JPH08276469A - Mold for thermoplastic resin gear and hollow injection molding method - Google Patents

Abstract

PURPOSE: To suppress the shrinkage of resin and to easily obtain a thermoplastic resin gear having excellent accuracy by radially forming a protruding state on the surface perpendicular to the driving direction of the protruding part into a cavity in the state that the movable part to be telescopically moved in the cavity is advanced. CONSTITUTION: The mold for a thermoplastic resin gear has at least one movable part 2, which has a cylindrical shape at the part protruding into a cavity 1 of the part 2 in the state that the part 2 is advanced into the cavity 1, and predetermined number of protrusions 5 each having a width W1 of 0.5 to 10mm are radially provided on the cavity surface of the end face of the part 2. The movable distance of the part 2 at the time of molding the part 2 is set to a range of 10 to 20% of the tooth width of the gear. The outer diameter D1 of the cylindrical shape of the protrusion into the cavity of the part 2 is set to 0.7 to one times as large as the tooth flank circle diameter of the gear and one to three times as large as the diameter of the shaft hole of the gear of the inner diameter D2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold for a thermoplastic resin gear and a hollow injection molding method. More specifically, it relates to a mold of a thermoplastic resin gear having excellent dimensional accuracy and gear accuracy, and a hollow injection molding method.

[0002]

Gears are used for automobiles, general machinery, precision machinery,
Widely used as mechanical parts in various fields such as electricity and electronics. As a recent trend, excellent mass productivity,
The range of applications of gears made of thermoplastic resin is expanding because they are lightweight and do not rust. Further, with the recent increase in technology in each field, demands for dimensional accuracy of the parts have become more sophisticated, and it is a technical subject to meet the demand.

However, when the injection molding is carried out, the thermoplastic resin largely shrinks in the process of solidifying from the molten state in the mold, so that a molded product cannot be obtained according to the size of the mold, and the product is manufactured accurately. It's very difficult to get. Therefore, various improvements have hitherto been made in terms of molding conditions, resin characteristics, mold design, and the like. For example, a mold design is designed in consideration of mold shrinkage, and the mold is designed mainly by using the following three methods. The cavity module should be the same as the product, allowing for mold shrinkage with pressure angle and transfer coefficient. The cavity pressure angle should be the same as the product and the module should allow for mold shrinkage. Expect mold shrinkage on both module and pressure angle.

However, both of the above and the above methods cause the deviation of the involute tooth profile itself due to contraction,
The contraction in the tooth trace direction was not taken into consideration, and when higher dimensional accuracy and gear accuracy were required, it was necessary to repeat the modification of the mold by trial and error. For this reason,
Since a lot of labor and time were required from product development to manufacturing, it was very difficult to commercialize an accurate gear in a short period of time.

Further, in a gear by injection molding, the accuracy is generally improved by providing a web portion. However, it is very difficult to avoid the reduction in strength of the gear itself due to the provision of the web portion. In the hollow injection molding method of a molded gear obtained by a resin gear molding die disclosed in Japanese Patent Laid-Open No. 4-299113, a gas injection nozzle is provided at a position corresponding to a molded product web portion of a mold cavity plate. A pressurized gas is injected into the molded product from the gas injection nozzle to form a hollow portion in the molded product. However, in the hollow molding method for gears of the same publication, a pressurized gas is injected from a nozzle provided on a cabinet plate, but a gate breakage occurs due to contraction when the resin solidifies. However, it is difficult to form a desired hollow portion inside the molded product because gas leaks out of the molded product through a gap formed between the nozzle and the molded product. Therefore, it can be said that the mold structure is not suitable for stable production. Further, in the mold structure shown in the publication, since the gas pressure feeding nozzle is provided in the vicinity of the gate part, it takes a lot of time to take out the runner after the molding process is completed, which is also unsuitable for stable production. It can be said to be a type structure.

In the thermoplastic resin gear disclosed in Japanese Patent Laid-Open No. 6-182821, which has a hollow portion at the entire circumference or part of the tooth root portion, the gear portion is formed by providing the tooth root with a hollow portion. It improves the accuracy. However, in the thermoplastic resin gear of the publication,
It is very difficult to effectively form the hollow portion over the entire circumference of the tooth root by the ordinary hollow injection molding. Further, the accuracy of gears at a portion where the hollow portion is not formed at the root portion of the tooth tends to be equal to or lower than that of a molded product manufactured by a normal injection molding method. Further, the thermoplastic resin gear of the same publication has a shaft hole portion and a tooth portion so that the pressurized fluid selectively forms a hollow portion at the root portion of the tooth in the injection process of the pressurized fluid in hollow injection molding. The space between the roots has a thin (web) structure. Therefore, the strength of the entire gear is equivalent to that of a gear manufactured by ordinary injection molding.

The high-rigidity gear made of thermoplastic resin disclosed in Japanese Patent Laid-Open No. 6-272748 has a characteristic that it does not have a web structure but has a hollow portion inside, so that dimensional accuracy and rigidity are balanced. Is to improve. However, in the thermoplastic resin gear of the same publication, like the gear disclosed in Japanese Patent Application Laid-Open No. 6-182821, a portion where a hollow portion effectively exists,
Since there is a difference in the dimensional accuracy of the gear from the other portion, it is difficult to maintain the uniform dimensional accuracy of the gear in the hollow portion over the entire circumference of the gear. In particular, the dimensional accuracy of a portion where the hollow portion is not formed tends to be equal to or lower than that of a molded product produced by a normal injection molding method. Further, in the thermoplastic resin gear of the same publication, the hollow portions are largely deviated in the cross section perpendicular to the axis, and the wall thickness is unevenly formed. Cooling and solidification of the thick portion requires a longer time than that of the thin portion, which is an obstacle to shortening the molding cycle.

