JP2005279949A - Injection molding method of ring-shaped rubber product - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for injection-molding a novel presice rubber product such as an O-ring or the like so as to reduce percent defective. <P>SOLUTION: In the method for injection-molding the ring-shaped rubber product such as the O-ring, the molten rubber molding material (hereinbelow referred to as "molten material") in an injection pot (injection barrel) 14 is injected in a mold 32 from an injection nozzle 16. At this time, the material flow in a gate 46 is divided to be allowed to flow to both side parts of a product cavity 48 by the flat sheet-like weir 49 formed on a movable mold (lower mold) 38 side. Then, shearing friction heat is applied by the thin-walled orifice 50a with a predetermined width connected to the product cavity of the gate to inject (charge) the material in the product cavity 48 while enhancing flowability. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an injection molding method for a ring-shaped rubber product, and is particularly suitable for manufacturing precision rubber parts such as O-rings, piston seals, piston cups, V-packings, and oil seals.

  Here, a case where an O-ring is injection-molded as a ring-shaped rubber product will be described as an example.

  O-rings, which are precision rubber products, are generally compression (compression molding). The reason is as follows.

  When trying to obtain a ring-shaped rubber product with sufficient accuracy and fusion strength by injection molding a precision rubber product, it is injected (injected) into the product cavity with good fluidity, and as much as possible It is necessary to prevent scorch (early vulcanization) from occurring during flow.

  However, if the material injection temperature is set to a relatively high temperature in order to inject with good fluidity, scorch is likely to occur before the rubber material (melted) reaches the product cavity gate. Also, if the fluidity is low, the flow velocity in the cavity is slow, and scorch (early vulcanization) occurs before fusing and fusion lines (weld marks) are likely to occur (it is difficult to obtain sufficient fusion strength).

  Therefore, in order to obtain a fusion strength, it is necessary to perform injection molding at a relatively high pressure (high speed).

  However, when trying to injection mold at a high pressure, it is necessary to relatively reduce the viscosity at a high temperature. When injected into the product cavity with the injection temperature kept high, the material flow resistance is increased by scorch in the cavity of the molding material, resulting in fusion joint / strength failure and filling failure (short shot). Cheap. That is, a lot of molded product defects occur.

  However, compression molding requires a longer vulcanization time (clamping time) than injection molding, and productivity is low.

  For example, when molding material: NBR rubber compound, mold temperature: 170 ° C., general mold clamping time is compression molding: 4-6 min, injection molding: around 2 minutes.

  In compression molding, burrs having a longer burr length than the injection molding (for example, 5 mm or more inside and outside at an inner diameter of 30 to 50 mm) are generated on the entire circumference. And, the deburring (handing) requires that the protrusion length (f) of the deburring mark be 0.10 mm or less (see Tables 1 to 3 of JIS B 2401), and the deburring man-hour is increased.

  Therefore, establishment of a method for injection molding a new precision rubber product such as an O-ring with a low defect rate is demanded.

  Although not affecting the inventiveness of the present invention, Patent Document 1 proposes a technique related to an injection mold having the following configuration for manufacturing a precision rubber product such as an O-ring by injection molding. This invention is based on the premise that a liquid rubber composition such as a liquid fluororubber composition is used as a molding material, unlike a normal kneaded rubber as in the present invention.

“After the molding material supplied from the molding material supply gate branches and flows in at least two branches, it merges and communicates with the cavity in an injection mold having a cavity where the molding material fuses at this junction. At least two air vent gates are arranged so as to be shifted from the merging point so that the fusion area is larger than the fusion surface of the molding material formed when these air vent gates are provided at the merging point. Injection mold characterized by. "
JP 2003-145588 A

  In view of the above, the present invention has an object (problem) to provide a method for injection molding a precision rubber product such as a novel O-ring, which is not described in the above-mentioned patent documents, with a low defect rate.

  The present inventors have found that the above-mentioned problems can be solved by devising the shape of the gate part of the product cavity in the process of diligently developing to solve the above-mentioned problems, and have arrived at the present invention having the following configuration. .

