GB1558648A

GB1558648A - Cleaning articles

Info

Publication number: GB1558648A
Application number: GB2285077A
Authority: GB
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 1976-05-28
Filing date: 1977-05-30
Publication date: 1980-01-09
Also published as: JPS52145475A

Description

(54) CLEANING ARTICLES (71) We, SUMITOMO CHEMICAL COM- PANY LIMITED, a Japanese Body Corporate, of No. 15, Kitahama 5-chome, Higashiku, Osaka-shi, Osaka-fu, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to methods for cleaning synthetic resin articles and for cleaning and dip-coating such articles and apparatus therefor.

Various kinds of synthetic resin articles are subjected to surface treatment for example printing, plating or coating. In these treatments, the cleanness of surfaces to be treated is very important since the efficacy of printing, plating and coating is affected by foreign substances on the sur face of substrate. When such foreign substances are not removed, they stay between the substrate surface and the printing, plating or coating film, and weaken the adhesion between the surface and the film, or promote the formation of pinholes and specks.

Various cleaning methods are employed industrially using organic solvents, emulsion liquors, detergents, acids or ultrasonic waves. Of these methods however, there are relatively few suitable for cleaning synthetic resin articles made of vinyl resins (e.g. polyvinyl chloride), acrylic resins (e.g.

Dolymethyl methacrylate), styrene resins (e.g. polystyrene, acrylonitrile-styrene-buta- dine terpolymers. acrylonitrile-styrene copolymers, methyl methacrylate-butadiene- styrene terpolymers) or polyearbonate resins. For example, solvent cleaning with an organic solvent such as benzine; gasoline, trichlene or perchloroethyleneq often causes the articles to dissolve in the solvent or produces cracking or blushing by contact with the solvent. Consequently, solvent cleaning is unevitable for synthetic resins.Emulsion cleaning is also improper when these solvents are used, because the same problems as above occur. Further more, the acid cleaning processes generally used for metals are also undesirable because this uses strong acids which easily damage the articles.

Detergent cleaning is relatively suitable for cleaning synthetic resin articles and results are improved when such cleaning is carried out using ultrasonic waves. When a detergent and ultrasonic waves are used together, dirt is removed from the surface of the articles, but the articles do not dissolve in the cleaning liquor nor produce cracking and blushing. When a detergent is used, it becomes necessary to completely remove the detergent from the surface of the articles. If the removal is insufficient, it rather results that adhesion between the surface and film becomes poor and surface smoothness is damaged. In addition, as the cleaning is carried out with water, drying after the cleaning is slow and dirt in the air easily adheres to the cleaned surface.

It is well known that materials of relatively simple shape, for example, a flat plate, may be satisfactorily coated by a method which comprises dipping them in a a coating liquor and pulling them up slowly thereby forming a coating film on the surface. In this case, the coated surface forms a flat or relatively gently curved surface so that, when foreign substances such as dust are present in the coating liquor, the poor smoothness of the coating film becomes easily noticeable. This becomes a serious problem when the coating film is thin. The foreign substances in the coating liquor include not only those contained in the liquor from the beginning, but also those produced by gelatin or drying of a Part of the liquor, those coming into the liquor from the atmosphere and those attached to the surface of articles to be coated.

In the so-called dip-coating process in which articles to be coated are dipped in a coating liquor and pulled up slowly whereby a coating film is formed, it is general that the articles are dipped in and pulled up from the coating liquor kept stationary. In this process, foreign substances in the liquor come into the coating film. In many cases, however, the smoothness of film surface is not much affected because the film thickness is relatively large in general, or the smoothness of film surface is not essential to the performances of the products. When the coating film is thin, the smoothness of film surface is adversely affected by the foreign substances and the value as goods is largely damaged.The commonly used, well-known filtration process for removing dust from a coating liquor is as follows: a coating liquor is supplied to a dip-coating vessel at one side of the top, allowed to flow on the surface layer of the liquor in the vessel and to overflow the other side of the top. The liquor which overflows is filtered and circulated to the vessel.

