JP2001026032A - Method for injection molding of polypropylene foam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for stably and continuously molding a polypropylene foam with beautiful appearance and a magnification of foaming of at least 1.5. SOLUTION: A method for injection molding of a polypropylene foam wherein a fixed mold connected with a nozzle of an injection molding machine and a movable mold movable forward and backward to an arbitrary position to the fixed mold are provided and under mold clamped condition, a polypropylene with an MFR of 0.5-1,000 g/10 min and with a foaming agent is injected at a temp. being at least 30 deg.C higher than m.p. of the polypropylene and after injection is completed, the movable mold is retreated to expand the vol. of a cavity to a specified vol. to perform foaming and in addition, an unfoamed layer is generated on the surface of the foam and magnification of foaming is made at least 1.5, is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to instant food containers, food trays and the like and automobile interior parts.
The present invention relates to an injection molding method for obtaining a polypropylene foam molded article having an expansion ratio of 1.5 times or more, a relatively small thickness, and a beautiful appearance.

[0002]

2. Description of the Related Art In the field of injection molding, attempts have been made for a method of molding a resin foam. Recently, methods for adjusting the movement of the mold during the foaming process have been proposed (for example, Japanese Patent Application Laid-Open Nos. 62-246710, 8-267526 and 8-300391).
No.). However, the technical hurdles for forming a highly foamed material having a relatively small thickness, a beautiful appearance, and 1.5 times or more in a stable and continuous manner are considered to be extremely high, and are seen in the market. Nothing seems to be on the market as long as it is available. The reason is that, even if polystyrene having a relatively high melt viscosity is used, unlike extrusion molding of a sheet or the like, the resin temperature and / or the injection pressure (in-mold pressure) increase during injection, and as a result, This is probably because it was difficult to stably obtain a foamed product having a beautiful appearance and a foaming ratio of 1.5 times or more that can withstand actual use. In fact, according to experiments by the inventors, it is possible to mold one molded article having an expansion ratio of about 1.5 to 2.0 times, but it is very difficult to continue 10 shots, and 100 shots are required. The technical hurdles for continuous molding were very high. Further, even if the expansion ratio is increased to 1.5 times or more, there are appearance problems such as coarse bubbles and large surface irregularities. It was not able to withstand the use of containers and the like.

[0003]

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and has a beautiful appearance suitable for various food containers and automobile parts, and has a relatively small thickness. It is another object of the present invention to provide a method for continuously and continuously injection-molding a polypropylene foam having an expansion ratio of 1.5 or more.

[0004]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that the above object can be achieved by molding specific polypropylene under specific temperature conditions. That is, the present invention comprises a fixed mold coupled to a nozzle of an injection molding machine, and a movable mold capable of moving forward and backward to an arbitrary position with respect to the fixed mold. A foaming agent-containing polypropylene of 0.5 to 1000 g / 10 min is injected at a temperature of at most 30 ° C. higher than the melting point of the polypropylene, and after the injection is completed, the movable mold is retracted to reduce the volume of the cavity to a predetermined value. The present invention provides an injection molding method for a polypropylene foam, which is expanded to a volume and foamed, furthermore, an unfoamed layer is generated on the foam surface, and a foaming ratio is 1.5 times or more. .

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The polypropylene used in the present invention includes propylene homopolymer and propylene and other α.
-Copolymers with olefins. Examples of the α-olefin include ethylene, butene-1 and the like.
Obtained by block or random copolymerization of olefin and propylene. The copolymerization ratio of the α-olefin is generally 40% by weight or less, preferably 2 to 20% by weight, more preferably 2 to 15% by weight.

The above-mentioned polypropylene is a catalyst system obtained from a transition metal compound (for example, a titanium compound) or a carrier (for example, a magnesium compound, a processed product thereof) and an organometallic compound (for example, an organoaluminum compound). (So-called Ziegler-Natta catalyst). The production method and various physical properties of polypropylene are well-known,
It is manufactured industrially and used in various fields.

