JPH06218805A - Forming method of foamed polypropylene sheet - Google Patents

Abstract

(57) [Summary] [Structure] The foamed PP sheet 21 is placed in the oven 10 in which the ambient temperature is kept in the range of 155 ° C. to 200 ° C.
Is gradually raised. The surface temperature of the sheet 21 is 160 ° C ±
Molding is performed while maintaining the temperature range of 5 ° C. [Effect] By maintaining the ambient temperature within the above range, the foamed PP sheet 21 is heated by using both radiant heating and heat transfer heating. This shortens the heating time of the foamed PP sheet 21 and improves the productivity. By molding while maintaining the surface temperature of the foamed PP sheet 21 in the above range, insufficient heating or overheating of the foamed PP sheet 21 is prevented. As a result, a defective appearance of the molded product 23 is prevented, and a molded product 23 having excellent quality is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for molding a foamed polypropylene sheet, and more particularly to a molding method for obtaining a molded product with high productivity and high quality.

[0002]

2. Description of the Related Art Conventionally, a method for molding a foamed polypropylene sheet (hereinafter referred to as "foamed PP sheet") has been
A method is known in which a foamed PP sheet is preheated in an oven to be softened to a moldable state and then molded in a molding zone to obtain a molded article. The oven normally uses radiant heating by an infrared heater or the like that is in a non-contact state with the foamed PP sheet.

When the expanded PP sheet is heated, the polypropylene resin has a melting point near 160 ° C. Therefore, if the temperature is greatly exceeded, the expanded PP sheet itself will melt and the foam layer will be lost. When the foamed PP sheet is molded in this state, thin parts are formed in places on the molded product, and a molded product of good quality cannot be obtained. Therefore, when the expanded PP sheet is heated by the oven or the like, it must be heated so that the temperature of the expanded PP sheet does not greatly exceed 160 ° C.

However, unlike the non-foamed PP sheet, the foamed PP sheet has a thickness and is heated by heating.
Since the secondary foaming is involved, the heat conduction in the thickness direction is poor, and it takes a long time to heat the foamed PP sheet. Therefore, when a foamed PP sheet is molded to obtain a molded product, the molding cycle becomes long and the productivity is lowered. Therefore, in order to improve the productivity, in order to shorten the heating time of the foamed PP sheet, the heater temperature of the oven is set to, for example, 300.
It has been attempted to heat to a high temperature around ℃.

[0005]

However, when the heater temperature of the oven is raised to a high temperature, there is a problem that the following molding defects occur. First, foam P
If the P sheet is heated at a high temperature, the PP foamed in proportion to this.
The heating rate of the sheet will increase. At this time, uneven heating easily occurs in the foamed PP sheet depending on the arrangement and performance of the heaters attached to the oven and the temperature control state thereof. When the foamed PP sheet is molded in the state where the heating is uneven, the expansion ratio of the foamed PP sheet varies from place to place, and cracks may occur during molding and stretching, resulting in poor appearance of the molded product.

[0006] Further, when the temperature of the heater of the oven due to radiant heating is high, but the ambient temperature in the oven is low, and in the case of a semi-open oven shape, the inflow rate of outside air is increased. Therefore, the surface temperature of the foamed PP sheet is lowered by the flow of air, which makes it difficult to stably heat the foamed PP sheet. When the surface temperature of the foamed PP sheet becomes lower than 155 ° C., cracks may occur during molding and stretching, resulting in poor appearance of the molded product, as described above.

That is, when the foamed PP sheet is heated to a high temperature, if the ambient temperature is low, the surface of the foamed PP sheet is cooled by the flow of air, so that there occurs a phenomenon that the temperature in the central portion becomes higher. Therefore, when the central portion of the foamed PP sheet reaches a temperature suitable for molding, the surface layer portion is lower than that temperature, and when the surface layer portion reaches a temperature suitable for molding, the central portion is already higher than that temperature. Will end up. Therefore, it is difficult to find a state in which the foamed PP sheet suitable for molding is in a uniformly heated state and has a small temperature difference between the surface layer portion and the central portion of the foamed PP sheet.
Particularly, as a characteristic of the foamed PP sheet, since the surface layer portion is also formed of a foamed film, it is strongly affected by the outside air temperature. Also,
In the case of polypropylene resin, if it is heated at a high temperature, the softening rate of the foamed PP sheet rapidly increases with respect to the time axis of the heating time, so that the softened range becomes very narrow in a state suitable for molding. It depends.

