AU9144198A

AU9144198A - Process for producing tubs of thermoplastic resin

Info

Publication number: AU9144198A
Application number: AU91441/98A
Authority: AU
Inventors: Satoru Funakoshi; Ryuichi Ishitsubo; Koji Ogura; Keiki Yamamoto
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 1997-11-17
Filing date: 1998-11-10
Publication date: 1999-06-03
Anticipated expiration: 2018-11-10
Also published as: KR19990045319A; JP3828649B2; AU734041B2; JPH11147260A; DE19852414A1

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT 4* II

I

Applicant(s): SUMITOMO CHEMICAL COMPANY, LIMITED A.R.B.N. 007 509 999 Invention Title: PROCESS FOR PRODUCING TUBS OF THERMOPLASTIC

RESIN.

a 0 The following statement is a full description of this invention, including the best method of performing it known to me/us: PROCESS FOR PRODUCING TUBS OF THERMOPLASTIC RESIN BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This invention relates to a process for producing tubs of thermoplastic resin.

2. DESCRIPTION OF RELATED ART it.

I Heretofore, bathtubs of thermoplastic resin have been known. Such bathtubs have a structure in which a surface layer of thermoplastic resin is

III

laminated on a reinforcing layer of another thermoplastic resin, because the surface layer itself does not have enough strength.

The Japanese unexamined patent publication No. Hei 5-176855 discloses a process for producing bathtubs of thermoplastic resin by heating and softening a thermoplastic resin sheet for forming a surface layer and another thermoplastic resin sheet for forming a reinforcing layer, separately, I followed by forming them into predetermined bathtub-forms utilizing a conventional molding method in which a mold is used such as vacuum molding and stamping molding. Both sheets formed are then laminated and united. This process, however, requires to heat and soften the thermoplastic sheet for the surface layer and the thermoplastic sheet for the reinforcing layer and then forming them separately, accordingly it does not have good productivity. Additionally, if the temperatures of moldings obtained from the sheets are low, the process is also problematic in that the -2moldings can not be adhered firmly and there causes unevenness in adhesion strength.

The Japanese unexamined patent publication No. Hei 7-250772 discloses a process for forming a laminated board into a bathtub-form by vacuum molding, etc., wherein the laminated board is obtained by sticking a cast acrylic sheet for forming a surface layer onto a thermoplastic resin sheet for forming a reinforcing layer with adhesive or by welding a heated and softened cast acrylic sheet onto a sheet-like thermoplastic resin extruded from an extruder. However, this process has the same problem as that mentioned above in the case of using adhesive in preparation of the S laminated board. In addition, when the laminated board is produced by welding of the sheets, the cast acrylic sheet is deteriorated in hot-water resistance and chemical resistance, which are important properties for bathtubs, due to two heating, in total, conducted in the preparation of laminated board and in forming it into the bathtub-form. Those properties S are important for bathtubs.

Under these circumstances, the present inventors have intensively studied to develop a process for producing bathtubs having a good adhesion between a surface layer and a reinforcing layer, the process being able to maintain the performance of the thermoplastic resin for forming the surface layer during it, and as a result, have accomplished this invention.

-3- SUMMARY OF THE INVENTION This invention provides a process for producing a tub comprising a surface layer of the first thermoplastic resin and a reinforcing layer of the second thermoplastic resin laminated on the surface layer, the process comprising a combination of the following steps of: making the first mold comprising the first and second mold halves into an open state, wherein the second mold half has a molten resin supply opening in its mold surface and at least one of the mold halves is movable in a mold clamping direction; supplying a sheet of the first thermoplastic resin between the mold halves; supplying the second thermoplastic resin in a molten state between the sheet of the first thermoplastic resin and the mold surface of the second mold half;

*II

clamping the first mold to fill a cavity with the sheet of the first thermoplastic resin and the second thermoplastic resin; S(e) cooling the first and second thermoplastic resins in the cavity while clamping the first mold with a predetermined clamping force; opening the first mold to remove a laminated board comprising the sheet of the first thermoplastic resin and the second thermoplastic resin laminated thereon; heating and softening the laminate board obtained; and -4molding the laminated board softened into a tub-form by using the second mold.

