into a cooled room. This room is cooled with highly cooled air to a low temperature at which the wax solidifies. On its emergence from the cooled room, the wax is un oulded by means of a removal mechanism, or through the container being inverted. The wax can now be packed further, and the empty mould is automatically refilled. The advantage of this method is that it involves a minimum of work; the disadvantage is that a very large amount of energy is required to achieve a reasonable output. Besides, in the case of more plastic waxes the capacity is very soon reduced by half, and this method is not suitable for sticky types of wax. In the production of wax compounds a distinction must be made between two groups of products, viz: A) the harder, non-sticky products, and B) the more plastic to soft products which are more or less sticky. The processing of the types specified in A) can take place very well without the use of slabbers, by means of prilling or pelleting. Pelleting, for example, is carried out by applying liquid wax drop by drop to a cooling belt. Both prilling and pelleting can be used for sticky waxes, but the problems arise thereafter. Due to the sticky nature of the products, the prills or pellets stick to each other again after packaging, thereby forming large conglomerates which are difficult to handle and contain large quanties of trapped air. The limit at which a wax can still be pelleted or prilled lies approximately at 25°C at a penetration of 22-30, measured by the ASTM T 1321-86 method. Moreover, this limit depends on the composition of the wax compound. The object of the present invention is to eliminate the disadvantages which the wax production methods described above have (with the exception of the pelleting or prilling of the harder wax types), and which are increasingly unpleasant as the wax (compound) becomes more plastic/stickier. This object is achieved according to the invention by a process of the type mentioned in the preamble, which is characterized in that the wax serving as the starting material is poured onto the bottom conveyor belt over the full width or a considerable part thereof, lateral leakage of the wax is prevented, preferably by means of one or more side guide belts running along with the conveyor belts, and the slab discharged from the conveyor device is cut into pieces of the desired dimensions. In the context of the present invention, wax also includes wax compounds and wax-type materials. For a definition of wax, you are referred to "Wax, an introduction" by R. Sayers, written on behalf of the European Wax Federation (1983) ' A definition of wax is given on p. 16. Although the present invention is particularly concerned with waxes from petroleum, mixtures thereof, and mixtures thereof with synthetic resins and/or plastics, the invention can in principle be used for the whole range of waxes, wax compounds and thermoplastic materials, i.e., for example, also materials such as hot-melt adhesives and materials with similar melting properties. The term "wax" used in this description and the accompanying claims covers all such materials. The process according to the invention is preferably carried out continuously through making the wax flow without interruption onto the bottom conveyor belt. Depending on the composition, the wax used as the starting material has a value which lies in the region of the solidification point of the wax. In general, a temperature range of 6θ-l4θ°C can be specified. It is, however, preferable here for the temperature to lie in a range 10°C above to 10°C below, in particular 5°C above to 5°C below, the solidification point of the wax. In practice, the wax to be produced is heated in mixing pans. In the process, the starting material is brought to a temperature which is often more than 10°C above the solidification point.. It is preferable to discharge the solidification heat of the pourable wax into a separate device as far as possible until the "ideal" pouring temperature has been achieved. As stated above, this temperature lies around the solidification point of the wax, for example in a range of 5°C above to 5°C below the solidification point of the wax. For coolin _ to the ideal pouring temperature, it is preferable to use a scraped or scraper cooler. The wax is conveyed from the scraper cooler via a filling station to the conveyor device, for example a double steel belt cooler. The filling station must be made such that the conveyor device is filled in the optimum manner. In the process according to the invention use is made of conveyor belts with adjustable spacing between them. It is essential in the process according to the invention for the wax applied to the bottom conveyor belt to be also in contact with the top conveyor belt. In order to prevent sticking of the wax to cooling belts and side guide belt, it is possible to moisten belts and guide with a solvent. In general, an aqueous solution of a surfactant or a mineral oil is used for_ this, although solvents of other types, which are known per se, can also be used. Since the wax leaves the filling station at as low a temperature as possible, solidification will occur fairly quickly. It must in any case take place in the part of the conveyor device which is provided with the sealing side guide system. After leaving this part of the conveyor device, no leakage can occur any more at the sides. The belt contact with the top and bottom cooling belt can be retained by the top belt resting on the solidified wax. At the end