Abstract:
A device for degassing melts, especially plastic melts, which has a melt nozzle ( 1 ) with an inlet connection ( 11 ) for connecting to a melt supply, a melt outlet opening and a vacuum chamber ( 20 ) defined by essentially vacuum-tight walls, which communicates with a vacuum source and which opens into a discharge opening for an essentially vacuum-tight connection to a melt processing system, the melt outlet opening of the nozzle extending into the degassing chamber. The melt outlet opening ( 13   a ) of the nozzle ( 1 ) is configured as a slit of a length which is a multiple of its width and which has a curved or folded configuration.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a device for degassing melts, in particular plastic melts, comprising a melt nozzle which has an inlet connection for connecting to a melt supply and a melt outlet opening, as well as a degassing chamber defined by essentially gas-impermeable walls, communicating with a vacuum source and opening into a discharge opening for essentially tight connection to a melt processing system, the melt outlet opening of the nozzle extending into the degassing chamber. 
     Such a device for degassing plastic melts is known from an extruder system. In this, granular plastic is molten under heat and pressure in an extruder, and this melt is discharged from the extruder. However, the melt is not homogenous, but contains air bubbles, which make further processing of the melt difficult. For this reason the melt has to be degassed, i.e. freed from air and other gas bubbles. This can be achieved most easily by connecting a vacuum portion to the extruder discharge opening, through which vacuum portion the melt flows. Because of the negative pressure in the vacuum portion, gas bubbles are drawn out of the melt and let off. Because of the high viscosity of the melt this process only works quite inadequately and furthermore necessitates a high vacuum so as to make it possible to draw gas bubbles even out of the central region of the melt. High-vacuum apparatus are very expensive and sensitive, however, and need frequent maintenance. Besides, the gases drawn out are hot (typically 250° C. to 260° C.), and they often contain aggressive vaporized plastic additives that attack the apparatus so that it may be necessary to replace it several times a year. 
     Thus, in order to be able to reduce the necessity to employ high vacuum, a device of the above type, as taught in U.S. Pat. No. 3,410,938 or the corresponding CH Patent No. 425 207 A, has been developed, wherein the melt flow initially flows through a nozzle connected to the extruder outlet, which nozzle is provided with a nozzle plate having a plurality of through-holes or slits dividing the melt flow into a plurality of “spaghetti-like” or ribbon-like sub-streams. A degassing chamber is connected downstream of the nozzle plate, which is under vacuum and through which the melt sub-streams flow after leaving the nozzle. As the diameter of the melt sub-streams is smaller than that of the flow as a whole it is not necessary to employ a vacuum as high as that employed for directly degassing the melt flow as a whole. However, practical use of this device also has disadvantages, as research carried out by the inventors of the subject matter of the present application has resulted in the discovery that the melt sub-streams have a strong tendency of adhering to each other after leaving the nozzle plate, thus again combining to form a thicker stream, so that the vacuum employed will not be enough to draw the gas bubbles out of the interior of the melt. 
     SUMMARY OF THE INVENTION 
     The present invention is based on this discovery and offers a solution for degassing melts in a satisfying way that avoids the disadvantages of the state of the art, while at the same time making it possible to further reduce the vacuum required. The invention emanates from the approach that dividing the melt into sub-streams cannot bring about the desired result as the subsequent reunification of the sub-streams can only be prevented at unduly great efforts. Thus the invention continues to employ an undivided melt flow, which is, however, brought in a shape having a cross-section of very small thickness, by forming the melt outlet opening of the nozzle as a single slit whose length is a multiple of its width and which has a curved or folded configuration in order to accommodate the required length of the slit in the nozzle plate. This measure according to the invention makes it possible to degas the melt to a satisfying degree already at very slight vacuum, for the gas bubbles only have to be drawn to the surface of the melt from a very small depth thereof or even burst by themselves because of the small thickness of the melt flow. 
     Conveniently the slit has a length that is at least 50 times, preferably at least 500 times, larger than its width. In some applications it may even be considered appropriate to increase the ratio between length and width to 5000:1 and above. 
     In order to further reduce the amount of vacuum necessary it may be provided for the width of the slit to be less than 2 mm, preferably less than 0.1 mm. 