Further, the polypropylene in the present invention has a temperature of 230 according to MFR (JIS K7210).
C., measured under the condition of a load of 2.16 kgf).
It needs to be 00 g / 10 minutes. The MFR is
Preferably 3 to 800 g / 10 min, more preferably 5
５００500 g / 10 min. Especially, the thickness of the molded product is 3m
m or less, more preferably 2 mm or less, MFR is preferably 50 to 1000 g / 10 min, more preferably 60 to 900 g / 10 min, and even more preferably 70 to 800 g / 10 min. Minutes. MFR is 0.5
If it is less than g / 10 minutes, in addition to the problem of the torque of the injection molding machine, the problem of insufficient filling in the mold will occur, and continuous stable molding cannot be performed. Further, since the heat generation is intense, the resin temperature increases, and not only the foaming ratio of the obtained foam decreases, but also the appearance deteriorates. On the other hand, MFR
Exceeds 1000 g / 10 minutes, the viscosity is too low, so that the foamed cells are broken in the mold. As a result, not only the expansion ratio does not rise, but also the mechanical strength of the molded product is too low. It is not preferable.

In the present invention, one or more kinds of thermoplastic resins may be blended with the above polypropylene for other purposes such as improvement of cold resistance.
For example, polymers or copolymers such as polyethylene, polystyrene, polyvinyl chloride, and ethylene-vinyl acetate copolymer can be used. The amount of addition is at most 40% by weight, preferably 20% by weight or less.

In the method of the present invention, it is necessary to control the temperature at which the temperature is higher than the melting point of at least 30 ° C. in the entire process in which the polypropylene is melted and injected in the injection machine, that is, from the bottom of the hobber to the nozzle. Preferably 2
Temperatures within 5 ° C, more preferably within 20 ° C, are preferred. When the temperature in the molding machine exceeds 30 ° C. from the melting point, even if the resin temperature is lowered near the nozzle, the foaming ratio of the obtained foam is remarkably reduced, and the molding is stably and continuously performed. Can not. Unless the resin temperature is raised to the melting peak temperature, the pressure increases and a torque is required, so that ordinary injection molding becomes difficult. Further, even if injected, it is not sufficiently kneaded with a foaming agent, so that practical and beautiful foaming is not performed. If the temperature of the resin injected from the nozzle is not uniform, a foam in a uniform foaming state cannot be obtained, and the foaming state deteriorates and the foaming ratio decreases. The temperature of the resin at the time of injection is not always the same as the set temperature of the injection cylinder or nozzle of the injection molding machine, and differs depending on the kneading conditions of the resin. Many injection molding machines cannot measure the resin temperature inside the injection cylinder, but use an injection molding machine that can measure the resin temperature by inserting a thermometer into the injection cylinder in advance to grasp the resin temperature and molding conditions. Need to be kept.

[0010] The injection molding machine used in the present invention comprises:
Although a generally used molding machine may be used, a mechanism having a gate valve or the like at the tip of the injection cylinder is particularly preferable when high flow polypropylene is used.

The melting point of polypropylene is JIS
The melting peak temperature measured according to K7121 was defined as the melting point. The melting point depends on the type of polypropylene,
Generally, a polypropylene homopolymer and a block copolymer have a temperature of 160 to 168 ° C, and a random copolymer has a temperature of 125 to 168 ° C.
１５155 ° C. Therefore, the molding temperature is 190 to 1 for polypropylene homopolymer and block copolymer.
98 ° C., preferably 180 to 188 ° C., 155 to 185 ° C. for a random copolymer, preferably 145 to 175 ° C.
Need to be kept.

The unfoamed layer in the present invention means a layer having a foaming ratio of less than 1.1 on the surface of the foam. In the present invention, the total thickness of the unfoamed layer is preferably 2 to 0% of the total thickness of the foam, and more preferably 4 to 0%.
It is preferably about 20%. Here, the expansion ratio is represented by a value obtained by dividing the density of the resin by the apparent density of the obtained foam. The density is JIS K
The measurement was carried out in accordance with the method A (underwater substitution method) 7112.

