JP4156473B2 - Filter for preventing pellet contamination - Google Patents

Description

  The present invention relates to a filter for preventing mixing of pellets into a circulating cooling water tank, which is used in a granulation method in which a molten polymer is cooled and solidified in water and then cut.

Conventionally, the underwater cutting granulation method has been widely used for thermoplastic resins and the like as an excellent method for producing a large amount of pellets.
The granulation method is a method of cooling and solidifying the molten polymer discharged from the nozzle in water, and discharging the molten polymer sent from an extruder or gear pump into the circulating cooling water from a circular die having many small holes. Cooled and solidified, cut into pellets with a rotary blade close to or in contact with the surface of the circular die, and the pellets were transported together with circulating cooling water to be completely cooled and solidified, and then using a centrifugal dehydrator or the like Dehydrate.

  According to this method, since the molten polymer is cut before it is completely solidified to the inside, there is an advantage that the burden on the cutter can be reduced, the power consumption is small, and the life of the cutter blade can be extended. At the same time, since the floss (fine thread form) generated by peeling off the burrs at the polymer cutting part at the time of cutting is washed away from the product pellets with circulating water, homogeneous product pellets can be obtained.

However, in this method, since the pellets are transported together with the circulating cooling water as described above, floss and some pellets are mixed in the circulating cooling water system.
If different types of pellets staying in the circulating cooling water system are mixed with the product pellets, they become foreign matter and cause fish eyes of films, laminate products, and the like. In particular, in the case of pellets that cross-link if retained for a long period of time, even if a small amount is mixed, a fish eye is caused and the performance and appearance of the product are deteriorated.

In order to solve this problem, conventionally, a filter is provided at the inlet of the circulating cooling water tank to capture floss and pellets, particularly floss. However, in this method, when the filtration accuracy is increased in order to capture the floss, the pressure loss of the circulating cooling water gradually increases, and the filter is clogged due to the attachment of floss and the circulating cooling water overflows outside the system. In order to avoid this, it is necessary to stop the operation of the apparatus and clean the filter each time, which causes a significant hindrance to continuous operation.
The frequency of cleaning differs depending on the type of thermoplastic resin, and there was a large variation of 1/3 hours to 1 time / several weeks. Many studies have been made to reduce the frequency of this cleaning and to make the continuous operation time as long as possible (see, for example, Patent Documents 1 and 2).

In Patent Document 1, it is proposed to provide a plurality of filters having a smaller opening as the lower filter in order to improve operability, but the filter used is 100 to 200 mesh and has a very large opening. It is small and not sufficient to reduce the frequency of clogging.
Further, in Patent Document 2, in order to enable continuous operation, a screen (filter) that rotates in an endless state is used, and resin powder adhering to the screen is removed by spraying liquid or gas onto the screen. However, the apparatus is complicated and there are problems in terms of maintenance and cost.
In order to solve these problems, there has been a demand for the development of a filter having a simple structure in which a product pellet does not contain foreign-grade pellets that become foreign matters and can be continuously operated.

Japanese Patent Laid-Open No. 11-207734 (first page) JP-A-10-151624 (first page)

  The present invention has been made under such circumstances, and is intended to provide a pellet mixing prevention filter having a simple structure in which product grade pellets are not mixed with foreign-grade pellets that are foreign substances and can be continuously operated. It is the purpose.

As a result of intensive research to achieve the above object, the present inventor has made the filter configuration into a two-stage composite filter comprising a removable upper stage filter and a fixed lower stage filter, and the upper filter It has been found that the above object can be achieved by making the opening smaller than the opening of the lower filter, and in particular by reliably capturing the pellets.
The present invention has been completed based on such findings.