[0008]

DISCLOSURE OF THE INVENTION The present invention provides a molding die for a thermoplastic resin gear having uniform and good dimensional accuracy over the entire circumference of the gear and excellent productivity, and a hollow injection molding method. To do.

[0009]

The present invention is as follows. 1. It has at least one movable part that can be arbitrarily advanced into the cavity and retracted from the cavity, and in a state where the movable part is advanced, a part of the part protruding into the cavity that is perpendicular to the driving direction. A molding die for a thermoplastic resin gear, characterized in that at least one convex shape having a width of 0.5 to 10 mm is radially formed on the surface. 2. The mold for molding a thermoplastic resin gear according to the above 1, wherein the movable distance of the movable portion during molding is within a range of 10 to 90% with respect to the tooth width of the thermoplastic resin gear. 3. With the movable portion advanced into the cavity, the shape of the portion of the movable portion protruding into the cavity is cylindrical, and the outer diameter D1 of the cylindrical shape is 0.7 of the root circle diameter of the gear. -1 times, and the inner diameter D2 is 1-3 times the diameter of the shaft hole of the gear. 4. On the surface perpendicular to the driving direction of the part protruding into the cavity in the state where the movable part has advanced,
The molding die for a thermoplastic resin gear according to the above 1, which has a movable core that can be projected from the surface and retracted from the projected position.

5. Using the molding die described in 1 above, the injection of the pressurized fluid into the resin in the cavity is started during the injection of the molten resin or 10 seconds after the completion of the injection, and the cavity is injected at the same time as or during the injection of the pressurized fluid. The hollow of the thermoplastic resin gear is characterized in that the pressure of the pressurized fluid injected into the resin in the cavity is maintained for 5 to 90 seconds in a state where the volume in the cavity is started to be expanded and the volume in the cavity is completed to be expanded. Injection molding method. 6. Using the molding die having the movable part and the movable core described in 4 above, the injection of the pressurized fluid into the resin is started during the injection of the molten resin or 10 seconds after the completion of the injection, and the injection of the pressurized fluid is started. Simultaneously with or during the injection, the volume of the cavity is expanded by driving the movable part, the movable core is driven during the driving of the movable part, and the movable part is moved within 20 seconds after the driving of the movable part is started. The step of expanding the cavity internal volume by driving the movable core is completed, and the pressure of the pressurized fluid injected into the resin is increased to 5 to 5 with the cavity internal volume being expanded.
A hollow injection molding method for a thermoplastic resin gear, comprising a step of holding for 90 seconds. 7. The pressurized fluid is injected from a nozzle of a cylinder, a sprue portion of a molding die, or a runner portion so that the pressurized fluid passes through a gate and reaches the resin in the cavity. Hollow injection molding method for thermoplastic resin gears.

The present invention will be described in detail below. In the present invention, “at the time of molding” means from the start of the resin injection process,
It refers to the time until the resin cooling step is completed. In hollow injection molding, it refers to the period until the step of releasing the pressure of the pressurized fluid is completed. In the present invention, "hollow injection molding method"
Is an injection molding method in which a hollow molded article is obtained by injecting a thermoplastic resin into a mold cavity during injection molding or by injecting a pressurized fluid into the resin within 10 seconds after the completion of injection.

In the present invention, the term "cavity" refers to a portion of the depression provided on the parting surface of the die, which is filled with resin to be a product after the completion of the molding process. In the present invention, the “hollow part” does not mean a hollow part or a hollow part formed by a foaming agent. In the present invention, the "movable core" is a part of the parts constituting the mold that can be projected into the cavity from the cavity surface perpendicular to the driving direction of the movable part and can be retracted from this projected position. Refers to.

The molding die of the thermoplastic resin gear according to the present invention has at least one movable part, and when the molten resin is injected, a convex shape is formed on the cavity surface perpendicular to the driving direction of the movable part. Therefore, when the movable part is driven to increase the volume in the cavity, the internal rib can be formed at the position in the gear where the convex shape exists. From this, the position and number of internal ribs formed in the gear are determined by the position and number of convex shapes. The number of internal ribs and the number of gates are preferably equal. When the number of internal ribs and the number of gates are equal, a gear having the same number of internal ribs, gates and hollows can be obtained. Further, since it is preferable that the inner ribs have a uniform thickness over the entire length thereof, it is preferable that the pressure applied to the inner ribs is uniform in the thickness direction when the inner ribs are formed.

Therefore, when there is one convex shape formed on the cavity surface perpendicular to the driving direction of the movable portion, the gate is arranged on the opposite side of the convex shaft portion provided in the movable portion. By doing so, pressure is evenly applied to the inner rib surface, which is preferable. Further, when a plurality of convex shapes are provided on the cavity surface perpendicular to the driving direction of the movable portion, it is preferable to dispose the convex shapes at equal intervals because pressure is evenly applied to each internal rib surface. Therefore,
The number of gates or inlets for pressurized fluid is the same as the number of convex shapes,
It is preferable to provide it between the inner ribs, and more preferably to arrange the ribs at the same distance from each inner rib.

