JPH0623780A - Injection molding method - Google Patents

Abstract

PURPOSE:To form a hollow part without adversely affecting surface characteristics in the vicinity of a gas channel of a molded item and generating an uneven gloss, esp., on a grained molded item by a method wherein after a hollow area is formed in a molding space filled with an injected melt resin by blowing a gas at a relatively low pressure, a gas is blown at a relatively high pressure. CONSTITUTION:After the start of the charging of a melt resin in a molding cavity by injection, a resin pressure increases along a curve Lp to reach a maximum value Pmax. At approximately the same time, gas blowing is started at a pressure Pg1 lower than a normal value as shown by a curve Lg for forming a hollow part (gas channel) We in a thick-wall part Wa of a molded item W. After a surface layer is set by a predetermined thickness after the elapse of a time Ta, a gas is blown at a normal gas blowing pressure Pg2 for expanding the gas channel Wc in diameter. In this manner, since the gas is blown at the secondary pressure Pg2 after the surface layer is set, the surface layer is prevented from displacing with respect to a molding surface.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for injection molding a molded product in which a predetermined portion is hollow.

[0002]

2. Description of the Related Art Conventionally, as a method of molding a molded product having a hollow portion at a predetermined portion by injection molding, a predetermined amount of gas measured in advance after or during injection of a molten resin into a mold molding space. By injecting into the molding space with a predetermined pressure, a hollow portion is formed inside the resin portion forming the predetermined portion of the molded product, and the resin is cooled to some extent so that the surface layer has self-supporting property. There is known a method of obtaining a molded article in which a predetermined portion is formed in a hollow shape by keeping the inside of the hollow portion in a pressure state and then releasing the gas in the hollow portion (for example, Japanese Patent Publication No. 3-47171). reference).

By adopting such a molding method, the inside of the hollow portion is kept in a pressure state until the resin surface layer of the predetermined portion obtains self-supporting property, and this pressure causes the resin surface portion of the predetermined portion to be molded. Since it is possible to press against the surface, it is possible to effectively generate the so-called “sink” due to the volume shrinkage of the resin due to cooling after injection, which has been a problem especially in the thick part of the molded product. It is possible to prevent this, and it is possible to solve the problem of the defective appearance due to the “sink”. Further, it is possible to form a hollow portion inside a thick portion or the like according to the amount of gas injected into the resin, and it is possible to reduce the product weight (and therefore the amount of resin material used) by that amount. Also has.

By the way, at least a part of the surface of the resin molded product is formed into a very fine unevenness (diaphragm) shape, thereby suppressing the gloss of the surface and making it a matte state. Molded articles are generally well known in the past. When injection-molding this squeeze-molded product, by applying the above-mentioned injection molding method, a specific portion such as a thick-walled portion is made hollow and the occurrence of "sink" at the portion is prevented,
Alternatively, the weight of the molded product can be reduced.

[0005]

However, when the above injection molding method is applied to such a squeezed molded article, the squeezed surface is in the vicinity of a hollow portion (so-called gas channel) formed by injecting gas. There is a problem that the part becomes glossy compared to the surrounding squeezing surface, and uneven gloss may occur. That is, in this injection molding method, after the injection and filling of the molten resin into the mold molding space is started, as shown in the graph of FIG. 6, for example, the injection step of the molten resin (the curve in the graph of FIG. 6 is performed. Lp ') is completed and the resin pressure reaches its maximum value Pmax' (for example, about 200 kg / cm ² ), as indicated by the curve Lg 'in the graph of FIG. The gas injection by the pressure Pg '(for example, 70 kg / cm ² ) is started. And
When high-pressure gas is injected into the molten resin, a high shearing force acts on the resin surface layer portion in contact with the molding surface in the direction along the molding surface. If this high shearing force acts before it solidifies to a certain extent, the resin surface layer moves slightly (for example, about several microns [μm]) with respect to the mold surface that molds the drawing surface and slips. Occurs, and the microscopic surface condition becomes smoother than other parts. As a result, it is considered that this portion becomes more glossy than the surrounding squeezing surface.

Even when the molded product is not the above-mentioned drawn product, when the wall thickness of the hollow part is relatively thin or the gas injection pressure is relatively high, the hollow part is still required. Before the resin surface layer of is solidified to a certain extent or more, due to the high shearing force due to the injection of high pressure gas,
It is considered that the molding of the hollow portion becomes difficult, or the surface quality of the portion near the gas channel of the molded product is adversely affected.

The present invention has been made to solve the above problems, and when molding a molded product having a hollow predetermined portion, the hollow portion can be formed without adversely affecting the surface properties and moldability of the predetermined portion. It is an object of the present invention to provide an injection molding method capable of forming a hollow portion and, in particular, forming a hollow portion in a drawn molded product without causing uneven gloss on the pressed surface.