In the present invention, after the mold is completed, the cavity is completely filled with the molten polypropylene containing the dissolved foaming agent, and then the movable mold is retracted to reduce the volume of the cavity to a predetermined volume. It is important to spread. When the molten polypropylene containing the blowing agent is not completely filled, the movable mold is retracted, the volume of the cavity is expanded to a predetermined volume, and the molten polypropylene containing the dissolved blowing agent can be foamed depending on the shape. In some cases, however, a short shot results in a large variation in the expansion ratio, and in particular, the appearance is deteriorated, and the molded product cannot be put to practical use. Therefore, after the cavity is completely filled with the molten polypropylene containing the foaming agent, it is very important to retract the movable mold and expand the volume of the cavity to a predetermined volume.

On the other hand, as the foaming agent in the present invention, a chemical foaming agent (also called a thermal decomposition type foaming agent) and / or a physical foaming agent are preferable. The chemical blowing agent is one or a mixture of two or more of various organic or inorganic chemical blowing agents. Examples of the organic chemical blowing agent include azodicarbonamide, N, N′-dinitrosobentamethylenetetramine, P-P'-oxybisbenzenesulfonyl hydrazide and the like. Examples of the inorganic chemical blowing agent include sodium bicarbonate, ammonium carbonate, ammonium bicarbonate, calcium azide and the like. Also, a mixture of an inorganic blowing agent and a fatty acid, such as a mixture of sodium bicarbonate and citric acid, can be used. Among them, a mixture of sodium bicarbonate and citric acid is preferred. Also,
These chemical blowing agents may be used in combination with a physical blowing agent described later. In that case, it may play a role as a foam nucleating agent. When an organic chemical foaming agent is used, urea, a fatty acid metal salt, zinc oxide, or the like is used as a foaming aid for adjusting the decomposition temperature to a lower temperature side in an amount of 0.1 to 50 parts by weight based on 100 parts by weight of the organic foaming agent. It can be used by adding parts by weight.
In selecting a chemical foaming agent, it is important to select a foaming agent and a foaming assistant so that the foaming agent is decomposed below the resin molding temperature.

On the other hand, as the physical foaming agent, an alternative fluorocarbon (generally, HC
FC or HFC-based Freon gas), butane,
Preferred are lower aliphatic hydrocarbons such as pentane, carbon dioxide, water, nitrogen and the like. Specifically, HCFC-22 (CF ₂
HCl), HCFC-32 (C ₂ F ₃ HCl ₂ ), HCF
C-124 (C ₂ H ₄ HCl), HCFC-141b (C
₂ FH ₃ Cl ₂ ), HCFC-142b (C ₂ F ₂ H ₃ C
l), HCFC-225ca (C ₃ F ₅ HCl), HCF
C-225cd (C ₃ F ₅ HCl ₂ ) and HFC-23
(CHF ₃ ), HFC-32 (CH ₂ F ₂ ), HFC-1
25 (C ₂ HF ₅ ), HFC-134 (CHF ₂ CH
F ₂ ), HFC-134a (CH ₂ FCF ₃ ), HFC-
_{152a (C 2 H 4 F 2} ), HFC-143 (CHF 2 CH
_{2 F), HFC-143a (} CF 3 CH 3), HFC-2
27ea (C ₃ HF ₇ ), HFC-236fa (C ₃ H ₂ F
₆ ), chlorofluorocarbons such as HFC-245ca (C ₃ H ₃ F ₅ ),
Halogenated hydrocarbons such as methyl chloride and ethyl chloride, methane, ethane, propane, n-butane, I-butane, butylene, n-pentane, I-pentane, neopentane, n
-Hexane, cyclohexane, lower hydrocarbons such as n-heptane, and carbon dioxide gas, nitrogen gas, water and the like. Among them, carbon dioxide, nitrogen and HF
C-134a is preferred. These various foaming agents can be used in combination. Although the total amount of the foaming agent varies depending on the type of the foaming agent and the desired expansion ratio, the amount of the foaming agent added is 0.0 with respect to 100 parts by weight of the resin.
The amount is 5 to 20 parts by weight, preferably 0.1 to 15 parts by weight, but can be appropriately selected depending on the type of the resin, the shape of the molded product, and the desired expansion ratio.