In the present invention, the shape of the internal rib is determined by the shape of the convex shape formed on the surface perpendicular to the driving direction of the movable part of the molding die. When the convex shape is provided radially, the thickness of the partition wall formed inside the thermoplastic resin gear becomes uniform over the whole, and while suppressing the reduction of gear strength, the gear accuracy is maintained over the entire circumference of the gear. It is possible to improve it uniformly. The width of the convex shape is 0.5 to 10
mm is preferable, and more preferably 0.5 to 5
mm. This is because when the width of the convex shape exceeds 10 mm, the thickness of the internal ribs formed inside the gear exceeds 5 mm, so that the molding shrinkage of the gear becomes uneven. As a result, it becomes difficult to maintain the gear accuracy evenly over the entire gear. The height of the convex shape is
(Tooth width of the thermoplastic resin gear-moving distance of the movable part)
× 0.2 to 0.8 is preferable. The length of the convex shape is preferably ((D1-D2) / 2) × 0.4 to 1. When the dimension of the convex shape is as described above, it is possible to form an internal rib inside the thermoplastic resin gear to improve gear accuracy while suppressing reduction in gear strength. However, the both ends of the convex shape may be rounded in a semicircular shape as shown in FIG. 1, may be sharp, or may be straight. Further, both sides of the convex shape may be rounded or may be straight. When both sides of the convex shape are rounded, the height of the convex shape indicates the height of the apex.

The molding die of the thermoplastic resin gear according to the present invention is characterized in that a convex shape is formed on a surface perpendicular to the driving direction of the movable part, and the convex shape is the perpendicular shape. The convex portion 5 may be formed on the surface in advance, or the movable core 3 may be formed by protruding from the vertical surface. In the mold having the movable core 3, a structure capable of radially forming a convex shape by the protrusion of the movable core 3 on a surface in the cavity of the movable portion and perpendicular to the driving direction is provided. By doing so, the thickness of the partition wall (internal rib) formed inside the molded product becomes uniform, and the gear accuracy can be improved uniformly over the entire circumference. Further, by retracting the movable core 3,
Since the convex shape disappears from the cavity surface which is perpendicular to the driving direction of the movable part 2 and the surface becomes a flat surface, it is possible to obtain a hollow molded gear having no surface irregularity after the completion of the molding process. From this, a molding die having the movable core 3 is preferable. It is presumed that it is possible to maintain the accuracy of the gear evenly over the entire circumference of the gear, because the shrinkage of the product after molding progresses uniformly over the entire product due to the lack of surface irregularities. To be done.

In a mold having a movable part and a movable core, a protrusion is formed radially on the surface in the cavity of the movable part and perpendicular to the driving direction by the protrusion of the movable core. The width of the convex shape is preferably 0.5 to 10 mm, more preferably 0.5 to 5 mm. In the hollow injection molding method using the molding die of the thermoplastic resin gear of the present invention, in order to form a hollow portion effective in improving the gear precision inside the thermoplastic resin gear, It is preferable that the moving distance of the movable portion at the time of molding is 10 to 90% with respect to the tooth width of the plastic resin gear. In order to form the hollow portion inside the thermoplastic resin gear while suppressing the reduction in the strength of the gear, it is preferable that the moving distance of the movable portion with respect to the tooth width is 30 to 80%.

Since the molding die according to the present invention has the movable part, it has a step of opening the mold after the completion of the molding step and projecting the movable part to a position beyond the parting surface, thereby taking out the product from the cavity. Is possible. Therefore, it is not necessary to provide an ordinary molding die with an ejection pin (also referred to as an ejector pin, EP) provided to eject the product from the cavity. In addition, the portion of the movable portion that is projected into the cavity by the advance of the movable portion has a tubular shape, and the outer diameter D1 of the tubular shape is 0.7 to 1 times the diameter of the root circle of the thermoplastic resin gear. That the inner diameter D2 of the tubular shape is 1 to 3 times the diameter of the shaft hole of the thermoplastic resin gear, improving the gear accuracy inside the thermoplastic resin gear while suppressing the reduction in gear strength. It is preferable because the hollow portion for effecting this can be effectively formed.

A hydraulic cylinder can be considered as a drive source for the movable part, but any device or method may be used as long as it has a structure in which the movable part is not retracted due to the injection pressure of the molten resin or the resin pressure and the cavity internal volume is not expanded. However, it can be implemented. In addition, a method of interlocking with the movable portion can be considered in the drive source for protruding and retracting the movable core. It can also be implemented by using a drive source that is separate from the movable part and is similar to the drive source of the movable part. In either method, it is necessary that the movable core does not retract from the advanced position due to the injection pressure of the molten resin or the resin pressure.

An example of a cross-sectional structure of a molding die for a thermoplastic resin gear according to the present invention is shown in FIGS. 1 (a) and 2 (a).
In the example of the molding die structure shown in FIG. 1A, in the state where the movable part 2 is advanced into the cavity 1, the shape of the part of the movable part 2 protruding into the cavity 1 is cylindrical, A predetermined number of convex portions 5 are formed on the cavity surface perpendicular to the driving direction of the movable portion of the portion protruding into the cavity.
It shows an example of arranging radially.

FIG. 1 (b) shows an example of the shape of the movable part 2 in the example of the molding die structure shown in FIG. 1 (a). D1 and D2 shown in the same figure indicate the diameter and inner diameter of the part (A part in the figure) of the movable part that is projected into the cavity when the movable part is advanced into the cavity 1. Is. The portion B in the figure is a portion that is not exposed in the cavity regardless of whether the movable portion moves forward or backward, and the shape of this portion is the shape required for the movable portion to smoothly drive (for example, a key groove). It can also be implemented by having it.

In the shape of the movable part 2 shown in FIG. 1 (b), the width of a part of the part which is protruded into the cavity 1 in the state where the movable part is advanced is formed on the surface perpendicular to the driving direction. Three convex parts 5 with W1 0.5 to 10 mm
Shows an example in which are arranged radially. The shape of the convex portion may be any shape as long as the hollow portion can be effectively formed inside the thermoplastic resin gear. The example of the molding die structure shown in FIG.
In the state where the movable part 2 is advanced into the cavity 1, the shape of the part of the movable part 2 protruding into the cavity is cylindrical, and the part of the part protruding into the cavity is perpendicular to the driving direction of the movable part. It shows an example having a movable core that can be ejected from another surface and retracted from this protruding position.