Therefore, an object of the present invention is to shorten the heating time of a foamed PP sheet, prevent the occurrence of molding defects and obtain a molded product of excellent quality.
It is to provide a method for forming a sheet.

[0009]

In order to solve the above-mentioned problems, a method for molding a foamed PP sheet of the present invention is a foamed PP sheet.
After the sheet is heated in an oven to soften it to a moldable state and then molded into a desired shape in the next molding zone, the atmosphere temperature in the oven is set to a temperature range of 155 ° C. or higher and 200 ° C. or lower. The above foamed PP while kept
While gradually raising the temperature of the sheet, raise the surface temperature to 160 ° C.
Molding is performed in the above molding zone while maintaining the temperature range of ± 5 ° C.

[0010]

According to the method for forming a foamed PP sheet of the present invention having the above-mentioned structure, the atmospheric temperature in the oven is 155 ° C.
As described above, by keeping the temperature in the range of 200 ° C. or less, the foamed PP sheet can be heated by using both the radiant heating by the heater and the heat transfer heating by the convection air. By gradually raising the temperature of the foamed PP sheet while maintaining the ambient temperature in the oven within this temperature range, the foamed PP sheet is heated and softened without insufficient heating or overheating of the foamed PP sheet. Thereby, the foamed PP sheet is efficiently heated in a short time. That is, the ambient temperature is 15
If the temperature is lower than 5 ° C, the air flow in the oven will be fast and the surface temperature of the foamed PP sheet will be lowered. On the contrary, if the temperature is higher than 200 ° C, the softening rate of the foamed PP sheet will be rapidly increased. It becomes difficult to find a softened range that is too much and can be molded.

Further, by gradually raising the temperature of the foamed PP sheet, the proportion of secondary foaming of the foamed PP sheet can be suppressed to a low level, and a large temperature difference is not generated between the surface layer portion and the inside of the foamed PP sheet. The foamed PP sheet is heated. Furthermore, the surface temperature of the foamed PP sheet is 160 ° C ±
By molding while maintaining in the temperature range of 5 ° C, foamed P
The extending ratio of the P sheet can be made substantially uniform, and the foam layer can be prevented from being broken due to melting of the resin. As a result, it is possible to prevent a crack pattern or a phenomenon of partial thinning of the molded product molded in the molding zone and obtain a molded product of good quality. That is, when the surface temperature of the foamed PP sheet is lower than 155 ° C (160 ° C-5 ° C),
The expansion rate of the foamed PP sheet may be different, and conversely, if it exceeds 165 ° C. (160 + 5 ° C.), the foamed PP sheet may melt above the melting point of the PP resin and the foamed layer may be destroyed. is there.

[0012]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a schematic view showing a molding step of a method for molding a foamed PP according to an embodiment of the present invention. The molding process shown in FIG. 1 has a preheating zone A for heating and softening the foamed PP sheet 21 to a moldable state, and a molding zone B for molding the foamed PP sheet 21 heat-softened in the preheating zone A. . Although not shown, the foamed PP sheet is fed to each zone without error in each zone by using a feeding mechanism with high accuracy.

