A METHOD AND A DEVICE FOR CHANGING THE TEMPERATURE OF A PLASTIC MELT BY COOLING OR HEATING THE SAME

Abstract: The present invention refers to a method for controlling the melt temperature of a plastic raw material in the manufacture of plastic articles as well as a device for performing this method. Increased melt temperature is reached and/or maintained by adding or removing heat from the melt with the aid of this device according to the invention.

Description: A METHOD AND A DEVICE FOR CHANGING THE TEMPERATURE OF A PLASTIC MELT BY COOLING OR HEATING THE SAME. The present invention refers to control of temperature of a molten plastic material used for the manufacture of plastic products, in particular such a control so that a desired temperature is obtained throughout the melt independent of the temperature of the melt prior to exposure to this control, as well as a device for performing this control. The raw material used for the manufacture of plastic products is supplied to the machine in the form of . granules, powder or in any other subdivided form, possibly together with additives such as dies, anti-oxidants, UV-stabilizors and the like. In the machine the raw material is mechanically kneaded, normally by means of a screw or the like rotating in a cylinder, said screw normally also providing the homogenization, the pressure build-up and the feeding of the molten raw material to a nozzle, die, mould or another tool giving the product its final shape prior to any additional working such as lon¬ gitudinal and transverse stretching. The most common form of kneading is performed by means of a screw rotating within a cylinder. The kneading is performed in a so-called working section, from which the viscous plastic material is fed via a screen pack assembly through an inter¬ mediate tube, normally designated as adapter, to the tool and via ducts or grooves therein out through the founding device or out into the mould. In addition to the supply of mechanical energy by means of the screw and cylinder the machine is provided with devices for heating and cooling. The heating device is so designed that parts or zones of the machine may be individually heated and controlled to the desired temperature independently of each other. The cooling is normally provided by means of fans which also cool one section or zone of the machine each. The various zones are provided with temperature sensors and the sensing or measuring is performed in the metal jacket of the cylinder. The control of the temperature in the various cylinder zones therefore fundamentally will be a control of the temperature in the cylinder itself, which means a control of the metal temperature. In addition to the cylinder also the screw within the cylinder of the working section may be provided with devices enabling the temperature to be changed, for example in the form of cooling ducts and/or immersion heaters. The primary reason for controlling the temperature of the working section is to arrive at a desired melt-temperature. On the other hand, the temperature control of the screen, adapter and die and any additional equipment is intended to maintain the temperature within the molten plastic material unchanged during the transmission of the material into the die or corresponding part of the process. In some of the processes a control of the nozzle temperature is performed for the purpose of obtaining a desired surface structure or optical properties of the manufactured product. Parts of the working section are a hopper for supplying the raw material, a screw rotating within a cylinder and a screen pack assembly comprising a filter screen removing impurities from the melt. As mentioned above, there are a number of temperature-controlling zones where the metal temperature is controlled to the desired value by heating or cooling. One can easily be made to believe that the addition of heat from the heating elements increasing the metal temperature would lead to an increased temperature of the melt, but this is definitely not the case. Actually, an increased supply of heat energy from the heating elements may sometimes result in a lower temperature of the melt and a lower production capacity at otherwise constant production conditions. It is easy to understand that the main part of the supplied energy is mechanical energy. The short dwell time, normally some minutes, in the working section in relation to the production capacity would require a very high capacity of the heating elements to perform the task to melt the raw material all together, a capacity which certainly would burn the raw material adjacent the heat elements and thereby destroy it. Friction and thereby frictional heat is produced during the rotation of the screw between the granulate and the screw, between the granulate and the cylinder, between the various raw material granules and in the melt. A change of the friction coefficient and the viscosity thus brings about a change of the frictional heat supplied and thereby of the melting temperature. Another factor of importance in this connection is the return flow around the screw. This depends on the fact that the screw does not work with an absolute seal against the cylinder wall, causing a return flow of part of the melt in the interspace between the ridges of the screw and the cylinder wall. The pressure ahead of the screen pack assembly and the viscosity of the melt are the factors directly influencing the return flow. If an increased production is desired, .the pressure must be increased which directly influences