FIG. 2 (b) shows an example of the shape of the movable part 2 in the example of the molding die structure shown in FIG. 2 (a). Among the parts that are projected into the cavity in a state where the movable part is advanced, the movable part has a movable core that can be projected from a plane perpendicular to the driving direction of the movable part and can be retracted from this projected position. It is an example of a structure. A and B shown in the figure are similar to those in FIG. 1B, the portion (A) that is projected into the cavity by the advance of the movable part 2 and the inside of the cavity regardless of the advance and retreat of the movable part 2. It refers to the part (B) that is not exposed. Also, D1, D
2 is an outer diameter (D1) and an inner diameter (D2) of the portion A which is a tubular shape
Are shown respectively.

The example of the movable core shown in FIG. 2B has three widths W2 on a plane perpendicular to the driving direction of the movable part 2 when the movable core 2 is projected from the movable part 2. It shows an example of forming a convex shape radially. The convex shape formed by the movable core, its various dimensions, and the amount of protrusion can be implemented in any shape, size, and amount of protrusion so long as the hollow portion can be effectively formed inside the thermoplastic resin gear. is there.

FIG. 3 shows an example of a molding method for carrying out the present invention. The first step of the method is shown in FIG.
Shown in In the first step, the molten resin is injected into the cavity 1 with the movable part 2 advanced. The filling amount of the molten resin at this time is 100 when the movable part 2 is in the advanced state, that is, the cavity internal volume before expanding the cavity 1 internal volume is 100.
%, An amount corresponding to 40 to 100% is preferable, and an amount corresponding to 80 to 100% is more preferable. This is because if the filling amount of the resin is less than 40%, the pressurized fluid injected into the resin in the next second step easily breaks through the resin and easily flows out of the resin. As a result, it is difficult to obtain a desired shape, which is not preferable.

The second step is shown in FIG. 3 (b). After the molten resin is injected and flows into the cavity 1, a pressurized fluid for forming a hollow portion in the molded product is injected into the resin. The injection of the pressurized fluid is started during the injection of the resin until 10 seconds after the completion of the injection, preferably during the injection and 5 seconds after the completion of the injection. This is because when 10 seconds or more has passed from the completion of injection of the molten resin, the resin in the cavity is gradually cooled and solidification starts from the surface in contact with the cavity surface, effectively forming a hollow portion in the molded product. This is because it becomes difficult.

The third step is shown in FIG. 3 (c). At the same time as the injection of the pressurized fluid is started or during the injection, the movable part 2 is started to retreat to expand the volume of the cavity 7. From the start of retreat of the movable part 2 to 20 seconds later, preferably 10
By completing the retreat before the elapse of seconds, it is possible to form an effective hollow portion for improving the dimensional accuracy of the molded product. This is because, as described above, the molten resin filled in the cavity is cooled from the surface of the mold cavity and solidifies as time passes. Therefore, in order to form the hollow portion in the resin, the volume expansion operation of the cavity needs to be completed before the resin filled in the cavity is cooled and solidified.

As shown in FIG. 3D, the fourth step is a step in which the pressure of the pressurized fluid injected into the resin is maintained for a certain period of time with the volume of the cavity expanded. In the present invention, the time required to hold the pressure of the pressurized fluid necessary to effectively provide the hollow portion is 5 after the completion of the retreat of the movable portion.
~ 90 seconds, preferably 10-60 seconds.
Further, by adjusting the pressure of the pressurized fluid injected and the pressure holding time of the pressurized fluid injected into the resin, it becomes possible to easily adjust the dimensions of the molded product.

FIG. 4 shows an example of a molding method for carrying out the present invention using a mold having a movable core. The first step of the method is shown in FIG. In the first step, the molten resin is injected into the cavity 1 with the movable part 2 and the movable core 3 advanced. The filling amount of the molten resin at this time corresponds to 40 to 100% when the movable part 2 and the movable core 3 are in the advanced state, that is, when the cavity internal volume before the cavity 1 is expanded is 100%. The amount is preferably, more preferably 80-100%.

The second step is shown in FIG. 4 (b). This is a process of injecting a pressurized fluid for forming a hollow portion in the molded product into the resin during injection of the molten resin or until 10 seconds after the completion of injection. The third step is shown in FIG. At the same time as the injection of the pressurized fluid is started, or during the injection, the movable part 2 is started to retract, and at the same time as the movable part 2 is started to retract, or while the movable core 3 is retracted, the internal volume of the cavity is reduced. It is a process of expanding. The retreat of the movable part 2 and the movable core 3 is preferably completed within 20 seconds after the start of the retreat of the movable part 2, and more preferably within 10 seconds after the start of the retreat of the movable part 2. It is to be.
This is because, as described above, the molten resin injected into the cavity is cooled and solidified from the portion in contact with the cavity surface over time. Therefore, in order to effectively form the hollow portion in the resin, it is necessary to complete the expansion operation of the cavity before the resin filled in the cavity is cooled and solidified.