The preheating zone A is provided with an oven 10 for heating the foamed PP sheet 21 fed from the original fabric 20. The oven 10 has a closed shape excluding the inlet and outlet of the foamed PP sheet 21. Inside the oven 10, there is provided a heater 12 that is not in contact with the foamed PP sheet 21. As the heater 12, for example, a far infrared heater or the like is used, and the heater 12 is arranged above and below the foamed PP sheet 21, respectively. The foamed PP sheet 21 is heated and softened by the radiant heat from the heater 12 to a moldable state. The heater temperature at this time is set to 22 so that the heating of the foamed PP sheet 21 is shortened and the foamed PP sheet 21 is not melted.
The temperature is set to about 0 to 260 ° C. The temperature of the heater 12 may be different between upper and lower sides, and in this case, the temperature of the upper heater 12 is preferably set high. That is, the heat of the heater 12 arranged on the upper side may be increased by the convection of air in the oven 10 and may not be easily transmitted to the foamed PP sheet 21. A cover 11 covering the entire oven 10 is attached to the side surface of the oven 10, although the oven 11 is partially cut away in FIG. The cover 11 can prevent the inflow of outside air by keeping the oven 10 substantially closed, so that the atmospheric temperature in the oven 10 is adjusted. The ambient temperature is 1
The temperature is set to 55 ° C to 200 ° C, preferably 155 ° C to 175 ° C. This ambient temperature is 155 ° C
If the temperature is less than the above value, the air flow in the oven 10 will be accelerated due to the convection of air, and the surface temperature of the foamed PP sheet 21 will be lowered. On the contrary, if it exceeds 200 ° C., the foamed PP sheet 21 will be softened. The speed becomes too steep and it becomes difficult to find the formable width. Here, the moldable width refers to a heating time width in which a cracked pattern or a phenomenon of partial thinning does not occur in a molded product when the foamed PP sheet softened to a moldable state is molded. In addition, as the measurement position of the ambient temperature, 10 points above and below the foamed PP sheet were used.
It indicates a temperature around 0 to 200 mm. The foamed PP sheet 21 is heated gradually so that the surface temperature thereof is in the range of 160 ° C ± 5 ° C. If the surface temperature of the foamed PP sheet 21 is less than 155 ° C. (160 ° C.-5 ° C.), the extending ratio may be different when molding the foamed PP sheet 21, and a crack pattern may occur.
Conversely, if the temperature exceeds 165 ° C (160 + 5 ° C), foam P
There is a risk that the P sheet will melt 21 and the foam layer will be destroyed, resulting in a thin portion in the molded product. This preheated sone A
The foamed PP sheet 21 that has been heated and softened by is then conveyed to the molding zone B.

The molding zone B is equipped with a molding die for molding the foamed PP sheet 21 which has been heated and softened in the preheating zone A. The molding dies are a pair of molding dies in which a plug mold 31 having a convex portion and a cavity mold 30 having a concave portion are combined, and in FIG. 1, two mold dies having the same shape are provided. This molding die is a cold press die, and the die temperature may be kept within a range from room temperature to about 40 ° C. or less. The molding die is a match mold in which the plug die 31 and the cavity die 30 are in contact with or in contact with the foamed PP sheet 21. The plug mold 31 and the cavity mold 30 are each a cylinder mechanism 3 that can move back and forth.
It is fixed to the molding press machines 32 and 33 to which 4, 35 are attached. In the molding die, by driving the cylinder mechanisms 34 and 35, the plug die 31 and the cavity die 30 facing each other are vertically moved to be fitted or separated from each other. The cavity mold 30 has a vacuum suction function, and a vacuum suction pipe 36 is attached. The vacuum suction pipe 36 is connected to a vacuum pump (not shown). By operating this vacuum pump, the softened foamed PP sheet 21 is vacuum-sucked by the cavity mold 30 and is closely adhered along the mold cavity surface. As for the operation of this molding die, first, the cavity mold 30 moves up and the foamed PP sheet 21 is vacuum-sucked and brought into close contact with the mold cavity surface, and then the plug mold 31 is lowered to mold the foamed PP sheet 21. Is done.

In this way, the foamed PP sheet 21 which has been heated and softened in the preheating zone A is molded into a molded product 22 having a desired shape by a molding die. Then, the sheet molded product 22 molded by the molding zone B is cut into each molded product 23 by the pair of cutters 4 and collected. As described above, in the preheating zone A, the atmosphere temperature in the oven 10 is kept in the temperature range of 155 ° C. or higher and 200 ° C. or lower.
The radiant heating and the heat transfer heating by convection air can be used together to gradually raise the temperature of the foamed PP sheet 21. As a result, the heating time of the foamed PP sheet 21 can be shortened, so that the molding productivity can be improved.