A method of injection molding a ring-shaped rubber product,
A melted molding material (rubber compound) in the injection pot (chamber) (hereinafter “molten material”) is injected from the injection nozzle into the mold, and the material flow at the gate is applied to both sides of the product cavity. Further, the material is injected into the product cavity by applying shear friction heat through a thin-walled orifice having a predetermined width connected to the product cavity of the gate.

  By diverting the material flow to both sides of the ring-shaped product cavity at the gate, the time to reach the fusion part (position opposite to the gate part) is shortened, and scorch is unlikely to occur in the product cavity. Further, since shear frictional heat is applied by a thin orifice having a predetermined width connected to the product cavity of the gate, the viscosity of the material flowing into the product cavity is reduced, and the fluidity is improved. These synergistically make it easy to obtain a ring-shaped rubber product with excellent dimensional accuracy, and it is easy to ensure the strength of the fusion part (weld line), and furthermore, a product with an outer peripheral burrless can be obtained.

  Therefore, productivity is significantly improved as compared with the case of manufacturing a ring-shaped rubber product by compression molding.

  In the above, the injection of the molten material into the mold is provided with an injection pot provided with an injection nozzle at the tip, and provided with a molten material inlet on the tip side, and a plunger slidable in the injection pot. Temperature distribution of the molten material in a cross-section in the injection pot, and a pre-plastic type injection molding machine having a plasticizing extruder in which the molten material charging port and the tip discharge port communicate with each other In addition, it is desirable to perform injection into the mold with the viscosity distribution made as uniform as possible. By adopting the above-described configuration, it becomes possible to inject a uniform rubber material having a reduced viscosity into the mold, and the above-mentioned operation and effect are more sure.

  The injection pot has a plunger fitting portion with a tip portion continuous with the injection nozzle corresponding to the tip shape of the plunger, and is insulated on each inner surface of the injection nozzle and the plunger fitting portion of the injection pot. It is desirable to form a coating layer. Since heat radiation from the injection pot wall surface of the molten rubber in the injection pot is blocked, the temperature and / or viscosity of the molten rubber in the pot can be easily stabilized.

  The dimension specification of the thin orifice is usually set to a height (gap at the time of clamping) of about 0.05 to 0.1 mm and a land length (width) of 0.02 to 0.05 mm.

  As a molding material, it is desirable to apply to the product which consists of a NBR rubber compound. This is because the NBR rubber compound has a relatively high viscosity and is a material applied to precision rubber products such as an O-ring.

  And the structure of the rubber injection mold applied to the manufacturing method of the present invention is as follows.

A rubber injection mold for molding a ring-shaped rubber product,
It comprises a fixed mold and a movable mold.
A fan gate is provided, a flat weir is provided at the center of the product cavity side of the fan gate to guide the material flow direction to the left and right sides of the product cavity. At least one of the mold dividing surfaces is a flat concave surface so that a thin orifice with a width can be formed when the mold is clamped.

  And in the said metal mold | die, while making the height (gap at the time of clamping) of a thin orifice into about 0.05-0.1 mm, the land length (width) shall be about 0.02-0.05 mm.

  In the injection mold according to the present invention, for example, when molding an O-ring or the like, the product cavity is annular, and the hand groove is formed only on the outer peripheral surface of the orifice product cavity. The configuration is as follows.

  In addition, the rubber injection mold according to the present invention preferably includes a reduced pressure suction path that can communicate with the product cavity when the mold is clamped. This is because, in the injection molding method, suction and pressure reduction can be performed prior to injecting the rubber material into the product cavity.

  Hereinafter, one of the best modes according to the present invention will be described.

  FIG. 1 shows a partial cross-sectional view of a rubber injection molding die 14 incorporated in a rubber injection molding machine 12 used in this embodiment. Here, a vertical injection molding machine will be described by taking a multiple die as an example, but the same applies to a horizontal injection molding machine or a single die. However, in consideration of the effect of gravity on the material flow, a vertical (竪) type injection molding machine is desirable, and from the viewpoint of productivity, it is desirable to pick a large number.