As a method of mixing the resin composition in the present invention, a resin and a foaming agent are mixed using a general blender, a mixer, or the like, and the mixture is put into a hopper of an injection molding machine.
At this time, commonly used pigments, antioxidants, lubricants, antistatic agents, weatherability improvers, fillers, and the like can be appropriately added within known use amounts. However, when a physical foaming agent is used, it is necessary to use a system that presses directly into the injection cylinder.

[0017]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but it should not be construed that the invention is limited thereto. 1) Resin used PP-1: HIPP MFR 60g / 10min, melting point
163 ° C. (manufactured by Nippon Polyolefin Co., Ltd.) PP-2: HIPP MFR 7 g / 10 min, melting point 1
62 ° C. (manufactured by Nippon Polyolefin Co., Ltd.) PP-3: Homo PP MFR 20 g / 10 min, melting point
163 ° C. (manufactured by Nippon Polyolefin Co., Ltd.) PP-4: Homo PP MFR 400 g / 10 min, melting point 162 ° C. (manufactured by Montell JPO) PP-5: Random PP MFR 42 g / 10 min, ethylene content 1.8% by weight, Melting point 150 ° C (manufactured by Japan Polyolefin) PP-6: HIPP MFR 0.3 g / 10 min, melting point 159 ° C (manufactured by Japan Polyolefin) PP-7: Homo PP (high melt strength type)
MFR 4g / 10min, melting point 162 ° C (Montel J
2) Foaming agent Servon P manufactured by Eiwa Kasei Kogyo Co., Ltd. having a decomposition start temperature of 160 ° C. was used as a decomposable foaming agent.

3) Injection molding machine The following injection molding machine was used for polypropylene foam injection molding using a decomposable foaming agent. A mold clamping control mechanism and a mold were modified and used based on an injection molding machine manufactured by Nippon Steel Corporation with a mold clamping pressure of 150 tons. In addition, a molding experiment was performed by attaching a tray-shaped mold having an outer diameter of about 150 mm shown in FIG. The mold used was capable of controlling the mold temperature from 20 to 5 ° C. The screw used had a volume compression ratio of 2.6. A gate valve was attached to the tip of the injection cylinder and used for the experiment. As for molding conditions, molding was performed only by primary injection.

The mold opening amount referred to in the present invention is a value indicating what percentage of the cavity volume before the mold is opened is increased. For example, if the cavity volume is 50 cc and the mold opening amount is 200
%, The cavity volume after opening the mold is 150 cc
It means that it became. A mold clamping amount of 10% means that the mold is clamped for 10% of the increased volume of the cavity, that is, 10 cc in the above example.

Examples 1 to 15 Using the resins shown in Table 1, foam injection molding was carried out in a tray-shaped mold shown in FIG. 1 (the molded product thickness was 3.0 mm when the mold was clamped). FIG. 2 shows a state where the movable mold is retracted after the injection is completed. A of FIG. 2 represents a mold opening distance. Table 1 shows the molding conditions, the expansion ratio of the obtained foam, the appearance, and the percentage of defective products after 100 shot molding. In addition, the appearance was evaluated visually by the following four grades of the molded article. ◎ ··· Beautiful with skin layer (level on which curved surface printing is possible) 〇 ··· ◎ Not as good, but at a level that can be used as a product △ ··· As a certain application, there may be fields where it can be used X: A level that cannot be used as a product. Further, the defective rate was evaluated by the following four steps by molding continuously for 100 shots. ◎… Defective rate is 2% or less ○… Defective rate is more than 2 and 5% or less △… Defective rate is more than 5 and 10% or less ×… Defective rate is more than 10%

Examples 16 and 17 Using PP-1, foam injection molding was attempted in a tray-shaped mold (having a thickness of 1.0 mm when the mold was clamped) shown in FIG. The volume was then expanded after a 2 second waiting time before expanding the mold volume. Table 1 also shows the conditions of other molding experiments, the expansion ratio of the obtained foam, the appearance, and the ratio of defective products after 100 shot molding.