In this preheating zone A, foam P
Since the surface temperature of the P sheet 21 is maintained in the temperature range of 160 ° C. ± 5 ° C., due to insufficient heating or overheating of the foamed PP sheet 21 in the molding zone B, the surface of the molded product 23 becomes cracked or partially thinned. It is possible to prevent the occurrence of such as. Further, by gradually raising the temperature of the foamed PP sheet 21, it is possible to make the ratio of the secondary foaming accompanying the foamed sheet smooth without abruptly increasing, and the foamed PP sheet 21 has a large surface layer portion and the inside thereof. It can be heated without causing a temperature difference. As a result, it is possible to prevent the appearance defect of the molded product 23 from occurring, and it is possible to obtain the molded product 23 having excellent quality.

Therefore, in the method of forming the foamed PP sheet 21, the ambient temperature and the foamed P in the preheating zone A are set.
By setting the surface temperature of the P sheet 21 within a certain range, it is possible to improve the productivity and obtain a molded product 23 having excellent quality. The method for molding the foamed PP sheet of the present invention is not limited to the above embodiment. For example, in the preheating zone A, hot air having a constant temperature may be blown into the oven 10 as a means for adjusting the ambient temperature.

In the molding zone B, the operation of the molding die is such that the foamed PP sheet 21 is sandwiched between the cavity mold 30 and the plug mold 31 and the mold is clamped, and then the foamed PP sheet 21 is vacuum-sucked from the cavity mold 30. You may be like this. Further, the molding die may have a built-in clamp for holding the sheet molded product 22 at a fixed position so that the sheet molded product 22 can be smoothly released. Further, although the number of surfaces of the molding die is two in the above embodiment, it may be one or a large number of two or more. In this case, productivity can be further improved by forming a large number of molding dies having a large number of surfaces. Furthermore, as the molding method, plug assist molding, double-sided vacuum molding having a vacuum suction function also in the plug die 31, pressure air molding or simple pressure molding instead of vacuum, and other known methods can be adopted.

In addition, air or the like may be blown from above and below so that the sheet molded product 22 after passing through the molding zone B can be cooled quickly, and various types can be used without changing the gist of the present invention. It is possible to make design changes. Next, the following experiment was conducted. As shown in Figure 2,
Using a molding machine, experimental examples and comparative examples were performed under the conditions shown in Table 1 below.

[0020]

[Table 1]

<Experimental Method> A foamed polypropylene sheet (s) having a basis weight of 350 g / m ² and a primary thickness of 1.5 mm is fixed to the clamp base (a), and the handle (m) extended from the square bar (b) is fixed. Grab it and slide it on the rail (i) and put it in the heating furnace. At the top and bottom of this heating furnace, a heater (h)
Is attached, and the foamed PP sheet (s) is radiantly heated by the heater (h). An openable / closable cover (n) is attached to the side surface of the heating furnace, and the atmospheric temperature in the heating furnace can be adjusted by opening / closing the cover (n).

After the set heating time, the square bar (b) attached to the clamp table (a) is moved to the proximity switch (c).
Is detected and the surface thermometers [infrared radiation thermometer manufactured by Keyence Corporation, model number "IT-1600"] (d) arranged above and below the foamed PP sheet (s) are Start surface temperature measurement. The ambient temperature is measured by thermocouples (e) arranged above and below the foamed PP sheet (s) in the heating furnace.

On the other hand, the forming method in the forming zone is the upper plug (f) and the lower cavity (g), in which the vacuum molding is started prior to the cavity (g), and then the upper plug (f) is closed at the timing of perfect match molding. is there. (J)
Are the upper plug (f) and the lower cavity (g), respectively.
Is a hydraulic cylinder that raises and lowers. Further, (k) is a tube connected to a vacuum pump (not shown) for vacuum forming the lower cavity (g).

