JPH028892B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a stamping molding method that eliminates spring back. Conventionally, in order to obtain molded products using thermoplastic polymer materials, injection molding is often not only stable against variations in dimensional accuracy and product strength, but also excellent in processing cycleability. However, for products with large molding areas, products without ribs or slits reduce processing cycles, reduce material strength due to welds during multi-point gates, and cause twisting due to difficulty in uniformly cooling the molded product. Vacuum forming is often used instead. However, a disadvantage of vacuum forming is that the sheet is heated above its melting point to form the sheet, resulting in large thickness deviations, and in order to eliminate thin-walled areas, consideration must be given to increasing the thickness of the sheet over the entire surface, which is uneconomical. In addition, if the wall is thick or has a large area, the sheet will be heated above its melting point, so the drawdown due to the sheet's own weight will limit the time from handling to molding and the heating temperature range, which may reduce the product's fabric weight. There is also a limit to the amount. As described above, in the product area where injection molding and vacuum forming overlap, each has its own drawbacks and is not sufficient, but stamping molding is a forming method characterized by being able to compensate for these points. Stamping molding is a method in which the sheet is softened at a low temperature, for example, just below its melting point, and then molded using a press mold.As with metal press molding, the thickness changes significantly due to the drawing process being performed at a low temperature. few. Therefore, unlike vacuum forming, there is no need to consider in advance the portions that will become thinner due to uneven thickness. In addition, vacuum forming requires reheating the sheet to bring it above its melting point, but stamping molding allows stamping of a sheet that has been softened by preheating during sheeting, so the processing step can be omitted. Therefore, it is suitable for energy saving. However, in stamping molding, when the material is removed from the mold after molding, there is a phenomenon in which the material tends to return to its original shape in order to release the molding strain, and this phenomenon is called spring back. Furthermore, even if the product is completed as a molded product, it is inevitable that the product will warp and twist due to the release of residual strain during molding due to heat cycles. The present invention provides a stamping molding method with excellent moldability as a result of extensive research into suppressing this spring back and residual strain during molding to obtain a predetermined molded product shape. The sheet materials used in the present invention are crystalline polypropylene resin (hereinafter referred to as "polypropylene resin") and crystalline polyethylene resin (hereinafter referred to as "polyethylene resin"), and the polypropylene resin is a polypropylene homopolymer. , random copolymers, block polymers containing amorphous ethylene propylene copolymers,
Random block polypropylene, polyethylene resins such as medium-low pressure high density polyethylene, α-olefin and ethylene copolymers such as ethylene propylene copolymer, ethylene butene copolymer, ethylene hexene copolymer,
Examples include ethylene-4-methylpentene copolymer and high-pressure polyethylene. Even if these polypropylene resins or polyethylene resins are used alone, the spring back is large and the forming temperature range is narrow, which not only causes whitening and necking, but also makes it difficult to draw complex shapes and deep drawing. is extremely difficult. However, if a high melting point resin made of one or more of the above polypropylene resins and a low melting point resin made of one or more polyethylene resins are combined, and the temperature is between the melting points of the high melting point resin and the low melting point resin, Only by stamping molding with the same method can an extremely good effect on spring back, which cannot be observed when used alone, can be observed. For example, the melting end temperatures of polypropylene resin and polyethylene resin are approximately 160℃ and