BRIEF DESCRIPTION OF DRAWINGS In the drawings, FIG. 1 is a schematic sectional view of one example of a mold for producing a laminated board according to the process of this invention; FIG. 2 is a schematic view illustrating one step carried out in the production of a laminated board; FIG. 3 is a schematic view illustrating another step carried out in the S production of the laminated board; FIG. 4 is a schematic view illustrating still another step carried out in the production of the laminated board; FIG. 5 is a schematic sectional view of a vacuum molding mold for IL' producing tubs by molding a laminated board; I, FIG. 6 is a schematic view illustrating one step carried out in the production of tubs by molding a laminated board.

FIG. 7 is a schematic view illustrating one step carried out in the production of tubs by molding a laminated board.

FIG. 8 is a schematic view illustrating one step carried out in the production of tubs by molding a laminated board.

FIG. 9 is a sectional view of one example of a tub which can be obtained according to the process of this invention; FIG. 10 is a partial sectional view of one example of a laminated board in which the thickness is partially changed; and FIG. 11 is a schematic sectional view of a press mold for producing tubs by molding a laminated board.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Tubs produced according to the process of this invention have a structure in which a surface layer, which is a designed surface (an inner surface) of a tub, formed from the first thermoplastic resin is laminated and I i I firmly united with a reinforcing layer of the second thermoplastic resin.

The surface layer is required to have properties such as hot-water resistance, chemical resistance and resistance against scratching. The 4 :reinforcing layer is mainly required to have a function of reinforcing the surface layer. When a tub has a transparent or semitransparent surface of layer, the tub can be decorated by providing decoration such as coloration to the reinforcing layer.

4 The first thermoplastic resin for forming the surface layer may be any thermoplastic resins which can be softened by heating and can be moderately extended in the step of molding the laminated board into a tubform. Examples of the first thermoplastic resin include thermoplastic resins such as acrylonitrile- styrene copolymer resin (AS resin), acrylonitrilebutadiene styrene terpolymer (ABS resin), polystyrene resin (PS resin), -6polycarbonate resin (PC resin) and (meth)acrylic resin and polymer alloys comprising these thermoplastic resins. Considering processability of the laminated board, amorphous thermoplastic resins having wide processable temperature range are preferred. Additionally, transparent or semitransparent thermoplastic resins can be preferably employed in view of design of tubs.

(Meth)acrylic resin can be preferably utilized because it can provide a deep tint to the appearance of tubs. Especially, (meth)acrylic resins having a gelation rate from about 40% to about 95% are preferred. (Meth)acrylic resins of such gelation rate can provide good hot-water resistance and good chemical resistance to the tubs and have good processability. Here, the gelation rate is measured by immersing a small chip of (meth)acrylic resin in plenty of chloroform at room temperature for 48 hours and represented by a ratio by weight) of residue insoluble to chloroform to the initial chip. A iI laminated board in which a sheet of (meth)acrylic resin is laminated with the Ssecond thermoplastic resin is usually heated to about 180°C to be softened and is molded into a tub-form. If tensile elongation of the (meth)acrylic resin sheet at that temperature is too small, there are occasions when the sheet breaks on the way of forming. In addition, if haze of the surface layer becomes high by to contacting hot water, the surface of the tub..whitens and the appearance thereof is extremely deteriorated. It is accordingly preferable that (meth)acrylic resins having tensile elongation at 180°C of at -7least 700% and difference of haze between before and after the 1000 hours immersion test in 90°C water of at most 20 are used. The tensile elongation and the haze are measured by an elongation test according to JIS K7113 and a test according to ASTM D1003, respectively. The sheet of (meth)acrylic resin may be various types of known (meth)acrylic resin sheets. Among them, an acrylic resin sheet which is obtainable by mixing an acrylic monomer, a mercaptan, a polyfunctional monomer and a radical polymerization initiator and conducting polymerization as disclosed in the all Japanese unexamined patent publication No. Hei 9-31109 is preferably used it, because of its good hot-water resistance and hot-moldability.

