JP2741648B2 - Polyacetal resin molded product - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a molded article of a polyacetal resin. More specifically, the present invention relates to a molded product of a polyacetal resin having a small molding shrinkage, good dimensional accuracy, and excellent mold reproducibility.

[0002]

2. Description of the Related Art Conventionally, in injection molding, as a method of suppressing shrinkage (molding shrinkage) due to cooling of a molded article and improving mold reproducibility of the molded article, a resin holding pressure is applied after completion of injection of a molten resin. The dwelling method is known.

Also, a hollow injection molding method is known as a molding method which also suppresses molding shrinkage and improves mold reproducibility of a molded product.

The above-mentioned hollow injection molding method is performed by combining an ordinary injection molding machine and a gas injection device. A pressurized gas is injected into a mold during or after injection of a molten resin, and the mold is injected. A hollow part is formed by a pressurized gas in the molten resin in the cavity, and the molded article obtained is a hollow molded article by a molding method in which the molded article is cooled while appropriately maintaining the pressure of the pressurized gas in the hollow part. Become.

[0005] In the resin pressure-holding method, after the gate sealing, the resin is solidified at the gate portion and the supply of the molten resin is stopped, so that the resin pressure-holding cannot be applied. There is also an advantage that the pressure can be maintained by the pressurized gas.

Accordingly, in the hollow injection molding method, the pressure-holding state is more reliable than in the resin pressure-holding method, and the shrinkage of the resin due to cooling is compensated for by the expansion of the hollow portion. There is an advantage that reproducibility is good and excellent dimensional accuracy is easily obtained.

[0007]

By the way, polyacetal resin is an engineering resin having excellent slidability, heat resistance, chemical resistance and creep resistance, and is used for automobiles, general machinery, precision machinery, electric / electronic equipment, etc. Is widely used for products in various fields. With the recent advancement of technologies in various fields, demands for dimensional accuracy of polyacetal-based resin molded products have been increasing, and responding to this has been a technical problem.

However, since the polyacetal resin is a crystalline thermoplastic resin, it shrinks greatly when solidifying from a molten state. Therefore, in the case of a polyacetal-based resin molded product, measures to prevent molding shrinkage from only the molding method such as the above-mentioned hollow injection molding method are insufficient, and there is a problem that dimensional accuracy sufficient to meet requirements cannot be obtained.

The present invention has been made in view of the above problems, and has as its object to provide a molded product made of a polyacetal resin having excellent characteristics as an engineering resin and having excellent dimensional accuracy.

[0010]

In order to achieve the above object, according to the present invention, a polyacetal resin having a melting point of 150 to 161 ° C. and comprising a polyacetal resin as a main component is formed by a hollow injection molding method. And a polyacetal resin molded article having a hollow portion.

In the present specification, the polyacetal resin containing a polyacetal resin as a main component refers to a polyacetal resin alone or a mixed composition of a polyacetal resin and another resin containing 50% by weight or more of a polyacetal resin. In addition, additives, fillers, coloring agents and the like which are usually used may be added.

As the polyacetal resin,-(CH
₂ O) - homopolymer consisting oxymethylene units represented by the oxymethylene _{units, - (C 2 H 4 -O} )
A copolymer of oxyethylene units represented by-, in which the oxymethylene units are 50 mol% or more, oxymethylene units and-[(CR ₁ R ₂ ) _m -O]-
(Where R ₁ and R ₂ are selected from hydrogen, an alkyl group, and an aryl group, and may be the same or different, and m is an integer of 3 to 6). Any of those having 50 mol% or more of oxymethylene units may be used. In any case, the terminal group may be a group other than a hydroxyl group, for example, an alkoxy group, a hydroalkoxy group, a formate group, an acetyl group, or the like.