Foreign substances deposited to the bottom are discharged from the bottom. This process is suitable for preventing foreign substances in in the atmosphere from entering the liquor, but when once the foreign substances enter the liquor, those having a specific gravity close to that of the coating liquor and those of fine size are dispersed in the liquor without settling to the bottom. Consequently, they can not be removed.

The present invention provides a method of cleaning a synthetic resin article, which comprises subjecting the article to ultrasonic vibration in an upward-flowing stream of an aqueous detergent solution in a first vessel, transferring the article to a first upward-flowing stream of warm water in a second vessel, and then to a second upward-flowing stream of warm water in a third vessel and extracting the article from the water to drain off the water attached to the article.

The invention includes an apparatus for cleaning synthetic resin articles comprising a mechanical conveyor for the articles cleaning vessel provided with means for applying ultrasonic vibrations to its contents and - having such a structure as to give an upward flow to an aqueous detergent solution, a rinsing vessel to which warm water is supplied through a filter and through which the warm water flows upward, and a draining vessel to which warm water is supplied through a filter and through which the warm water flows upward, the conveyor being arranged and constructed to carry the articles successively from the cleaning vessel through the rinsing vessel to the draining vessel and to pull the articles up from the draining vessel.

A preferred apparatus comprises three open-topped tanks, each tank having an inlet at a bottom portion thereof, and an outlet formed by overflow tray around at least a portion of a top edge thereof, each tank being provided with a pump and a filter, to filter liquid overflowing into the overflow tray and return it to the inlet, means being provided for mechanically conveying the articles successively from one tank to another, means being provided for supplying ultrasonic vibrations to a liquid in the first tank and means being provided for withdrawing the articles from the last tank.Preferably the means for conveying the articles comprises an endless belt or chain, having means for attaching an article thereto, cogs or pulleys being provided for driving the belt or chain, and means being provided for moving at least one such cog or pulley transversely of the direction of motion of the belt or chain, to dip an article into a tank.

The invention includes an apparatus for cleaning synthetic resin articles comprising a mechanical conveyor for the articles cleaning vessel provided with means for applying ultrasonic vibration to its contents and having such a structure as to give an upward flow to an aqueous detergent solution, a rinsing vessel to which warm water is supplied through a filter and through which the warm water flows upward, and a draining vessel to which warm water is supplied through a filter and through which the warm water flows upward, the conveyor being arranged and constructed to carry the articles successively from the cleaning vessel through the rinsing vessel to the draining vessel, and to pull the articles up from the draining vessel, in combination with dip-coating apparatus which comprises an open topped tank having an inlet at a bottom portion thereof, a foraminous plate disposed within the tank above the level of the inlet, whereby when the apparatus is in use a coating liquor flowing up from the tank from the inlet flows through the plate, an overflow tray around at least a portion of the top edge of the tank a pump for pumping the coating liquor from the overflow tray to the inlet to circulate the coating liquor, and a filter for removing foreign particles from the liquor.

The invention accordingly also provides a method of coating an article comprising cleaning the article by a method which comprises subjecting the article to ultrasonic vibration in an upward-flowing stream of an aqueous detergent solution in a first vessel, transferring the article to a first upward-flowing stream of water in a second vessel, and then to a second upward-flowing stream of warm water in a third vessel and extracting the article from the water to drain off the water attached to the article, dipping the article in a dip-coating apparatus as defined above and withdrawing the article from the coating composition.

As the aqueous detergent solution, there may be used aqueous solutions containing a common detergent for cleaning, for example, anionic or nonionic surface active agents. As to the concentration of the detergent in water, a conventionally used concentration is sufficient, but some detergents become sometimes vigorous in foaming and poor in processability when used in too high concentrations.

While, when the concentration is lowered to less than a critical micelle concentration, the cleaning effect decreases sharply.

A preferred concentration is 0.5-10 g/liter of water in general. A preferred temperature of the aqueous detergent solution is 20 to 50"C, more favorably around 40"C.