[0008]

Therefore, according to the first invention of the present application, in a method of injection molding a molded product in which a predetermined portion is hollow, a molding space formed by closing a pair of molding dies is provided. By injecting the molten resin and then injecting a gas into the molding space at a relatively low first pressure, a hollow region is formed in a predetermined portion of the resin filled in the molding space, and thereafter, The gas is injected into the hollow region at a relatively high second pressure at a predetermined timing with respect to the gas injection work.

The second invention of the present application is characterized in that, in the first invention, the molded product is a squeezed molded product in which at least the surface of the predetermined portion is a squeezing surface. is there.

[0010]

According to the first invention of the present application, a gas is injected into the molding space at a relatively low first pressure (low pressure) to form a hollow region in a predetermined portion of the resin filled in the molding space. After forming the, at a predetermined timing for the gas injection work,
Since the gas is injected into the hollow region at a relatively high second pressure (high pressure), after the gas is injected at the low pressure, the resin surface layer portion in the hollow region is solidified to a predetermined thickness at the high pressure. By setting the gas injection timing to perform the gas injection of, by the action of shearing force due to the gas injection at high pressure,
It is possible to prevent the surface properties and moldability of the predetermined portion from being adversely affected.

Further, according to the second invention of the present application, basically, the same effect as that of the first invention can be obtained,
In particular, in the case where the molded product is a squeezed molded product in which at least the surface of the predetermined portion is a squeezed surface, after the gas injection at the low pressure, the resin surface layer portion in the hollow region is solidified to a predetermined thickness. By setting the gas injection timing so as to perform the gas injection at the high pressure, it is possible to prevent the resin surface layer from moving with respect to the mold molding surface due to the action of the shearing force due to the gas injection at the high pressure. That is, the surface of the molded product is not rubbed by the molding surface of the part corresponding to the gas channel on the narrowed surface, and this part is more glossy and uneven in gloss than the surrounding narrowed surface. Disappears. This can prevent the appearance of the squeezing surface from being impaired.

[0012]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. FIG. 1 is an overall configuration diagram for explaining the outline of the injection molding apparatus according to the present embodiment. As shown in this figure, the injection molding apparatus 1 is fixed to a stationary mold plate 5 by fixing. The mold apparatus 2 is configured with a mold 3 and a movable mold 4 attached to a movable mold plate 6 as main parts. The fixed mold plate 5 is fixed to the injection molding machine (not shown) side, while the movable mold plate 6 is joined to, for example, a hydraulic cylinder device (not shown), and the cylinder device is driven. Thus, the movable die 4 can be moved back and forth with respect to the fixed die 3. Then, the movable mold 4 is advanced to the fixed mold 3 side to close the two molds 3 and 4 to form a predetermined molding cavity 7. In addition, at the time of injection molding, a runner portion (not shown) and a gate portion (not shown) forming a passage of the molten resin to the molding cavity 7 are simultaneously formed.

Inside the fixed mold 3, a gas injection nozzle 11 (gas nozzle) for injecting gas into the molding cavity 7 is provided. The gas nozzle 11 is formed by closing the molds 3 and 4 together. In this state, the tip end portion is positioned so as to face a runner portion (not shown) connected to the molding cavity 7, for example. The gas nozzle 11
A hydraulic cylinder 12 for driving a nozzle that moves the gas nozzle 11 forward and backward with respect to the fixed die 3 is attached to the. In addition, a heater 13 for heating is provided near the tip of the gas nozzle 11 for the purpose of heating the gas passing through the inside of the gas nozzle 11 to minimize the temperature difference from the molten resin.
Is attached.

Although not specifically shown, an injection molding machine (not shown) is provided inside the fixed mold 3 at a position separated from the gas nozzle 11 by a predetermined distance in the paper surface direction in FIG. Is provided with a resin supply nozzle for supplying a molten resin into the molding cavity 7, and the molten resin injected from the injection molding machine (not shown) is hot runner (not shown) in the fixed mold 3. And the resin supply nozzle to sequentially flow into the runner portion (not shown), and the molding cavity 7 is filled from this runner portion through the gate portion (not shown). Since both the resin supply nozzle and the gas nozzle 11 are the same as those well known in the related art, detailed description and illustration of the internal structure thereof are omitted.