     In order not to limit the throughput of melt from the melt supply, e.g. from an extruder, to the subsequent processing means by the interposed degassing device, the cross-sectional area of the slit may not be smaller than a predetermined value. As the width of the slit may not be increased without negatively affecting the degassing performance of the device according to the invention, it is thus necessary to adjust the length of the slit to a suitable value. In order to do so without at the same time unduly increasing the total size of the nozzle, an embodiment of the invention is characterized in that the slit is formed in zigzag or corrugated form in its longitudinal direction. 
     A particularly favorable embodiment of the device according to the invention is obtained by forming the slit as a ring, as in this case the nozzle may be produced by turning it on a lathe and its total dimensions may be kept small. If it is necessary to further increase the length of the slit, it is of course also possible to form the annular slit in corrugated or zigzag configuration. 
     In order to avoid the disadvantage that a vacuum is only applied to the exterior surface of the melt hose thus produced while it flows through the degassing chamber, a particularly preferred embodiment of the invention is characterized in that the nozzle has a through-hole extending into the interior of the ring and communicating with a vacuum source. By this measure a vacuum is applied to the melt hose both from the outside and the inside at the same time, which because of the small wall thickness of the melt hose ensures high efficiency of the degassing device according to the invention. The construction of the device is simplified if the through-hole of the nozzle opening into the interior of the ring of the slit communicates with the same vacuum source as the degassing chamber. 
     In order to be able to vary the thickness of the melt leaving the slit in accordance with the intended use and material properties of the melt, the invention furthermore provides for the nozzle to be equipped with replaceable inserts for slits of different shapes and/or dimensions. 
     Providing the nozzle and/or the degassing chamber with heating means makes it easier for the melt to flow through the degassing device. 
     In order to monitor the degassing process, at least one viewing window may be provided in the walls defining the degassing chamber. 
     Preferably a filter for filtering foreign matters out of the melt flow is provided at the inlet connection of the nozzle. Thus is it possible to prevent foreign matters from entering the degassing device, which would be in particular harmful if they were inflammable at the temperatures in the degassing device, as undesirable combustion gases would be formed during their combustion. 
     According to a particularly advantageous embodiment the device according to the invention comprises a Venturi fluid entrainment pump as its vacuum source. Such a pump does not comprise any movable parts but produces a negative pressure in a Venturi tube by a fluid flowing therethrough, which negative pressure sucks fluid from a side entrance into the tube, i.e. in the present case the gases to be withdrawn from the degassing chamber. The Venturi tube may be provided easily with a corrosion-resistant coating on the inside, so that it withstands the aggressive components of the gas to be withdrawn. 
    
    
     The invention will now be described in detail by way of a non-limiting example with reference to the attached drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal section of an embodiment of the device according to the invention for degassing melt; and 
     FIGS. 2 to  6  show various embodiments of the slit of the nozzle in a schematic plan view. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiment of the device according to the present invention for degassing melts shown in longitudinal section in FIG. 1 is formed as a degassing head which is connected downstream of a plastic extruder in order to free the plastic melt produced by the extruder from gas bubbles. The device has a nozzle generally indicated at  1  as well as a degassing chamber generally indicated at  20  downstream thereof. The nozzle  1  has a body  10  generally taking the form of a hollow cylinder, into which a hollow sleeve  11  opens radially, which sleeve may be connected to the discharge opening of a plastic extruder or the like not shown. The nozzle body  10  as well as the sleeve  11  is surrounded by heating elements  5  on the outside. Furthermore a filter  14  is provided at the sleeve so as to remove foreign matters included in the melt stream, in particular inflammable foreign matters like paper, thus effectively preventing the foreign matters from igniting in the degassing device or blocking the nozzle channels. The nozzle body is provided with an axial through bore  12 , which has a first upstream portion  12   a  and a second portion  12   b  downstream thereof having a radius increased with respect to that of the first portion  12   a . A nozzle insert  2  taking the form of a hollow cylinder is inserted into the through bore  12  from the downstream end of the through bore  12 , a through bore of the nozzle insert  2  being designated by reference numeral  2   a . The outer radius of the jacket of nozzle insert  2  corresponds to the radius of portion  12   a  of the through bore  12  of nozzle body  10 , so that the nozzle insert may fit snugly in the portion  12   a  in the nozzle body. However, an annular gap  13  is formed between the wall of the enlarged portion  12   b  of the through bore  12  of the nozzle body  10  and the outer periphery of the jacket of nozzle insert  2 , as the radius of the jacket of the nozzle insert is initially constant and only increases close to its downstream end (shown at  2   b ) so as to reduce the annular gap  13  to the width desired for the melt outlet opening  13   a . Sleeve  11  opens into this annular gap  13 . Melt  6  entering into nozzle  2  through sleeve  11  is distributed in annular gap  13  and leaves it as a melt hose  6   a  of very small thickness at its downstream end, i.e. at melt outlet opening  13   a . It is to be noted that the nozzle insert  2  may be replaced by inserts of different dimensions so as to adjust the size of the annular gap and of the melt outlet opening, respectively. 