The fourth step is shown in FIG. 4 (d). That is, this is a process of maintaining the pressure of the pressurized fluid injected into the resin for a certain period of time while the volume of the cavity 1 has been expanded. As the pressurized fluid used to obtain a hollow shape inside the molded article in the present invention, a gas or liquid at room temperature is used. Further, those which do not react or be compatible with the molten resin used for molding under the temperature and pressure of injection molding are preferable. For example, nitrogen, carbon dioxide, air, helium, glycerin,
Liquid paraffin and the like can be mentioned. Usually, a pressurized gas is used, and it is particularly preferable to use an inert gas such as nitrogen, helium, neon or argon. Further, although the use of these gas bodies usually contains impurities, if too much impurities are contained, the resin may be decomposed or burned during molding, which is not preferable. Nitrogen gas is industrially preferable in consideration of economy. Hereinafter, a general example of the hollow molding method using a pressurized gas will be further described.

Hollow injection molding using a pressurized gas is carried out by a combination of an ordinary injection molding machine and a gas injection device. The gas injection device is a device that injects a gas into the resin through a pipe after starting the injection of the resin and holds the gas for a set time. For this, the gas to be injected is compressed to high pressure in advance,
It is possible to store it in an accumulator and introduce high-pressure gas through a pipe when injecting gas, or to measure a fixed amount of gas and continuously feed it with a pump to pressurize it. Any method is possible as long as a gas body is sent to. At this time, the gas inlet is a cylinder nozzle, a mold sprue, a runner,
It is possible to provide it in the product part, but more preferably, an injection port is provided at any position of the nozzle of the cylinder, the sprue part of the mold, and the runner part, and the pressurized gas passes through the gate and enters the cavity. To reach.

The thermoplastic resin used in the present invention includes polyethylene, polypropylene, polystyrene and ABS.
Resin, polyvinyl chloride, polyamide, polyacetal,
Polycarbonate, modified polyphenylene ether, polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, polyimide, polyamideimide, polyetherimide, polyarylate, polysulfone, polyethersulfone, polyetherketone, liquid crystal polymer, polytetrafluoroethylene, thermoplastic elastomer However, any thermoplastic resin can be used as long as ordinary injection molding is possible. In particular, the polyacetal resin and the polyamide resin have high heat resistance, excellent mechanical properties, and excellent sliding properties, and are therefore widely used as resins for gears and are preferably used in the present invention.

Suitable fillers include glass fiber, carbon fiber, metal fiber, aramid fiber, calcium titanate,
Asbestos, Silicon Carbide, Ceramic, Silicon Nitride, Barium Sulfate, Calcium Sulfate, Kaolin, Clay, Pyrophyllite, Bentonite, Sericite, Zeolite, Mica, Mica, Nepheline Cinite, Talc, Atalpulgite, Wollastonite, PMF , Ferrite, calcium silicate, calcium carbonate, magnesium carbonate, dolomite, zinc oxide, titanium oxide, magnesium oxide, iron oxide, molybdenum disulfide, graphite, gypsum, glass beads, glass powder, glass balloon, quartz, quartz glass, etc. Mention may be made of reinforcing fillers, which may be hollow. Also, these reinforcing fillers are 2
It is possible to use a combination of two or more of them, and if necessary, they can be used after pretreatment with a coupling agent such as a silane coupling agent or a titanium coupling agent.

The molded product obtained according to the present invention has a hollow portion, and the preferred hollow ratio is 10 to 90%, more preferably 20 to 80%. This has a hollow ratio of 90%
Hollow molded products that exceed the limit will be extremely thin.
It is not preferable because it causes a decrease in strength of the product. In addition, if the product is made extremely thin, the pressurized fluid that is injected to form the hollow inside the product may flow out of the product from the thin part, and it may not be possible to obtain a hollow molded product of the desired shape. Not good for Further, in a hollow molded product having a hollow ratio of less than 10%, the effect of the pressurized gas is weakened, and improvement in dimensional accuracy and gear accuracy, which is the object of the present invention, cannot be expected. The hollow ratio is defined by the following equation. Hollow ratio (%) = {(V × ρ−M) / (V × ρ)} × 100 where V is the volume when the hollow portion is filled with the same resin, ρ is the specific gravity of the resin used, M is the mass of the hollow molded product.

The thermoplastic resin gear obtained by hollow injection molding according to the present invention guides the pressurized gas to the inside of the root circle to effectively form the hollow portion, thereby suppressing the shrinkage of the resin. In the hollow injection molding method, a pressurized gas is supplied into the resin, so that the resin can be effectively brought into close contact with the mold by compensating for the shrinkage of the resin.
Gear accuracy is expected to improve. Even in normal injection molding, it is possible to compensate for the shrinkage of the resin by applying a holding pressure, but since the resin is solidified at the gate portion after the gate sealing, the effect of pressure cannot be expected. However, in the hollow injection molding method, pressure is applied from the inside of the molded product by the pressurized gas even after the gate sealing, and therefore it is presumed that the mold transfer property is improved. However, it is difficult to improve the accuracy of the gear by simply injecting the pressurized fluid into the product.

For the thermoplastic resin gear of the present invention, a mold having a structure capable of expanding the volume of the mold cavity is prepared.
The molten resin is injected into the mold cavity, and pressurized gas is injected into the molded product during the injection of molten resin or until 10 seconds after the completion of injection, at the same time as or during the injection of pressurized gas. By enlarging the volume in the cavity, it is possible to effectively form the hollow portion for improving the gear precision of the thermoplastic resin gear.

In the present invention, a pressurized gas is injected into the molten resin to expand the volume in the cavity 1 to obtain a hollow portion in the molded product, but until the resin in the cavity 1 is cooled. Meanwhile, it is necessary to stop the movable portion 2 at the retracted position while maintaining the pressure of the gas injected into the hollow portion. The time period during which the movable portion 2 is kept stopped at the retracted position while maintaining the gas pressure in the hollow portion necessary to effectively form the hollow portion is 5 after the movable portion is retracted.
The time is preferably 90 seconds to 90 seconds, more preferably 10 to 60 seconds.