The ultrasonic wave applied to the aqueous detergent solution can be generaged by commonly used ultrasonic generators (e.g. frequency, 25-45 Kilohertz; output, 0.5-2.0 Watt 1cm2).

As to the temperature of the warm water for rinsing off the detergents, the higher the more effective the rinsing will be. But, the upper limit of the temperature is limited because synthetic resin articles are deformed by heat when the temperature is too high. The upper limit is determined by the thermal properties of the articles so that it depends upon the kind of synthetic resin and the processing condition. In general, however, one standard to determine the upper limit is a temperature 10 C lower than the heat distortion temperature of the articles which is measured according to the test method specified by ASTM (American Society for Testing and Materials) D-648. A preferred temperature of the warm water for rinsing off detergents on synthetic resin articles molded by the usual conditions is within a range of 40 to 700C.But, a temperature of 50 to 600C is more desirable to suffi ciently remove the detergents with no deformation of the articles. When this warm water contains many dust particles, the dust adheres to the surface of the articles, and enters the warm water used for the draining step, so that particles remain on the articles after drying. Con sequently, warm water for the rinsing is, if necessary, filtered before it is supplied to the vessel.

As the filter, commonly used ones are sufficient. A preferred filter is those of fine mesh, but filters having a mesh of less than 1 u in size are not desirable for practical purposes because they are expensive and high in resistance to filtration, which leads to the enlargement of equipment and increase of equipment cost. On the other hand, the size of the dust which must be kept off from synthetic resin articles depends upon the kind of treatments to which the article will be subjected such as printing, plating and coating, and the thickness of coating film.

In general, however, dust of more than 5 u in size should be removed. Consequently, filters having a mesh of 1 to 5 u on the average are preferred for filter ing warm water for the rinsing. Further, as to the quality of the warm water for rinsing off the detergents, water of such a quality that passes the quality standard of city water (authorized by Ministry of Health and Welfare) is sufficient. But, when superior surface smoothness and adhesion properties are particularly required, it is desirable to further lower the contents of organic substances, chlorine and metal in the water. Water having a required purity can be obtained using a sand filter, active carbon filter, ion-exchange filter, iron-removing equipment, manganese-removing equipment or a combination thereof.Another vessel of fresh warm water is preferably used for dipping the articles before the water attached on the articles is drained off. This vessel is used for removing any trace amount of detergent which may possibly remain on the surface of synthetic resin articles to be cleaned, and at the same time for preventing the attachment of dust to the surface and drying the surface rapidly by pulling up the articles slowly out of the fresh warm water. A a higher temperature is therefore desirable for increasing the rate of drying, but it must not deform the articles as in the case of the warm water for the rinsing. One standard to determine the upper limit of the temperature is also a temperature 10 C lower than the heat distortion temperature of the articles.

When the heat distortion temperature of the articles is less than 70"C, the tem perature of the water should be less than 60"C. In case of synthetic resin articles molded by the usual conditions, the tem perature of the water is 60 to 700 C. The size of dust contained in this warm water is decreased to the same degree as or less than that of dust in the warm water for the rinsing of detergents. As the filter, those having a mesh of 1 to 5 u, prefer ably 1 to 3 u, in size are suitable for use.

The purity of the warm water is suitably of the same degree as or higher than that of the warm water used for the rinsing.

Further, it is preferred that the articles are pulled up more slowly out of the warm water in the third vessel, because water drops then find it difficulty to remain on the surface of the articles and a uniform surface is easily obtained. But, the rate of pulling-up can be increased with increasing of the temperature of the warm water.

When the temperature is 60 to 700 C, the rate is preferably 20-30 cm/min.

The articles cleaned by the method of the invention give good results when they are dried and subjected to treatments such as printing, plating and coating. The coating can effectively be carried out using the method or apparatus described hereinafter.