The other resin which can be used by mixing with the above-mentioned polyacetal resin is not particularly limited as long as it is compatible with the polyacetal resin. For example, polyethylene, polypropylene, polystyrene, ABS resin,
Polyvinyl chloride, polyamide, polycarbonate, modified polyphenylene ether, polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, polyimide, polyamideimide, polyetherimide, polyarylate, polysulfone, polyethersulfone, polyetheretherketone, liquid crystal polymer, poly Examples include tetrafluoroethylene and thermoplastic elastomers.

When a mixed composition of a polyacetal resin and another resin is used, the content of the polyacetal resin is 5%.
It is necessary to be 0% by weight or more, preferably 70% by weight or more. If the content of the polyacetal resin is too small, the characteristics of the polyacetal resin excellent as an engineering resin cannot be obtained.

The polyacetal resin may be used alone or in combination with another resin.
It is necessary that the melting point is 150 to 161 ° C, and preferably, the melting point is 151 to 160 ° C. If the melting point is less than 150 ° C., the mechanical properties, particularly tensile strength, of the obtained molded article will be impaired. If the melting point exceeds 161 ° C., the molding shrinkage will increase, and the dimensional accuracy of the obtained molded article will be impaired.

Usually, most of the commercially available polyacetal resins have a melting point of 162-175 ° C. In the present invention, a resin having a lower melting point than such a commercially available polyacetal resin is used as the polyacetal resin.

As a method for preparing a polyacetal resin having a low melting point, a method for adjusting a molecular weight, an additive (a plasticizer, etc.)
, A method of increasing the ratio of oxyethylene units in the molecular chain, and a combination thereof. Among them, the method of increasing the ratio of oxyethylene units in the molecular chain is the most effective, and this adjustment method can be adopted. Therefore, the above-described copolymer is preferable as the polyacetal resin used in the present invention.

In the polyacetal resin of the present invention, usual additives such as an antioxidant, a flame retardant, a release agent, a coloring agent, a lubricant, and a heat-resistant stabilizer are used as long as they do not affect the melting point of the polyacetal resin. Agents, weathering stabilizers, rust inhibitors and the like can be added.

The molded article according to the present invention is a hollow molded article formed by the hollow injection molding method using the above-mentioned polyacetal resin.

Here, the hollow injection molding method is, as described in the "prior art", a pressurized gas is injected into the mold during or after the injection of the molten resin, and the molten resin is injected into the mold cavity. A molding method in which a hollow portion is formed in a molten resin by a pressurized gas, and cooling of a molded article is performed while appropriately maintaining the pressure of the pressurized gas in the hollow portion.

The hollow injection molding method will be further described.
The injection amount of the molten resin in the hollow injection molding may be either a full shot for injecting a sufficient amount to fill the mold cavity or a short shot for injecting an insufficient amount to fill the mold cavity. In the case of a full shot, a pressurized gas is injected with the contraction of the resin due to cooling, and in the case of a short shot, the shortage of the resin is compensated for and the pressurized gas is injected with the contraction of the resin.

As the pressurized gas, any gas may be used as long as it is inert to the polyacetal resin, and examples thereof include inert gases such as nitrogen, helium, neon, and argon. Further, in order to prevent the decomposition and burning of the polyacetal resin at the time of molding, it is preferable to use a gas containing less impurity components. Considering economics, nitrogen gas is industrially preferable. The pressurized gas may be injected by introducing the pressurized gas stored in the accumulator into the mold or by continuously supplying the pressurized gas to the mold with a pump. In addition, there are a method of performing the process from a gas nozzle built in the injection nozzle and a method of performing the process from a gas passage provided in a mold. When using a gas nozzle built into the injection nozzle, the pressurized gas is pressed into the mold cavity via the gate, like the molten resin. In the case of performing from a gas passage provided in the mold, if the gas passage is opened to a sprue or a runner, pressurized gas will be injected into the mold cavity through a gate. And the pressurized gas can be directly injected into the mold cavity.

