JPS595094B2 - Container manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing thin-walled containers from plastic sheets.

The object of the present invention is to improve wall thickness distribution without any scrap.
To provide a method for thermoforming a thin-walled thermoplastic resin container with good stiffness and appearance.

Another object of the invention is to provide an economically advantageous thermoforming process.

In recent years, with the deepening recognition of the finite nature of petroleum resources, a method of thermoforming thin-walled containers from plastic sheets without generating any scrap has been attracting attention as a resource-saving technology.

Conventional sheet thermoforming methods such as vacuum forming and pressure forming5 are:
Molding machines and molds are relatively inexpensive and have the advantage of being able to mold multiple thin-walled products, so they are widely used. However, in conventional sheet thermoforming methods, 40% to 60% of It has a major disadvantage in that it generates a large amount of scrap.

The development of a molding method for thin-walled containers that does not produce any scrap has been attracting attention in this sense, and it has already been developed to mold a diaphragm by compressing molten resin injected into a compression mold using an injection molding machine, and then molding the diaphragm. A method of manufacturing a thin-walled container by forming the diaphragm of the container under pressure has been proposed (for example, Japanese Patent Publication No. 47-5427).

However, with this method, (1) the degree of orientation over the entire surface of the diaphragm is not sufficiently homogeneous, so the wall thickness distribution, strength, etc. of the thin-walled container manufactured by pressure forming may be uneven depending on the position; (2) injection molding Since the resin is supplied by the method, there are disadvantages such as: traces of the gate remain on the bottom of the container, spoiling the appearance, and (3) multi-layered containers cannot be formed. In addition, as a method for forming thin-walled containers that does not generate any scrap, cut-sized tablets are made from plastic sheets, lubricating oil is applied to the surface of the tablets, and the tablets are heated at temperatures above the softening point and below the melting point of the resin. A method has been proposed in which the tablet is preheated and fed into a pair of upper and lower forging molds to form the tablet into a thin heat-formable sheet, and then this forged sheet is formed into a thin-walled container by air pressure forming or the like. (For example, Japanese Patent Application Laid-Open No. 47-4588, etc.).

By this method, thin-walled containers can be formed from single-layer or multi-layer extruded sheets without producing any scrap. However, with this forming method, when a rectangular tablet is coated with lubricating oil and forged after preheating to form a semi-formed forged sheet, the thickness distribution of the forged sheet becomes uneven.
Even through forging, the resin at the four corners of a square tablet does not spread evenly over the entire surface of the sheet, resulting in four areas with stronger uneven thickness than other areas around the forged sheet as a semi-formed product. . As a result, the finish of the flange and the upper part of the side wall of the final product container deteriorates. When a tablet is forged using this method, it is difficult for the peripheral edge of the forged sheet of the semi-formed product to be press-fitted into the sheet transport body. If this forging pressure is insufficient, the resin will not fully enter the gap of the transfer body, and the forge sheet will come off from the transfer body in the next thermoforming process, resulting in molding defects. more likely to occur. In order to make it difficult for the forged sheet to come off the transfer body, special measures are required for the shape of the transfer body, which has the drawback that the shape of the flange portion of the container is limited. The present inventors have found that the forging pressure of the tablet, which could not be achieved with the previously proposed method, can be reduced, the thickness distribution of the forged sheet is uniform, the shape of the three flange parts of the container is not limited, and the shape of the container is As a result of research into a method for manufacturing thin-walled containers using a scratch press that has better mechanical properties and appearance than those of molded products made using the conventional thin-walled container manufacturing method using a scratch press, we have developed a rectangular shape with the cut size to be fed to a forging die. While the central part of the tablet is preheated to a temperature below the melting point of the resin, the peripheral part of the tablet is preheated to a temperature above the melting point and kept in a molten state, thereby reducing the pressure required for forging. They found that it was possible to significantly reduce the φ and improve the finish of the forged sheet, and based on this knowledge, they proceeded with various studies and completed the present invention.

