JP2000000865A - Method for injection molding plastic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for injection molding to obtain a plastic product without weld line without impairing a molding time. SOLUTION: The method for injection molding a plastic product comprises the steps of rapidly heating a mold surface brought into contact with the product to a softening temperature or higher of a material resin in a short time, filling a resin in a mold 1, holding the temperature while a resin inner pressure for pressing the resin surface to the mold surface exists, and then controlling the mold temperature to rapidly cool the mold surface in a short time, thereby obtaining a plastic product having a high strength without weld line at a low pressure.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plastic injection molding method for preventing a weld line from being formed on the surface of a molded product. More specifically, the present invention relates to a plastic injection molding method for performing molding by heating the surface temperature of a mold to a temperature equal to or higher than the softening temperature of a material resin in order to prevent the occurrence of weld lines.

[0002]

2. Description of the Related Art In conventional plastic injection molding in which a mold surface is controlled at a low temperature and a constant temperature, when two or more flow ends of a material resin such as a molten thermoplastic resin join together, the resin flow ends are joined to each other. Due to the incompatibility,
A defect called a weld line having a linear appearance occurs, which causes insufficient strength or poor appearance of the product. This is because the hard solidified layer on the resin surface hinders the deformation of the resin due to the internal pressure and prevents the resin surface from sticking to the mold surface.

[0003] The weld line portion of such a product has low strength because the resins are not fused together, and cannot satisfy the strength required for the product, making the product worthless. In addition, since the product appears linearly on the product surface, the appearance of the product is impaired and the product value is reduced.In addition, even when the product surface is applied, uniform painting is impaired and the aesthetic appearance is impaired. There is also a problem that the cost for this is high. Also,
In an extreme case, the surface smoothness or appearance required for the product cannot be satisfied, and the product is rendered worthless.

[0004] The decrease in product strength and appearance due to such a weld line may be caused by increasing the pressure of the resin in the mold to increase the pressure of the resin against the surface of the mold (hereinafter referred to as "pressure keeping").
It can also be improved to some extent by improving the material resin itself, such as increasing the amount of fillers, rubber particles, etc., contained in the material resin.

However, increasing the internal pressure of the mold to a high pressure requires a mold having a higher strength and a large-sized molding machine capable of generating a high pressure, which increases the cost and causes the product itself to undergo internal distortion and deformation. Raises new problems. Further, an increase in the amount of fillers, rubber particles, and the like added to the resin causes a problem that the material cost is soared or the appearance of the molded article is significantly deteriorated.

The most effective effect on the weld line is to heat the mold to a temperature higher than the softening temperature of the material resin on the surface in contact with the product. This is because by setting the mold surface to a high temperature, the solidified layer of the resin in contact with the mold surface is thin and easily deformed, and the close contact with the mold surface tends to reduce the width and depth of the weld. is there.

However, simply heating to a high temperature causes new problems such as warpage and deformation of the product,
The disadvantage is that the molding cycle is extended and the product cost is increased. Conventionally, the following various devices have been proposed as devices which improve the mold temperature response and shorten the mold heating time.

[0008] A mold in which an induction coil is provided on the outer periphery of a mold (Japanese Utility Model Laid-Open No. 62-111182). A high-frequency induction coil made of a copper pipe is provided in a mold so that cooling water flows in the pipe (Japanese Patent Laid-Open No. 63-1).
No. 5707). One in which a movable insert provided with an electric heater can be taken in and out of a mold (JP-A-63-15719). A device for rapidly heating and cooling the mold temperature adjusting medium or for switching the heated / cooled medium to the mold and supplying the medium to the mold outside the mold, and supplying the supplied heating and cooling medium to the temperature adjusting medium in the mold. A mold which can be heated and cooled by passing through a circulation path (Japanese Patent Application Laid-Open Nos. 62-15707 and
JP-A-2-208918, JP-B-7-25115).

[0009]

As described above, the weld line of a product can be improved by heating the surface through which the resin material of the mold flows to a high temperature. However, if the heating and cooling take time, the molding cycle can be improved. As a result, the productivity is reduced and the product cost is increased. In addition, a new defect such as warpage occurs in the product itself. In addition, if the area where the mold can be heated and cooled is limited, the difference in the temperature response during heating and cooling depending on the area of the mold is large, and if the mold surface temperature distribution is not uniform, the area of the weld line should be changed. Not enough heating,
When the position of the weld line shifts, it is impossible to cope with changes in molding conditions, wall thickness, and the like.