the melt temperature and thereby the viscosity. Various raw products have different friction coefficients and in addition different viscosities at the same temperature. A higher viscosity requires an increased pressure in order to obtain a constant production capacity. According to its definition it also contributes by a higher internal friction to the desired frictional heat. However, friction coefficient melt/metal, granulate/metal and/or granulate/granulate also contributes to increased frictional heat. A high surface temperature of the screw and/or the cylinder melts the material contacting the metal. This molten material acts as a lubricant and decreases thus the friction against the cylinder wall and/or the screw and thereby the frictional heat. The raw material normally is a poly ere and has a molecular weight distribution varying between various qualities and also between various supplies of a raw material of identical specifications. The low molecular weights melt at a lower temperature than the high molecular weights and is thus acting as lubricant reducing the friction coefficient. From the above considerations it will appear that the tempera¬ ture addition from heating elements in a high degree will change the friction conditions and thereby the supplied frictional heat. It is also obvious that frictional heat practically is responsible for all the energy supplied to the melt and thus for the melt temperature achieved. From the above explanations it also appears that a change of the pressure immediately after the screw directly influences the frictional heat and thereby the melt temperature. It also appears that the normal variations of the raw material, the production capacity etc. directly changes the frictional heat. The pressure after the screw may be changed by changing the number of revolutions of the screw, the through-flow area of the screen or the discharge area with or without changing the screw rpm. A change of the pressure thus yields an immediate change of the amount of mechanically supplied energy to a certain amount of raw material and thereby a change of the melt temperature. From the above statements it will appear that the temperature of a melt is influenced by a number of different factors in connection with the operation and it is difficult with the technique used to obtain an even, desired temperature of the melt and the temperature variations of the same easily changes due to changes of some or several of the parameters of the operation. The trend of increasing the production capacity in todays technique involves an increase of the number of revolutions of the screw which in turn requires the pressure ahead of the screen and ahead of the discharge to be increased, all these factors contributing to an increase of the melt temperature. The result thereof will be an increased cooling demand in order to cool the final product and in many cases it is just this cooling which constitutes the production- limiting factor. As mentioned before, the cooling technique normally used is cooling by means of fans blowing on machine sections and the finished plastic product. Here it is no question of temperature control but only of cooling. If the melt has a lower tempera¬ ture than desired this temperature cannot be increased in a quick and simple way. It has been found that feeding of the die, such as a nozzle for the manufacture of plastic film, with a plastic melt of a determined temperature which in the specific case was lower than the melt temperature from the extruder has yielded a production increase of 20 to 25%. As mentioned, the cooling as applied today, has certain disadvantages and the consequence will be that the melt not reliably has the same temperature throughout. In order to enable an increase of the production to be obtained which also yields a product of a good quality, it is necessary that the plastic melt has the same homogenious temperature. The theories about this technique are discussed in an article on pages 125-133 in Journal of Plastic Film & Sheeting, vol. 3 - April 1987. As appears from this article, the increase of production by decreasing the temperature of the melt is surprising; it is not explained how this temperature decrease is to be performed. The present invention is intended to overcome the above- mentioned problem and to obtain temperature control of the plastic melt to arrive at the same temperature throughout the melt in a simple and effective way. This purpose is achieved by a method and a device of the type indicated in the claims from which also the characteristic features of the invention appear. Hereafter, the invention will be described by reference to the attached drawing in which FIGURE 1 is a schematical sideview and cross section of an element forming part of the temperature control device according to the invention, FIGURE 2 is a schematical sideview and cross section of a second element forming part of the temperature control device and FIGURE 3 is a schematical view of the labyrint-shaped flow path formed by the elements according to Figs. 1 and 2 for the melt through the temperature control device according to the invention. In order to change the melt temperature and thereby also attempting to maintain an even temperature of the melt, according to this invention, the melt is passed through a device in which the melting temperature is changed by means of temperature-controlled machine element or tubes in which a flow of temperature-adjusting medium in gas or liquid form is circulating or passing and threby