The method for producing a container of the present invention involves cutting a thermoplastic resin sheet consisting of a single layer or a plurality of layers into square tablets, and determining the temperature (Tc) at the center of the tablet as the Vikato softening point (Tv) of the resin. The tablet is preheated to a temperature within the range of the melting point (Tm), and the temperature (TE) of the periphery of the tablet is set to (Tm).
After being preheated and melted to a temperature of m+10)°C or higher, it has a pair of forged surfaces facing each other, and (Tc-10)
Disposed in the gap of a press forging die heated to a temperature within the range of °C to (Te + 10) °C and at a position around the forging die,
The tablet is introduced into the space formed by the gap between the pair of annular transfer bodies held at a lower temperature than the forging die, the tablet is placed in the center of the forging die, and then the tab V-shaped is pressed by the forging die to reduce the gap between the forging die until a predetermined thickness is reached, and the central part of the tablet is forged, and the peripheral edge of the molten tablet is press-fitted into the gap of the transfer body. The method is characterized in that the flange portion of the container is formed by using the forged sheet, and then the forged sheet is transferred onto a mold by the transfer body, and the forged sheet is thermoformed into the mold and cooled.

The method of the present invention will be explained below with reference to the drawings.

FIG. 1 is a side view of a molding device implementing the present invention,
A and b are forging devices for tablets, and c is a thermoforming device for forged sheets. In the figure, 1 and 2 are the upper and lower dies of a forging mold, respectively, 3 is an annular transfer body, 4 is a rectangular tablet, and 5 is a gap between the transfer bodies. The center of the square tablet 4 is shaded, and the periphery is filled in black. beginning,
A square tablet 4 whose central part is preheated to a temperature within the range of Tv to Tm and whose peripheral part is preheated to a temperature of (Tm+10)°C or higher and melted is surrounded by press forging molds 1 and 2 and an annular transfer body 3. The tablet 4 is placed in the center of the forging surface of the lower die 2, and the tablet 4 is press-pressed by the forging dies 1 and 2 until it reaches the predetermined thickness shown in b. 2, the central part of the tablet shown with diagonal lines is forged, and the peripheral part of the tablet in the molten state shown in black is press-fitted into the gap part 5 of the transfer body 3 to form the flange part of the container. Then, as shown in c, the forged sheet 7 is transferred onto a mold 8 by the transfer body 3, and the forged sheet 7 is thermoformed into the mold and cooled to produce a container 9. In the present invention, thermoplastic resins include crystalline resins such as polyethylene, polypropylene, and polyethylene terephthalate, and amorphous resins such as polystyrene, polyvinyl chloride, and ABS, and crystalline plastics such as polyethylene and polypropylene are particularly preferred.

A thermoplastic resin sheet consisting of a single layer or multiple layers is preferably a sheet that is extruded from a T-die using a melt extrusion method and then cooled and solidified, and when taken off with a cooling roll after exiting the T-die, it is preferably symmetrical from the front and back. It is preferable to cool the tablet and cut it flat without rolling it up to obtain a square tablet. If a melt-extruded sheet is cooled asymmetrically from the front and back, warping will occur during the process of reheating and forging, which will cause problems with the molding equipment. Similarly, when the sheet is rolled up, curling occurs, which causes warping when reheated. For crystalline plastics, forging and thermoforming 1!
In order to improve processability, it is preferable to produce an extruded sheet with a low degree of crystallinity.