[0039]

EXAMPLES The present invention will be described in more detail below with reference to examples. In Examples and Comparative Examples, polyacetal resin (Tenac-C 4520 manufactured by Asahi Kasei Co., Ltd.)
Was used for hollow injection molding.

[0040]

Examples 1 to 4 have a mold structure as shown in FIGS. 5 (a) and 5 (b), and have a module 1, 60 teeth, and teeth as shown in FIGS. 6 (a) and 6 (b). We prepared a mold that can mold spur gears with a width of 8 mm and a shaft hole diameter of 8 mm. Of the part of the movable part 2 protruding into the cavity 1, on the surface perpendicular to the driving direction, from the shaft hole of the gear toward the tooth part, the width is 2 mm,
One convex portion 5 having a height of 2 mm and a length of 20 mm was provided. Gate 1
6 is one point on the opposite side of the protrusion 5 with the center of the gear interposed,
It was provided at a position 15 mm from the center. The cylinder temperature of the molding machine was set to 200 ° C and the mold temperature was set to 90 ° C. The pressurized fluid injected to form the hollow portion in the product was nitrogen gas, the gas pressure was 150 kg / cm ^2, and the gear having the shape shown in FIG. 6 was obtained by hollow injection molding.

3 (a), 3 (b), 3 (c), and 3 (d) show the molding process for carrying out the process. The first step is shown in Figure 3 (a).
As shown in (1), the process is to inject the molten resin into the cavity 1 so that the unfilled portion does not remain in the cavity 1 with the movable portion 2 advanced by 4 mm. The second step is shown in Figure 3.
As shown in (b), it is the process of injecting the pressurized gas for forming the hollow portion in the molded product into the resin after the injection of the molten resin is completed. The pressure of the pressurized gas at this time is 1 as described above.
It was set to 50 kg / cm ² . The third step is a step of expanding the internal volume of the cavity 1 by retracting the movable portion 2 during the injection of the injection of the pressurized gas, as shown in FIG. 3C. In Examples 1 to 4, hollow injection molding was performed by changing the retraction start timing of the movable part 2. That is, the retreat of the movable part was started 0.5, 1, 2, 10 seconds after the start of the injection of the pressurized gas. The moving distance of the movable portion at this time is 4 mm, which corresponds to 50% with respect to the gear tooth width of 8 mm. The fourth step is shown in Fig. 3 (d).
As shown in, the process is to hold the pressure of the pressurized fluid injected into the resin for 30 seconds while the volume of the cavity is expanded. After the fourth step, the gas pressure in the product was released and the product was taken out of the mold.

A cross-sectional view of the obtained gear is shown in FIG.
It shows in (b). For the accuracy measurement of the obtained gear, the tooth profile error and the tooth trace direction error of the JIS gear accuracy standard (JIS B 1702) were used. Any of the errors is a dimensional deviation from the ideal involute gear, and it can be said that the smaller the error value, the more accurately the gear operates. Moreover, the thickness of the partition wall 15 was measured at each point of and shown in FIG. The more uniform the thickness at each point is, the more the molding shrinkage of the gear progresses uniformly in the entire gear, and thus it can be said that the gear is made of a thermoplastic resin capable of maintaining gear precision. The measurement results are shown in Table 1.

[0043]

COMPARATIVE EXAMPLE 1 In Examples 1 to 4, except that the surface of the movable portion 2 projecting into the cavity 1 perpendicular to the driving direction has a flat structure without a convex portion. Comparative Example 1 was carried out using a mold having basically the same structure as that of the existing mold. In addition, each temperature setting condition of molding machine, molding condition,
The pressure of the pressurized gas, the holding time of the gas pressure, etc. were in accordance with Example 2. The obtained gear cross section is shown in FIGS. 8 (a) and 8 (b). The gear accuracy and the thickness of the partition wall were measured, and the measurement results are shown in Table 1.

[0044]

[Comparative Example 2] In a mold similar to Comparative Example 1, the movable part 2 was fixed at a retracted position without being driven, and molten resin injection, pressurized gas injection, gas pressure retention, gas pressure release, and product removal. Molding was carried out in the same process as the ordinary hollow injection molding. In addition, the temperature setting conditions of the molding machine, the molding conditions, the pressure of the pressurized gas, the holding time of the gas pressure, etc. were in accordance with Example 2. The obtained gear cross section is shown in FIGS. 9 (a) and 9 (b). The measurement results are shown in Table 1.

[0045]

Embodiments 5 to 8 A mold having a mold structure as shown in FIG. 10 (a) and a parting surface 9 as shown in FIGS. 10 (b) and 10 (c). , (B), a mold capable of forming a spur gear having a module 1, 36 teeth, a tooth width of 35 mm and a shaft hole diameter of 6 mm was prepared. Figure 10
As shown in (b), of the portion of the movable portion 2 protruding into the cavity 1, on the surface perpendicular to the driving direction, from the shaft hole of the gear toward the tooth portion, and from the shaft hole to the tooth. Radial projections 5 with a width of 3 mm, a height of 3 mm, and a length of 12 mm toward the part
Three pieces were provided at intervals of 20 degrees. The gate 16 is shown in FIG.
As shown in, three points are provided at the position corresponding to the center of the convex portion 5,
It was provided at a position 10 mm from the center. The cylinder temperature of the molding machine was set to 200 ° C and the mold temperature was set to 90 ° C. The pressurized fluid injected to form the hollow portion in the product was nitrogen gas, and the gas pressure was 150 kg / cm ² .