That is, the present invention
(1) In a granulation method in which molten polymer made of a thermoplastic resin is cooled and solidified and cut in cooling water circulated through a circulating cooling water tank, it is installed at the inlet of the circulating cooling water tank and is detachable. A filter for preventing mixing of pellets, which is a two-stage composite filter composed of an upper filter and a fixed lower filter, wherein the opening of the upper filter is smaller than the opening of the lower filter,
(2) The aperture of the upper filter is 0.5 to 2.0 mm, the aperture of the lower filter is 1.2 to 2.4 mm, and the ratio of the aperture of the lower filter to the aperture of the upper filter is 1 The filter for preventing mixing of pellets of (1) above, which is .1 to 2.4, and (3) the thermoplastic resin is low density polyethylene, linear low density polyethylene, ethylene / vinyl acetate copolymer and ethylene / methyl (1) and (2) pellet mixing prevention filters, which are polyethylene resins selected from methacrylate copolymers,
Is to provide.

  According to the present invention, the structure of the filter is a two-stage composite filter composed of a removable upper filter and a fixed lower filter, and the opening of the upper filter is smaller than the opening of the lower filter, By preventing the mixing of pellets into the circulating cooling water tank, it is possible to provide a simple structure pellet mixing prevention filter that can prevent the mixing of foreign grade pellets, which are foreign substances, into the product pellets and can be operated continuously. At the same time, pellets with excellent product performance can be provided.

The present invention is described in detail below.
First, the pellet mixing prevention filter according to the present invention is a granulation method in which a molten polymer made of a thermoplastic resin is cooled and solidified in cooling water circulated through a circulating cooling water tank, and the circulating cooling water is cut. This is a composite filter installed at the tank entrance. The composite filter has a two-stage structure composed of an detachable upper filter and a fixed lower filter, and has a feature that the opening of the upper filter is smaller than the opening of the lower filter.
The thermoplastic resin used in the present invention is not particularly limited as long as it can be granulated by an underwater cutting method. For example, low density polyethylene, linear low density polyethylene, ethylene / vinyl acetate copolymer, Examples thereof include polyethylene resins such as ethylene / methyl methacrylate copolymer, among which low density polyethylene is preferable.

The filter for preventing contamination of the pellets of the present invention firstly needs to prevent the pellets from being mixed into the circulating cooling water tank. Both the upper filter and the lower filter are designed so that the pellets can be reliably captured. The opening is set.
On the other hand, although it is not necessary to capture all of the floss only with the filter, the size of the upper filter is set smaller than that of the lower stage so that more floss can be captured in the upper stage.
However, the amount of floss in the product pellet should be as small as possible, and there are many means for capturing other than the filter, which will be described in detail later.

The second object of the present invention is not only to prevent the aforementioned pellets from being mixed into the circulating cooling water tank, but also to continuously operate the pellet production apparatus for as long as possible.
Therefore, the upper filter is detachable, that is, can be easily attached and detached in order to quickly replace the filter clogged with floss and pellets without stopping the operation of the apparatus.
In addition, a spare is prepared for the upper filter, and the time for replacing the upper filter is completed in a very short time of about 5 to 30 seconds.
While the upper filter is removed for replacement, the pellets are captured by the lower filter. Since the lower filter is fixed, pellets do not enter the circulating cooling water tank through the gap between the filters.

The aperture of the filter in the present invention is not particularly limited, but usually it is preferably 0.5 to 2.0 mm for the upper filter, and 1.2 to 2.4 mm for the lower filter, and with respect to the aperture of the upper filter. The aperture ratio of the lower filter is preferably 1.1 to 2.4.
On the other hand, the particle size of the pellet can be captured not only by the upper filter but also by the lower filter by increasing the average particle size by 1 mm or more based on the opening of the lower filter.
Further, the length of the floss is not particularly correlated with the size of the pellets, and is a fine thread shape of 1 to 5 mm.

Hereinafter, the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic process diagram including one embodiment of the present invention. 1 is a kneading granulator, 2 is a three-way valve, 3 is a dewatering screen, 4 is a centrifugal dehydrator, 5 is a filter, 6 is a circulating cooling water tank, 7 is a circulating pump, 8 is a pellet conveying path, 9 is a vibrating sieve, Reference numeral 10 denotes a pellet outlet.
Moreover, a, b, c, d, e, and d respectively show the paths of the circulating cooling water.