  The rubber injection molding machine used in the present embodiment is a pre-plastic type, and for example, the one described in Japanese Patent Application Laid-Open No. 2003-11189 can be suitably used.

  Basically, an injection machine 12 and a plasticizing extruder 24 are provided.

  The injection machine 12 includes an injection pot 18 having an injection nozzle 16 attached to the tip and a molten material inlet 20 on the tip side, and a plunger 22 slidable in the injection pot 18.

  The plasticizing extruder 24 includes an extrusion barrel 25 and a kneading extrusion screw 26 in the extrusion barrel 25. The tip discharge port 27 of the extrusion barrel 25 communicates with the molten material charging port 20 of the injection pot 18 via the communication path 28.

  A check valve 30 is incorporated in the tip discharge port 27 so that the molten material put into the injection pot 18 does not flow backward.

  In the present embodiment, the injection pot 18 is a plunger fitting portion 22 a in which the tip portion continuous with the injection nozzle 16 attached to the tip is a fitting shape corresponding to the tip shape of the plunger 22. ing. And the heat insulation coating layer (not shown) is formed in each inner surface of the injection nozzle 16 and the plunger fitting part 22a. Further, a temperature sensor 19 can be provided on the tip end side of the plunger fitting portion 22a to send a signal to the temperature control heater 31 for control.

  Here, each shape of the plunger tip end portion 22a and the plunger fitting portion 18a of the injection pot 18 is preferably a longitudinal section taper as shown in the figure. This taper angle is usually selected as appropriate in accordance with the molding material characteristics within a range of 30 to 60 °.

  The heat insulating coating layer is usually formed of a ceramic spray coating layer. For example, it is assumed that the surface is polished with a total thickness (undercoat, topcoat) of 80 μm with alumina / titania 50/50 (see paragraph 0033).

  In addition, the temperature control means 31 is each attached to each outer periphery of the plasticizing extruder 24 and the injection barrel 14. In the illustrated example, the temperature adjusting means 31 is a heat medium (oil or superheated steam), but may be resistance heating, induction heating, or the like.

  The details of the rubber injection mold 32 in this embodiment are shown in FIGS. 2 to 3, and a plan sectional view of the O-ring molded with the mold is shown in FIG. This mold is for molding a ring-shaped rubber product (O-ring) 32. The size of the O-ring (molded product) 34 to which the present invention can be applied is usually an inner diameter of 10 to 500 mmφ and a thickness of 5 to 150 mmφ (refer to the attached table of JIS B 2401), but an inner diameter of 30 to 200 mmφ and a thickness: The effect of the present invention becomes remarkable when applied to a material having a diameter of 2 to 10 mm.

  The metal mold 32 basically includes a fixed mold (upper mold in the illustrated example) 36 and a movable mold (lower mold in the illustrated example) 38. Here, the fixed mold 36 and the movable mold 38 have holes and depressions so that a flow path (sprue 42, runner 44, gate 46) and a product cavity (product shaping part) 48 of the molten material are formed when the mold is closed. Is formed. Although not shown, the fixed mold and the movable mold are provided with a heat medium flow path so that the mold can be vulcanized. The following description is based on the time of mold clamping unless otherwise specified.

  The gate 46 is a fan gate (a divergent shape), and a flat plate weir (a crescent shape in the illustrated example) 49 that guides the material flow direction to the left and right sides of the product cavity 48 at the center on the product cavity side of the fan gate 46. The product cavity inflow ports 50a, 50a of the branch inflow passages 50, 50 formed on both sides of the flat plate weir 49 are formed into slit-like thin orifices. That is, in the illustrated example, at the product cavity inlets 50a and 50a, both surfaces of the mold dividing surface are flat concave surfaces, and the product cavity inlets 50a and 50a are thin orifices.

  In the example shown in the figure, the shape of the flat plate weir 50 is a crescent shape, but it is arbitrary as long as it can be formed in a triangular shape (two-dot chain line), a trapezoidal shape, or the like to form a branch inflow path.