An example of the method and apparatus of the present invention will be explained in detail with reference to the accompanying drawings wherein: Fig. 1 is a schematic view of the apparatus for cleaning articles; Fig. 2 is a perspective view of a vessel in which a liquor overflows the vessel at the whole of the top; Fig. 3 is a perspective view of a vessel in which a liquor overflows the vessel at a part of the top; Fig. 4 is a cross-sectional view of a vessel in which a liquor overflows the vessel at the whole of the top and a perforated plate is located at the lower part; Fig. 5 is a cross-sectional view of a vessel in which a liquor overflows the vessel at a part of the top and a perforated plate is located at the lower part; Fig. 6 is a flow sheet showing the flow of liquor when the whole liquor is circulated;; Fig. 7 is a flow sheet showing the flow of liquor when a part of the liquor is discharged and a fresh liquor is supplied; Fig. 8 is a schematic diagram showing one embodiment of the dipping apparatus; Fig. 9(a) is a perspective view of a dipping vessel in which a liquor overflows the vessel at the whole of the top; Fig. 9(b) is a cross-sectional view of Fig. 9(a); Fig. 10(a) is a perspective view of a dipping vessel in which a liquor overflows the vessel at one side of the top; Fig. 10(b) is a cross-sectional view of Fig. 10(a); and Fig. 11 is a perspective view of a dipping vessel having a wave-like overflow dam.

Throughout these figures, 1 is a pneumatic or hydraulic cylinder for vertical motion, 2 is an endless conveyer for carrying the synthetic resin articles to be cleaned, 3 is a hanger for holding the articles, 4 is an ultrasonic cleaning vessel, 5 is a rinsing vessel, 6 is a draining vessel, 7 is an ultrasonic generator, 8 is an overflow dam of the ultrasonic cleaning vessel, 9 is an overflow dam of the rinsing vessel, 10 is an overflow dam of the draining vessel, 11 is a cleaning solution inlet, 12 is a warm water inlet, 13 is a warm water inlet, 14 is an aqueous cleaning solution outlet, 15 is a warm water outlet, 16 is a warm water outlet, 17 is a filter for warm water, 18 is a filter for warm water, 19 is a perforated plate, 20 is a circulating equipment, 21 is a fresh liquor inlet and 22 is an old liquor outlet.Further, 23 is an overflow dam, 24 is a perforated plate, 25 is a circulating equipment, 26 is a filter, 27 is a dipping vessel and 28 is a storage tank for coating liquor.

More specifically, in Fig. 1, 1 is an air cylinder or hydraulic cylinder for vertically moving a feed gear for articles to be cleaned, and 2 is an endless conveyer for carrying the articles which is equipped with a hanger 3 for holding a device to which one or more articles are fixed. 4 is an ultrasonic cleaning vessel equipped with an ultrasonic generator 7 at the bottom.

The vessel is constructed so that the aqueous cleaning solution enters the vessel through an inlet 11 at the bottom, flows upward in the vessel, and flows down from an overflow dam 8 to an outlet 14. 5 is a rinsing vessel and warm water passed through a filter 17 enters the vessel through an inlet 12 at the bottom, flows upward in the vessel, and flows down from an overflow dam 9 to an outlet 15. 6 is a draining vessel and warm water passed through a filter 18 enters the vessel through an inlet 13 at the bottom, flows upward in the vessel, and flows down from an overflow dam 10 to an outlet 16. Since a cleaning vessel 4, rinsing vessel 5,draining vessel 6, and filters 17 and 18 are always brought into contact with water, it is preferred to make them of anti-corrosive materials and if necessary to cover them with heat-insulating materials. Suitable anti-corrosive materials include anti-corrosive metals (e.g. stainless steel) and synthetic resins.

The overflow dam may be mounted either on the whole or a part of the top of the vessel. Fig. 2 shows a draining vessel with an overflow dam mounted on the whole of the top. The flow of liquor becomes more uniform by this mechanism of overflow. Fig. 3 shows a draining vessel, as one example, with an overflow dam mounted on a part of the top. In this vessel (Fig. 3), an inlet 13 is made widened at the bottom, that is, warm waters first fed to the upper part of the left side and then enters the vessel through a wide inlet 13 at the lower part. Such a mechan ism of warm water supply is more desirable for the purpose of the draining because air bubbles can be removed from warm water so well that warm water entering the vessel through an inlet 13 contains no air bubbles.Further, it is often difficult to keep the whole of the top of the vessel at the same level, and therefore it is very effective to make the shape of an overflow dam wave-like, because a uniform overflow becomes possible even if the level is somewhat different. The abovementioned dam is very effective to make the upward flow of liquor uniform in the ultrasonic cleaning vessel, rinsing vessel and draining vessel.

