JP2008000911A - Upright boss molding method and molded product - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a molding method capable of certainly preventing the sink of a molded product corresponding to a boss part from an aspect of appearance in a case that the boss part is provided to the appearance back of the product in a resin injection molding method, and the molded product. <P>SOLUTION: The boss part free from a sink or cast wrinkles and having high bending strength, that is, folding resistance is molded by a casting pin and a gas assisting molding method. Further, a gas assisting method is employed to enhance the folding resistance of the boss part by providing an R-rib to the root part of the boss part. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to plastic molding having a straight boss portion by a gas assist injection molding method.

Plastic molded products are used in many fields such as transportation equipment, home appliances, and so on. Plastic molded products are mainly manufactured by injection molding. FIGS. 1 and 2 show a process of injection molding of a plastic molded product and a schematic view thereof. Plastic injection molding is the process of heating and melting a plastic material (hereinafter sometimes referred to as resin) into a fluidized state, and injecting it into a cavity (cavity) 7 of a closed mold to solidify it. It refers to a method of making a shape corresponding to a mold cavity. This injection molding is performed by an injection molding machine having a function of melting a resin, opening and closing a mold, a mold clamping mechanism, and the like. After the mold is closed and tightened, the resin 8 melted in the heating cylinder (cylinder) 5 is pressed into the mold through a cylinder nozzle 6 (hereinafter referred to as a nozzle) by a screw in the cylinder. The resin press-fitted into the mold is molded into a shape corresponding to the cavity and cooled and solidified by the mold. Next, the nozzle is retracted, the mold is opened, and the plastic molded product is released from the mold by the ejecting device. As described above, the mold has a function of cooling and solidifying the molten resin as well as molding the molten resin. Therefore, the mold is always cooled by providing cooling water holes. The mold is composed of a fixed mold 2 and a movable mold 4. The movable mold 4 has a core 10 for forming the inner surface of the molded product. The fixed side mold has a space partial cavity corresponding to the molded product and forms the outer surface of the molded product. When the mold is opened and closed, the movable mold is retracted, and when the mold is opened, the molded product is always left on the movable mold 4 side, and then the mold is released with an ejection pin (eject pin or eject sleeve). An apparatus including such a movable mold, a fixed mold, a cooling water hole, an eject pin, and the like is referred to as a mold apparatus. In order to increase the strength of the plastic molded product, plate-like ribs are attached or bosses for joining to other parts as parts are provided. These ribs and bosses are also integrally formed by a mold.

Sink marks and voids are generated by the shrinkage of the resin and impair the appearance of the product. When the resin is cooled by the mold, the resin is hardly shrunk and hardly resembles the cavity. Usually, the shrinkage of the central portion of the plane where the cooling is delayed is larger than the corner where the cooling is early, and sink marks occur. The voids are those where the surface solidifies quickly and the shrinkage amount of sink marks appears inside. FIG. 3 shows examples of sink marks and voids. Sink marks and voids tend to occur on thick ribs and bosses. In order to avoid sink marks and voids, the holding pressure is increased in the injection process of FIG. However, increasing the holding pressure increases the size of the mold and injection molding machine. In order to avoid sink marks and voids, it is desirable that the thickness of the ribs and bosses is designed to be ３ or less if it can be ½ of the plate thickness. However, if the rib or boss is thicker than that, use a slide core as shown in FIG. 4 to form a box boss, or use an inclined core as shown in FIG. 5 to prevent the occurrence of sink marks and voids. It is out. However, attaching a slide core or an inclined core to the mold apparatus complicates the mold and increases the manufacturing cost of the mold apparatus. In addition, at the base of the box boss, it is necessary to reduce the thickness of the base by calling it “meat thinning” to prevent sink marks. Also, ribs are attached to the bosses to increase the strength, but the thickness of the ribs is limited to prevent sink marks.
Here, the boss means a cylindrical shape, a polygonal column shape, or a protruding shape projecting in the shape of a cylinder on the back side of the basic thick portion, that is, the inside of the plastic molded product. It is for fastening or positioning with a counterpart part as a part. The basic thick part refers to the plate-shaped part that forms the overall shape of the plastic molded product and is formed with a substantially constant thickness. Necessary for increasing the strength of the basic thick part or for gas channels, etc. Accordingly, the thickened portion is the portion where the plate thickness is increased. The boss includes the box boss and the straight boss. A box boss is a box-shaped protrusion formed using a slide core to prevent sink marks and voids, and a direct boss is a protrusion formed directly without using such a slide core. A shape. Note that the straight boss is not a cylinder but a solid rod that is clogged, but here, the straight boss refers to a cylinder as described above unless otherwise noted. FIG. 6 shows a molded product of a straight boss. (1) is a solid straight boss, and (2) is a cylindrical straight boss formed by a cast pin. FIG. 7 shows the names of the straight bosses. The straight boss portion means a straight boss and a basic thick portion in the vicinity thereof. The external appearance surface 171 of the molded product refers to the surface opposite to the surface from which the straight boss protrudes, and the surface from which the direct boss protrudes is the inner surface 172. The reason why the external surface is used as the reference surface is to indicate the distance from the external surface to zero and the tip of the core pin.

