JPS6254650B2 - - Google Patents

Description

Detailed Description of the Invention Extrusion molding of plastic sheets or films is carried out using T-dies, coated hanger dies, etc., but as the extrusion speed increases, the extrudate (product) may have shark skin, melt fractures, etc. A problem occurs. For this reason, there is a limit production rate, which hinders speeding up, and this tendency is noticeable in high molecular weight, high viscosity resins.

Therefore, in order to increase the productivity of sheets and films, there is a trend toward widening the product, but even widening the product width promotes non-uniformity in quality in the width direction of the product. Conventionally, these causes are due to abnormal flow within the die due to increased speed, or differences in local viscoelastic effects due to non-uniformity in shear rate, residence time, temperature, etc. in the width direction of the die. better known.

For this reason, it is necessary to correct these non-uniformities by using a choke bar inside the die, a lip opening at the die exit, etc.
In other words, the cross-sectional area of the flow path has been made smaller. Although this method has the effect of making the thickness of the product uniform, reducing the cross-section of the flow path increases the shear rate locally, causing the product to become thinner. glossiness, haze,
There was a drawback that quality could not be made uniform in terms of Young's modulus, yield strength, etc.

When considering high productivity, increasing the width is one direction, but it is difficult to say that it is always effective because the molding equipment becomes larger and the above-mentioned problems cannot be completely eliminated. Therefore, increasing the production amount per die unit width, that is, increasing the speed, is considered to be the essential direction for improvement.

The factors that inhibit production per unit width of the die are, as described above, the excessive shear stress of the molten plastic in the flow path within the die and the non-uniformity of the shear history.

Therefore, the present invention makes the land length in the die width direction variable in order to make the shear rate constant at the die lip and reduce the shear stress on the die wall surface at the die lip, and also to heat the variable section. The purpose is to provide extrusion molding dies for the

Embodiments of the present invention will be described below with reference to the drawings. FIGS. 1 and 2 are schematic diagrams of extrusion molding dies showing embodiments of the present invention. First of all, FIG. 1 is a front partial sectional view of the die, in which 1 is the die body, 2 is the die lip, 3 is the manifold, 4 is the inlet from the manifold 3 to the lip 2, and 5 is the melt-kneaded plastic from the extruder. The inflow portion, 6, is an adjustment bolt. Further, FIG. 2 is a cross-sectional view taken along the plastic flow line in the die from A to A at the center of the die in FIG. 1, and 7 in the figure is a lip length adjustment and heating section.

Next, to explain the operation, the plastic that has been uniformly melted and kneaded in a plasticizing device such as an extruder enters the plastic inflow section 5, passes from the manifold 3 to the die lip section 2, and is shaped into a sheet outside the die and cooled. Although it is solidified, the molten plastic develops a defective phenomenon based on the viscoelastic effect at the flow path change portion within the die.

Specifically, when it flows from the manifold 3 to the die lip 2 part, shark skin, melt fractures, etc. occur due to the elastic effect due to shearing, and die swell occurs due to the balance effect at the die exit, which limits the production rate.

Therefore, based on these backgrounds, the inventors of the present invention investigated whether the above-mentioned defective phenomenon could be suppressed and the production speed could be increased.As a result, the present inventors made the shear velocity distribution in the widthwise shearing direction uniform in the flow path inside the die. thing,
We also found that it is fundamental to reduce shear stress.

In other words, methods such as using a choke bar or adjusting the lip opening that have been used to make the thickness of the product uniform have been used to make the thickness uniform by reducing the cross section of the flow path and thereby imparting resistance to the flow path. However, if the cross section of the flow path is reduced, the shear rate (stress) will naturally increase in order to obtain the same flow rate as the others. In areas where the shear rate (stress) is high, the viscoelastic effect is more pronounced, promoting non-uniformity in the width direction. However, in order to solve this problem, it is necessary to make the land length of the lip portion longer than necessary.

Another method for reducing shear stress is to raise the temperature of the resin in the extruder and feed it to the die, but with this method, the resin deteriorates due to the rise in resin temperature, resulting in a decrease in the physical properties of the product.

In the present invention, as shown in the embodiment of the drawings, the lip length adjustment and heating section 7 shown in FIG. This method lowers the viscosity of the resin in the surface layer and lowers the shear stress.

The lip length adjustment and heating section 7 is fixed to the die body 1 at both ends A and B in the resin flow direction, or only at the end A on the manifold side. The shape of the reprint length adjustment and heating section 7 can be changed using the adjustment bolt 6. The adjustment bolt 6 can be adjusted to the land length in the die width direction (L+△
L) can be adjusted freely.

In addition, the Rippland length adjustment and the heating of the heating section 7 are performed by flexible heaters such as surface heating elements, and the heaters are divided and arranged in a grid pattern in the flow direction and in the direction perpendicular to the flow, so that the temperature of each section can be controlled separately. desirable. Due to this mechanism, there is a pressure difference in the width direction of the molten plastic flowing from the manifold 3 to the die lip 2 section, and when the flow rate changes, the lip land adjustment in the section with a large flow rate and the adjustment bolt 6 of the heating section 7 are tightened as shown in Fig. 2. As shown by the broken line, the land length can be increased as indicated by L' in FIG. 2 while keeping the lip opening degree t constant.

Therefore, in the present invention, by keeping the shear rate of the molten plastic constant as it passes through the die lip and by increasing the land length, the flow resistance can be increased and the flow rate can be adjusted. By lowering the shear stress, it is possible to obtain a uniform and good extruded product with no uneven shear history in the die width direction. Although FIG. 2 shows the ripple length adjustment and heating unit 7 disposed on one side of the die, it goes without saying that the same effect can be obtained even if the unit is disposed on both sides of the die.

[Brief explanation of the drawing]

FIG. 1 is a partially sectional front view of an extrusion molding die showing an embodiment of the present invention, and FIG. 2 is a sectional view taken from A to A in FIG. 1. Brief explanation of the drawings: 1...Die body, 2...Die lip, 3...Manifold, 4...Inflow section from the manifold to the nip, 5...Inflow section for molten plastic, 6...Adjustment bolt (lip land length) (adjustment means), 7... Rippland length adjustment and heating section.

Claims (1)

[Claims] 1. In a molding die that has an inlet for melt-kneaded plastic and a die lip for shaping and extruding the plastic, the land length is variable between the manifold and the lip land while keeping the lip opening constant. What is claimed is: 1. A die for extrusion molding, characterized in that the die is provided with a lip length adjustment and heating means that can heat the lip inflow portion from the land portion.