For crystalline polypropylene, extruded sheets with a crystallinity of 40% to 55% are preferred. T2 is a method for producing polypropylene sheets with low crystallinity by melt extrusion.
(There is a known method in which the molten polypropylene sheet immediately after exiting the die is put into the gap between the cooling rolls and rapidly cooled. However, in the case of a sheet with a thickness of 1 mm to 5 mm used in the method of the present invention, It is difficult to produce a sheet with a low degree of crystallinity because a sufficient cooling effect to the inside of the sheet cannot be obtained by quenching alone.As a method for producing a polypropylene sheet suitable for use in the method of the present invention, The resin temperature immediately after leaving the T-die is 210℃~2
By passing the molten sheet at 70°C through a sheet slow cooling device,
31'C/sec or less by cooling the sheet gradually to a temperature within the range of 140°C to 180°C, and then by contact with the surface of one or more cooling rolls.
``It is preferable to rapidly cool the sheet to a sheet temperature of 40° C. to 100° C. at a sheet cooling rate of C/sec or more. By this method, it is possible to produce a sheet with good moldability, which is suitable for the molding process of the present invention and has a crystallinity of 40% to 55%. By producing a multilayer coextruded sheet mainly composed of polypropylene by the method described above, a sheet with good moldability suitable for the molding method of the present invention can be obtained.

The structure of the multilayer sheet is not particularly limited, but for example, it is a three-layer sheet of polypropylene/saponified ethylene-rezo-vinyl acetate copolymer/polypropylene, where the surface polypropylene layer is sufficiently thicker than the saponified ethylene-rezo-vinyl acetate copolymer layer at the center. In some cases, forging and thermoforming are possible under the molding conditions of the main component polypropylene.

As a multilayer sheet structure with good moldability, a five-layer coextruded sheet of polypropylene/adhesive polyolefin/saponified ethylene-vinyl acetate copolymer resin or polyamide/adhesive polyolefin/polypropylene is used for transparent containers with good gas barrier properties. It is particularly suitable for the molding method of the present invention.

The thickness of the extruded sheet is preferably about Im to 5 mm.

When the thickness is less than 1 meter, it becomes increasingly difficult to forge the tablet to make it even thinner, and when the thickness is 5 mm or more, it takes a long time to preheat the tablet and make the temperature in the center of the tablet uniform all the way to the inside.

When cutting the extruded sheet into square tablets, the square tablets are cut to a size that will fit into the forging surface of the forging die. For example, in the case of a round forging die, the diagonal length of the upper and lower surfaces of a square tablet is preferably about 90(f) to 50% of the diameter of the forging surface. When the size of the rectangular tablet approaches the size of the forging surface, the corner portions of the tablet cannot be sufficiently wrapped around the periphery of the forging die even during forging, which tends to result in poor wall thickness distribution.
Although the method of the invention considerably improves the wall thickness distribution of the forged sheet, it is still preferred that the diagonal length of the prismatic tablet is less than 90% of the diameter of the forged surface.
The central part of the square tablet is located at the Vikatsu softening point (T
v) to the melting point (Tm), and preheat the peripheral edge of the tablet to a temperature of (Tm+10)°C or higher to melt it.
It was measured by the method of STMD-1525:58T, and the melting point (Tm) of the resin is the crystal melting point for crystalline resins, and the flow start temperature for amorphous resins.

The tablet consists of a central part and a peripheral part, (Tm+10)
It is preferable that the volume of the peripheral portion to be preheated to a temperature of .degree.

Figure 2 shows the central part 10 and peripheral part 11 of a square tablet, and the volume (E) of the peripheral part 11 is defined by the length of the two sides of the square tablet as A, and the width of the peripheral part as measured from the end. If △A and △b are the thickness of the tablet, then
VE={2(△Alb+△b-a)-4△a・△b}c
It is.

On the other hand, the volume (Vc) of the central portion 10 of the tablet is Vc-
It is expressed as (a-2Δa)(b-2Δb)·c.

FIG. 3 shows the central part 12 and flange part 13 of a disc-shaped forged sheet. The volume (r) of the flange portion 13 is approximately expressed by the following equation, where the width of the flange portion 13 is Δd and the thickness of the forged sheet is t.

7'EL-πt・△d-d(Tm+10)℃
It is preferable that the volume of the peripheral portion of the tablet to be preheated is 〉VE-V/E.

2 In FIG. 3, the volume (V5c) of the central portion 12 of the disc-shaped forged sheet excluding the flange portion 13 is as follows, and it is preferable that Vc≦v'c.