On the other hand, there is a gas assist molding method as a method for preventing sink marks and voids. The gas assist molding method is an injection molding process for producing a molded part having a hollow structure by assisting the resin pressure at the time of injection molding with the pressure of an inert gas enclosed in the resin, and the resin pressure in the mold is low. Since the pressure distribution is uniform, warping and cracking of the molded product due to residual stress can be prevented, and the occurrence of sink marks on the ribs and bosses can be avoided. This gas assist molding method is also called gas assist injection molding method, gas assist method, gas injection molding, etc., and a molten resin is press-fitted into the cavity, and then high-pressure nitrogen gas is injected into the molten resin to gas the cavity. The pressure-holding is performed to prevent the sink and warpage of the molded product by making the thick part of the molded product hollow. The process is shown in FIG. 8 and the schematic diagram is shown in FIG. In FIG. 9, molten resin is injected into the cavity 7 (1), nitrogen gas as an inert gas 19 is injected into the molten resin from the gas inlet nozzle 18 (2), and then the gas pressure of the inert gas. While being held, the molten resin is solidified and a molded product having a cavity is completed. Since the volume of the molten resin is increased at the boss portion, cooling is likely to occur later than the surroundings, and a device has been devised to prevent sink marks, voids and the like by using a gas assist molding method.

As prior arts related to the gas assist molding method and the boss portion, there are the following. A method of using a gas assist molding method while making a thick wall part to prevent sink marks in the box boss, straight boss and rib part (Patent Document 1), pressurizing the molten resin in the cavity at a low pressure There are a method of injecting a fluid to provide a high-quality gas-assist molded product (Patent Document 2), a gas-assisted molding method (Patent Document 3) for preventing sink marks around a nozzle insertion mark of the molded product, and the like.
JP-A-10-95027 JP 2001-269967 A JP 2001-138354 A Hirohiro Arikata "Plastic molding processing basics and practice" published by Nikkan Kogyo Shimbun, 2005

  In the above prior art and the like, a method for preventing sink marks by a gas assist molding method has been proposed, but the actual situation is that the sink marks have not yet been eliminated. The present invention has been made in view of such circumstances, and when a boss portion is provided on the back surface of the external appearance of the product in the resin injection molding method, it is possible to reliably prevent the sink of the external surface of the product corresponding to that portion. It is an object to provide a molding method that can be used.

The invention of claim 1 is a straight boss formed by using a cast pin that forms a hole of a straight boss, and forming a gas-assisted cavity in the base portion of the straight boss and the basic thick portion in the vicinity thereof by a gas assist method. This is a method for molding a plastic molded product having a portion.
The invention according to claim 2 is the plastic molded product having the straight boss portion according to claim 1, wherein the base portion of the straight boss and the basic thick portion in the vicinity thereof do not have an annular cavity by gas assist, but are formed as one cavity. This is a molding method.
According to a third aspect of the present invention, the outer surface of the plastic molded product is used as a reference surface, and the distance between the reference surface and the tip of the die forming pin for forming the hole of the straight boss is adjusted to form the hole of the straight boss. This is a molding method of a plastic molded product having two straight boss portions.
The invention of claim 4 is a method for molding a plastic molded article having the straight boss portion according to claim 1, 2, or 3, wherein a predetermined dimension R is imparted to the root portion of the straight boss portion.
A fifth aspect of the present invention is a method for molding a plastic molded article having the straight boss portion according to the first, second, third, or fourth aspect, wherein a rib is provided at a base portion of the straight boss portion.
The invention of claim 6 is a plastic molded article having a straight boss produced by the method of claim 1, claim 2, claim 3, claim 4 or claim 5.