Usually, after the necessary cooling is completed, the molded product is discharged from the mold after discharging the pressurized gas in the hollow portion and opening the hollow portion to atmospheric pressure. Therefore, the hollow portion of the molded article according to the present invention is usually at atmospheric pressure. However, a molded product in which a pressurized gas is sealed in a hollow portion may be used.

The molded article according to the present invention is formed by the hollow injection molding method as described above, and thus has a hollow portion. The hollow ratio is preferably from 3 to 50%, and more preferably from 5 to 40%. If the hollow ratio is less than 3%, the pressure of the pressurized gas is difficult to propagate throughout, and it is difficult to obtain sufficient dimensional accuracy. When the hollow ratio exceeds 50%,
Poor appearance such as hesitation marks is likely to occur.

The hollow portion in the present invention refers to a hollow portion formed by press-in of a pressurized gas in hollow injection molding, and is different from a so-called hollow portion or a hollow portion formed by foaming.

[0026]

EXAMPLES First, measurement methods and conditions of measurement items in the following Examples and Comparative Examples will be described.

(1) Melting point Measured using a differential scanning calorimeter (DSC).

5 mg of the sample was heated from 30 ° C. to 200 ° C.
After the temperature was raised at 20 ° C./min and held for 2 minutes, the temperature was lowered to 130 ° C. at 10 ° C./10 minutes, and further from 130 ° C. to 2.5 ° C.
The temperature rose in minutes. When the temperature rises last, an endothermic peak is observed along with crystallization, and the peak top temperature at this time was defined as the melting point.

(2) Tensile strength The tensile strength was measured based on ASTM D638.

(3) Hollow Ratio The apparent volume V of the obtained molded product, the density ρ of the polyacetal resin used, and the mass M of the obtained molded product were calculated by the following equation.

Hollow ratio (%) = ｛(V × ρ−M) / (V ×
ρ)｝ × 100

(4) Difference between Dimensions of Mold and Dimension of Molded Product As will be described later, a rectangular molded product having a length of 150 mm and a square shape of 10 mm is molded by a mold to be molded, and the central axis length of the obtained molded product is obtained. The length L (mm) was measured, and the center axis length L (m
m) was subtracted.

(5) Molding Shrinkage Rate From the center axis length L (mm) of the molded article obtained above and 150 mm which is the length in the mold, it was calculated by the following equation.

Molding shrinkage (%) = ｛(150−L) / 1
50｝ × 100

Examples 1 to 10 The polymerization of a polyacetal resin was carried out by the usual polymerization method of a copolymer, but the polymerization was carried out under different conditions in order to obtain polyacetal resins having different melting points.

Specifically, bulk polymerization was carried out using trioxane and ethylene oxide as monomers, methylal as a molecular weight regulator, and a BF ₃ catalyst.

The molar ratio of trioxane to ethylene oxide was varied in the range from 1: 0.06 to 1: 0.20. The molar ratio of trioxane to the molecular weight regulator and the molar ratio of trioxane to the catalyst were 1: 0.7 × 10 ⁻³ ,
1: polymerized at 0.41 × 10 ^-4 .

As a polymerization apparatus, a twin-screw kneader is used, and a liquid high-purity monomer and a catalyst are supplied.
Polymerization was performed in the range of 00 ° C. A bulk polymer was continuously obtained from the polymerization apparatus, and polyacetal resins having different melting points were obtained depending on the monomer component ratio supplied to the polymerization apparatus. Table 1 shows the obtained polyacetal resin.
・ No. Shown as 1-6.

POM No. Using each of the polyacetal resins 1 to 6, hollow injection molding was performed in a prism-shaped mold cavity of 10 mm square and 150 mm length.

The gate was provided at one point, and was provided on one of the four peripheral surfaces of the molded product at the center (center of gravity) of the surface. The mold temperature is 80 ° C, the cylinder set temperature is 200 ° C,
Further, the molding was performed while changing the hollow ratio by changing the measured value of the molten resin.