Hollow injection molding was carried out by the molding method shown in FIGS. 3 (a), 3 (b), 3 (c) and 3 (d). The injection amount of the resin was set to an amount corresponding to 80% when the internal volume of the cavity before the expansion of the internal volume of the cavity 1 was 100%. The pressure of the pressurized gas injected to form the hollow portion in the molded product was 100 (kg / cm ² ). The process of retracting the movable part 2 and expanding the internal volume of the cavity is
It started from 1 second after starting the pressurized gas injection. The moving distance of the movable portion 2 at this time is 25 mm, which is a value corresponding to 71.4% of the tooth width of the gear shown in FIG. With the volume inside the cavity being expanded, the setting of the pressure holding time of the pressurized gas injected into the molded product was changed in Examples 5 to 8 to be 10, 20, 30, and 60 seconds, respectively. 12A and 12B are sectional views of the obtained gear. The gear accuracy and the thickness of the partition wall were measured, and the results are shown in Table 2. In Examples 5 to 8, since a plurality of partition walls were formed, the thickness of each partition wall was measured at three points, and the average value at each point is shown in Table 2.

[0047]

[Comparative Example 3] The gold used in Examples 5 to 8 except that the surface of the movable portion 2 projecting into the cavity is not a convex portion on a surface perpendicular to the driving direction but is a flat surface. It was performed using a mold having basically the same structure as the mold. In addition, the temperature setting conditions of the molding machine, the molding conditions, the pressure of the pressurized gas, the holding time of the gas pressure, etc. were in accordance with Example 7. An example of a cross section of the obtained gear is shown in FIGS. 13 (a) and 13 (b). The gear precision and the thickness of the partition wall were measured, and the results are shown in Table 2.

[0048]

[Comparative Example 4] In a mold similar to Comparative Example 3, the movable part was fixed at the retracted position without being driven, and the molten resin was injected.
Molding was carried out in the same process as ordinary hollow injection molding, such as pressurized gas injection, gas pressure retention, gas pressure release, and product removal. In addition, each temperature setting condition of molding machine, molding condition,
The pressure of the pressurized gas, the holding time of the gas pressure, etc. were in accordance with Example 6. The cross section of the obtained gear is shown in FIGS.
Table 2 shows the measurement results.

[0049]

Embodiments 9 to 12 The mold structure shown in FIG. 15A and the parting surface 9 shown in FIG.
Module 1 shown in (b), 60 teeth, tooth width 8 mm,
We prepared a mold that can mold spur gears with a shaft hole diameter of 8 mm. Of the part of the movable part 2 protruding into the cavity 1, the movable core 3 is capable of protruding from a plane perpendicular to the driving direction and retracting from the protruding position.
15 in a state of being projected from the movable part 2.
As shown in (b), FIG. 16 (a), and (b), width (w) 2 mm, height (h) 2 mm from the shaft hole of the gear toward the tooth portion,
Radial convex shapes with a length (l) of 20 mm at intervals of 120 degrees,
The structure is such that three can be formed. The gate 16 is a movable core 3
Three points are provided at a position corresponding to the center of the convex shape formed by the forward movement of, and the distance from the center of the gear is 15 mm.

FIGS. 4 (a), (b), (c) and (d) show the molding process for carrying out Examples 9-12. In the first step, as shown in FIG. 4 (a), the movable part 2 is advanced by 4 mm, and the movable core is advanced by 2 mm, and is melted in the cavity 1 so that no unfilled part remains in the cavity 1. This is the process of injecting resin. In the second step, as shown in FIG. 4B, the injection of the molten resin is completed, and a pressurized gas for forming a hollow portion in the molded product is injected into the resin. The pressure of the pressurized gas at this time was changed in Examples 9 to 12. That is, 80, 100, 150, 200 kg
/ cm ² . As shown in FIG. 4C, the third step is a step of starting the injection of the pressurized gas and retracting the movable part 2 and the movable core 3 during the injection to expand the internal volume of the cavity 1. . One second after the injection of the pressurized gas was started, the movable part and the movable core started to retreat. The moving distance of the movable portion 3 at this time is 4 mm, which is a value corresponding to 50% with respect to the gear tooth width of 8 mm. Also,
The moving distance of the movable core 3 is 2 mm. The fourth step is Figure 4
As shown in (d), it is a process of maintaining the pressure of the pressurized fluid injected into the resin for 30 seconds in a state where the volume of the cavity is expanded. After the fourth step, the gas pressure in the product was released and the product was taken out of the mold. 17A and 17B are cross-sectional views of the obtained gear, and Table 3 shows the measurement results.

[0051]

COMPARATIVE EXAMPLE 5 Using the molds used in Examples 9 to 12, the movable part 2 and the movable core 3 were not driven, but were fixed at the retracted positions and ordinary hollow injection molding was carried out. That is,
The steps are injection of molten resin, injection of pressurized gas, retention of gas pressure, release of gas pressure, and removal. The temperature setting conditions were as described above, and the pressure of the pressurized gas and the pressure holding time were the same as in Example 10. 18A and 18B are cross-sectional views of the obtained gears, and Table 3 shows the measurement results.

[0052]

[Table 1]

[0053]

[Table 2]

[0054]

[Table 3]

[0055]

The gear manufactured according to the present invention is a thermoplastic resin gear with reduced resin shrinkage, improved mold transferability, small error and good accuracy as an involute gear. Therefore, it can be said that the gear is a gear that can be accurately operated as a mechanical component. Therefore, it can be said that the gear is very useful in industry.

[Brief description of drawings]

1A shows an example of a cross section of a molding die having a movable part 2 according to the present invention, and FIG. 1B shows the movable part 2 of the mold in FIG. 1A.

2A shows an example of a cross section of a molding die having a movable part 2 and a movable core 3 according to the present invention, and FIG. 2B shows (a).
The movable part 2 and movable core 3 of the mold in FIG.