According to the first aspect of the present invention, by forming a cavity by gas assist in the base portion of the straight boss portion and the basic thick portion in the vicinity thereof by a cast pin and a gas assist method, the straight boss portion is formed by gas holding pressure of the cavity. Can prevent sink marks on the appearance of products that comply with. According to the second aspect of the present invention, the appearance of the product corresponding to the straight boss portion can be prevented from sinking by forming the gas assist cavity into one cavity instead of being annular. According to invention of Claim 3, while adjusting the distance of a reference surface and the front-end | tip part of a casting pin, the volume of the molten resin of a boss | hub part is reduced, and the thickness of the resin layer directly under a casting pin is made appropriate. It can prevent hot water wrinkles and sink marks on the external surface corresponding to the straight boss. According to the invention of claim 4, even if R is added to the base portion of the straight boss and the volume of the molten resin is increased, the sink can be prevented by the gas assist cavity, and the bending resistance can be further improved. According to the invention of claim 5, the volume of the molten resin is increased by adding a rib to the base portion of the straight boss portion, but the thickness of the rib can be increased without worrying about sink marks due to the gas-assisted cavity. Can be determined, and the bending resistance can be further improved. According to the invention of claim 6, it is possible to provide a plastic molded product having a clean straight boss portion that does not sink on the appearance surface of the product corresponding to the straight boss portion by any one of the methods described above.

  Examples and results of the present invention are shown below.

  FIG. 10 shows the invention according to claim 1. When the straight boss is provided in the basic thick portion, the volume of the resin in that portion is greatly increased as compared with that in the surrounding basic thick portion although it depends on the size. That is, at the base portion of the straight boss, while the molten resin flows into the boss molding cavity having a large volume, a part of the molten resin temporarily stays at the inlet portion, so that the temperature of the molten resin in that portion is locally As a result, the phenomenon of hot water wrinkles may occur on the appearance of the molded product. In order to reduce the increase in the volume of the resin, the inventor extends the cast pin to near the base of the straight boss as shown in FIG. 10 to reduce the volume of the straight boss and forms a gas assist cavity in the straight boss portion. By doing so, it was possible to prevent generation of hot water wrinkles and sink marks on the external surface of the straight boss. The thickness of the basic thick portion 17 of the molded product is about 2 mm. A straight boss in which a hole is formed by a cast pin, and a cylindrical straight boss. The height of the straight boss is 70 mm from the reference surface 171. The gas channel 21 refers to a nitrogen gas passage inside the resin. In the mold, a thick wall portion 3 mm wide and 1 mm thicker than the basic thick wall portion is provided on the inner surface (boss side) from the gas injection port 211 to the straight boss in advance, and as shown in FIG. Is injected into the cavity and held by the cylinder and the mold. The molten resin is cooled and solidified by a mold, but the thick part is thicker than the basic thick part by about 1 mm, and solidification is slowed accordingly. Therefore, the nitrogen gas that has entered the molten resin from the gas injection port reaches the base portion of the straight boss while forming the gas channel 21 inside the thick wall portion, and gas flows to the base portion of the straight boss and the basic thick portion in the vicinity thereof. A cavity is formed by assist. While the molten resin is cooled and solidified by the mold, the gas pressure is maintained to apply pressure from the inside of the mold to the inner surface of the mold to prevent sink marks. A gas assist cavity refers to a cavity formed inside a resin by a gas assist molding method. FIG. 8 shows a process of injection molding by gas assist, and FIG. 9 shows a schematic diagram of gas assist molding.

FIG. 11 (1) is an implementation of the invention of claim 2 and claim 3. The distance between the reference surface of claim 3 and the tip of the core pin will be described with reference to FIG. In the first embodiment, the point of extending the core pin near the base of the straight boss in order to reduce the volume of the straight boss has been described. However, it has been found that there is a problem even if it is too close.

That is, as shown in FIG. 11 (2), if the distance 122 between the reference surface 171 and the tip of the core pin 12 is too short, the resin layer between the two becomes too thin and the molten resin is locally cooled by the core pin and the mold. In order to solidify the part quickly, the gas-assisted cavity 16 becomes annular as shown in FIG. 11 (2), even if gas is subsequently injected by the gas-assist method. Thus, it has been found that sink marks 91 are generated on the appearance surface corresponding to the central portion. Therefore, the inventor appropriately adjusts the distance between the reference surface 171 and the tip of the core pin 12 according to the thickness of the basic thick portion, the boss diameter, etc. as shown in FIG. In (1), when the thickness of the basic thick portion is 2 mm, the distance is appropriately 5 mm, and gas assist was performed while suppressing local cooling as described above by adjusting the distance. As a result, the gas also entered directly under the core pin 12 to form one disk-like cavity 16 as shown in FIG. 11 (1), and a clean appearance surface free from hot water wrinkles and sinks could be obtained.

