JPH0450167B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an injection molding method for small industrial parts, such as gears, made by injection molding a raw material of ultra-high molecular weight polyethylene or a raw material obtained by combining ultra-high molecular weight polyethylene with an inorganic material such as carbon fiber. This invention relates to an injection molding machine for molding parts.

Ultra-high molecular weight polyethylene with a molecular weight of 2 million to 6 million is extremely suitable as a material for small industrial parts such as gears because it has excellent chemical resistance, impact resistance, and cold resistance, as well as self-lubricating properties.

Injection molding, compression molding, and extrusion molding are conceivable methods for molding small industrial parts etc. using ultra-high molecular weight polyethylene, but as mentioned above, ultra-high molecular weight polyethylene has an extremely high molecular weight, so it is difficult to heat it. However, the viscosity does not decrease and it does not exhibit fluidity. For this reason, even if injection molding is performed, it is not possible to fill the cavity with ultra-high molecular weight polyethylene to the smallest detail, resulting in poor precision and the surface of the molded product cannot be made smooth. Furthermore, when ultra-high molecular weight polyethylene is plasticized using a screw, high shear occurs in the cylinder, oxidation and main chain scission occur due to heating, and the molecular weight decreases, resulting in ordinary polyethylene.

Therefore, conventionally, ultra-high molecular weight polyethylene is currently molded by compression molding or extrusion molding. However, these molding methods cannot efficiently produce molded products with high precision and at low cost.

On the other hand, in order to increase the rigidity of gears and the like, it is conceivable to combine ultra-high molecular weight polyethylene with an inorganic material such as carbon fiber. In the case of compression molding, compounding is possible because the composite material is fused to the surface of each particle of powdered ultra-high molecular weight polyethylene during compression.
In normal injection molding, each particle of ultra-high molecular weight polyethylene is melted and composited, resulting in a decrease in molecular weight and loss of the basic properties of ultra-high molecular weight polyethylene.

The present invention was made in view of the above-mentioned conventional problems, and uses an injection molding method to produce high-precision injection molding from ultra-high molecular weight polyethylene or a raw material made by adding an inorganic composite material to this ultra-high molecular weight polyethylene. The purpose is to obtain products efficiently.

In order to achieve this objective, the first invention, the injection molding method, softens the particle surface of ultra-high molecular weight polyethylene using a powdered ultra-high molecular weight polyethylene raw material or high-speed mixing, and attaches an inorganic composite material to this surface. The gist of the invention is to melt a raw material consisting of a material in a non-oxidizing atmosphere in a short period of time, and to inject this molten raw material into a cavity before starting thermal crosslinking. The molding machine is an in-line screw type molding machine, and the hopper is connected to an atmosphere gas supply pipe for melting the ultra-high molecular weight polyethylene raw material in a non-oxidizing atmosphere, and the compression part of the cylinder containing the screw and The gist is that a heater is provided only in the nozzle section, and a portion of the feeder section is used as a heat dissipation section.

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a sectional view of an injection molding machine according to the present invention. The injection molding machine of the present invention has a cylinder 2 in which a screw 1 is disposed, a hopper 4 for supplying a raw material 3 into the cylinder 2, and a molding cavity 5 inside which the tip of the cylinder 2 faces. A mold 6, a vacuum tank 7 and a rotary pump 8 for evacuating the cavity 5.
The main parts are:

An atmospheric gas supply pipe 9 such as nitrogen gas is connected to the hopper 4 so that the raw material 3 supplied into the cylinder 2 is melted in a non-oxidizing atmosphere. Further, at the lower part of the hopper 4, there is provided an adjustment plate 10 having a circular or rectangular slit 10a for adjusting the supply amount of the raw material 3.

On the other hand, the compression ratio of the screw 1 rotated by the vane motor 11 is preferably 2.3 or less, especially 1.3 to 2.0, and L/D (L: length, D: diameter)
is preferably 10 to 25, particularly preferably 15 to 20. Also, the pitch P of the screw is determined by P=πDtanθ, but θ
10° to 18°, especially 11.5° to
Preferably it is 14.5°.