FIG. 3 is an explanatory diagram of a molding process when performing hollow injection molding using the molding die shown in FIG.

FIG. 4 is an explanatory view of a molding process when performing hollow injection molding using the molding die shown in FIG.

5A is a cross-sectional view of a molding die used when executing Examples 1 to 4 and Comparative Examples 1 and 2, and FIG.
A cross section, that is, a parting surface in the movable die is shown.

6 is a front view (a) and a side view (b) of the gears obtained in Examples 1 to 4 and Comparative Examples 1 and 2. FIG.

7 (a) shows a cross section perpendicular to the axial direction of the gears obtained in Examples 1 to 4, and FIG. 7 (b) shows an AA cross section.

8A shows a cross section perpendicular to the axial direction of the gear obtained in Comparative Example 1, and FIG. 8B shows an AA cross section.

9A shows a cross section perpendicular to the axial direction of the gear obtained in Comparative Example 2, and FIG. 9B shows a BB cross section.

10 (a) is a cross-sectional view of a molding die used in carrying out Examples 1 to 4 and Comparative Examples 1 and 2, and FIG.
Section A, that is, the parting surface in the fixed side mold, (c) shows the section BB, that is, the parting surface in the movable side mold.

FIG. 11 shows a front view (a) and a side view (b) of gears obtained in Examples 5 to 8 and Comparative Examples 3 and 4.

FIG. 12 (a) shows a cross section perpendicular to the axial direction of the gears obtained in Examples 5 to 8, and FIG. 12 (b) shows an AA ′ cross section,
(C) shows an AA "cross section.

13A shows a cross section perpendicular to the axial direction of the gear obtained in Comparative Example 3, and FIG. 13B shows an AA cross section.

14 (a) shows a cross section perpendicular to the axial direction of the gear obtained in Comparative Example 4, and FIG. 14 (b) shows a BB cross section.

15 (a) is a cross-sectional view of a molding die used for carrying out Examples 1 to 4 and Comparative Examples 1 and 2, and FIG.
The A section, ie, the parting surface in the movable mold, is shown.

FIG. 16 (a) is a front view and FIG. 16 (b) is a side view showing a state where a movable core is projected from a movable mold component.

17 (a) shows a cross section perpendicular to the axial direction of the gears obtained in Examples 9 to 12, FIG. 17 (b) shows an AA ′ cross section, and FIG. 17 (c) shows an AA ″ cross section. Show.

FIG. 18 (a) shows a cross section perpendicular to the axial direction of the gear obtained in Comparative Example 5, (b) shows a cross section taken along the line AA ′,
(C) shows an AA "cross section.

[Explanation of symbols]

 1 Cavity 2 Movable part 3 Movable core 4 Sprue and runner part 5 Convex part 6 Resin 7 Pressurized fluid flow path 8 Hollow part 9 Parting surface 10 Cavity surface 11 Pitch circle 12 Tip circle 13 Tooth part 14 Shaft hole part 15 Partition wall 16 Gate 17 Recess

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location B29D 15/00 7726-4F B29D 15/00 // B29K 101: 12 B29L 15:00 22:00

Claims (7)

[Claims] 1. A driving direction of at least one movable part capable of advancing into a cavity and retracting from the cavity, and in a state where the movable part is advanced, a part of the part protruding into the cavity is driven. At least 1 convex shape with a width of 0.5 to 10 mm on the surface perpendicular to
A molding die for a thermoplastic resin gear, which is formed in a radial pattern as described above. 2. The thermoplastic resin gear according to claim 1, wherein the movable distance of the movable portion during molding is within a range of 10 to 90% with respect to the tooth width of the thermoplastic resin gear. Mold for. 3. The shape of a part of the movable part projecting into the cavity is cylindrical when the movable part is advanced into the cavity, and the outer diameter D1 of the cylindrical part is the root circle of the gear. The molding die for a thermoplastic resin gear according to claim 1, which has a diameter of 0.7 to 1 times and an inner diameter D2 of 1 to 3 times the shaft hole diameter of the gear. 4. A movable member which is capable of protruding from a surface perpendicular to the driving direction and retracting from the protruding position on a surface of the portion protruding into the cavity when the movable portion is advanced, A molding die for a thermoplastic resin gear according to claim 1, further comprising a child. 5. Using the molding die according to claim 1, the injection of the pressurized fluid into the resin in the cavity is started during the injection of the molten resin or 10 seconds after the completion of the injection, and the injection of the pressurized fluid is started. Simultaneously or during the injection, the volume inside the cavity is started to expand, and the pressure of the pressurized fluid injected into the resin inside the cavity is maintained for 5 to 90 seconds while the expansion of the volume inside the cavity is completed. Hollow injection molding method for plastic gears. 6. Using a mold having a structure having a movable part and a movable core according to claim 4, injecting a pressurized fluid into the resin during injection of the molten resin or 10 seconds after the completion of injection, Simultaneously with or during the injection of the pressurized fluid, the movable part is driven to expand the internal volume of the cavity, the movable core is driven during the driving of the movable part, and 20 seconds after the start of the driving of the movable part. In the meantime, the step of expanding the internal volume of the cavity by driving the movable part and the movable core is completed, and the pressure of the pressurized fluid injected into the resin is maintained for 5 to 90 seconds in the state where the internal volume of the cavity is completely expanded. A hollow injection molding method comprising the steps of: 7. A pressurized fluid is injected from a nozzle of a cylinder, a sprue portion of a molding die, or a runner portion, so that the pressurized fluid reaches a resin inside a cavity through a gate. A hollow injection molding method for a thermoplastic resin gear according to claim 5 or 6.