The sheet material made of these blends has an extremely wide softening temperature range of about 25 to 30°C just below the melting point. This is because the polyethylene resin phase in the sheet exists as a phase close to a molten state, and the linear chains of polypropylene resin do not move almost completely due to thermal motion. The addition of the polypropylene resin allows drawing processing to be carried out without causing residual distortion during molding, and springback does not occur.Also, since the polypropylene resin is still thermokinetically rigid at this temperature, it is possible to draw the sheet material into the drawing part without uneven thickness. Since it is extremely easy to attract fish, there is almost no unevenness of meat. In addition, the wide temperature range of the softened sheet not only works well for deep drawing and complex shapes, but also takes a long time for the temperature to drop due to solidification even when rapidly cooled in the mold, resulting in solidification during the drawing process. The occurrence of whitening and netting is also eliminated compared to when used alone. When combining a high melting point resin and a low melting point resin, add the high melting point resin to the total amount.
It should contain 20-70wt%. Even if the amount is less than 20 wt% or exceeds 70 wt%, various problems will occur during molding. Note that the melting point in the present invention refers to the end temperature of the melting point when measured by a differential scanning calorimeter. When polypropylene is used alone, the molding temperature range is about 3 to 5°C, and the sheet immediately enters the solidification temperature range, which tends to cause spring back, and furthermore, whitening occurs during necking and mold drawing. Not only is it extremely difficult to form, but deep drawing is nearly impossible. Further, block polypropylene containing an amorphous ethylene propylene copolymer shows some improvement, but it is insufficient for molding. On the other hand, the same is true for polyethylene alone, but the spring back is large due to rapid crystallization. Also, deep drawing is extremely difficult. In such a combination of a high melting point resin and a low melting point resin, block polypropylene and high density polyethylene or block polypropylene and linear low density polyethylene are preferably used from the viewpoint of blendability. In combinations of homo or random polypropylene and polyethylene, blendability can be improved by adding amorphous ethylene propylene copolymer (hereinafter abbreviated as EPR) or ethylene vinyl acetate copolymer (hereinafter abbreviated as EVA). You can.
The weight part ratio of polypropylene/polyethylene is
30/70 to 70/30, preferably 40/60 to 60/40. In addition, to improve blendability, EPR,
EVA may be added in an amount of 3 to 20 parts by weight. On the other hand, linear low-density polyethylene may be used instead of high-density polyethylene, and the number of branches is determined to be 10 or more at CH ₃ /1000C using a method of quantifying the carbon in the main chain of polyethylene using an infrared absorption spectrum, MI = 1. Below, those with a density of 0.930 or less have excellent moldability. Examples of means for blending the two include an extruder, a Banbury mixer, a continuous mixer, etc., and among them, a Banbury mixer and a continuous mixer are preferred. Example 1 Showa Denko Co., Ltd. Showa Allomer MK111E (MFI
(230℃) = 1.8g/10min, melting point: 168℃) High-density polyethylene 6002B (MI (190℃)) made by Showa Denko Co., Ltd.
=0.2g/10min, density: 0.96g/cc, melting point: 137
Weigh out 50 parts by weight of each of the antioxidants (°C) in advance.
After adding 0.05 parts by weight of BHT, the mixture was thoroughly mixed using a tumbler type blender. Thereafter, using a continuous mixer, the jacket temperature of the mixer was adjusted to 160°C with steam and water so that the resin temperature did not exceed 230°C, and the mixture was kneaded and pelletized. The blend thus prepared was extruded to a sheet thickness of 1.5 mm using a T-die sheet forming machine with a screw diameter of 65 φ mm and a width of 1100 mm, and both ends of the sheet were trimmed to form a sheet with a width of 800 mm. did. Even after this handling step, the sheet temperature remains at 145°C and remains in a softened state. The sheet in this state was drawn onto a mold mounted on a vertical press and stamping molding was performed. After molding, the ram of the press was raised and a device was used to grasp both sides of the sheet and pull out the molded product. One cycle until the next molding takes 15 seconds, which is much faster than other molding methods. The molded products were sent to the trimming process while still connected by sheets, and the unnecessary parts were crushed and recycled as recovered raw materials. Table 1 shows the basic physical properties of the sheet before stamping. In addition, the stamping formability is shown in Table 2. In order to grasp the drawing formability, a male mold with a cylindrical shape of 150 mm diameter and a corner radius of 6 mm so that the depth can be changed and a corresponding female mold are used. If the diameter of the male mold can be squeezed by more than twice the diameter of the male mold, it is considered "very good," if it can be squeezed by 1 to less than 2 times, it is "good," and if it can be squeezed by 0.2 times to less than 1, it is classified as "slightly poor."