When the process is stable and in a steady state, the pellets containing partly floss cut underwater by the kneading granulator 1 are transferred to the dewatering screen 3 from the path b while being cooled with the circulating cooling water via the three-way valve 2. The floss is sent together with the cooling water from the path c to the circulating cooling water tank 6 via the filter 5. Floss is partially captured by the filter 5. The filter 5 is composed of two upper and lower filters, and the opening of the lower filter is larger than the opening of the upper filter, and the upper filter is detachable, that is, can be removed and attached. The filter is fixed. By fixing the lower filter, it is possible to prevent the pellets from entering the circulating cooling water tank 6 from the gap between the filters.
Since the floss that has entered the circulating cooling water tank 6 stays mainly on the surface portion of the circulating cooling water tank 6 due to the specific gravity, the surface portion is caused by overflowing a part of the circulating cooling water (not shown). A portion of the floss remaining in the tank flows out of the system together with the circulating cooling water. Subsequently, the circulating cooling water containing a small amount of froth returns to the kneading granulator 1 from the path f via the circulation pump 7.

On the other hand, pellets partially containing froth are dehydrated from the dehydration screen 3 via the path d by the centrifugal dehydrator 4 and separated into pellets and floss by the vibrating sieve 9 via the pellet conveyance path 8.
Further, the pellets are pneumatically transported, and the product pellets and floss are further separated by a classifier (not shown) provided at the outlet.
The cooling water separated by the centrifugal dehydrator contains a small amount of froth and pellets and flows into the filter 5 via the path e, and the pellets are captured by the upper filter.

When the upper filter is clogged due to continuous operation for a long time, the upper filter is replaced with a spare filter in a short period of several seconds without stopping the operation. During the replacement period, the pellets are removed with the fixed lower filter. To capture. Since the lower filter is fixed, there is no inflow of pellets from the gap into the circulating cold water tank, and it serves as a pellet stopper.
Although continuous operation for about one month is possible, the fixed lower filter is also cleaned around one month, and at the same time, the punching plate of the dewatering screen is cleaned.

Next, the filter 5 and the unsteady state will be described with reference to the drawings.
FIG. 2 is a perspective view showing an embodiment in which each part of the filter of the present invention is disassembled. 11 is a top cover, 12 is an upper filter (removable), 13 is a lower filter (fixed), and 14 is a pedestal. FIG. 3 is a perspective view showing an embodiment of the filter of the present invention with the front cover removed, and FIG. 4 is a perspective view showing an embodiment of the filter of the present invention with the front cover attached. .
The front cover is attached during operation and can be easily removed. It is also possible to open only the frontmost surface for inspection.
Further, the front cover is effectively used in the following unsteady state. Immediately after the underwater cutting operation is started, it is in an unsteady state. During that time, the three-way valve 2 shown in FIG. 1 causes the pellets to flow directly to the circulating water tank through the paths a and c together with the circulating cooling water. At that time, a large amount of pellets are captured by the filter, but the pellets can be easily removed from the filter 5 by removing the front cover of the filter. When the steady state is reached, the three-way valve is switched to the path of the dewatering screen 3.

Here, a floss capturing method will be described. In the present invention, since the filter 5 is intended to reliably capture pellets, a small amount of floss stays or circulates in the circulating cooling water path. Normally, the amount of floss generated is when the resin is hard and difficult to cut, such as when the melt viscosity of the resin is low, the temperature of the circulating cooling water is low, or when the core of the granulator's cutting device is misaligned. It tends to increase.
In a steady state, the amount of froth generated in the vicinity of normal cutting is several hundred ppm by mass or less with respect to the mass of the pellet.
As described above, the froth is captured by (1) the filter 5, particularly the upper filter 12. (2) The froth staying on the surface portion of the circulating cooling water tank 6 is partially overflowed and discharged out of the system. (3) The vibrating sieve 9 separates the pellet from the floss. (4) Sort pellets and floss with a classifier.
By capturing and separating the floss in the above four steps, the amount of floss generated at the time of cutting in the product pellet becomes several mass ppm or less, and the influence on the product performance can be ignored.