[0010] In addition, the method of heating the mold surface has the following problems in the conventional method. In the case where the induction coil is provided, there are many restrictions on assembling in the mold, and the mold becomes large. In the case where the movable insert having the electric heater is provided, the insertion portion of the movable insert must be provided in the mold, which complicates the mold structure. Further, there is no versatility of the heating means. Since cooling is based on natural cooling, the molding cycle becomes longer. Heating and cooling medium from the external device through the temperature control medium circulation path in the mold to heat and cool the mold surface
It is versatile and the mold structure does not require special processing.
However, in the case of using a temperature adjusting medium circulation path in a mold in a normally provided category, heat loss occurs due to a large heat capacity of a mold portion to be heated and cooled, and cooling takes time. In addition, since the response of the mold surface temperature is finally slow, there is a problem that the temperature control accuracy is deteriorated.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method of injection molding a plastic product for obtaining a high-strength product without a weld line at a low pressure.

[0012]

Means for Solving the Problems To solve the above problems, the present inventors have proposed a molding method in which the temperature of a mold in contact with a product is raised to a high temperature, and a mold structure having good temperature responsiveness and uniform temperature distribution. , Earnestly studied the manufacturing method. As a result, in injection molding, the mold surface in contact with the product is quickly heated to a temperature higher than the softening temperature of the material resin in a short time, the resin is filled in the mold, and the resin internal pressure that presses the resin surface against the mold surface is increased. A plastic product that maintains this temperature while it is present and performs a mold temperature cycle to rapidly cool the mold surface in a short time to obtain a high-strength product without weld lines without extending the molding cycle Completed the injection molding method.

[0013] The structure and manufacturing method of a mold that enables rapid heating and cooling of the mold surface and uniform temperature distribution of the mold surface have been disclosed in the past by the present inventors in a patent application (Japanese Patent Application No. 8-184441). In addition, this molding method is not only a general injection molding method,
It is also applicable to gas assist molding, compression molding and the like.

[0014] To achieve the above object, the present inventors
According to the first aspect of the present invention, in injection molding of a plastic product, the surface temperature of the mold in contact with the product is rapidly heated to a temperature equal to or higher than the softening temperature of the material resin in a short time, and the resin is filled in the mold. This temperature is maintained as long as there is an internal resin pressure that presses the resin surface against the mold surface, and mold temperature control is performed to quickly and quickly cool the mold surface temperature, and high-strength plastic products without weld lines Is obtained at a low pressure.

Further, in the invention according to claim 2,
According to the first aspect of the present invention, the mold surface portion for controlling the heating and cooling temperatures at high speed is limited to only the mold surface in contact with a portion where a weld line is formed on the product surface.

Further, according to the third aspect of the present invention,
In the invention according to claim 1 or 2, when the temperature of the mold surface is rapidly heated and cooled, the range of the mold surface to be temperature-controlled is divided into a plurality of areas, and different heating temperatures, cooling temperatures or temperature control times are set. It is characterized by being molded by.

The resin applicable to the molding by the mold of the present invention is not limited to general-purpose resins such as polyolefin resin, polystyrene resin and ABS resin, industrial resins such as polycarbonate resin and polyamide resin, but also various resins. Or reinforcing material, or metallic as a design,
Organic and inorganic fillers for giving a stone-like feeling, and those containing a foaming agent can also be used.

[0018]

As described above, in the injection molding, the mold surface in contact with the product is rapidly heated to a temperature higher than the softening temperature of the material resin in a short time to fill the mold with the resin, and the resin surface is molded. When a molding method that maintains this temperature while there is internal resin pressure that presses the surface and cools quickly in a short time is used, it is heated to a higher temperature than the molding surface at the conventional low mold surface temperature. The solidified layer on the resin surface that is in contact with the surface of the mold is difficult to develop and is easily deformed thinly. Therefore, the resin surface easily adheres to the mold surface due to the internal pressure of the resin, and a high-strength product having no weld line can be obtained.

Moreover, since the resin surface is easily deformed,
The internal pressure of the resin required to press the resin surface against the mold surface and adhere to it is low, and the resin solidified layer is thin, so the pressure loss of the resin is small and molding with a lower mold internal pressure than conventional molding methods Is possible.

In particular, the reason why it is essential to rapidly heat and cool the mold surface, which is a feature of the present invention, to minimize the time required for the resin to be exposed to the high-temperature mold surface is as follows. Details will be described. If the material resin is exposed to the high-temperature mold surface for an unnecessarily long time, the amount of shrinkage of the material resin further increases, and due to post-shrinkage, fillers such as glass, rubber particles such as butadiene, etc. are mixed into the resin. Is left on the resin surface, and poor transferability recurs. Further, when the mold surface is molded at a high temperature, if the cooling is slow, the product is likely to be deformed such as warpage. Moreover, if the heating and cooling time of the mold surface cannot be achieved in a short time, the molding cycle is extended, mass productivity is impaired, and the cost is increased.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The construction of a mold used in the present invention according to the first and second aspects will be described in detail below. FIG. 1 is a schematic view showing one embodiment of a plastic injection mold for performing the molding method of the present invention. The plastic injection mold 1 shown in FIG. 1 is roughly divided into a core 2 (product invisible surface side) and a cavity 3 (product visible surface side), and both are connected to an injection molding machine (not shown). The core 2 and the cavity 3 can be opened and closed. A recess 5 corresponding to the outer shape of the product 4 is formed in the core 2 and the cavity 3.
The molten plastic is poured into the concave portion 5 from an injection molding machine (not shown) through a sprue 6 provided at a predetermined position of the cavity 3, and the product 4 is molded.