  The height (gap at the time of mold clamping) and the land length of the thin orifice 50a vary depending on the material characteristics and the size of the O-ring. For example, when an O-ring having an inner diameter of 30 to 200 mm and a thickness of 2 to 10 mm is formed using a medium / high nitrile type NBR rubber compound, the setting is performed as follows.

Orifice height h: Usually 0.05 to 0.1 mm (preferably 0.06 to 0.08 mm)
Land length (width): 0.02 to 0.05 mm (preferably 0.02 to 0.03 mm)
Further, the mold dividing surface (PL) gap (at the time of mold clamping) in the plate-like weir 48 site is such that the molten material does not flow in and mold clamping does not occur at the time of mold clamping. Although it varies depending on the material characteristics and the size of the O-ring, it is usually set to 0 to 0.03 mm when the O-ring having the above setting is formed.

  Further, in this mold, the hand groove 52 is formed only on the outer peripheral surface of the connection portion of the gate 46 in the cavity 48 from the viewpoint of gate removal workability.

  Further, in the present embodiment, the mold is provided with a vacuum suction path 53 that can communicate during mold clamping in order to improve the inflow characteristics of the molten material into the product cavity 48. The vacuum suction path 53 includes a vacuum suction pipe 53a and a vacuum accumulator 53b.

  Further, in order to ensure suction under reduced pressure, an urging means is provided to ensure a clearance for suction under reduced pressure around the outer periphery of the O-ring of the product cavity when the mold is closed. Specifically, a plurality of pairs of spring seat protrusions 36a and spring seat recesses 38a are formed between the fixed mold (upper mold) 36 and the movable mold (lower mold) 38, and the protrusions 36a and the recesses 38a A concave compression coil spring (biasing means) 51 is interposed therebetween. Of course, the compression coil spring 51 has a spring constant that is easily compressed by a clamping load. Reference numeral 54 denotes an O-ring for ensuring sealing performance.

  Next, the case where an O-ring is manufactured using the injection molding machine and the molding die will be described.

  The molds 32 and 38 are heated by the upper and lower heating plates 37 and 39.

  First, a rubber material for molding (for example, in the form of a ribbon) is put into a material charging port of a plasticizing extruder (screw pre-plastic type) 24, and is kneaded with a screw while being heated by a temperature control heater 31, thereby plasticizing (melting). ) The melted material is put into the injection pot 18 of the injection machine (plunger type) 12 through the communication passage 28 from the tip discharge port 26 of the plasticizing extruder 24.

  When the tip of the injection pot (injection chamber) 18 is filled (stored) with a predetermined amount or more, the plunger 22 is operated to inject the molten material into the mold 32 from the injection nozzle 16. .

  At this time, since it is a pre-plastic injection molding machine, the molten material of the injection pot 18 is not affected by the shearing heat generated by the kneading in the plasticizing extruder 24, and the tip 18a and the injection nozzle 16 of the injection pot 18 are not affected. The inner surface is hardly affected by external heat due to the heat insulating coating layer. Accordingly, the molten material can be injected into the mold with the temperature distribution and / or viscosity distribution of the molten material made as uniform as possible.

  The molten material temperature and injection pressure here differ depending on the material characteristics, the size of the O-ring, and the mold temperature (mold vulcanization temperature). When the size of the material and the O-ring is the mold vulcanization temperature 160 to 180 ° C. in the above setting, for example, the molten material temperature (injection chamber temperature): about 85 to 120 ° C. (desirably about 90 to 110 ° C.), Injection pressure: 150 to 180 MPa (desirably 160 to 170 MPa).

  If the injection pot temperature (melting material temperature) is too low, it is difficult to obtain a predetermined fluidity (Mooney viscosity) at the time of injection. Conversely, if the injection pot temperature is too high, the Mooney time will be short, and the inside of the chamber or mold There is a risk of scorching in

  More specifically, the injection material temperature shows a minimum Mooney viscosity (Vm) of about 45 or less, preferably about 30 in the Mooney scorch test (JIS K 6300), and Mooney scorch time (ts: minimum Mooney viscosity). The time from the start of heating until the temperature rises to 5M from 20 to 20 minutes or more, preferably 30 minutes or more. Such temperature becomes each said temperature.