Fig. 4 and Fig. 5 show cross-sections of the draining vessels having a perforated plate at the lower part. In Fig. 4, 19 is a perforated plate and warm water entering the vessel at an inlet 13 flows upward through this plate. Because of perforated plate 19, the upward flow of liquor is more uniform and the effect of rinsing off a detergent is further increased. Fig. 5 shows a vessel in which a perforated plate is mounted on the lower part of a vessel as shown in Fig. 3. The upward flow of liquor is more uniform in this modified vessel.

Installment of perforated or other foraminous plates is very effective for making uniform the upward flow of liquor in a draining vessel and rinsing vessel. Foraminous dates suitable for use include perforated anti-corrosive metal plates (e.g.

stainless steel plate) and synthetic resin plates, and porous plates produced by sintering the fine powder or small particles of anti-corrosive metals or synthetic resins.

The pore diameter may be 100 u to 20 mm, preferably 600 u to 10 mm, and the plates having a porosity of 10 to 50 % are suitable because they have a relatively low pressure. loss and act to make the flow of liquor uniform.

Fig. 6 is a flow sheet showing the flow of warm water in a draining vessel combined with circulating equipment 20. Warm water coming out of a draining vessel 6 is sent to a filter 18 by means of a circulating equipment 20 and is again supplied to the draining vessel. This circulation system can be employed to reuse the aqueous detergent solution. warm water for the washing vessel and warm water for the draining vessel. If necessary, it is effective to discharge a waste liquor while supplying a fresh liquor. This is shown in the flow sheet shown in Fig. 7 which shows the flow of warm water in a draining vessel combined with a circulating equipment 20, and the discharge of a part of the waste warm water coming out of a draining vessel 6 at an outlet 22 with recirculation of the remainder while fresh warm water is supplied at an inlet 21.By employing this circulation system, the liquor is effectively used, dust can be removed from water with a high efficiency, and the treatment of water quality can be carried out at a low cost. In employing this circulation system, it is particularly necessary to maintain the water temperature in a suitable range and this may be attained by combining the above apparatus with a heating equipment.

In the ultrasonic cleaning vessel, rinsing vessel and draining vessel of the present invention, it is desirable to properly select the rate of the upward flow of liquor and the correct rate is dependant upon the cleaning (rinsing) time. When the cleaning (rinsing) time is short, a relatively high rate is preferred. The typical example of averate rate are 5 - 500 mm/min, 100 1000 mm/min and 50 - 500 mm/min for the ultrasonic cleaning vessel, rinsing vessel and draining vessel, respectively.

In the dipping vessel to be used in the cleaning and dip-coating method of the present invention shown in Fig. 8, a coating liquor inlet is set at the bottom of a dipping vessel 27, and a coating liquor supplied to the inlet passes through a perforated plate 24 which acts to uniformalize the flow of liquor, flows upward uniformly in the vessel, and overflows a dam 23 mounted on the top of the vessel. The liquor which overflows is collected by circulating equipment 25 and sent to a filter 26 in order to remove foreign substances from the liquor. The coating liquor passed through filter 26 is again supplied to a dipping vessel 27 at the inlet. If necessary, the circulation system may be connected with a storage tank 28 for coating liquor.

Further, desirable results pan be obtained by furnishing the dipping vessel and piping system with heating and cooling equipment and heat-insulating barriers which are useful for controlling the temperature of the coating liquor.

The dipping apparatus shown is constructed so as to allow the whole liquor to circulate therethrough. For this purpose the flow of liquid is changed from the conventional horizontal flow (the liquor which enters flows on the surface layer of the coating liquor in the vessel) to the vertical flow (from bottom to top), and perforated plate is mounted in the vessel to minimize local stagation and uneven rate of flow.