We used a criterion that defines a lens that can be narrowed down by less than 0.2 times as "defective." The results shown in Table 2 were obtained regarding deep drawability, whitening, netting, uneven thickness, springback resistance, etc. during stamping molding. There is a concern that the residual strain that occurs during molding will be released by the heat aging that is applied later, resulting in deformation of the molded product.For this reason, we measured the amount of deformation after leaving it in an oven at 100℃ for 10 hours, but no results were found. No deformation was observed in the molded product, and it was found that there was little residual strain during molding. At the same time, the shrinkage rate was measured at 20/1000.
It showed ~25/1000, which was the same as injection molded products and vacuum molded products. Examples 2 to 4 Shorex MX0438-5 manufactured by Showa Denko K.K. as high-density polyethylene (MI (190°C) = 0.45 g/
The procedure was the same as in Example 1 except that 10 min, density: 0.95 g/cc, melting point: 139°C) was used, but the blend ratio was polypropylene/polyethylene = 50/
The basic physical properties of the sheets are shown in Table 1 for Examples 2 to 4 in the case of 50, 45/55, and 40/60. Stamping formability was good as shown in Table 2. Examples 5 and 6 Showa Denko K.K.'s Showa Allomer SK711 (MFI (230°C) = 0.7 g/min, melting point: 168°C) was used as the polypropylene; Showa Denko K.K.'s Showa Rex C510 3- was used as the high-density polyethylene. D (MFI
(190°C) = 0.3g/10min, density: 0.95g/cc, melting point: 137°C) was carried out in the same manner as in Example 1, but the blend ratio was polypropylene/polyethylene = 60/40. , 50/50 is used as Examples 5 and 6, and the basic sheet properties are shown in Table 1. Stamping formability was good as shown in Table 2. Examples 7 and 8 Polypropylene is SK711 manufactured by Showa Denko K.K.
(MFI (230℃) = 0.7g / 10min, melting point: 168℃) and Showa Denko K.K.'s Shorex MX0438-5 (MI (190℃) = 0.45g /
Example 7 was carried out in the same manner as in Example 1 except that 10 min, density: 0.95 g/cc, melting point: 139°C) was used, but the blend ratio was 60/40, 50/50. Table 1 shows the basic physical properties of the sheet. Stamping formability was good as shown in Table 2. Comparative Example 1 Polypropylene was Showa Denko K.K.'s Showa Allomer MK111E (MFI (230°C) = 1.8 g/10 min,
After sheeting (melting point: 168°C) alone, stamping molding was performed in the same manner as in Example 1, but deep drawing was nearly impossible, and necking at the corners,
Whitening is large, thickness deviation is significant, and the required shape of the molded product cannot be obtained due to spring back. Furthermore, when this product was left in a gear oven at 100°C for 10 hours and then observed, the residual strain generated during molding was released, resulting in significant twisting and warping of the product. Comparative Examples 2 and 3 In Comparative Example 2, Shorex 6002B (MI (190°C) = 0.2 g/10 min, density: 0.96 g/cc, melting point: 138°C) manufactured by Showa Denko Co., Ltd. was used in Comparative Example 3. is the company's MX0438-5 (MI (190℃) = 0.45
g/10min, density: 0.95g/cc, melting point: 139℃)
was carried out alone in the same manner as in Comparative Example 1, but the crystallization rate was higher than that of polypropylene and the spring back was extremely large, making it difficult to form into a product, and the corner portions were torn off during stamping. For this reason, the product is not worthy of evaluation.

【table】

【table】

[Table] However, ◎…Very good ○…Good △…
Slightly poor ×...poor As described above, the stamping molding method according to the present invention has many advantages such as excellent drawing processing, no occurrence of spring back, twist, or whitening, and less uneven thickness.

Claims (1)

[Claims] 1 Consisting of a high melting point resin made of one or more types of crystalline polypropylene resin and a low melting point resin made of one or more types of polyethylene resin, and the difference in melting point temperature between the high melting point resin and the low melting point resin is 25℃ or higher and the proportion of high melting point resin is 20~
A stamping molding method characterized in that a sheet having a concentration of 70 wt% is molded at a temperature between the melting point of the high melting point resin and the melting point of the low melting point resin.