The boundary between the periphery of the tablet, which is preheated to above (Tm+10)°C, and the central part of the tablet, which is preheated to a temperature within the range of Tv to Tm, does not necessarily have to be sharp. If the tablet is forged with a temperature difference between the center and the periphery, uneven stretching will occur between the center and the periphery of the tablet during forging, and the degree of orientation, mechanical strength, etc. of the forged sheet will be affected. There is a concern that this will result in non-uniformity between the center and the periphery, resulting in forming defects in the subsequent thermoforming process, but in reality, the appearance, degree of orientation, and mechanics of the forged sheet will be better than if the tablet was heated uniformly over the entire surface. The properties, etc. become uniform, and in particular, the finish of the peripheral edge of the forged sheet forming the flange of the container becomes good, and a good forged sheet can be obtained with a forging pressure of 50% or less of that in the case of uniform heating over the entire surface.

This point is a remarkable effect of the method of the present invention, and this effect is thought to be produced for the following reasons. When a tablet is forged that has been preheated so that the central part is in a solid state and the peripheral part is in a molten state, the entire surface of the tablet is compressed uniformly in the thickness direction during the forging process, and is uniformly stretched in the direction parallel to the forging surface. As the peripheral edge of the tablet travels along the forging surface until it reaches its end, it is then pressed into the gap in the carrier to form the flange of the container.

Thus, during the forging process, the tablet actually undergoes two different types of deformation: forging between the forging surfaces and press-fitting into the gap of the carrier. The central part of the tablet is compressed and deformed between the forged surfaces and only expands toward the periphery, but the periphery of the tablet is compressed and deformed between the forged surfaces and then press-fitted into the gap of the transfer body. is necessary. In the process of press-fitting into the gap of the transfer body, the resin is not subjected to pressure in the thickness direction of the sheet, and the resin flows toward the flange part due to the resin pressure from the center of the sheet to the peripheral edge. Therefore, in such a flow process, it is advantageous for the resin to be in a molten state, and by melting the peripheral edge of the tablet, a forged sheet with a good finish can be obtained. Furthermore, when the peripheral edge of the tablet in a fully molten state is press-fitted into the gap between the conveying bodies, the elasticity of this part is small, so elastic recovery does not occur after forging, and the forged sheet is subject to contractile force from the conveying body. This eliminates the problem of desorption and facilitates subsequent thermoforming. By simply heating the peripheral portion, which is about 10% of the total volume of the tablet, to a temperature of (Tm+10)°C or higher, it becomes possible to form a forged sheet with a uniform thickness distribution, which was previously difficult, at a low forging pressure. Also, since the sheet can be easily press-fitted into the gap of the conveying body, the problem of the forged sheet coming off from the conveying body is solved, and the degree of freedom in the shape of the flange portion of the container is increased.

Of the entire tablet, the volume of the peripheral portion to be preheated to (Tm+10)°C or higher is preferably about 5% to 30%.

If it is less than 5%, the uneven thickness of the forged sheet originating from the four corners of the square tablet will not be sufficiently eliminated, and the forging pressure will gradually increase.

If the content exceeds 30%, the momme appearance of the forged sheet and even the side wall of the thermoformed container deteriorates. Particularly in the case of crystalline resins such as polypropylene, if the temperature of the peripheral edge of the tablet heated above (Tm+10)°C exceeds 3001), the transparency of the side wall of the container deteriorates and the rigidity of the container decreases. The advantages of this method gradually disappear.

The press forging die has a pair of forging surfaces arranged facing each other, and it is preferable that both of the pair of forging surfaces are flat.

However, the forging surface does not necessarily have to be flat, and when forming deep-drawn containers, a forging die with a slightly concave forging surface in the center is used to create a forged sheet that is thicker at the center than at the periphery. Preferably, the tablet is forged in a.
When a deep-drawn container is manufactured by thermoforming a forged sheet with a thick center portion made using such a forging die, a sturdy container with an appropriately thick bottom portion can be obtained. The material of the forging die is preferably a metal such as iron or aluminum, but in order to improve the lubricity of the forging surface, it is preferable to highly polish the surface and chrome plate it.