The dipping apparatus shown can be used for applying common surface-coating agents (e.g. paints, coating agents, surface treating agents) and primers used prior to the application of these surfactcoating agents. The superiority of the dipping apparatus is particularly great when the size of foreign substances which can be tolerated on the surface is small because the thickness of coating film is small.

As to the flow of liquor over the top of the dipping vessel, it becomes more uniform when the liquor overflows the whole of the top, as shown in Fig. 9 (a) and Fig. 9 (b). But, uniform circulating of liquor can be attained even when the liquor overflows a part of the top, as shown in Fig.

10 (a) and Fig. 10 (b).

Referring to the shape of overflow dam, when the whole of the top is used as overflow dam, it is difficult to keep the whole dam at the same level and therefore it is difficult to ensure the uniform flow over the whole dam. The wave-like dam as shown in Fig. 11 is effective for uniform overflow in this respect, because it gives some allowance to the level of the dam.

The higher the rate of flow in the vessel, the more rapidly the liquor circulates and the larger the amount of dust removed but, the surface of the coating liquor becomes turbulent and smooth coating surface becomes difficult to obtain. Consequently, it is necessary to employ a proper rate depending upon the composition of coating liquors, the shape of vessels and the film thickness required.

The perforated plate for the dipping apparatus may be made of various materials by various methods. For example, perforated plates of metals, inorganic substances (e.g. glass) and synthetic resins, and those produced by sintering the fine powder or granules of metals, inorganic substances (e.g. glass) and synthetic resins, are suitable. Particularly preferred ones are those produced by sintering, because they are low in pressure loss and produce a uniform flow.

When pressure drop is a particular problem particularly, nonwoven fabric-like felts of metallic fibers, inorganic fibers (e.g. glass fibers) or synthetic resin fibers, or sintered products thereof are effective.

The filter used in the present invention may be any of those usually used but the size of mesh is preferably selected having regard to the thickness of coating film.

For example, when the thickness of dried coating film is about 10 u, the size of mesh is preferably such that foreign substances of 3 u or more in particle size can be filtered off.

The circulating equipment referred to can suitably be a pump. The circulating equipment is selected depending upon the pressure drop of filter and the desired rate of circulation which is determined by the composition of coating liquor, the shape of vessel and the thickness of coating film.

The materials used for the dipping vessels, perforated plates, filters, circulating equipment and nipes should be such that they have no interaction with the coating liquor. Generally, stainless steel, glass, synthetic resin and ceramics are used alone or in combination. Further, since the coating apparatus is used for making a smooth coating film, it is often desirable to pull up the coated articles slowly from the dipping vessel in order to minimize thickness variations. The rate at which the coated articles are pulled up depends upon the composition of coating liquor, but the rate is preferably 5 to 100 cm/min.

Further, it is desirable that the atmosphere in which coating is carried out is kept clean, and it is more preferred to use the apparatus of the present invention in a place furnished with an equipment for removing dust from the air.

WHAT WE CLAIM IS: 1. A method of cleaning a synthetic resin article, which comprises subjecting the article to ultrasonic vibration in an upward-flowing stream of an aqueous de- tergent solution in a first vessel, transferring the article to a first upward-flowing stream of warm water in a second vessel, and then to a second upward-flowing stream of warm water in a third vessel and extracting the article from the water to drain off the water attached to the article.

2. A method as claimed in claim 1, wherein the temperature of the first and second streams of warm water is about 10 C below the heat distortion temperature (ASTM D-648) of the article.

3. A method as claimed in claim 1 or claim 2 wherein the average rate of upward flow of the detergent solution is from 5 to 500 mmlminute.

4. A method as claimed in any one of claims 1 to 3 wherein the average rate of upward flow of the first steam of warm water is from 100 to 1000 mm/min.

5. A method as claimed in any one of claims 1 to 4 wherein the average rate of upward flow of the second stream of warm water is from 50 to 500 mm/min.

6. A method as claimed in any one of claims 1 to 5 wherein the temperatures of the first and second streams of warm water are each from 40"C to 700 C.