of the conveyor device an endless wax slab emerges and is cut into manageable formats. The use of professional cutting equipment such as knives (stationary, rotary or acting as a guillotine) and wires, which may or may not be heated, is possible in principle, although heavy soiling of equipment and product occurs, and the risk of breakdowns increases. In the process according to the invention the endless wax slab is preferably cut with a liquid jet. The advantage of this is that no soiling of wax slabs and cutting mechanism occurs, no heat need be fed in, and the spray nozzles of the liquid jet are easy to change. It is also preferable to cut the wax slabs by means of a laser beam, in particular if one wishes the process to be liquid- free. Water or oil, for example, can be used as the liquid. It is also possible to cut by means of compressed air. The cutting device cuts the wax slab in the lengthwise and crosswise directions into slabs of manageable dimensions. After this operation, the slabs are conveyed further to a packing line, which stacks and packs them, either with or without dividing sheets (for example, siliconized paper or film). The invention will be explained in greater detail below with reference to an example of an embodiment shown in the figures. Fig. 1 shows a side view of the device according to the invention. Fig. 2 shows a top view of a part of the device of Fig. 1. Fig. 3 shows cross-section III-III of Fig. 2, on a larger scale. Fig. 4 shows the nozzle with container in perspective. Fig. 5 shows cross-section V-V of Fig. 2 on a larger scale. Fig. 6 shows a partial view of the device in perspective. The device shown in Fig. 1 comprises an endless bottom belt 1 and an endless top belt 2, which are guided around over rollers 3- A feed element 4 for molten wax is provided at the rear end of the conveyor belt 1, 2, viewed in the direction of rotation of the parts facing each other. The molten wax is taken from the container 5 via holes 6 and baffle 7 through the nozzle 8 and deposited between the conveyor belts 1,2. The wax is then carried along between the moving conveyor belts 1, 2 and solidifies in the process. The conveyor belts are cooled with a suitable liquid 11 by means of the spray elements 9. 10. The cooling liquid is fed in through the pipes 12. The liquid sprayed on the bottom conveyor belt 1 is collected in tank 13 and discharged through pipe 14. The liquid sprayed onto the top conveyor belt 1 cannot flow away, due to raised longitudinal edges 16 in the form of V-belts on the conveyor belt, and is extracted through pipe 15. The device is also ΛΛ , Λ ^ n 90/06219 provided on either side of the conveyor belts 1, 2 with side guide belts, one 17 of which can be seen in Fig. 1. The side guide belts are guided around over pulleys 18 and move at the same speed as the conveyor belts 1, 2. A solidified wax slab 19, which is subsequently further processed in the known manner, is discharged from the device. In the top view shown in Fig. 2 the manner in which the side guide belt 17 is guided round can be seen. The side guide belt comprises a part 20 which is designed as a V-belt, and a part 21 which is a strip of flexible material such as polyurethane attached thereto. The V-belt part 20 is accommodated in the peripheral groove of pulley 18, and the belt part 21 projects relative to said pulley 18. As can also be seen in Fig. 5» the belt part 21 located near the pulley 18, which is curved, is supported at its top and bottom side by correspondingly curved roller guides 22, 23. These roller guides 22, 23 each comprise a U-shaped section 24 containing rotary rollers 25. As can be seen in Fig. 2, the roller guides 22, 23 run through almost to the conveyor belts 1, 2. In this way, the side guide belt 17 can be well secured against flipping over from its plane. As shown in Fig. 3t the side guide belt is also supported between its guided parts. At the part facing away from the conveyor belts 1, 2 it rests on guides 26, also comprising U-shaped sections 24 and rollers 2 . The part facing the conveyor belts 1, 2 is pressed, against the pressure of the liquid wax, between said conveyor belts 1, 2 by guides 27. The latter also comprise a U- shaped section 24 and rollers 25, but are turned through 90° relative to the preceding guides 26. These guides 22, 23, 26 and 27 are supported on frame leg 2 of the device by means of adjustable supports 28. Fig. 4 shows the feed element 4 in perspective. The molten wax flows out of the holes 6 around the baffle 7, as indicated by the arrows 30. A good distribution is thereby obtained in the width direction over the nozzle slit 31• The container is also provided with a number of baffles 32. The baffles 32 have recesses 33. so that the molten wax can spread uniformly over the entire container 5- The illustration in perspective in Fig. 6 shows that the solidifying wax is confined between bottom and top conveyor belts 1, 2, and the left and right side guide belts 17• For the sake of clarity, a number of elements such as roller guide 22 and the U- shaped channel 24 of the roller guide 27 have been omitted from this figure. It can also be seen that the rollers 3 are provided with suitable V-shaped grooves 34 for the V-belts 16 on the conveyor belts 1, 2. If this is necessary for obtaining a good seal between conveyor belts 1, 2 and side guide belts 17, provision can be made for pressure rollers (not shown) acting on the V-belts 16 and pressing the conveyor belts 1, 2 against the part 21 of the side guide belts 17.

