JPH0712635B2 - Injection molding method for cylindrical molded products - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a molding method for obtaining a highly accurate cylindrical molded article by injection molding a thermoplastic resin.

[Conventional technology and its problems]

Molded articles made of thermoplastic resin are widely used in place of metal because they can be manufactured very easily and efficiently by injection molding and the manufacturing cost of molded articles is low.

However, resin injection-molded products generally have the disadvantage that they are inferior in dimensional accuracy compared to machined products, and in particular due to their application, they have a high degree of dimensional accuracy, especially roundness (deviation from a geometrically correct circle). Size; deviation of each diameter of a constant cross section circle and cylindricity (size of deviation from a geometrically correct cylinder; difference in size (diameter) of each cross section when viewed in the longitudinal direction of the cylinder) This is often a serious problem in a cylindrical molded product for which (1) is required.

On the other hand, as a means for improving the accuracy of the conventionally known cylindrical molded product by injection molding, the temperature of the mold is adjusted to be as uniform as possible, and in particular, the heat released from the resin is easily accumulated. Generally, the core portion corresponding to the inner diameter portion is temperature-controlled (cooled) with a heat pipe, air or the like.

Even with such a conventionally known method, the core that easily accumulates heat is cooled smoothly, so that voids, sink marks, etc. are reduced, and a molded product with a certain degree of accuracy can be obtained. In particular, it is one of the important accuracy factors for cylindrical molded products. It is quite effective in improving the roundness, which is one of the two factors, but has little effect in improving the cylindricity, which is another important precision factor. The phenomenon of becoming larger than the outer diameter cannot be solved. Specifically, as shown in FIG. 1 (C), the opening at the tip of the cylinder is protruding. That is, in many cases, the current injection molding method cannot meet the demand for a cylindrical part having a high degree of dimensional accuracy, particularly excellent cylindricity. In such a case, a cylindrical molded product once molded by injection molding. The method is extremely disadvantageous in terms of cost, such as cutting later. Therefore, there has been a strong demand for improvement of the molding method for obtaining a highly accurate cylindrical molded product only by economically advantageous injection molding.

[Means for solving problems]

In view of such problems, the present inventor has arrived at the present invention as a result of intensive studies for the purpose of obtaining a highly accurate cylindrical molded product only by injection molding, particularly a molded product excellent in cylindricity.

That is, the present invention, in injection molding a cylindrical molded article,
It is characterized in that molding is performed by using a mold having a temperature difference such that the surface temperature of the mold contacting the opening of the tip of the cylinder is higher than the surface temperature of the mold contacting the center of the cylinder by 3 ° C. or more. The present invention relates to an injection molding method for a cylindrical molded product.

Hereinafter, the present invention will be described with reference to the accompanying drawings.

First, FIGS. 1 (A) and 1 (B) show the most basic cylindrical molded product. Further, FIG. 1 (C) is a mold made for the purpose of producing a cylindrical molded product as shown in FIGS. 1 (A) and (B), and the mold is uniformly cooled by a conventionally known method, and the core is FIG. 3 schematically shows the shape of an actual molded product obtained when the portion is cooled. As described above, the cylindrical tip opening portion is flipped up, and the outer diameter in the vicinity of the cylindrical tip opening portion 1 is It is inevitable that the outer diameter will be larger than the outer diameter and the cylindricity will be insufficient.

On the other hand, according to the present invention, it is possible to mold using a mold provided with a temperature difference such that the surface temperature of the mold contacting the opening at the tip of the cylinder is higher than the surface temperature of the mold contacting the center of the cylinder. This is a feature, and by doing so, a cylindrical molded product having extremely good cylindricity can be obtained.

The temperature difference on the surface of the mold provided here is at least 3 ° C. or higher, preferably 5 ° C. or higher. There is no particular upper limit for the temperature difference, but in practice it is about 60 ° C.

In the present invention, means for providing such a mold temperature difference is
Although not particularly limited, for example, a method of heating by providing a heater, for example, an electric heater, in a position close to the cavity at the tip of the mold is provided with a liquid or gas flow at a position close to the cavity at the tip of the mold. A method in which a passage is provided and a temperature-controlled liquid or gas is flowed so that the surface temperature of the mold contacting the opening at the tip of the cylinder is higher than the surface temperature of the mold contacting the center of the cylinder in the flow path, That is, a method of flowing a liquid or gas having a temperature of 5 ° C., preferably 10 ° C. or higher, more preferably 15 ° C. or higher from a cooling medium which is dominant in the temperature control of the central part of the cylinder in the flow path. A liquid or gas flow path is provided at the position where the temperature is adjusted so that the surface temperature of the mold contacting the opening at the tip of the cylinder is lower than the surface temperature of the mold contacting the opening at the end of the cylinder. Liquid or A method of flowing a gas, that is, a method of flowing a liquid or gas having a temperature of 5 ° C. or higher, preferably 10 ° C. or higher, more preferably 15 ° C. or higher from a cooling medium which is dominant in the temperature control of the opening at the tip of the cylinder, is preferable. It is more effective and preferable to use two or more of these, particularly or together.