Rather than capturing floss only with a filter as described above, by ensuring the capture of pellets with the filter, the opening of the filter can be increased by about an order of magnitude or more compared to the conventional conventional technology, By making the opening of the lower filter 13 larger than that of the upper filter 12, a pellet mixing prevention filter having a simple structure according to the present invention corresponding to the underwater cutting granulation method for mass production of pellets of usually 1 to 10 ton / hr. Can be provided.
By using a filter with a large opening in the present invention, the frequency of clogging of the filter is reduced due to adhesion of floss and the like, which has been a problem in the past, and the manufacturing apparatus can be continuously operated and manufactured using pellets. It is possible to provide a product pellet having excellent performance without degrading the quality of the product to be produced.

EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited at all by these examples.
Example 1
Using a horizontal single screw extruder manufactured by Kobe Steel Co., Ltd., various low density polyethylene of film grade, laminate grade and molding grade with MFR (melt flow rate) of 0.3 to 20 g / 10 min by the method shown in FIG. The granulation operation of pelletizing at -9 ton / hr was performed continuously for about 1 month. At this time, the circulating cooling water amount is 80 to 90 m ³ / hr, the upper filter uses an opening of 1.8 mm, the lower fixed filter uses an opening of 2.0 mm, and the average particle size of the pellet is 3.5 to It was 4 mm. During this period, cleaning of the pellets staying in the upper filter was performed about once / day on average, but no mixing of the pellets into the circulating cooling water tank was confirmed. (Visual confirmation)
In addition, the measurement of MRF was performed based on JISK7210.

Example 2
Using a horizontal single-screw extruder manufactured by Kobe Steel, 2 types of ethylene / vinyl acetate copolymers of film grade, laminate grade and molding grade with MFR of 0.3 to 20 g / min are used in the manner shown in FIG. The granulation operation for pelletizing at a rate of ˜3 ton / hr was performed continuously for about one month. At this time, the amount of circulating cooling water is 70 to 80 m ³ / hr, the upper filter uses an opening of 1.7 mm, the lower fixed filter uses an opening of 2.2 mm, and the average particle size of the pellet is 3.5 to It was 4 mm. During this time, the pellets staying in the upper filter were cleaned on average about once / day, but no mixing of the pellets into the tank was confirmed. (Visual confirmation)
In addition, the measurement of MRF was performed according to JIS K 7210 like Example 1.

It is a schematic process drawing containing one embodiment of the present invention. It is a perspective view which shows one embodiment which decomposed | disassembled each part of the filter of this invention. It is a perspective view which shows one embodiment of the state which removed the front cover in the filter of this invention. It is a perspective view which shows one embodiment of the state which attached the front cover in the filter of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Kneading granulator 8 Pellet conveyance path 2 Three-way valve 9 Vibrating sieve 3 Dehydration screen 10 Pellet taking out port 4 Centrifugal dehydrator 11 Top cover 5 Filter 12 Upper filter 6 Circulating cooling water tank 13 Lower filter 7 Circulating pump 14 Base

Claims (3)

In a granulation method for cooling, solidifying and cutting a molten polymer made of a thermoplastic resin in cooling water circulated through a circulating cooling water tank, the upper stage filter installed at the inlet of the circulating cooling water tank and detachable A filter for preventing mixing of pellets, wherein the filter is a two-stage composite filter composed of a fixed lower filter and the upper filter has a smaller opening than the lower filter. The opening of the upper filter is 0.5 to 2.0 mm, the opening of the lower filter is 1.2 to 2.4 mm, and the ratio of the opening of the lower filter to the opening of the upper filter is 1.1 to The pellet mixing prevention filter according to claim 1, which is 2.4. The pellet mixture according to claim 1 or 2, wherein the thermoplastic resin is a polyethylene resin selected from low density polyethylene, linear low density polyethylene, ethylene / vinyl acetate copolymer and ethylene / methyl methacrylate copolymer. Prevention filter.

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