  The inside of the product cavity 48 of the mold 32 is previously in a reduced pressure state by sucking under reduced pressure after closing the mold and before closing the mold. The degree of decompression at this time is 70 to 74.5 kPa (absolute pressure).

  The molten material injected into the clamped mold 32 reaches the gate 46 via the sprue 42 and the runner 44.

  Here, the gate 46 is a fan gate, and the molten material diffuses vigorously in a laminar manner in a divergent form, becomes a branch flow by the flat plate weir 38, and flows from the branch inflow path 50 to the product cavity inlet (thin orifice) 50a. And flows into the product cavity 48 to fill the product cavity 48. Then, after the molten material is cured (added), the mold is opened to release the molded product.

  The vulcanization time at this time mainly depends on material characteristics, injection pot temperature (molten material temperature), and mold temperature.

  For example, when a general-purpose medium / high nitrile rubber compound for O-ring molding is used and the injection pot temperature (melting material temperature) is 90 to 110 ° C. and the mold temperature is 165 to 170 ° C., the vulcanization time (clamping time) ) About 15 to 90 s.

  The O-rings manufactured under the above conditions, as shown in the examples described later, hardly generate burrs on the outer periphery, and the vulcanized state (no fusion line) is also used in the conventional compression molding. It was not inferior to that produced by

  The reason is considered as follows.

  Compared to the case where a molten material with a uniform viscosity or temperature distribution is not split by the material flowing in the ring-shaped product cavity from the gate toward the maximum bulge on both sides of the ring (the vertical center line position of the ring). Time to material fusion is shortened. In addition, since the molten material is squeezed by the thin-walled orifice 50a and is subjected to a shearing action, the temperature of the molten material is increased and the fluidity is improved (viscosity is decreased), and the time until material fusion is further shortened. From the gate to the fusion part, it reaches the fusion part while scorching in synergy with the heat from the shaping surface (outer peripheral surface) side of the product cavity.

  In the above description, the case where a nitrile rubber (NBR) compound is used as the molding material has been described as an example. However, other hydrogenated NBR, fluorine rubber (FKM), silicone rubber (Q), etc. are also used. Is possible.

  Hereinafter, in order to confirm the effect of this invention, the Example performed with the comparative example is described.

The dimensional specifications of the O-ring were an inner diameter (reference dimension d ₁ ): 133.5 mm and a thickness (reference dimension d ₂ ): 4.5 mm.

<Example>
Using a general-purpose medium / high nitrile rubber compound for gasket injection molding, the injection chamber temperature (melting material temperature): 105 ° C., mold temperature: 170 ° C., and the mold clamping time were varied as shown in Table 1, and the sample (O The ring) was injection molded (set injection pressure: 170 MPa).

  The fan gate specifications of the mold used were as follows.

Fan gate: Connection part runner (rectangular cross section): 4 mm width x 1.5 mm height, fan taper angle: 60 °, wide part full length (material flow direction central axis position) L: 5 mm, full width W: 60 mm ,
General part height: 0.7 mm, Product cavity inlet (thin orifice) height: 0.08 mm, Land length: 0.03 mm,
Flat plate weir (Crescent shape): Product cavity side R: 73, 25 mm, weir crest R: 2, 5 mm, material flow direction length L ₂ : _2, 5 mm, height (thickness) 0.55 mm
<Conventional example>
Using a general-purpose medium / high nitrile rubber compound for gasket compression molding, compression molding was performed under conditions of a mold temperature of 170 ° C. and a mold clamping time (vulcanization time) of 6 minutes.

<Test method>
Using the molded product (O-ring) obtained above as a sample (test piece), the following swelling test (solvent: toluene first grade, immersion time: 24 h) was performed. At that time, the weight before and after the test was measured, and “100 × (weight after test / weight before test)” was determined as the swelling ratio (arithmetic average value of n = 3).