Then, the above Experimental Example 1 and Comparative Examples 1 to 4
Under the conditions described above, the moldable width, which is the width of the heating time at which molding was performed in the molding zone and did not cause a defective appearance, was searched for every predetermined heating time, and the time up to this molding width was investigated. Further, the sheet surface temperature and the secondary thickness were measured for each heating time. Regarding these relationships, the horizontal axis represents the heating time (second), the vertical axis represents the sheet surface temperature (° C.) and the secondary thickness (mm), and the sheet surface temperature is drawn by the dotted line and the secondary thickness is drawn by the solid line. The graphs are shown in FIGS. 3 to 7, and the time (second) to the moldable width, the moldable width (second) and the sheet surface temperature (° C.) are shown in Table 2 together with the ambient temperature.

[0025]

[Table 2]

From the results of Table 2 and FIGS. 3 to 7, Comparative Examples 1 and 2 have long moldable widths and can be stably molded, but the moldable time is long and the molding cycle is slow. . On the contrary, in Comparative Example 3, the time to the moldable width is short and the generation cycle is fast, but the moldable width is short and stable molding cannot be performed. In Comparative Example 4 in which the ambient temperature was low, a moldable width that could obtain a good molded product was not found.

On the other hand, the ambient temperature is 155 ° C.-2.
In Experimental Example 1 in the range of 00 ° C., it was found that the conditions were such that a productable and high-quality molded product having a short moldable width and a long moldable width was obtained.

[0028]

As described above, according to the method for molding a foamed PP sheet of the present invention, the atmospheric temperature in the oven is set to 15
By gradually raising the temperature of the foamed PP sheet in the temperature range of 5 ° C. or higher and 200 ° C. or lower, the foamed P sheet
The heating time of the P sheet can be shortened. As a result, the productivity in molding the foamed PP sheet can be improved.

Further, the surface temperature of the foamed PP sheet is set to 160
By molding while maintaining the temperature range of ℃ ± 5 ℃, it is possible to prevent the appearance defect of the molded product due to insufficient heating or overheating of the foamed PP sheet, so molded product of excellent quality Can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic view showing a molding process of a method for molding a foamed polypropylene sheet according to an embodiment of the present invention.

FIG. 2 is a schematic view showing a molding machine used in an experiment of a method for molding a foamed polypropylene sheet of the present invention.

FIG. 3 is a graph showing the relationship between the secondary thickness of the expanded polypropylene sheet and the sheet surface temperature with respect to the heating time of Experimental Example 1, which was tested by the molding machine shown in FIG.

FIG. 4 is a graph showing the relationship between the secondary thickness of the expanded polypropylene sheet and the sheet surface temperature with respect to the heating time of Comparative Example 1, which was tested by the molding machine shown in FIG.

5 is a graph showing the relationship between the secondary thickness of the expanded polypropylene sheet and the sheet surface temperature with respect to the heating time of Comparative Example 2, which was tested by the molding machine shown in FIG.

FIG. 6 is a graph showing the relationship between the secondary thickness of the expanded polypropylene sheet and the sheet surface temperature with respect to the heating time of Comparative Example 3, which was tested by the molding machine shown in FIG.

7 is a graph showing the relationship between the secondary thickness of the expanded polypropylene sheet and the sheet surface temperature with respect to the heating time of Comparative Example 4, which was tested by the molding machine shown in FIG.

[Explanation of symbols]

 A Preheating zone B Molding zone 10 Oven 11 Cover 12 Heater 21 Foamed PP sheet 22 Sheet molded product 23 Molded product 30 Cavity mold 31 Plug mold

Claims (1)

[Claims] 1. A method of heating a foamed polypropylene sheet in an oven to soften it to a moldable state, and then molding it into a desired shape in the next molding zone, wherein the atmospheric temperature in the oven is 155 ° C. or higher. 200 ° C
The surface temperature of the foamed polypropylene sheet is kept at 160 ° C. while gradually raising the temperature in the following temperature range.
A method for molding a foamed polypropylene sheet, which comprises molding in the above molding zone while maintaining a temperature range of ± 5 ° C.