FIG. 12 is an explanatory view of the invention in which a predetermined dimension R is added to the base portion of the straight boss of claim 4. R means rounding around the root part of the straight boss, and R of a predetermined dimension means the radius of the rounded circle and changes depending on the thickness of the basic thick part. When forming protrusions (bosses) directly on the inner surface (boss side) of plastic molded products that require no appearance of harmful sink marks on the exterior, use a slide core as a measure to reduce the thickness of the base of the boss. The box boss is configured to be about 1/2 to 1/3 of the thickness of the thick part (FIG. 4), but in this case, the thickness of the base of the boss is thinner than other parts. Its bending strength decreases. By forming the straight boss by the gas assist method, a method using a slide core becomes unnecessary, and an R shape of about 0.5 mm to 5 mm can be formed at the root of the straight boss. At this time, thanks to the gas assist, there is no appearance detrimental sink. Further, since the base portion has a predetermined R shape, the bending strength as a boss is increased.

FIG. 13 is an explanatory view of the invention in which the rib 20 is provided at the base portion of the straight boss 11 of claim 5. In some cases, a rib structure is arbitrarily given to the box boss in order to increase the bending strength of the box boss. At this time, the plate thickness is limited in the rib shape structure. That is, in order to prevent the occurrence of harmful sink marks in the appearance portion, the thickness of the rib base portion in contact with the basic thick portion must be equal to or less than that of the box boss. Even if a rib is added to the box boss, the bending strength does not increase greatly. On the other hand, when the rib 20 is applied to the root portion of the straight boss 11 using the gas assist method, the thickness of the rib 20 is not limited, and the root of the rib 20 has a predetermined dimension similar to the root portion of the straight boss 11. The bending strength of the entire straight boss can be increased by attaching R (about 0.5 mm to 5 mm). When the box boss with ribs and the straight boss with ribs were compared, the bending strength of the latter was 1.2 to 2.5 times that of the former.

FIG. 14 shows a prototype for the invention of claim 6. In FIG. 14, the heights of the straight bosses A, B, and C are 75 mm, 35 mm, and 35 mm, respectively. The rib angle 21 is 20 °, 15 °, and 15 ° in this order. There was no sink mark on the appearance of this prototype. The plastic molded product of claim 6 includes motorcycle covers, such as an engine cover and a side cover.

  Many plastic molded products require a boss for assembly. The present invention prevents sink marks and hot water wrinkles, which are problems in appearance when the boss part is injection-molded, and increases the bending strength of the boss part, and the method of the present invention is considered to be widely adopted.

Plastic injection molding process. Schematic diagram of plastic injection molding process Sink and void Box boss and slide core Inclined core Direct boss molding Direct boss name, etc. Gas assist injection molding process Schematic diagram of gas assist molding Direct boss with gas assist Distance between reference surface and core pin tip R at the base of the straight boss Straight boss with ribs Prototype

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fixed side mounting plate 2 Fixed side metal mold 3 Movable side mounting plate 4 Movable side metal mold 5 Cylinder 6 (Cylinder) nozzle 7 Cavity 8 Molten resin 9 Molded product 91 Sink of molded product 92 Void of molded product 10 Core
11 Straight boss 111 Straight boss hole 112 Straight boss root
12 Cast Pin 121 Lead Tip of Cast Pin 122 Distance of Cast Pin Tip and Reference Surface 13 Eject Sleeve 131 Eject Pin
14 Box boss 141 Slide core 15 Inclined core 16 Cavity by gas assist
17 Basic thick part (of molded product) 171 External surface (reference surface) 172 Internal surface (boss side)
18 Gas inlet nozzle 19 Inert gas 20 Rib 21 Rib angle 21 (gas assist method) gas channel 211 Gas inlet (by gas assist molding method)

Claims (6)

  A plastic molded product having a straight boss formed by forming a cavity by gas assist at the base part of the straight boss and the basic thick part in the vicinity of the base part of the straight boss by using a cast pin that forms a hole of the straight boss. Molding method.   The method for molding a plastic molded article having a straight boss portion according to claim 1, wherein the gas-assisted cavities of the base portion of the straight boss and the basic thick portion in the vicinity thereof are not formed in an annular shape, but are formed as one cavity.   3. The straight boss portion according to claim 2, wherein the straight boss hole is formed by adjusting the distance between the reference surface and the tip of the die forming pin for hole formation of the straight boss, with the appearance surface of the plastic molded product as the reference surface. Molding method for plastic molded products.   The molding method of the plastic molded product which has the straight boss part of Claim 1, Claim 2, or Claim 3 which gave R of the predetermined dimension to the base part of the straight boss part.   5. A method for molding a plastic molded article having a straight boss portion according to claim 1, wherein a rib is provided at a root portion of the straight boss portion. A plastic molded article having a straight boss produced by the method of claim 1, claim 2, claim 3, claim 4 or claim 5.

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