<Test results and discussion>
From Table 1 showing these results, the O-ring obtained by the injection molding method of this example is not inferior in swelling ratio to that obtained by compression molding, and the vulcanization time is about 25 s. If it is more than it, it turns out that a vulcanization | cure state is favorable. In addition, as the vulcanization time becomes longer, the swelling rate increases. The reason is that, in this injection molding method, vulcanization is completed to the extent before reaching 60 s, and therefore, further vulcanization, conversely, rubber performance may be deteriorated by vulcanization reversion.
Moreover, in each Example, the burr | flash revealed on the outer periphery did not generate | occur | produce.

It is a fragmentary sectional view of the metal mold for rubber injection molding built in the rubber injection molding machine used for one embodiment. FIG. 3 is a schematic partial cross-sectional view of the rubber injection mold when the mold is closed just before the molten material is injected. It is a schematic partial sectional view at the time of mold clamping at the time of completion of molten material injection of the same mold for rubber injection molding. FIG. 4 is a schematic cross-sectional view taken along line 4-4 of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 12 ... Plunger type injection machine 14 ... Plasticizing extruder 16 ... Injection nozzle 18 ... Injection pot 22 ... Plunger 32 ... Mold for rubber injection molding 36 ... Fixed mold ( Upper mold)
38 ... Movable type (lower type)
42 ... Sprue 44 ... Runner 46 ... Gate 48 ... Product cavity 49 ... Flat plate weir 50 ... Branch inflow channel 50a ... Product cavity inlet (thin orifice)

Claims (10)

A method of injection molding a ring-shaped rubber product,
A melted molding material (rubber compound) in the injection pot (hereinafter referred to as “molten material”) is injected from the injection nozzle into the mold, and the material flow at the gate is divided into both sides of the product cavity. Furthermore, a ring-shaped rubber product injection molding method is characterized in that material is injected into the product cavity by applying shear frictional heat with a thin orifice having a predetermined width connected to the product cavity of the gate portion.   Injection of the molten material into the mold includes an injection pot provided with an injection nozzle at the tip, a melt material inlet at the tip, and a plunger slidable in the injection pot. And a temperature distribution of the molten material in a transverse section in the injection pot and / or a pre-plastic type injection molding machine provided with a plasticizing extruder in which the molten material charging port and the tip discharge port communicate with each other 2. The injection molding method for a ring-shaped rubber product according to claim 1, wherein the injection into the mold is performed with the viscosity distribution made as uniform as possible.   The injection pot is a plunger fitting part having a shape corresponding to the tip shape of the plunger, the tip part continuous with the injection nozzle, and each of the injection nozzle and the plunger fitting part of the injection pot. The ring-shaped rubber product injection molding method according to claim 2, wherein a heat insulating coating layer is formed on the inner surface.   The ring-shaped rubber product injection molding method according to any one of claims 1 to 3, wherein the inside of the cavity is sucked and decompressed prior to injection of the rubber material into the product cavity.   2. The height of the thin orifice (gap at the time of mold clamping) is set to about 0.05 to 0.1 mm, and the land length (width) is set to 0.02 to 0.05 mm. The ring-shaped rubber product injection molding method according to any one of -4.   6. The ring-shaped rubber product injection molding method according to claim 1, wherein the molding material is an NBR rubber compound. A rubber injection mold for molding a ring-shaped rubber product,
Comprising a fixed type and a movable type,
The gate is a fan gate, and includes a plate-like weir that guides the material flow direction to the left and right sides of the product cavity at the center on the product cavity side of the fan gate, and branch inflow passages formed on both sides of the plate-like weir. A mold for rubber injection molding, characterized in that the product cavity inlet is a slit-like thin orifice.   8. The rubber according to claim 7, wherein the height of the thin orifice (gap at the time of mold clamping) is 0.05 to 0.1 mm, and the land length (width) is 0.02 to 0.05 mm. Injection mold.   The rubber injection mold according to claim 7 or 8, wherein the product cavity has an annular shape, and a hand groove is formed only on an outer peripheral surface of the orifice product cavity. The rubber injection mold according to any one of claims 7 to 9, wherein the product cavity is provided with a vacuum suction path that can communicate with the product cavity during mold clamping.

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