Comparative Example 1 Using PP-2, molding was performed without opening the mold. Conditions of other molding experiments and expansion ratio, appearance,
Table 1 shows the percentage of defective products when the 00 shot molding was performed. The expansion ratio was 1.1 times, and almost no foaming occurred.

Comparative Example 2 Using PP-2, molding was performed at a molding temperature of 230 ° C. Table 1 shows the other molding conditions, the expansion ratio of the obtained foam, the appearance, and the ratio of defective products after 100 shot molding.
Shown in The foaming ratio varied widely from 1.3 to 1.6 times, the appearance also varied greatly, and the defective rate exceeded 30%, and continuous stable molding could not be performed.

Comparative Example 3 Using PP-6, molding was carried out at a molding temperature of 190 ° C. Table 1 shows the other molding conditions, the expansion ratio of the obtained foam, the appearance, and the ratio of defective products after 100 shot molding.
Shown in Due to short shots (insufficient filling), molding experiments could not be performed.

Comparative Example 4 Using PP-6, molding was carried out at a molding temperature of 230 ° C. Table 1 shows the other molding conditions, the expansion ratio of the obtained foam, the appearance, and the ratio of defective products after 100 shot molding.
Shown in Although the short shot (insufficient filling) was resolved, the expansion ratio was only 1.4 times, and the appearance was poor.
The defect rate was high, and the molding was not stable.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

The polypropylene foam molded article obtained by the production method of the present invention has an expansion ratio of 1.5 times or more, has a relatively small thickness, and has a beautiful appearance.
Particularly, it is suitable and useful for instant food containers, food trays and the like, and automobile interior parts.

[Brief description of the drawings]

FIG. 1 is a mold sectional view showing a mold clamping state in a method of the present invention.

FIG. 2 is a mold sectional view showing a mold open state when a movable mold is retracted in the method of the present invention.

[Description of Signs]: Fixed mold: Movable mold: Injection nozzle A: Indicates the mold opening distance after the completion of injection.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kenji Takemura 1-7-13 Shiba Park, Minato-ku, Tokyo Showa Denko Plastic Products Co., Ltd. (72) Mitsunobu Machida 1-7- Shiba Park, Minato-ku, Tokyo 13 F-term in Showa Denko Plastic Products Co., Ltd. (reference) 4F202 AA11 AB02 AG03 AG07 AG20 AH26 AH55 AH56 AH58 CA11 CB30 CK18 CK19 4F206 AA11 AB02 AG03 AG07 AG20 AH26 AH55 AH56 AH58 JA04 JL22 JN26 JN25

Claims (5)

[Claims] 1. A fixed mold coupled to a nozzle of an injection molding machine, and a movable mold capable of moving forward and backward to an arbitrary position with respect to the fixed mold, and having an MFR of 0 in a clamped state. 0.5 to 1000 g / 10 min of a blowing agent-containing polypropylene at a temperature of at least 30 ° C higher than the melting point of the polypropylene.
After the injection is completed, the movable mold is retracted, the volume of the cavity is expanded to a predetermined volume and foamed, and further, an unfoamed layer is generated on the foam surface, and
An injection molding method for a polypropylene foam, wherein the expansion ratio is 1.5 times or more. 2. The injection molding method for a polypropylene foam according to claim 1, wherein a waiting cooling time after the completion of the injection until the movable mold is retracted is 0 to 20 seconds. 3. The injection molding method for a polypropylene foam according to claim 1, wherein the volume of the cavity is reduced so that the foaming volume is reduced by 2 to 30% after the foaming is completed. 4. The injection molding method for a polypropylene foam according to claim 1, which is a thin foamed product having a thickness of 3 mm or less. 5. A food container obtained by the method according to claim 1.