The second thermoplastic resin which forms the reinforcing layer is suitably selected, considering properties required. Ones which can be welded with the sheet of first thermoplastic resin are preferred. If there is a large difference between the heat distortion temperatures of the surface layer of the first thermoplastic resin and the reinforcing layer of the second I I thermoplastic resin, the laminated board including these layers is difficult to be molded into the tub-form. It is accordingly preferred that the difference between the heat distortion temperatures of the first thermoplastic resin and the second thermoplastic resin is under about 301C, particularly under about For example, if the heat distortion temperature of the first thermoplastic resin is too higher than that of the second thermoplastic resin, the second thermoplastic resin will foam or decompose in the case of setting heating conditions based on the properties of the first thermoplastic resin.

On the other hand, when the heating conditions are set based on the properties of the second thermoplastic resin, the first thermoplastic resin sheet will have insufficient elongation.

The second thermoplastic resin which forms the reinforcing layer is preferred to have good hot-water resistance and to be easy to be designed.

With respect to the hot-water resistance, a bending modulus of elasticity and a heat distortion temperature of the resin are considered. As regards the

I,I

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S ease of design, the ease of coloration, formability and the like of the resin are iI |I C considered. Furthermore, the creep property of the second thermoplastic if resin at high temperature is also important. The second thermoplastic resin, therefore, is selected according to the pressure which will be applied to I I the reinforcing layer. In order to achieve a sufficient reinforcing effect, the higher bending modulus of elasticity of the reinforcing layer is preferred. In Sview of hot-water resistance of the reinforcing layer, it is preferred that the t second thermoplastic resin has a heat distortion temperature measured 4 L according to ASTM D-648 of higher than about 801C, particularly higher than about 90°C. In order to increase the bending modulus of elasticity and the heat distortion temperature, it is effective to compound fillers such as glass fibers with the resin.

On the other hand, in view of moldability, any thermoplastic resins which can be employed in injection molding, extrusion molding and the like -9can be used and ones which can be supplied by conventional injection machines are preferably used. Especially, thermoplastic resins which have a wide range of moldable temperature are highly preferred. From those viewpoints, amorphous thermoplastic resins such as acrylonitrile* styrene copolymer (AS resin), acrylonitrile butadiene -styrene terpolymer (ABS resin) and polystyrene resin and polymer alloys thereof are particularly preferably used as the second thermoplastic resin. Concretely, the second thermoplastic resin is chosen considering the type of the first thermoplastic ll resin to be used. For example, when a (meth)acrylic resin sheet is used as the first thermoplastic resin sheet, heat-resistant ABS resin having a heat distortion temperature from 90 C to 125 °C such as, for example,

LI

KRALASTIC KU-600-R3 manufactured by SUMIKA A&L is suitably used, considering weldability with the sheet, heat resistance and moldability.

i' i S, In the process of this invention, the tubs can be produced by molding a laminated board, which is obtained by laminating the sheet of first S* thermoplastic resin with the second thermoplastic resin, into a tub-form by a method, such as vacuum molding and press molding, in which a mold is used.

Thickness and shape of the laminated board are suitably selected according to the form of the desired tub. Although the ratio in thickness of the sheet of the first thermoplastic resin to the layer of the second thermoplastic resin in the laminated board can be selected depending on use of the desired tub, thickness of the laminated board is, in general, from about 5 mm to about mm and the above ratio is from about 1:9 to about 9:1.

In the process of this invention, the laminated board is first prepared by laminating and uniting the sheet of the first thermoplastic resin, which will form the surface layer of the tub, with the second thermoplastic resin, which will form the reinforcing layer of the tub, and then the laminated board is molded to produce the tub.

Referring to FIG. 1 which illustrates a schematic sectional view of one example of the apparatus used in the process of this invention, a mold is (the first mold) for producing the laminated board comprises the first mold half and the second mold half The second mold half has a molten resin supply opening through which the molten second thermoplastic resin is supplied, and a molten resin path leading to the molten resin supply opening. The first mold half is a female mold half and the second mold half is a male mold half. The molten resin path is connected with an injection machine or the like for injecting a molten S thermoplastic resin. The molten thermoplastic resin injected from the ,1 ~injection machine is supplied into the mold through the molten resin supply opening via the molten resin path. The molten resin supply opening (4) may have a switching valve for controlling conditions of supplying the second thermoplastic resin. At least one mold half can move in the opening/closing direction of the mold (the mold clamping direction), and in general, one mold half is fixed and the other is movable. In FIG. 1, the second mold half (the male mold half) is fixed and the first mold half (the female mold half) (2) II can move in the mold clamping direction (the up-and-down direction, in this example).