In the case of aluminum molds, it is preferable to oxidize the forged surface to make it porous, and then process this surface with a fluororesin to improve slipperiness. Applying a lubricant such as silicone oil or glycerin to the tablet or forged surface in advance will improve the finish of the forged sheet.

In this case, since the lubricant adheres to the surface of the forged sheet, it is necessary to remove the lubricant after molding. The temperature of the forging die is Tc, which is the temperature at the center of the tablet (Tc-1).
0)°C to (Tc+10)°C.

If the temperature of the forging die is below (Tc-10)℃, the forged sheet will become too cold and the next thermoforming will be difficult, and if the temperature of the forging die is above (Tc+10)℃, the forging sheet may stick to the surface of the forging die. There is. The pair of annular transfer bodies is heated to a lower temperature than the forging die. If the temperature of the transfer bodies is too high, the sheet retention force of the flange of the forged sheet press-fitted into the transfer body will be weakened, causing the sheet to contract and be transferred. There is a risk of it coming off the body.

It is preferable to lower the temperature of the transfer body to such an extent that the peripheral edge of the molten tablet is press-fitted into the transfer body to form a flange and the sheet does not detach from the transfer body. Cooling is not necessary. After reducing the gap between the forging dies to a predetermined thickness of the forging sheet, it is preferable to maintain the distance between the forging dies constant for 0.2 seconds to 3 seconds.

Through this operation, the central part of the tab V-piece is forged, and the peripheral edge of the tablet, which is in a molten state, is sufficiently press-fitted into the gap of the transfer body, forming a well-finished flange of the container. I can do it.

The method of the present invention has made it possible to produce thin forged sheets with an average thickness of approximately 0.5 m or less.

The forged sheet is transferred onto a mold, thermoformed onto the mold surface, and cooled.

As the thermoforming method, straight pressure forming, plug assist pressure forming, etc. are preferable. By the method of the present invention, the press pressure during forging can be lowered to 50 (fl) or less compared to the conventional method, making it possible to manufacture a thin forged sheet, and also making it possible to reduce the gap between the resin transfer bodies during forging. Because it is easier to press fit into the part,
The formability of the flange portion has been significantly improved, and there is no longer any fear that the forged sheet will separate from the transport body. The present invention will be explained in more detail below with reference to Examples. Example 1 40m7! A crystalline polypropylene resin sheet with a thickness of 2.1 m7fL and a width of 150m7fL produced by the T-die melt extrusion method was cut into 75 sinus squares to make a square tablet using an Lφ extruder, and this tablet was placed between two hot plates.
Heat to 50 to 160°C, then blow hot air around the periphery of the tablet and heat to 170°C to 2000°C all around the circumference to a point 3mm from the edge to melt it. The tablet is placed into a pair of forging molds having a round forging surface with a diameter of 100rfLTfL heated to ~155°C, pressurized and forged, and the peripheral edge of the tablet is press-fitted into the gap of the transfer body. The forged sheet with a thickness of 111t7It, held by the transfer body with a gap opened, is transferred onto the pressure molding mold, and an auxiliary plug heated to approximately 135°C is inserted from above the forged sheet, and then compressed air is introduced to remove the surface of the female mold. A sheet was formed.

The dimensions of the molded container were a flange diameter of 110 m and a depth of 100 mm. For comparison, a similar tablet of 75 degrees Celsius was uniformly heated over the entire surface to 150 to 160 degrees Celsius, and this tablet was forged in the same forging die heated to 145 to 155 degrees Celsius to make a forged sheet with a thickness of 11Lm. This was plug-assisted air pressure molded to form a container of the same size.

Table 1 shows the forging conditions of the method of the present invention and the conventional method, and the evaluation results of the performance of the forged sheet and container.