Further, in order to improve the heat radiation and cooling of the central part of the cylinder, it is extremely possible to form the mold contacting the central part of the cylinder with a material having good thermal conductivity and the mold contacting the opening at the tip of the cylinder with a material having poor thermal conductivity. It is valid.

In the present invention, it is preferable that the heater and the liquid or gas passage are arranged along the outer circumference of the cylinder in order to efficiently control the temperature of the desired portion.

Also, since the cylindricity sharply decreases in the vicinity of the opening at the tip of the cylinder (for example, 1 in FIG. 1), the mold temperature is raised only in this part as much as possible, and the temperature is wide and almost constant in the center of the other cylinder. It is preferable to keep the temperature, and therefore, it is preferable to provide a heat insulating layer in the mold between the opening of the tip of the cylinder and the center of the cylinder, which require high temperature.

Further, in the present invention, it is also possible to use temperature control of the entire mold, which is usually used for molding a cylindrical molded product, cooling of the core, etc. in combination, and an excellent cylindrical molding having both good cylindricity and roundness. It is more preferable because a product can be obtained.

Next, some embodiments of the cylindrical molded article to which the present invention can be applied will be illustrated and described.

That is, the cylindrical molded product of the present invention is not limited to one having only a complete cylindrical shape as shown in FIG. 1, but a plurality of cylindrical bodies are combined to form one molded product as shown in FIGS. 2 and 3. Forming part of the cylinder as shown in Fig. 4, especially 1/4 of the circumference
With the following notches, as shown in Figs. 5 and 6.
At least one of the ends of the cylinder has a cylindrical portion, such as one in which one end of the cylinder is blocked or one in which the cylindrical body is integrally incorporated into a part of the molded article as shown in FIG. Anything that is open on one side is included.

The present invention is effective for improving the dimensional accuracy of the cylindrical portion, especially the cylindricity for any of the molded products.

In particular, as shown in FIG. 1, the one having only the cylindrical portion having the same inner diameter and the outer diameter and the one having a shape very close to this have remarkable effects because the temperature control of the desired portion is easy as described above. .

Further, as the wall thickness of the cylinder portion increases, the deformation due to warpage or sink marks at the center portion of the cylinder increases, which controls the dimensional accuracy, and the effect of the present invention is relatively small.
Therefore, the present invention is preferably applied to a cylinder having a wall thickness / outer diameter ratio of 1/3 or less.

Further, if the cylinder length is too short, it will be extremely difficult to provide a temperature difference between the mold at the tip of the cylinder and the mold at the center, so the present invention is applied to a cylindrical molded product having a cylinder length of approximately 10 mm or more. Is preferred.

As described above in the present invention, it is not always clear why providing the mold with a temperature difference is effective in improving the cylindricity of the cylindrical molded product, but the present inventors speculate as follows.

That is, considering the solidification process of the filled molten resin in the molding of a cylindrical molded product, the direction of heat radiation diffusion from the opening at the tip of the outer periphery of the cylinder (for example, part 1 in FIG. 1) is wide and this part solidifies. On the other hand, the heat radiation and diffusion direction from the central portion of the cylinder (for example, the portion of FIG. 1) is limited, but it is more difficult to solidify than the tip opening portion.

For this reason, if the entire mold is cooled to a constant temperature according to the conventional technique, the opening at the tip of the cylinder is first solidified in a state closer to the size of the mold, and the central part of the cylinder which is solidified later is sized due to resin molding shrinkage Will be small.

On the other hand, according to the method of the present invention, since the cylindrical tip portion is warmed from the central portion, the solidification speeds of the tip opening portion (particularly the outer peripheral tip portion) and the central portion are close to each other, and the shrinkage is uniform throughout. It is considered that excellent cylindricity is achieved.

The present invention is presumed to exhibit the effect by the mechanism as described above, and can be applied to any thermoplastic resin as long as it causes molding shrinkage, and it is possible to manufacture a highly accurate cylindrical molded product, In particular, it exhibits an excellent improving effect on a crystalline resin having a large molding shrinkage, such as a polyacetal resin, a polyester resin, a polyamide resin.

〔Example〕

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

Examples 1 to 3 and Comparative Example 1 As shown in FIG. 8, the heater 6 is embedded in the mold close to the one end opening of the cylindrical molded product 3, and the heat transfer from the heater to the center of the cavity is performed. In order to prevent this, a mold provided with a heat insulating material 7 is used to control the heating of the heater to change the surface temperatures of the mold contacting the opening at the tip of the cylinder and the mold contacting the center of the cylinder as shown in Table 1. A cylindrical molded product was molded. The resin used here is a polyacetal resin [DURACON M270 manufactured by Polyplastics Co., Ltd.].
And the mold size of the cylindrical molded product is 12 mm outer diameter, 6 mm inner diameter,
It is 30 mm long. The obtained molded product was evaluated for cylindricity according to JIS B0621, and Table 1 shows the deviation of the outer diameter of the opening at the tip of the cylinder from the center of the cylinder.

Further, for comparison, the same molding was performed using a mold without the heater 6 and the heat insulating material 7 and evaluated. The results are also shown in Table 1.