The mold is opened and then a sheet of the first thermoplastic resin is supplied between the mold halves, as shown in FIG. 2. At this time, if the mold can be opened and closed in the up-and-down direction, the sheet of the first thermoplastic resin should be just placed on the mold surface of the mold half located at a lower position. On the other hand, the mold can be opened and closed horizontally, it is preferred that the sheet (8) of the first thermoplastic resin is fixed to the mold surface of one of the mold 2 halves, generally of the movable mold half. Although a way of fixing the sheet is arbitrary, the sheet may be adsorbed onto the mold surface by sucking the space between the sheet and the mold surface through a suction 2. opening provided in the mold surface. In this case, providing the suction opening near positions corresponding to corners or periphery of the tub to be produced can make the marks caused by suction inconspicuous. Such fixing of the sheet of the first thermoplastic resin, of course, can be carried out in the case that the mold is opened and closed in the up-and-down direction, as well as in the case that it is horizontally opened and closed.

Although the sheet of the first thermoplastic resin to be supplied between the mold halves does not necessarily need to be preheated, deformation of the laminated board (11) can be controlled by preheating the sheet with, for example, a far-infrared ray heating furnace. When the deformation occurs, the laminated board is often warped so as to make a thermoplastic resin, etc.

Although the cavity clearance at the time of commencing the supply of the molten second thermoplastic resin may vary depending upon the type of the thermoplastic resin, a thickness of the laminated board desired, molding conditions, etc., in general, it preferably ranges from mm to mm, wherein T is the thickness (mm) of the sheet of the first thermoplastic resin supplied between the mold halves. If the cavity clearance is smaller than the above lower limit, the portion of the sheet of the first thermoplastic resin located at the right above of the molten resin supplying opening is damaged by heat and pressure of the molten second thermoplastic resin supplied and, in some occasions, a hole opens on the sheet. On the other hand, when the cavity clearance is over the upper limit, the molten resin supplied to the cavity involves air, gas, moisture, etc. As a result, the defect of appearance so-called "flash" arises on the surface of the product and the resin deteriorates. A concrete cavity clearance at the time when the second thermoplastic resin is supplied depends on the temperature of the sheet of the first thermoplastic resin supplied between the mold halves, the supplying temperature of the second thermoplastic resin, etc., and it can be suitably determined according to molding conditions.

The supplying temperature of the second thermoplastic resin varies depending upon the type of the second thermoplastic resin, the type of the sheet of the first thermoplastic resin to be laminated, etc. When a (meth)acrylic resin sheet is used as the sheet of the first thermoplastic resin -14and heat resistant ABS resin is used as the second thermoplastic resin, the supplying temperature of the heat resistant ABS resin ranges from about 220 0 C to about 280 0 C. In order to determine the combination of the first and second thermoplastic resins, it seldom needs to take the difference between their melting temperatures into consideration. It is generally preferred that the temperature of the second thermoplastic resin is lower than that of the first thermoplastic resin.

The mold clamping is carried out while or after supplying the molten second thermoplastic resin. In order for the sheet of the first thermoplastic resin and the molten second thermoplastic resin are welded firmly together, it is required for the surface of the molten second thermoplastic resin to be maintained at a temperature at which it can be welded with the sheet of the first thermoplastic resin.

In general, the temperature of the molten second thermoplastic resin falls gradually as the resin flows toward the edge of the cavity due to the mold clamping, and accordingly, a resin temperature at a flow end of the resin is a little lower than the temperature of the resin which has been just supplied. In order to make adhesion intensity (welding intensity) between the surface layer of the first thermoplastic resin and the reinforcing layer of the second thermoplastic resin good throughout the laminated board, it is important to make a fall of the temperature of the second thermoplastic resin at its flow end as small as possible. For that purpose, it is preferred that the mold clamping speed is made as fast as possible so as to make a period of time to be taken for the second thermoplastic resin supplied to reach the cavity end as short as possible.