If the compression ratio is 2.3 or more, more pressure than necessary is applied to the ultra-high molecular weight polyethylene powder, which causes it to remain in the cylinder, and as crosslinking begins, it no longer exhibits fluidity. Also, the compression ratio is 1.3
If it is below, it is insufficient to bring the surface of the ultra-high molecular weight polyethylene powder into a molten state.

Regarding L/D, if L/D is 20 or more, there is a risk that ultra-high molecular weight polyethylene will remain in the cylinder for an unnecessarily long time, and if L/D is 15 or less, heating control of ultra-high molecular weight polyethylene is required. accuracy becomes insufficient.

The screw pitch is determined by the flight angle θ, which determines the magnitude of the torque applied to the ultra-high molecular weight polyethylene.
When θ is 10° or less, sufficient torque can be obtained, but unnecessary residence time and frictional energy are added to the ultra-high molecular weight polyethylene powder. Also,
If θ is 18° or more, it will not be possible to apply sufficient torque to the ultra-high molecular weight polyethylene, putting a strain on the vane motor, and the screw will remain in the cylinder for a long time without being able to obtain the specified screw rotation speed. and is inappropriate.

Furthermore, the screw head has a flight cut to the tip without using a backflow prevention ring, and the screw diameter is 0.1 to 0.2 mm smaller than the inner diameter of the cylinder 2.

In addition, the rear half of the cylinder 2 is connected to the feeder section 2.
a, the front half is the compression part 2b, the tip part is the nozzle part 2c
The length of the heat radiation fins 12 formed in the feeder portion 2a is preferably 1/4 to 1/3 of the total length of the cylinder 2.

Ultra-high molecular weight polyethylene is conveyed by a screw in a cylinder, and as the compression ratio increases,
Heat is generated by friction between the polyethylene powders. This state occurs when the screw reaches its compression zone.
It is only in this section 2b that the individual powders are softened by external heating, and before that, in zone 2a, heating must be avoided as much as possible, heat due to friction must be dissipated, and crosslinking must be prevented.

When the heat dissipation fins 12 are not provided, the compression part 2b
It becomes impossible to control the heating, and the polyethylene powder ends up being crosslinked. A heater 13 is provided in the compression section 2b and the nozzle section 2c to melt the raw material 3 instantly.

In addition, FIG. 2 is a plan view of the cavity, FIG. 3 is a vertical cross-sectional view of the mold, and FIG. 4 is a cross-sectional view of the mold. As can be seen from these figures, one of the molds of the mold 6 A runner 14 is bored in the mold 6a, and a cavity 5 for gear molding is formed on the mold matching surface of the other mold 6b.
A vacuum path 15 having a diameter of approximately 5 mm to 10 mm is formed around the cavity 5, and the vacuum path 15 and a portion corresponding to the tooth tips of the cavity 5 are communicated through holes 16. The length of this hole 16 is 1 to 20 mm, preferably 3 to 7 mm. Also,
Seal rings 17 are provided at the barting portions of the molds 6a and 6b to maintain the degree of vacuum during mold matching. In addition, during molding, the temperature of the mold 6 is 30
The temperature is preferably 60°C to 120°C, particularly 60°C to 120°C.

A method for producing molded articles of ultra-high molecular weight polyethylene using the injection molding machine as described above will be described below.

First, a predetermined amount of powdered ultra-high molecular weight polyethylene having a particle size of about 30 μm to 100 μm is supplied as the raw material 3 into the hopper 4 . Unlike regular polyethylene, ultra-high molecular weight polyethylene is in the form of powder rather than pellets, and the coefficient of friction between particles is small, so the amount of raw material supplied per hour can be determined simply by adjusting the particle size. Adjusting the amount of raw material supplied to the feeder section 2a will relatively reduce the compression ratio of the screw 1, and as a result, when plasticizing ultra-high molecular weight polyethylene, the compression ratio will be abnormally increased. It is possible to prevent the raw material from staying in the cylinder 2.

The ultra-high molecular weight polyethylene supplied into the cylinder 2 is transferred from the feeder section 2a to the compression section 2b.
It is sent to the compression section 2b where it is instantaneously heated and melted (only the surface) at the nozzle section 2c. here,
Since nitrogen gas is supplied into the hopper 4, the inside of the cylinder 2 is maintained in a non-oxidizing atmosphere, thereby preventing a decrease in molecular weight due to oxidation. Moreover, once ultra-high molecular weight polyethylene is melted, crosslinking begins immediately. Therefore, if it is instantaneously melted in the compression part and injected into the cavity 5 from the nozzle part as in this embodiment,
Can be injected while still fluid.