The gas nozzle 11 has a gas supply device 2
A gas pipe 29 extending from 1 is connected. The gas supply device 21 includes, for example, a gas supply source 22 formed of a gas cylinder, a gas measuring cylinder 23 for measuring the amount of gas supplied to the mold device 2, and a driving device for driving the measuring cylinder 23. A cylinder chamber 24 and a main part, and a measuring chamber 23a of the gas measuring cylinder 23.
Is connected to the gas supply source 22 through the flow control valve 25 and the pressure adjusting valve 26 in this order. An opening / closing control valve 28 is provided on the outlet side of the gas metering cylinder 23.

As the above gas, for example, air can be used, but in the present embodiment, a relatively inexpensive and inert nitrogen gas is more preferably used. Theoretically, it is possible to use not only gas (gas) but also other kinds of fluid such as liquid as long as it is inert to the resin material. When injecting and filling the molten resin material into the molding cavity 7 of the mold apparatus 2, the gas measuring cylinder 23 controls the amount of gas injected into a predetermined portion of the portion constituting the product portion of the injected molten resin. The amount of gas is to be measured in advance, and the amount of gas is determined as, for example, an amount sufficient to compensate the shrinkage volume of the resin at the time of cooling after completion of injection, or an amount sufficient to reduce the weight of the molded product to a necessary degree. Further, the gas pressure in the measuring chamber 23a is sufficient to form a hollow portion in the molten resin material injected and filled in the molding cavity 7, and this gas is injected into the molten resin to form a hollow portion. Is set as a pressure that does not break the resin material surrounding the hollow portion when the pressure control valve 26 is formed. In the present embodiment, as will be described later in detail, this pressure setting can be switched in two steps depending on the timing.

Gas is injected into the molten resin in the molding cavity 7 by using the gas supply device 21 and the gas nozzle 11 to form a hollow portion at a predetermined portion such as a thick portion of the molded product and a surface layer of the resin. By keeping the inside of the hollow portion in a pressure state until the self-supporting property is obtained, the resin surface portion can be pressed toward the mold molding surface by this pressure. As a result, in particular, for the thick part of the molded product, the so-called, due to the volume contraction of the resin accompanying cooling after injection,
It is possible to effectively prevent the occurrence of a defective appearance due to "sink". Further, a hollow portion corresponding to the amount of gas injected into the resin can be formed inside the molded product, and the product weight (and therefore the amount of the resin material used) can be reduced by that amount.

By the way, in the present embodiment, at least a part of the surface of the molded product, for example, one surface formed on one side by the movable mold 4 is formed into a very fine uneven (diaphragm) shape, Suppressed the gloss of the surface and made it matte,
It is designed to be a so-called squeezing surface. Therefore, the molding surface of the movable die 4 is provided with a large number of irregularities whose size, height, etc. are limited within a predetermined range, for example, over substantially the entire surface. The shape, size, etc. of the unevenness are determined according to how much the matting state of the narrowing surface is to be set.For example, in the case of a bumper face of an automobile, generally, the surface side of the narrowing surface is The size of the convex portion to be formed is set to be minute, for example, on the order of hundreds of microns (μm) or tens of microns.

In this embodiment, as shown in FIG. 2, at least the upper surface Wf of the molded product W having a substantially rectangular parallelepiped shape is a drawing surface, and the peripheral edge portion on the upper surface Wf side has a relatively thick frame shape. Has been formed. Then, the thick portion Wa is formed into a hollow shape by gas injection. In addition, in the present embodiment, the thick portion Wa has a long length in place of its cross-sectional area, and as described below,
Gas injection work into the thick portion Wa has different pressures 2
It is considered that the smooth formation of the hollow portion (gas channel) is restricted due to the fact that the gas injection in the first stage is performed at a relatively low pressure because it is divided into stages. For this reason, more preferably, for example, a plurality of plurality provided at a predetermined location apart from each other.
Gas injection is performed by the (two) gas nozzles 11. In this case, gas injection can be performed at different timings depending on the region.

Next, the injection molding method for molding the molded article W shown in FIG. 2 using the injection molding apparatus 1 having the above-described structure, that is, the mold apparatus 2 and the gas supply apparatus 21,
This will be described with reference to the graph of FIG. First, movable type 4
Is moved forward to close the fixed mold 3 and the molding cavity 7
To form. Next, injection / filling of the molten resin into the molding cavity 7 is started. Then, as shown by the curve Lp in the graph of FIG. 5, the injection process of the molten resin is completed, and the resin pressure reaches its maximum value Pmax (for example, about 200 kg).
/ Cm ² ), as indicated by the curve Lg in the graph of FIG. 5, the primary pressure Pg1 (which is considerably lower than usual) by the gas nozzle 11 is almost simultaneously or immediately after (about 1 second in this embodiment). For example, gas injection at 20 kg / cm ² ) is started. By the gas injection at the primary pressure Pg1, a hollow portion Wc (gas channel) is formed in the thick portion Wa of the molded product W as shown in FIG. The gas channel Wc is somewhat thin at this point because the gas injection pressure is relatively low.