In Table 1, the resins are PP (polypropylene homopolymer, melting point: 165°C) and PP copolymer (polypropylene random copolymer, melting point: 155°C), each processed at the optimum forging temperature. As is clear from Table 1, according to the method of the present invention, the forging load can be significantly reduced, the forged sheet has a good finish at the flange, and the manufactured containers have no uneven thickness and are transparent. is also good.
In the comparative example, the forging load was high and the finish of the flange portion of the forged sheet was poor, so that the resin was not sufficiently press-fitted into the gap of the transfer body. Also, the container has noticeable uneven thickness at the top of the side wall. Example 2 One 65m1Lφ extruder and 40#! Thickness 2 manufactured by co-extrusion method using two 71φ extruders! N7lLl width 15
A tablet was made by cutting a 0mm 5-layer sheet (polypropylene/adhesive polyolefin/saponified ethylene-vinyl acetate copolymer resin/adhesive polyolefin/polypropylene) into 75 square pieces, and the surface was lubricated with silicone oil. Next, sandwich the tablet between hot plates and heat it to 150-160℃.
Heat to 0°C, and then heat the entire periphery of the tablet 4j from the edge.
Heat the tablet to a temperature of 18°C to 200°C until it reaches a molten state, place it in the center of the same forging mold as in Example 1, pressurize it, and forge it. Press it into the 1.0 immediately! ! The forged sheet with a thickness of LlL is transferred onto the pressure forming mold by a transfer body, and the diameter of the flange portion is reduced to 110m7 by plug-assisted pressure forming! L
A container with a depth of 60 mm was manufactured.

For comparison, a 75 mm square tablet made of this five-layer sheet was uniformly heated to 150 to 160 DEG C., and similarly forged and air-formed to produce a container with a flange diameter of 110 mm and a depth of 60 mm. Table 2 shows the performance of the forged sheet and container under the forging conditions of the method of the present invention and the conventional method. In Table 2, PP is a polypropylene homopolymer (MI=1.
0, crystal melting point 216 rC), EV is a saponified ethylene-vinyl acetate copolymer resin (crystal melting point 18 C), AD is a polypropylene-based adhesive polyolefin, and molding was carried out under optimal processing conditions.

As is clear from Table 2, the forging load can be significantly reduced by the method of the present invention, and there is no uneven thickness of the side wall of the formed container. Uneven thickness is noticeable on the upper side wall of the container.

[Brief explanation of the drawing]

FIG. 1 is a side view of a manufacturing apparatus that implements the method of the present invention, with A and b showing a forging apparatus for tablets, and C showing a thermoforming apparatus for forging sheets. In the figure, 1 and 2 are the upper and lower dies of the forging mold, respectively, 3 is the annular transfer body, 4 is the square tablet, 5 is the gap of the transfer body, 6 is the flange, 7 is the forged sheet, and 8 is the molded The mold, 9 is a container. Figure 2 shows a square tablet, with 10
is the center part, 11 is the peripheral part, the length of the two sides of the tablet is A, and the width of the peripheral part of the tablet is ΔA from the end.
, Δb1 The thickness of the tablet is c. FIG. 3 shows a disc-shaped forged sheet, with reference numeral 12 indicating its central portion and 13 indicating its flange portion.

Claims (1)

[Claims] 1 A thermoplastic resin sheet consisting of one or more layers is cut into square tablets, and the temperature (Tc) at the center of the tablet is within the range of the Picato softening point (Tv) to the melting point (Tm) of the resin. The temperature of the peripheral edge of the tablet (T_E) is preheated to a temperature of (Tm + 10) °C or higher and melted, and then the tablet has a pair of forged surfaces arranged facing each other, and (Tc- 10)°C to (Tc+10)°C
The gap is formed by a gap between a press forging die heated to a temperature within the range of Introducing the tablet into the space and placing the tablet in the center of the forging die, then pressing the tablet with the forging die to reduce the gap between the forging die until a predetermined thickness is reached, and At the same time, the periphery of the tablet in a molten state is press-fitted into the gap of the transfer body to form the flange of the container, and then the forged sheet is transferred onto the mold by the transfer body. . A method for manufacturing a container, comprising thermoforming the forged sheet into a mold and cooling it.