After completing to supply the molten second thermoplastic resin the mold is closed until a cavity clearance reaches the thickness of the laminated board desired to fill the space defined between the mold surface of the second mold half and the sheet of the first thermoplastic resin with the second thermoplastic resin, thereby filling the cavity with the sheet of at the first thermoplastic resin and the second thermoplastic resin. While maintaining such a state, the mold is continually clamped with a predetermined force (clamping force) and the first thermoplastic resin sheet and the second thermoplastic resin are weld together as shown in FIG. 4.

Although the clamping force at this time may varies depending upon types of the first and second thermoplastic resins, it usually ranges from about a kgf/cm 2 to about 150 kgf/cm 2 The first and second thermoplastic resins are cooled while the I pressurization state maintained. After temperatures of the first thermoplastic resin sheet and the second thermoplastic resin have, respectively, become lower than their heat distortion temperatures, the mold is opened and the laminated board (11) in which the first thermoplastic resin sheet and a layer (10) the second thermoplastic resin are welded and united together are removed. Although the laminated board removed may be warped so as to project toward the side of the first thermoplastic resin sheet, such deformation especially is not problematic in the step of molding the laminated board into a tub-form.

The laminated board (11) thus obtained is molded into the tub-form using the second mold. In the molding of the laminated board using the second mold, various known methods for molding sheet-like materials into a predetermined shape can be applied. Among them, vacuum molding using a vacuum mold and press molding using a press mold are representative.

The method for molding the laminated boards by vacuum molding using a vacuum mold will be described below.

In general, the vacuum mold used for vacuum molding is a mold (12) which is called a female mold whose schematic section is shown in FIG. At a predetermined position in a mold surface (13) designed in a tub-form, a 0 vacuum suction opening (14) for sucking the laminated board heated and !ft softened to stick it onto the mold surface. The vacuum suction opening is connected to a vacuum suction device (not shown) via a vacuum suction path .The laminated board (11) obtained in the previous step is heated and softened by a heating device (16) such as a far-infrared ray heater in advance of forming (FIG. At this time, the laminated board (11) must be heated so that the temperatures of both the sheet of the first thermoplastic resin and the layer (10) of the second thermoplastic resin become equal to or higher than their heat distortion temperatures respectively. If both difference in heat distortion temperature and difference in melting temperature between the first thermoplastic resin and the second 1 thermoplastic resin are not so large, the laminated board (11) may be equally heated from its both sides. If heat distortion temperatures of the first and second thermoplastic resins are different, it is preferable to give a difference between heating conditions of the side of the layer of the first thermoplastic resin and the side of the layer (10) of the second thermoplastic resin and to heat the layers so that their temperatures become equal to or higher than their heat distortion temperatures respectively, preferably 100 0 C higher than their heat distortion temperatures respectively.

Although upper limits of heating temperatures of those layers in the laminated board depend on melting temperatures and decomposition temperatures of the first and second thermoplastic resins, it is preferred that they are as low as possible in order to prevent deterioration of performances

I

of the layers of the first and second thermoplastic resins. Accordingly, when I f using (meth)acrylic resin for the sheet of the first thermoplastic resin and heat-resistant ABS resin for the second thermoplastic resin, a suitable heating temperature of the sheet of the first thermoplastic resin is from about 150 0 C to about 200 0 C and that of the second thermoplastic resin is from about 130 0 C to about 170 0 C. When the temperature of the sheet of the first thermoplastic resin is made different from that of the layer (10) of the second thermoplastic resin when the laminated board (11) is heated, it is required that each layer included in the laminated board is in a temperature at which the laminated board can be vacuum molded at each welding surface, as well as at each exposed surface. If the difference between temperatures of the resin layers suitable for vacuum molding is more than 60 0 C, it is difficult to heat the laminated layer to a temperature suitable for both the layers. Because, in the vicinity of the boundary of the two layers, if the layer having the higher heat distortion temperature is heated to a temperature suitable for the layer having the lower heat distortion temperature, the former layer tends to have an unsatisfactory elongation, and if the layer having the lower heat distortion temperature is heated to a temperature suitable for the layer having the higher heat distortion temperature, the resin of the former layer tends to decompose. Accordingly, in the production of the laminated board it is preferred that the first and the second thermoplastic resins are selected so that the difference in heat distortion temperature between the thermoplastic resins is at most The laminated board (11) which has been heated to a predetermined temperature to be softened is stuck on the upper edge (17) of a mold (12) designed to have a tub-form so that the layer of the second thermoplastic resin faces the mold surface (13) of the mold as shown in FIG. 7. By sucking through a vacuum suction opening the gas in the space surrounded by the laminated board (11) and the mold surface (13) is discharged. The heated and softened laminated board (11) is extended toward the mold surface and sticks to the mold surface with increasing the degree of vacuum in the above space, as shown in FIG. 8.