Nitrogen gas was used as the pressurized gas, and a shut-off valve for preventing gas from flowing back into the injection cylinder was provided. Press-fitting was performed from a gas nozzle incorporated in the injection nozzle.

The pressurized gas is injected at 150 kg of nitrogen gas.
/ Cm ² , stored in an accumulator, and injected into the mold from the gas nozzle through a pipe after injection of the molten resin. The conditions for pressurizing the pressurized gas are as follows: a gas press-in delay time (time from completion of injection of the molten resin to a start of pressurized gas press-in) of 0.5 seconds, and a gas press-in time (time of pressurized gas press-in) of 5 seconds. Second, the pressure holding time (the time obtained by adding the gas injection time to the time for stopping the injection of the pressurized gas and keeping the gas system closed) was set to 60 seconds.

The mold opening was performed 5 seconds after the end of the pressure holding time after the pressurized gas in the hollow portion was discharged.

FIGS. 1 and 2 show sectional views of the obtained molded product. In the figure, reference numeral 1 denotes a hollow portion.

Table 1 shows the measurement results of the molded product.

Comparative Examples 1 and 2 The polyacetal resin was obtained by the same polymerization method as in Examples 1 to 10, but the melting point was 16% of that of a commercially available polyacetal resin.
Polymerization was carried out under conditions where the ethylene oxide ratio in the monomer component was lower than in Examples 1 to 10 so as to be around 3.0 ° C.

Specifically, polymerization was carried out with the molar ratios of trioxane and ethylene oxide being 1: 0.02 and 1: 0.05. Table 1 shows the obtained polyacetal resin.
OM No. Shown at 7 and 8.

POM No. Using the polyacetal resins of 7 and 8, hollow injection molding was performed under the same mold and conditions as in Examples 1 to 10. Table 1 shows the measurement results of the obtained molded products.

Comparative Examples 3 and 4 The polyacetal resin was obtained by the same polymerization method as in Examples 1 to 10, but the ethylene oxide ratio in the monomer component was adjusted so that the melting point was lower than in Examples 1 to 10. Polymerization was carried out under larger conditions.

Specifically, polymerization was carried out with the molar ratios of trioxane and ethylene oxide being 1: 0.22 and 1: 0.25. Table 1 shows the obtained polyacetal resin.
OM No. Shown at 9 and 10.

POM No. Using the polyacetal resins 9 and 10, hollow injection molding was performed under the same mold and conditions as in Examples 1 to 10.

Table 1 shows the measurement results of the obtained molded articles.
Shown in

Comparative Examples 5 to 8 POM No. Using each of the polyacetal resins 2, 5, 7, and 8, normal injection molding (injection molding without pressurized gas injection) was performed in the same mold as in Examples 1 to 10.

The cylinder set temperature and the mold temperature were the same as in Examples 1 to 10, and the resin pressure-holding time was 600 kg / cm ^2.
The pressure was 60 seconds, and the cooling time was 35 seconds.

Table 1 shows the measurement results of the obtained molded products.
Shown in

[0056]

[Table 1]

[0057]

As is clear from the examples and comparative examples,
The molded article according to the present invention has not only a significantly small molding shrinkage but also no large decrease in mechanical properties (tensile strength). Therefore, the molded article according to the present invention is a molded article having a dimension extremely close to the mold dimension and having high dimensional accuracy, and also having the mechanical properties required for the application, and is extremely useful as various industrial parts. It is a useful molded article.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a molded product obtained in Examples 1 to 10 cut in an axial direction.

FIG. 2 is a cross-sectional view of the molded product obtained in Examples 1 to 10 cut in a direction perpendicular to an axis.

[Explanation of symbols]

 1 hollow part

Claims (1)

(57) [Claims] 1. A molded article of a polyacetal resin having a hollow portion and made of a polyacetal resin having a melting point of 150 to 161 ° C. and containing a polyacetal resin as a main component and molded by a hollow injection molding method.