7. A method as claimed in claim 6, wherein the temperatures of the first and second streams of warm water are each from 50"C to 60"C.

8. A method as claimed in claim 7, wherein the article is extracted from the second stream of warm water at a speed of from 20 to 30 em/min.

9. A method of cleaning a synthetic resin article substantially as hereinbefore described, with reference to and as illus

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

when the size of foreign substances which can be tolerated on the surface is small because the thickness of coating film is small.

As to the flow of liquor over the top of the dipping vessel, it becomes more uniform when the liquor overflows the whole of the top, as shown in Fig. 9 (a) and Fig. 9 (b). But, uniform circulating of liquor can be attained even when the liquor overflows a part of the top, as shown in Fig.

10 (a) and Fig. 10 (b).

Referring to the shape of overflow dam, when the whole of the top is used as overflow dam, it is difficult to keep the whole dam at the same level and therefore it is difficult to ensure the uniform flow over the whole dam. The wave-like dam as shown in Fig. 11 is effective for uniform overflow in this respect, because it gives some allowance to the level of the dam.

The higher the rate of flow in the vessel, the more rapidly the liquor circulates and the larger the amount of dust removed but, the surface of the coating liquor becomes turbulent and smooth coating surface becomes difficult to obtain. Consequently, it is necessary to employ a proper rate depending upon the composition of coating liquors, the shape of vessels and the film thickness required.

The perforated plate for the dipping apparatus may be made of various materials by various methods. For example, perforated plates of metals, inorganic substances (e.g. glass) and synthetic resins, and those produced by sintering the fine powder or granules of metals, inorganic substances (e.g. glass) and synthetic resins, are suitable. Particularly preferred ones are those produced by sintering, because they are low in pressure loss and produce a uniform flow.

When pressure drop is a particular problem particularly, nonwoven fabric-like felts of metallic fibers, inorganic fibers (e.g. glass fibers) or synthetic resin fibers, or sintered products thereof are effective.

The filter used in the present invention may be any of those usually used but the size of mesh is preferably selected having regard to the thickness of coating film.

For example, when the thickness of dried coating film is about 10 u, the size of mesh is preferably such that foreign substances of 3 u or more in particle size can be filtered off.

The circulating equipment referred to can suitably be a pump. The circulating equipment is selected depending upon the pressure drop of filter and the desired rate of circulation which is determined by the composition of coating liquor, the shape of vessel and the thickness of coating film.

The materials used for the dipping vessels, perforated plates, filters, circulating equipment and nipes should be such that they have no interaction with the coating liquor. Generally, stainless steel, glass, synthetic resin and ceramics are used alone or in combination. Further, since the coating apparatus is used for making a smooth coating film, it is often desirable to pull up the coated articles slowly from the dipping vessel in order to minimize thickness variations. The rate at which the coated articles are pulled up depends upon the composition of coating liquor, but the rate is preferably 5 to 100 cm/min.

Further, it is desirable that the atmosphere in which coating is carried out is kept clean, and it is more preferred to use the apparatus of the present invention in a place furnished with an equipment for removing dust from the air.

WHAT WE CLAIM IS: 1. A method of cleaning a synthetic resin article, which comprises subjecting the article to ultrasonic vibration in an upward-flowing stream of an aqueous de- tergent solution in a first vessel, transferring the article to a first upward-flowing stream of warm water in a second vessel, and then to a second upward-flowing stream of warm water in a third vessel and extracting the article from the water to drain off the water attached to the article.
2. A method as claimed in claim 1, wherein the temperature of the first and second streams of warm water is about 10 C below the heat distortion temperature (ASTM D-648) of the article.
3. A method as claimed in claim 1 or claim 2 wherein the average rate of upward flow of the detergent solution is from 5 to 500 mmlminute.
4. A method as claimed in any one of claims 1 to 3 wherein the average rate of upward flow of the first steam of warm water is from 100 to 1000 mm/min.
5. A method as claimed in any one of claims 1 to 4 wherein the average rate of upward flow of the second stream of warm water is from 50 to 500 mm/min.
6. A method as claimed in any one of claims 1 to 5 wherein the temperatures of the first and second streams of warm water are each from 40"C to 700 C.
7. A method as claimed in claim 6, wherein the temperatures of the first and second streams of warm water are each from 50"C to 60"C.
8. A method as claimed in claim 7, wherein the article is extracted from the second stream of warm water at a speed of from 20 to 30 em/min.
9. A method of cleaning a synthetic resin article substantially as hereinbefore described, with reference to and as illus