By cooling the laminated board (11) until the temperatures of the sheet of the first thermoplastic resin and the layer (10) of the second thermoplastic resin both constituting the laminated board (11) become lower than their heat distortion temperatures and subsequently taking out the board from the mold a tub (18) of thermoplastic resin can be obtained, as shown in FIG. 9.

When such vacuum molding is carried out, there is tendency that the laminated board is extended very much in the side wall near the bottom of 0 6 the tub and four corners of the bottom and the laminated board partially becomes very thin after molding, in general. In order to make the thickness ~of the tub at its portion easy to be extended as uniform as possible, it is effective to make, at the time of manufacture of the laminated board, the portion of the laminated board which is easy to be extended at the time of molding thicker than other portions, as shown in FIG. 10. In order to produce such a laminated board in which the thickness partially differs, it is .11.8.

just necessary to use the second mold half having a concave portion at a predetermined position of its mold surface, in manufacture of the laminated board previously described.

If the laminated board is comparatively thick, when forming a curved surface projecting toward the surface layer side between the upper portion of the inner wall and the flange portion of the tub it may be difficult to form the curved surface with a small curvature radius by vacuum molding. In 22 the first thermoplastic resin is molded into the tub by vacuum molding has been described above, another processes such as press molding using a press mold can produce similar tubs. In such a case, for example, a press mold comprising a pair of female and male mold halves (21, 22) designed to have a desired tub-form as shown in FIG. 11 is used. A laminated board which has been heated and softened is supplied between the female mold half and the male mold half, and subsequently, the press mold is clamped to form the laminated board into the tub-shape. Then, the laminated board is cooled.

The process of the present invention can be applied to production of tubs having various configuration and size such as bathtubs, shower pans, sinks, water tanks, etc.

The present invention will be further explained below by Example which should not be construed to limit the scope of the present invention.

EXAMPLE

Using a mold for producing laminated boards as shown in FIG. 1, a laminated board was prepared according to the steps illustrated in FIGS. 2, 3 and 4. The laminated board obtained was molded according to the steps illustrated in FIGS. 6, 7 and 8 in which a vacuum mold of FIG. 5 was used, and a bathtub shown in FIG. 9 was produced.

The mold for producing laminated boards was first opened, and a mm-thick transparent acrylic resin sheet (SUMIPEX SA, manufactured by Sumitomo Chemical Co., Ltd.) was then placed on the mold surface of a male mold half (FIG. Then a female mold half was lowered to start mold clamping. When a clearance between a mold surface of the female mold half and the transparent acrylic resin sheet became 15 mm, mold clamping was quitted and a molten heat-resistant ABS resin (KRALASTIC KU600-R3, manufactured by SUMIKA A L) heated to 2501C was supplied through a molten resin supply opening provided in the mold surface of the male mold.

At this time, the transparent acrylic resin sheet was pushed up toward the female mold half by the heat-resistant ABS resin supplied as shown in FIG.

3.

At the same time when the supply of the heat-resistant ABS resin was completed, mold clamping at a rate of 10 mm/sec was restarted to fill the cavity defined between the mold surface of the male mold half and the transparent acrylic resin sheet with the heat-resistant ABS resin (FIG. 4).

The mold continued to be clamped with a clamping force of about 100 kg/cm 2 for 120 seconds.

After that, the mold was opened and the laminated board in which a mm thick heat-resistant ABS resin layer was laminated on a 5 mm thick transparent acrylic resin sheet was taken out.

The laminated board obtained had changed into the saddle-form in which the laminated board has warped to project toward the transparent acrylic resin sheet side. The laminated board was set in a heater and heated so that the surface of the transparent acrylic resin sheet and the surface of the heat-resistant ABS resin layer, respectively, might become 1951C and 1351C, as shown in FIG. 6.