The product visible side of the product 4 is heated to a high speed and a high temperature during molding by a medium flowing in a mold temperature adjusting medium circulation path 8 in a mold surface high-speed heating and cooling insert 7. After cooling and solidifying, the plastic injection mold 1
Taken out of The mold temperature adjusting medium circulation path 8
The medium flowing through the inside is provided with a medium temperature controller that includes a high-frequency heater and a cooler (not shown) and can perform high-speed heating and cooling. Reference numeral 15 denotes a projection provided on a part of the core 2 for forming a boss on the molded product, and has a structure for pushing off the cavity 3.

FIGS. 2, 3 and 4 show the insert 7 for high-speed heating and cooling of the mold surface shown in FIG. The mold surface electroformed layer 9 that is in contact with the visible surface of the product is formed of Ni—Cr, and a mold temperature adjusting medium circulation path 8 is formed inside. The mold temperature adjusting medium circulation path 8 forms a conduit with a lost wax on the inner surface of the mold of the mold surface electroformed layer 9, coats the surface with the electroformed layer, and then elutes the lost wax. And is formed integrally with the mold surface electroformed layer 9.

The sectional shape of the mold temperature adjusting medium circulation path 8 is a square of 5 mm × 5 mm, and the pitch is 12 mm.
The distance between the mold temperature adjusting medium circulation path 8 and the surface of the mold insert 7 for high-speed heating and cooling of the mold surface in contact with the product was 2 mm. Also, on the back surface of the mold temperature adjusting medium circulation path 8,
A heat insulating layer 10 composed of epoxy and a reinforcing material having a thickness of about 2 mm was provided, and a heat capacity was reduced by thermally isolating a mold portion for rapid heating and cooling.

[0025]

EXAMPLE 1 The apparatus shown in FIG.
560, softening temperature 97 ° C, manufactured by Idemitsu Petrochemical Co., Ltd.).
Injection molding was performed by heating and cooling at 60 ° C. and 60 ° C. at high speed during cooling (Experiment 1). Also, the molding cycle (injection ~
65 seconds for holding pressure-cooling-removal), and the time required for heating is 2
0 seconds, and the time required for cooling was 25 seconds.

No weld line was visually observed on the obtained product surface. In particular, the weld line could not be seen on the downstream side of the boss where the weld line is likely to occur. FIG. 5 shows the measurement results of the weld depth of the obtained product. As is clear from FIG. 5, the depth of the weld line was almost 0 at any distance from the origin of the weld line, and it was confirmed that no weld line was generated. In addition, the condition was almost the same regardless of the presence or absence of the holding pressure, and a molded product that did not require a holding pressure and had a low pressure and no weld was obtained. When the weld line generated portion of the product was measured with an impact tester, the energy required for breaking was 3.1 kgf · cm.

[0027]

COMPARATIVE EXAMPLE 1 Injection molding was performed using the same experimental apparatus and resin as in Example 1, except that the surface temperature of the mold on the visible side of the product was set at 60.degree. At this time, the mold surface temperature was controlled to a constant temperature of 60 ° C. from filling of the resin to cooling. At this time, the molding cycle (injection-holding pressure-cooling-
Take-out) was 65 seconds. A weld line was clearly and visually confirmed on the obtained product surface. FIG. 5 shows the measurement results of the weld depth.

In the comparative example, the weld line was measured over a range of about 8 mm from the origin of the weld line. In addition, as a result of measuring the energy required at the time of destruction of the weld line generating portion of the product in the same manner as in Experiment 1,
It was 2.2 kgf · cm.

[0029]

COMPARATIVE EXAMPLE 2 An ordinary mold structure in which a mold surface high-speed heating / cooling insert 7 of Example 1 is made of steel material S55C and a mold temperature adjusting medium circulation path 8 is machined into two lateral holes. The same apparatus and resin as in Example 1 were used except that the steel material was replaced with a steel material insert 13 simulating the above.
Molding was performed under temperature control conditions of cooling to ℃.