trated in the accompanying drawings.
10. A method of cleaning and dipcoating a synthetic resin article, which comprises cleaning the article by a method as claimed in any one of the preceding claims, drying the article, and then immersing it in an upward-flowing steam of a coating liquor.
11. An apparatus for cleaning synthetic resin articles comprising a mechanical conveyor for the articles, a cleaning vessel provided with means for applying ultrasonic vibration to its contents and having such a structure as to give an upward flow to an aqueous detergent solution, a rinsing vessel to which warm water is supplied through a filter and through which the warm water flows upward, and a draining vessel to which warm water is supplied through a filter and through which the warm water flows upward, the conveyor being arranged and constructed to carry the articles successively from the cleaning vessel through the rinsing vessel to the draining vessel and to pull the articles up from the draining vessel.
12. An apparatus as claimed in claim 11, wherein said ultrasonic cleaning vessel, rinsing vessel and draining vessel all have their said inlet at the bottom and allow the liquor to overflow the whole of the top of the vessel to reach the outlet.
13. An apparatus as claimed in claim 11 or claim 12, wherein said rinsing vessel and draining vessel are furnished at the lower part with a foraminous plate for the water to pass through.
14. An apparatus as claimed in any one of claims 11 to 13 at least one of said ultrasonic cleaning vessel, rinsing vessel and draining vessel is furnished with a circulation system to reuse the liquor overflowed from the vessels.
15. Apparatus for cleaning synthetic resin articles, which comprises three opentopped tanks, each tank having an inlet at a bottom portion thereof, and an outlet formed by overflow tray around at least a portion of a top edge thereof, each tank being provided with a pump and a filter, to filter liquid overfLowing into the overflow tray and return it to the inlet, means being provided for mechanically conveying the articles successively from one tank to another, means being provided for supplying ultrasonic vibrations to a liquid in the first tank and means being provided for withdrawing the articles from the last tank.
16. Apparatus as claimed in claim 15, wherein the means for conveying the articles comprises an endless belt or chain, having means for attaching an article thereto, cogs or pulleys being provided for driving the belt or chain, and means being provided for moving at least one such cog or pulley transversely of the direction of motion of the belt or chain, to dip an article into a tank.
17. Apparatus for cleaning synthetic resin articles, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.
18. Apparatus for cleaning and dipcoating synthetic resin articles comprising an apparatus for cleaning synthetic resin articles as claimed in anyone of claims 11 to 17 and a dip-coating apparatus which comprises an open topped tank having an inlet at a bottom portion thereof, a foraminous plate disposed within the tank above the level of the inlet, whereby when the apparatus is in use a coating liquor flowing up the tank from the inlet flows through the plate, an overflow tray around at least a portion of the top edge of the tank, a pump for pumping the coating liquor from the overflow tray to the inlet to circulate the coating liquor and a filter for removing foreign particles from the liquor.
19. Apparatus as claimed in claim 18, wherein the overflow tray extends around the whole of the top edge of the tank.
20. Apparatus as claimed in claim 18 or claim 19, wherein the perforated or foraminous plate has been prepared by sintering in inorganic substance, a synthetic resin, or a powdery, granular or fibrous metal.
21. Apparatus for cleaning and dipcoating articles, substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.
22. A method of coating an article which comprises cleaning a synthetic resin article by a method claimed in any one of claims 1 to 9, dipping the article in an upward-flowing stream of a coating composition in a dip-coating apparatus as defined in any one of claims 18 to 21, and withdrawing the article from the coating composition.