After completion of heating, the heat-resistant ABS resin layer of the laminated board was stuck onto the upper end of a vacuum mold, and subsequently, by suction from the vacuum suction opening provided in the mold, the space defined by the mold surface and the laminated board was exhausted and the space was made to a depressurized state, as shown in FIG.

7.

The laminated board was extended and pulled toward the mold surface as the above space was reduced pressure, and as a result, the laminated board was stuck onto the mold surface, as shown in FIG. 8.

After cooling the product at room temperature for 300 seconds while maintaining the above state, vacuum suction was stopped and the bathtub was released from the mold (FIG. 9).

According to the process of the present invention, thermoplastic resin tubs in which adhesion between its surface layer and its reinforcing layer is good and performance of the thermoplastic resin of the surface layer is not deteriorated can be easily produced.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprising" is used in the sense of-."including", i.e. the features specified may be associated with further features in various embodiments of the invention.

Claims

1. A process for producing a tub comprising a surface layer of the first thermoplastic resin and a reinforcing layer of the second thermoplastic resin laminated on the surface layer, the process comprising a combination of the following steps of: making the first mold comprising the first and second mold halves into an open state, wherein the second mold half has a molten resin supply opening in its mold surface and at least one of the mold halves is movable in a mold clamping direction; supplying a sheet of the first thermoplastic resin between the mold halves; supplying the second thermoplastic resin in a molten state between the sheet of the first thermoplastic resin and the mold surface of the second mold half; clamping the first mold to fill a cavity with the sheet of the first thermoplastic resin and the second thermoplastic resin; cooling the first and second thermoplastic resins in the cavity while clamping the first mold with a predetermined clamping force; opening the first mold to remove a laminated board comprising the sheet of the first thermoplastic resin and the second thermoplastic resin laminated thereon; heating and softening the laminate board obtained; and molding the laminated board softened into a tub-form by using the second mold.

2. A process according to claim 1 wherein, in the step a vacuum mold is used as the second mold and the laminated board is molded into the tub-form by vacuum molding.

3. A process according to claim 1 wherein, in the step a press mold is used as the second mold and the laminated board is molded into the tub- form by press molding.

4. A process according to any one of claims 1, 2 and 3 wherein the sheet of the first thermoplastic resin is a transparent or semitransparent sheet.

A process according to any one of claims 1, 2 and 3 wherein the sheet of the first thermoplastic resin is a sheet of (meth)acrylic resin.

6. A process according to any one of claims 1, 2 and 3 wherein the sheet of the first thermoplastic resin is a sheet of (meth)acrylic resin having a gelation rate from about 40% to about

7. A process according to any one of claims 1, 2 and 3 wherein the sheet of the first thermoplastic resin is a sheet of (meth)acrylic resin having a tensile elongation at 18 0 °Cof at least 700% and difference in haze between before and after a 1000 hours immersion test in 90°C water of at most

8. A process according to any one of claims 1, 2 and 3 wherein, in the step the temperature of the sheet of the first thermoplastic resin ranges from about 60°C to about the heat distortion temperature of the first thermoplastic resin.

9. A process according to any one of claims 1, 2 and 3 wherein, in the step a cavity clearance defined at a time when the second thermoplastic resin is started ranges from mm to (T+30) mm wherein T is a thickness (mm) of the sheet of the first thermoplastic resin supplied between the mold halves.

A process according to any one of claims 1, 2 and 3 wherein the second thermoplastic resin can be welded with the sheet of the first thermoplastic resin and difference in heat distortion temperature between the first and second thermoplastic resins is at most

11. A process according to any one of claims 1, 2 and 3 comprising a further step of fixing the sheet of the first thermoplastic resin supplied between the mold halves to a mold surface of the first mold half.

12. A process according to any one of clams 1, 2 and 3 wherein partial difference in thickness of the laminated board ranges from about to about ±50% based on a standard thickness of the laminated board and in a portion of the laminated board which has a thickness different from the standard thickness an inclination of a sloping portion is at most 5 degrees to the surface of the standard thickness portion.

13. A process according to any one of claims 1, 2 and 3 wherein the tub is a bathtub. Dated this 10th day of November, 1998 SUMITOMO CHEMICAL COMPANY, LIMITED By its Patent Attorneys GRIFFITH HACK Fellows Institute of Patent Attorneys of Australia