JPH0939055A - Injection molding method for methacrylate resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an injection molding method for low strain methacrylate resin using a nozzle opening-closing type hot runner injection molding die. SOLUTION: A nozzle opening-closing type hot runner injection molding die is used. A temperature of a manifold is 300 deg.C or lower. A temperature of a nozzle is set within a range of 280-350 deg.C. After filling molten resin into the die by injection, the nozzle is closed within 15sec.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an injection molding method for methacrylic resin, and more particularly to a low distortion injection molding method for methacrylic resin.

[0002]

The conventional low-distortion injection molding method for methacrylic resin is generally a method in which the cylinder temperature and the mold temperature are set as high as possible and the low-pressure high-speed molding is performed. Molding with molds causes large cooling and pressure loss of the sprue / runner, and there is a limit to reducing molding strain due to molecular orientation of the resin near the gate, and there are few problems due to increase in birefringence and polarization in optical applications. Absent.

The present invention is intended to provide a low distortion injection molding method for a methacrylic resin, which uses a nozzle opening / closing hot runner injection molding die as a low distortion injection molding method for a methacrylic resin.

[0004]

According to the present invention, a nozzle opening / closing hot runner injection molding die is used, the manifold temperature is set to 300 ° C. or lower, and the nozzle temperature is set to a high temperature in the range of 280 to 350 ° C. A method for low-distortion injection molding of methacrylic resin, characterized in that the nozzle is closed within 15 seconds after injection molding the molten resin into the mold.

In the present invention, a nozzle opening / closing hot runner injection molding die is used, the hot runner manifold temperature is set to 300 ° C. or lower, and the nozzle temperature is set to a high temperature in the range of 280 to 350 ° C. The secondary process in the pressure-holding process is performed by reducing the melt viscosity of the resin to suppress the molecular orientation (orientation layer) of the resin in the injection process, and closing the nozzle within 15 seconds after injection-filling the molten resin in the mold. The flow can be suppressed, and therefore, the molecular orientation of the resin generated by shearing between the solidified layer of the resin formed by contact with the mold surface near the gate and the fluidized bed continuously injected and injected. (Alignment layer) can be made smaller, and molding distortion can be suppressed.

Generally, the molding conditions for reducing the molding strain due to the molecular orientation of the resin in the vicinity of the gate of the molded product are considerably high by increasing the cylinder temperature, the mold temperature, the injection speed, and the pressure holding time. Although the effect of is recognized, the birefringence and the polarized light are still large, which is not satisfactory.

In the present invention, a nozzle opening / closing type hot runner injection molding die is used, and the nozzle temperature is set to a high temperature of 280 ° C. or higher, and preferably to a higher temperature setting which allows molding without generation of silver due to thermal decomposition. Suppresses the molecular orientation of the resin near the molded product gate.

In order to reduce the molecular orientation of the resin in order to suppress the molding strain near the gate of the molded product, it is desirable to set the nozzle temperature of the hot runner as high as possible.
The setting range of the nozzle temperature is 280 ° C or higher, which corresponds to the lower limit temperature which is almost the upper limit temperature of general molding, and the upper limit temperature is the JIS K7120 test method for methacrylic resin (thermogravimetric measurement method).
The temperature is set to 350 ° C. or lower, which corresponds to a temperature of about 10% weight loss due to heating in nitrogen gas.

Further, the structure of the nozzle portion of the hot runner,
The upper limit settable temperature of the nozzle temperature is predicted to vary depending on the heating method, the temperature control method, etc., but the temperature should be set appropriately for each nozzle method and optimal for the molding cycle.

In the present invention, the manifold temperature is not so effective as the high temperature setting of the hot runner nozzle, but is set to a high temperature in order to suppress the pressure loss in the manifold in lower pressure high speed molding, and the upper limit temperature is 300.
Up to ℃.

In the present invention, the amount of retained resin in the nozzle of the hot runner of methacrylic resin is set to be smaller than the volume of one molded product obtained by injection molding, thereby
The molten resin in the nozzle is completely replaced every shot molding. Therefore, the heat history due to retention is small and the resin does not easily decompose, so it is possible to set a higher temperature than the cylinder temperature setting for general molding.