changes, normally reduces the melt temperature. In order to secure a homogeneous temperature throughout the entire melt, this device has a structure adapted to homogenize the melt simultaneously with the change of temperature. An embodiment comprises a device having sets of elements according to Figs. 1 and 2 in an alternating succession. These gear wheel-like elements with alternating inner and outer cogs, the cogs being shifted some distance in relation to each other, yields labyrint-shaped flow path according to Fig. 3. Thus, the portion of the melt positioned between the two cogs will be pressed against an opposite cog and will be divided into two portions which in turn combine with portions from adjacent paths. Subsequent to the cog wheel-like elements there are ducts for the temperature control medium such as gas or liquid. These ducts may pass through the gear like elements or in connection therewith. In one embodiment these ducts extend helically in the axial direction causing the medium in the outer helix to flow axially in the one direction and the medium in the inner helix to flow axially in the opposite direction. The tempera¬ ture control of the medium may be such that a heat-recovery is obtained. Another construction comprises temperature-controlled tur¬ bulence elements and alternatively a separate homogenizing unit may be provided subsequent to the temperature control device. Increased heat exchange surface and extended dwell time of the molten melt will obviously increase the efficiency of the unit. In order to increase the dwell time of the melt in the described labyrint-shaped device therein, it has preferably a through-flow area which is between two and three times larger than that of the supply pipe. In order to obtain a temperature of the heat exchange elements as uniform as possible and thereby improve temperature homogenity, the energy used for the heat exchange during the transmission to a material gap should be supplied to elements which penetrate the entire depth of the gap and should be supplied from the both sides of the element or alternatively every second element from the one side and every second element from the other side. Various modifications of the invention are obvious to an expert on the field. However, such modifications are intended to fall within the frame of the invention as defined in the attached claims.

Claims: CLAIMS 1. A method of controlling the melt temperature of a plastic raw material used for the manufacture of plastic products, the plastic raw material will be melted in a plastifying section from which the melt is fed into a die, characterized in that an adjusted desired value of the melt temperature is obtained by passing the melt through a heat exchanger. 2. The method as claimed in claim 1, characterized in that the melt is directed through the heat exchanger with heat transmitting elements in contact throughout the entire depth of the melt. 3. The method as claimed in claim 1, characterized in that the melt is directed through the heat exchanger with the heat exchange elements in contact with the melt surface. 4. The method as claimed in any of the preceding claims, characterized in that the energy transmission to the heat exchanger is performed from the outside, from the inside or from both sides. 5. The method as claimed in any of the preceding claims, characterized in that as a temperature control medium a substance is used in liquid or gas form circulating via a temperature control unit through the heat exchanger or caused to flow straight through the same. 6. The method as claimed in any of the preceding claims, characterized in that the desired temperature control effect is obtained by controlling the duration of the dwell time in the heat exchanger. 7. The method as claimed in any of the preceding claims, characterized in that existing machine elements such as screen pack assembly, static mixer, adapter or die is used as heat exchanger. 8. The method as claimed in any of the preceding claims, characterized in that the heat exchanger performs the homo- genization of the melt temperature or is succeeded by a homogenizing unit mixing the melt in such a way that a uniform temperature is obtained in all portions thereof. 9. A heat exchange device for temperature control of a plastic melt prior to its shaping into a product, charac¬ terized by an array of labyrint-shaped paths, in which every path is subdivided into sections each spreading into at least two susequent sections, the walls of the sections being temperature controlled. 10. The heat exchange device as claimed in claim 9, charac¬ terized in that the labyrint-shaped paths are formed by gaps between the cogs of gear like elements, several such elements being provided in axial succession, and that the cogs on every second element are directed radially inwardly and on every second element radially outwardly. 11. The heat exchange device as claimed in claim 9, charac¬ terized in that cooling ducts are provided subsequently to the elements, these cooling ducts being helical, such that an outer helical cooling duct extends about the one array of elements and an internal helical cooling duct extends within the other array of elements, the feeding of the cooling medium through the ducts being such that the cooling medium in the one cooling duct is moving axially in the one direction and in the other one in the opposite direction.

Inventors: NILSSON BO, NILSSON, BO
Applicants: NILSSON BO, NILSSON, BO
Application year: 1989
Main classification: B29B13
IPC codes: B29B13, B29C47