The raw material injected into the cavity 5 immediately starts crosslinking, and by promoting this crosslinking, the raw material does not re-melt even in the high temperature cavity. Furthermore, the cavity 5 is filled with molten ultra-high molecular weight polyethylene up to the tooth tips.

Next, a method for compounding the ultra-high molecular weight polyethylene to be fed into the injection molding machine described above will be described.

FIG. 5 is a longitudinal cross-sectional view of a jar for compounding ultra-high molecular weight polyethylene. The side wall of the jar 18 is filled with heating oil 19, and the bottom part has a stirring blade 21 rotated by a motor 20. Further, a supply pipe 22 for supplying a non-oxidizing atmosphere gas such as nitrogen gas is inserted through the upper part.

Ultra-high molecular weight polyethylene and a composite material are charged into the jar 18, and the composite material is preferably an inorganic material, such as CaCO ₃ Al ₂
(OH) ₃ , Al ₂ O ₃ , Ca(PO ₄ ) ₂ , glass fiber powder, carbon fiber powder, wood powder, paper powder, glass beads, metal particles, etc. And the amount added is 30wt% or less,
Particularly preferred is 3wt% to 30wt%.

Then, the temperature of the jar 18 is set to 80° C. to 120° C., preferably 85° C. to 100° C., the rotation speed of the stirring blade 21 is set to 1800 rpm to 3000 rpm, and the composite material 23 of ultra-high molecular weight polyethylene and the composite material is introduced. When the temperature of mixture 23 reaches 110°C to 120°C, reduce the rotation speed of the stirring blade to 90 rpm,
The composite ultra-high molecular weight polyethylene is placed in a cooling jar and cooled to room temperature at 60 rpm.

That is, if the particles of ultra-high molecular weight polyethylene are completely melted, they cannot be composited, but when stirring is performed as described above, the particles of ultra-high molecular weight polyethylene absorb heat from the wall surface of the jar 18 and also absorb friction due to stirring. The heat softens the surface of the particles. Then, the composite material is hammered into the softened particle surface by the acceleration caused by stirring and adheres to it.
Compositeization is thus achieved.

The composite raw material is put into the hopper 4 of the injection molding machine shown in Fig. 1, and the composite raw material is melted in a short time as described above, and the molten raw material is immediately injected into the cavity and molded. do.

Next, a specific example will be described.

First, 20wt% of carbon fiber powder (Torayca MLD-30, manufactured by Toray Industries, Inc.) was mixed with ultra-high molecular weight polyethylene (manufactured by Hercules Co., Ltd.) with a molecular weight of 6 million, and using a Hersiel mixer, the jar was heated with oil, and the stirring blade The resin was heated at a rotational speed of 2980 rpm for approximately 10 minutes until the resin temperature reached 120°C, and ultra-high molecular weight polyethylene was composited. This composite ultra-high molecular weight polyethylene was transferred to a cooling jar and cooled at 90 rpm for 30 minutes. Incidentally, both the heating jar and the cooling jar were filled with nitrogen gas to create a non-oxidizing atmosphere.

The composite raw material was put into the hopper of the injection molding machine according to the present invention, and the slit for adjusting the raw material supply amount at the bottom of the hopper was fully opened. Here, the screw has a diameter of 25 mm, a compression ratio of 1.8, a pitch of 18 mm, and L/
D was set to 20, and the screw rotation speed was set to 180 rpm. In addition, the temperature of the nozzle part (open nozzle) of the cylinder is 220℃, and the temperature of the compression part is 170℃.
When the feeder part was measured during molding without heating, it was 70
The temperature ranged from ℃ to 100℃. Furthermore, 1.0 for Hopper
Nitrogen gas was allowed to flow in at a rate of 1/min.

On the other hand, the mold temperature was set at 100℃, the source pressure of the cavity was increased to 0.5 atm, and the molded product had a module of 0.8
A small gear (P, C, D = 25.6 mm, number of teeth 32, thickness 3 mm) was molded using a two-point gate (gate diameter 0.4 mm).