     A degassing pipe  3  is vacuum-tightly connected to the outlet of nozzle  2  via a connection pipe  4 , and its interior defines a degassing chamber  20  communicating with a vacuum source not shown via a connection  3   a  of the degassing pipe  3 . It is to be noted that the term vacuum as used herein includes any suitable negative pressure. The melt hose  6   a  with gas bubbles therein leaving the nozzle  2  thus flows into the degassing chamber  20 , where gas bubbles are withdrawn from the interior of the melt to the surface and then through connection  3   a  to the outside by the negative pressure, as shown by the bubbles and small arrows in the degassing chamber and by the large arrow with bubbles next to it near connection  3   a , respectively. However, as the melt flowing therethrough has the configuration of a continuous hose, the interior space  20   a  of the melt hose  6   a  is sealed with respect to the degassing chamber  20 . It is, however, desirable for the interior space  20 a to be subject to the same negative pressure as the degassing space  20  in order to also carry out degassing of the melt starting from the interior  20   a  and prevent inflation of the melt hose by a difference in pressure between its inside and its outside. For this reason nozzle insert  2  was provided with through bore  2   a , which is connected to a vacuum source not shown. Conveniently the vacuum source is the same as the one connected to the connection  3   a  of the degassing pipe  3 , which is how differences in pressure between the inside and the outside of the melt hose are prevented. As the vacuum source need not produce a particularly high vacuum for degassing because of the small wall thickness of the melt hose, it is possible to employ a side fan or a Venturi fluid entrainment pump. The use of a Venturi fluid entrainment pump (also known as Venturi system or jet ejector pump) is preferred, as it contains no movable parts and may easily be coated so as to become corrosion-resistant, which is how it becomes preferable to other types of vacuum pumps because of its much longer service life. 
     The progress of degassing of the melt hose  6   a  may be monitored through viewing window  8  in degassing pipe  3 . Degassing pipe  3  is surrounded by heating means  5  as well. While melt hose  6   a  is flowing through the degassing chamber, gas bubbles are continuously withdrawn from the melt and discharged, so that the melt hose will have been degassed to a sufficient extent when it reaches the downstream end of the degassing chamber and thus may be subjected to further processing. For instance a discharge extruder may be connected in vacuum-tight fashion to the degassing pipe. In any event, however, the degassing pipe is sealed vacuum-tightly by a subsequent processing station. 
     Apart from the embodiment of the nozzle having a ring-shaped melt outlet opening  13   a , as described in detail and schematically shown in FIG. 2 in plan view from the downstream nozzle end, it is also possible to use different configurations of melt outlet openings. Examples thereof are shown in schematic plan view in FIGS. 3 to  6 , which in this order show a corrugated annular melt outlet opening  13   a - 2 , a straight melt outlet opening  13   a - 3 , which is not within the scope of the present invention, a generally straight but corrugated melt outlet opening  13   a - 4  and a zigzag melt outlet opening  13   a - 5 . These melt outlet openings may be also formed by providing replaceable nozzle inserts of corresponding shapes, the embodiments of FIGS. 4 to  6  making it necessary to use suitable designs, known to specialists in the field of extrusion, to ensure that the melt will be distributed all over the width of the melt outlet opening inside the nozzle.