Claims: 219 C L A I M S 1. Device for the production of slabs of wax or other thermoplastic material, provided with two endless conveyor belts placed opposite each other and one above the other between which the wax is fed, with side guides running along with and between the parts of the conveyor belts facing each other and with cooling means for cooling the wax, characterized in that the side guides are designed as endless belts. 2. Device according to Claim 1, characterized in that each side guide belt has two flat, parallel surfaces facing away from each other, each of which can mate in sealing fashion with the opposite surface of the corresponding conveyor belt part. 3. Device according to Claim 2, characterized in that the side guide belts are guided around over two pulleys whose centre line is at right angles to the surfaces of the conveyor belt parts facing each other. 4. Device according to Claim 3. characterized in that the pulleys are located next to and outside the conveyor belt parts facing each other, and each side guide belt in the plane of the pulleys projects relative to the pulleys, in such a way that the projecting section of the part of the side guide belts which faces the conveyor belts is between the conveyor belt parts facing each other. 5- Device according to Claim 1, 2, 3 or 4, characterized in that the dimension of the side guide belts is greater in the plane of the pulleys than at right angles to said plane. 6. Device according to Claim 3. 4 or 5. characterized in that provision is made at the guided part of the side guide belts outside the conveyor belts for stabilization means above and below the side guide belts for holding the guided part flat. 7. Device according to Claim 3t 4, 5 or 6, characterized in that each of the side guide belts comprises a V-belt which on the largest trapezium side is provided with a rectangular strip of bendable material. 8. Device according to Claim 7, characterized in that the V- belt and the strips are made of polyurethane and are attached to each other by means of adhesive. 9. Device according to one of Claims 1 - 8, characterized in that the conveyor belts are supported and/or held by pressure at the level of the region over which the side guide belts can extend. 10. Device according to one of Claims 1 - 7. provided with a container with nozzle for feeding in starting material between the side guides and the conveyor belts, characterized in that the nozzle is provided with a slit-type aperture which extends over the entire width or a considerable part of the nozzle, the width of the nozzle being approximately equal to the distance between the side guide belts. 11. Device according to Claim 10, characterized in that in the direction of flow of the starting material a baffle extends downstream of the aperture, in such a way that there is a slit or are slits between said baffle and the top wall and/or bottom wall of the interior space of the nozzle, for uniform distribution of the starting material over the press aperture. 12. Device according to Claim 11, characterized in that the container is approximately the width of the aperture and is provided with holes disposed over said width for the infeed of the starting material to the side of the baffle facing away from the aperture. 13- Device according to Claim 12, characterized in that the container is provided on the inside at regular intervals with baffles placed at right angles to the container wall in which the holes are located, while the container spaces formed by the baffles are connected to each other by recesses in the baffles. 14. Device according to Claim 12 or 13, characterized in that the diameter of the holes is in the range 2 - 20 mm. 15. Process for the production of slabs of wax or other thermoplastic material, with a device according to one of the preceding claims, characterized in that the wax serving as the starting material is poured onto the bottom conveyor belt over the full width or a considerable part thereof, and the slab discharged from the conveyor device is cut into pieces of the desired dimensions. 16. Process according to Claim 15, characterized in that the wax is made to flow continuously on the bottom conveyor belt. " 17- Process according to Claims 15 or 16, characterized in that the temperature of the starting material is maintained at a value which is in the region of the solidification point of the wax. 18. Process according to one of Claims 15 - 17, characterized in that the temperature lies in a range of 10°C above to 10°C below, preferably 5°C above to 5°C below, the solidification point of the wax. 19. Process according to one of Claims 15 - 18, characterized in that starting material is first heated to a temperature lying more than 10°C above the solidification point and is then cooled to the pouring temperature. 20. Process according to Claim 17, characterized in that cooling to the pouring temperature is carried out in a scraped cooler. 21. Process according to one of Claims 15 - 20, characterized in that the conveyor belts and/or the guide belts are treated with a solvent. 22. Process according to one of Claims 15 - 21, characterized in that the slab discharged from the conveyor device is cut by means of a liquid jet or laser beam.

Inventors: DE FOUW ADRIAAN LEENDERT, SIJPESTEIN PIETER, DE FOUW, ADRIAAN, LEENDERT, SIJPESTEIN, PIETER
Applicants: PARAMELT SYNTAC BV, PARAMELT SYNTAC B.V.
Application year: 1989
Main classification: B29K91
IPC codes: B29C35, B29C39, B29C67, B29K91