Furthermore, the molding conditions were as follows: plasticizing time by screw was 7 seconds, injection time was 2 seconds, and curing time was 10 seconds.
seconds, extraction time 1.5 seconds, (interval) 4
seconds, the total cycle time was 23.0 seconds.

The dimensional accuracy of the gear thus obtained is 3δ = 0.12 mm when the roundness is expressed as standard deviation (δ).
The molding shrinkage rate was a uniform 4%.

Then, the above gear is similarly modular (m=
0.8) in combination with Shinchiyu's spur gear to reduce the burden.
Durability tests were conducted at 2.5Kg/cm and a rotation speed of 280rpm. Similar durability tests were also conducted on gears made of polyacetal resin, which has already been used as a resin material, and gears made of polycarbonate resin, a type of engineering plastic.

Comparing the results, the gear according to the present invention had wear resistance equal to or better than polyacetal even after 300 hours, and the noise level was lower. On the other hand, polycarbonate gears
After 30 hours, abrasion particles were generated and the noise increased rapidly.

As explained above, according to the injection molding method and injection molding machine of the present invention, the composite ultra-high molecular weight polyethylene or the ultra-high molecular weight polyethylene itself is instantly melted, and the molten raw material is immediately injected into the cavity. As a result, it is possible to easily manufacture a high-precision gear molded product. Since the resulting gear has self-lubricating properties, it does not require lubrication, and if it is further compounded, the rigidity will be at least three times greater, making it possible to prevent tooth loss.

In addition, it has lower noise than conventional polyacetal gears, and although gears made of ultra-high molecular weight polyethylene have been considered unsuitable due to mutual wear, it is possible to combine them with gears made from the composite raw materials of the present invention. It exhibits many effects such as being able to have sufficient durability.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view of an injection molding machine according to the present invention;
Figure 2 is a plan view of the cavity in the mold, Figure 3 is a vertical cross-sectional view of the mold, Figure 4 is a cross-sectional view of the mold, and Figure 5 is a cross-sectional view of the mold.
The figure is a longitudinal cross-sectional view of a jar that performs compounding. 1... Screw, 2... Cylinder, 2a...
...Feeder part, 2b...Compression part, 2c...Nozzle part, 3...Ultra high molecular weight polyethylene raw material, 4...
Hopper, 5... Cavity, 6... Mold, 9...
...Atmosphere gas supply pipe, 13...Heater, 1
5...vacuum path, 16...hole, 18...jar.

Claims (1)

[Claims] 1. A powdered ultra-high molecular weight polyethylene raw material is supplied to an injection cylinder in a non-oxidizing atmosphere, and the compression ratio of the screw of the injection cylinder is set to 1.3 to 2.0,
An injection molding method characterized in that the raw material is melted in a short time with a length to diameter ratio of 15 to 20, and is injected into a cavity before thermal crosslinking begins. 2 The ultra-high molecular weight polyethylene raw material is made by injecting ultra-high molecular weight polyethylene into particles in advance through high-speed mixing, and when the surface of each particle is softened, an inorganic composite material is added to the particle surface to attach the inorganic composite material to the particle surface. The injection molding method according to claim 1, characterized in that the injection molding method is made in such a manner as to cause the injection molding to occur. 3 An in-line system in which the raw material in the hopper is supplied into the injection cylinder, and by rotating a screw placed in the cylinder, the raw material is melted in the cylinder and injected into the mold cavity from the nozzle of the cylinder. In the screw type injection molding machine, the hopper has an atmospheric gas supply pipe for melting powdered ultra-high molecular weight polyethylene or powdered high molecular weight polyethylene with an inorganic composite material attached to the particle surface in a non-oxidizing atmosphere. A heater is provided only in the compression part and nozzle part of the cylinder, at least a part of the feeder part is used as a heat radiation part, and the compression ratio of the screw in the injection cylinder is 1.3 to 2.0, and the length and An injection molding machine characterized by a diameter ratio of 15 to 20. 4. The cavity is in the shape of a gear, a vacuum path is formed around the cavity, the vacuum path and the cavity are communicated through a hole formed in the mold, and the vacuum path is connected to a vacuum source. The injection molding machine according to claim 3, characterized in that the injection molding machines are connected in series.