After starting the gas injection at the primary pressure Pg1 (pressurization time: for example, about 1 to 3 seconds), when the predetermined time Ta required for the resin surface layer portion to solidify to a predetermined thickness elapses, the gas injection pressure The setting has been changed, and the "thinning" of the thick part Wa
As a pressure sufficient to prevent this, gas injection at a secondary pressure Pg2 (for example, 70 kg / cm ² ) which is a normal gas injection pressure is started. As shown in FIG. 4, the gas channel Wc is expanded by the gas injection at the secondary pressure Pg2. In the case of the present embodiment, the solidification of the resin surface layer portion starts about 2 to 3 seconds after starting the gas injection at the above-mentioned primary pressure Pg1 and is about 15
Since the solidification of the surface layer ends after about a second, the predetermined time Ta may be set to an appropriate time during this period. In this way, by injecting the gas at the secondary pressure Pg2 in the state where the resin surface layer portion is solidified to a predetermined thickness, the resin surface layer acts on the molding surface of the mold by the action of the shearing force due to the gas injection. It is possible to prevent the movement.

After that, until a predetermined time (for example, about 20 seconds) elapses after starting the gas injection at the secondary pressure Pg2, that is, the resin is cooled to a certain extent or more, and the surface layer thereof has a self-supporting property. Up to this point, the inside of the hollow portion Wc is kept in a pressure state by the gas pressure. When the surface layer of the molded product W is cured to some extent and self-supporting property is obtained, at a predetermined timing, for example, the hydraulic cylinder 1 for driving the nozzle.
By activating 2 to retract the gas nozzle 11, the gas in the hollow portion Wc is released to the outside. After that, the movable mold 4 is retracted to open the mold, and the molded product W is taken out from the mold device 2.

As described above, according to this embodiment, a gas is injected at a low primary pressure Pg1 into the molding cavity 7 in which the molten resin is injected and filled, and the hollow region is formed in the thick wall portion Wa. After the formation of the gas, the gas injection timing was set so that the gas was injected at a high secondary pressure Pg2 in a state where the resin surface layer portion in the hollow region was solidified to a predetermined thickness. By this action, it is possible to prevent the resin surface layer from moving with respect to the molding surface of the mold. That is, the surface of the molded product is not rubbed by the mold molding surface in the portion of the squeezing surface Wf corresponding to the gas channel Wc, and this portion is more glossy and uneven in gloss than the surrounding squeezing surface Wf. It will not occur. This makes it possible to prevent the appearance of the squeezing surface Wf from being impaired.

Although the above-mentioned embodiment was applied to the case where the molded product was a so-called wrought molded product W, the method of the present invention can be applied even when the molded product is not such a wrung molded product. The effect of preventing the surface properties and the moldability of the hollow portion of the molded product from being adversely affected by the action of the shearing force due to the high-pressure gas injection can be obtained.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram schematically showing a mold device and a gas supply device of an injection molding device according to an embodiment of the present invention.

FIG. 2 is a perspective view schematically showing the outer shape of a molded product according to the above embodiment.

3 is a cross-sectional explanatory view in the AA direction of FIG. 2 showing a gas channel by injecting gas at a primary pressure.

FIG. 4 is a cross-sectional view taken along line AA of FIG. 2 showing a gas channel by gas injection at a secondary pressure.

FIG. 5 is a graph showing an example of gas pressure change characteristics in the injection molding method according to the present embodiment.

FIG. 6 is a graph showing an example of gas pressure change characteristics in an injection molding method according to a conventional example.

[Explanation of symbols]

 1 ... Injection molding device 2 ... Mold device 7 ... Molding cavity 11 ... Gas nozzle 21 ... Gas supply device Pg1 ... Primary pressure Pg2 ... Secondary pressure W ... Molded product Wa ... Thick part Wc ... Gas channel Wf ... Constricted surface

Claims (2)

[Claims] 1. A method for injection-molding a molded product having a hollow predetermined portion, wherein a molten resin is injected into a molding space formed by closing a pair of molding dies, and then the molding space is injected into the molding space. By injecting the gas at a relatively low first pressure, a hollow region is formed in a predetermined portion of the resin filled in the molding space, and then at a predetermined timing with respect to the gas injection operation, An injection molding method, characterized in that gas is injected into the hollow region at a relatively high second pressure. 2. The injection molding method according to claim 1, wherein the molded product is a squeezed molded product having at least the surface of the predetermined portion as a squeezing surface.