Molding conditions injection pressure 1400 kg / cm ² pressure keeping 750 kg / cm ² Injection speed 1 m / min resin temperature of 200 ° C. Mold temperature (coolant temperature) 60 ° C. molding cycle injection-holding time 10 seconds Cooling time 15 seconds intermediate time 3 seconds Examples 4 to 8 and Comparative Examples 2 and 3 As shown in FIG. 9, a mold 14 contacting the central portion of the cylindrical molded product 3 was used.
Is made of beryllium with good thermal conductivity, the mold 13 that comes into contact with the opening at the tip of the cylinder is made of stainless steel with poor heat conductivity, and is independently provided in the mold near the center of the cylinder and the opening at the end of the cylinder. Using a mold provided with liquid temperature controllable liquid channels (12 and 11), the temperature of the liquid flowing in the flow channels was changed as shown in Table 2 to form a cylindrical molded article.

The resin used here was a polyacetal resin (manufactured by Polyplastics Co., Ltd., DURACON VC-10), and the mold dimension of the cylinder was 6 mm in outer diameter, 4.8 mm in inner diameter, and 15 mm in length.

Further, as Comparative Examples 2 and 3, a mold having the same structure as that of FIG. 9 but having the same mold as a whole made of the same material (stainless steel) was used, and the cooling liquid temperatures were all the same to form a cylindrical molded product. did.

The cylindricity of the obtained molded product was measured in the same manner as in Examples 1 to 3. The results are shown in Table 2.

Molding conditions Resin temperature 200 ℃ Mold temperature (coolant) temperature (Refer to Table 2) Injection pressure 1200kg / cm ² Holding pressure 700kg / cm ² Injection speed 1m / min Molding cycle Injection / holding time 1.5 seconds Cooling time 2 seconds Intermediate time 2 seconds [Effect of the invention] As is apparent from the above description and the examples, according to the present invention, the surface temperature of the mold contacting the opening at the tip of the cylinder is 3 times less than the surface temperature of the mold contacting the center of the cylinder. A cylindrical molded product molded by using a mold having a temperature difference such that the temperature rises by ℃ or more is extremely excellent in dimensional accuracy, particularly cylindricity, and does not impair the physical properties and appearance of the molded product. For this reason, economically disadvantageous post-processing such as cutting is unnecessary, productivity is improved, and manufacturing cost is significantly reduced.

[Brief description of drawings]

FIG. 1 shows the most basic embodiment of the cylindrical molded product of the present invention, and (A) and (B) are a perspective view and a sectional view, respectively. Further, FIG. 1C is a cross-sectional view schematically showing a state of defective cylindricity which occurs when this is molded by a conventional molding method. 2 to 7 exemplify various embodiments of a cylindrical molded article to which the present invention is applied, and (A) and (B) are a perspective view and a sectional view, respectively. FIG. 8 is a schematic sectional view of the mold used in Examples 1 to 9,
The figure is a schematic sectional view of a mold used in Examples 4 to 8. 1: Cylindrical tip opening 2: Cylindrical center part 3: Cylindrical molded product 4: Sprue 5: Runner 6: Heater 7: Heat insulating material 8: Normal mold cooling liquid passage 9: Heat pipe 10: Parting surface 11: Liquid flow path (high temperature side) 12: Liquid flow path (low temperature side) 13: Stainless steel mold block 14: Beryllium mold block 15: O-ring

Claims (4)

[Claims] 1. A temperature difference such that the surface temperature of a mold contacting the opening at the tip of the cylinder is 3 ° C. or more higher than the surface temperature of the mold contacting the center of the cylinder in injection molding a cylindrical molded product. An injection molding method for a cylindrical molded article, which is characterized by molding using a metal mold having a mold. 2. The cylindrical molded product according to claim 1, wherein the temperature difference between the mold at the tip of the cylinder and the temperature at the mold at the center of the cylinder is given by any one or a combination of two or more of the following methods. Injection molding method. A method of heating by providing a heater in a position close to the cavity at the tip of the cylinder of the mold. A liquid or gas flow path is provided at a position close to the cavity at the tip of the cylinder of the mold, and the surface temperature of the mold in contact with the opening at the tip of the cylinder is higher than the surface temperature of the mold in contact with the center of the cylinder. A method of flowing a temperature-controlled liquid or gas so that the temperature becomes high. A liquid or gas flow path is provided in a position close to the center cavity of the mold, and the surface temperature of the mold contacting the center of the cylinder is lower than the surface temperature of the mold contacting the opening at the tip of the cylinder. A method of flowing a liquid or gas whose temperature is controlled so that 3. The mold according to claim 1, wherein the mold contacting the central part of the cylinder of the cylindrical molded product is made of a material having better thermal conductivity than the mold contacting the opening at the tip of the cylinder. The method for injection molding a cylindrical molded article according to item 2. 4. The mold according to any one of claims 1 to 3, wherein a heat insulating layer is provided in the mold between the opening at the tip of the cylinder and the center of the cylinder of the cylindrical molded product. The method of injection molding a cylindrical molded article according to item 1.