No weld line was visually observed on the obtained product surface. However, the time required for heating at this time was 39 seconds, the cooling time was 40 seconds, and the molding cycle was 84 seconds.
Seconds and 29% longer than in Example 1. When the molding cycle was set to 65 seconds as in Example 1, the mold surface temperature was as low as 82 ° C., and a weld line was clearly observed.

[0031]

Embodiment 2 This embodiment corresponds to claim 3, and FIG.
In the cavity 24 of FIG. 7 which is in contact with the gate side weld line generation site 22 and the terminal side weld line generation site 23 of the molded product 20 having the punched shape of FIG. Insert 25
Incorporating the insert 26 for high-speed heating and cooling of the die surface on the end side and impact-resistant polystyrene (HT560, softening temperature 9)
7 ° C., manufactured by Idemitsu Petrochemical Co., Ltd.), the surface temperature of the mold surface high-speed heating and the cooling mold pieces 25 and 26 at the time of resin filling are 80 ° C. and 100 ° C., respectively. Injection molding was performed by heating and cooling. The molding cycle (injection-holding pressure-cooling-extraction) at this time is 68
Seconds, heating time is 21 seconds, cooling time is 2
5 seconds. No weld line was visually observed on the obtained product surface. The weld depth of the obtained product was 0.

[0032]

Comparative Example 3 Using the mold of Example 2, except that the insert 27 for high-speed heating and cooling of the mold surface shown in FIG. In the same manner as in Example 2, molding was carried out under the temperature control conditions in which the surface temperature of the mold on the visible side of the product at the time of filling the resin was 100 ° C. or higher than the softening temperature, and the temperature was cooled to 60 ° C. No weld line was visually observed on the obtained product surface. However, the time required for heating at this time was 45 seconds, the cooling time was 38 seconds, and the molding cycle was 100 seconds, which was 32% longer than that of Example 2.

The molding cycle was set to 68
In the case of seconds, the mold surface temperature was as low as 80 ° C., and no weld line was visually observed at the gate side weld line generation site 22, but the weld line was clearly observed at the terminal side weld line generation site 23. Was done.

[0034]

According to the present invention, a high-strength product having no weld line can be obtained at a low pressure without extending the molding cycle, and there are great merits in terms of quality and cost. The effect of the reduction is extremely large.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of an injection mold according to an embodiment of the molding method of the present invention.

FIG. 2 is a cross-sectional view of the insert for high-speed heating and cooling of the mold surface as viewed from the back side (non-product side).

FIG. 3 is a sectional view taken along line AA '.

FIG. 4 is a sectional view taken along line BB ′.

FIG. 5 is an explanatory diagram of a comparison result of the presence or absence of a weld line.

FIG. 6 is an explanatory diagram of the second embodiment.

FIG. 7 is an explanatory diagram of a cavity used in the second embodiment.

FIG. 8 is an explanatory diagram of a cavity used in Comparative Example 3.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold for plastic injection molding 2 Core 3 Cavity 4 Product 5 Concave part 6 Sprue 7 Insert for high speed heating and cooling of mold surface 8 Medium circulation path for mold temperature adjustment 9 Mold surface electroforming layer 10 Heat insulation layer 11 Mold Temperature control medium inlet 12 Mold temperature control medium outlet 13 Steel material holder 14 Steel material 15 Weld line generation projection 20 Molded product 22 Gate side weld line generation site 23 Terminal side weld line generation site 24 Cavity 25 Gate side die surface Insert for high-speed heating and cooling 26 Insert for high-speed heating and cooling of the end side mold surface 27 Insert for high-speed heating and cooling of the die surface

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hirofumi Tateyama 1-1-11 Hourai-cho, Fukushima City, Fukushima Prefecture F-term in Tohoku Munekata Co., Ltd. 4F202 AE10 AK13 AM36 AR06 CA11 CB01 CN01 CN05 CN15 CN22 4F206 AE10 AM36 AR064 JA07 JN43 JQ81

Claims (3)

[Claims] 1. In injection molding of a plastic product,
The surface temperature of the mold in contact with the product is heated to a temperature higher than the softening temperature of the material resin in a short time at a high speed, the resin is filled in the mold, and the resin pressure is applied to press the resin surface against the mold surface. A plastic injection molding method for obtaining a high-strength plastic product without a weld line by performing mold temperature control for maintaining the temperature and rapidly cooling the mold surface temperature in a short time. 2. The plastic injection molding method according to claim 1, wherein the mold surface portion for controlling the heating and cooling temperatures at high speed is limited to only the mold surface in contact with a portion where a weld line is formed on the product surface. 3. When the temperature of the mold surface is rapidly heated and cooled, the mold surface area to be temperature-controlled is divided into a plurality of areas.
3. The plastic injection molding method according to claim 1, wherein the molding is performed at different heating temperatures, cooling temperatures or temperature control times.