In the case of the hot runner in the nozzle opening / closing hot runner injection molding die used in the present invention, the amount of resin retained in the nozzle is about 7 cm ³ and the molded product volume is 117 cm.
Corresponding to about 6% of ³ , there is no silver generation due to thermal decomposition for 180 seconds or more when the nozzle temperature is set at 310 ° C.
When the setting is 0 ° C., the molding cycle is 90 seconds, and when the setting is 340 ° C., the molding cycle is 30 seconds. Therefore, the higher the nozzle temperature setting of the hot runner, the shorter the molding cycle that can be molded. Therefore, it is necessary to set the molding cycle and nozzle temperature according to the wall thickness of the molded product and the sink condition.

On the other hand, the amount of resin retained in the manifold of the hot runner is smaller than the amount of resin retained in the nozzle, which is smaller than the volume of one molded product obtained by injection molding, and the heat history due to the residence time is smaller. This makes it possible to set a high temperature, which is also effective for high-speed molding at a lower pressure.

Since the cylinder temperature corresponds to the resin fluidized bed in the pressure-holding process near the gate of the molded product, the molecular orientation of the resin due to the secondary flow is suppressed and the molecular orientation of the resin in the cooling process is suppressed. Considering the effect of relaxation, it is preferable to set the temperature as high as possible.

It is assumed that the nozzle of the hot runner used in the present invention can be opened / closed and the opening / closing time can be set arbitrarily, and the holding time is applied during the injection process during the time the nozzle is open.

The present invention requires that the nozzle be closed within 15 seconds after the injection and filling of the molten resin into the mold, and suppresses the molding strain due to the molecular orientation of the resin due to the secondary flow in the pressure holding step, and It is possible to prevent the molten resin injected into the mold from flowing backward in the direction of the nozzle, and to suppress the sink mark of the molded product.

Since the wall thickness of the molded product is not always constant, it is necessary to set the time for closing the nozzle of the hot runner in accordance with the wall thickness of each molded product so as to suppress molding distortion and prevent sink marks. . For example, the thickness of the molded product is 4
In the case of mm, the nozzle may be closed within 15 seconds, in the case of 3 mm, the nozzle may be closed within 10 seconds, and in the case of 2 mm, the nozzle may be closed within 5 seconds.

The hot runner nozzle closing time (pressure-holding time) is set to 15 seconds, and it is set as short as possible while checking the thickness, sink marks and dimensions of the molded product. It is preferable to suppress the molecular orientation of the resin due to the secondary flow near the gate.

Furthermore, in the present invention, since the nozzle temperature of the hot runner is set to a high temperature of 280 ° C. or higher, cold slugs such as stringing and drooling are likely to occur at the time of mold opening, which causes molding distortion and Since it is feared that a defective appearance of the molded product may occur, it is preferable to adopt an opening / closing type nozzle, for example, a system in which the gate part of the molded product is completely sealed like a valve type nozzle.

The hot runner used in the present invention prevents the occurrence of molding distortion near the gate due to cold slug and the appearance defect of the molded product, and the molten resin injected and filled in the mold is directed toward the nozzle. Open and close to prevent backflow,
A valve-type nozzle with hydraulic pressure that allows the opening and closing time to be set arbitrarily is used.

The methacrylic resin used in the present invention is not particularly limited as long as it is one generally called a methacrylic resin. Specific examples thereof include methyl methacrylate homopolymers, copolymers composed of 50% by weight or more of methyl methacrylate units and 50% by weight or less of other vinyl monomer units copolymerizable with methyl methacrylate, and alkyl groups. To an acrylic rubber elastic body, a diene elastic body or an ethylene-propylene elastic body based on an acrylate having 2 to 6 carbon atoms, and methyl methacrylate alone or 50% by weight or more of methyl methacrylate.
Graft copolymer obtained by copolymerizing other vinyl monomer mixture copolymerizable with 0% by weight or less of methyl methacrylate in one step or in multiple steps, and further, such graft copolymer and the above methyl methacrylate alone Examples thereof include a polymer or a mixture of 50% by weight or more of a methyl methacrylate unit and 50% by weight or less of a methyl methacrylate unit and a copolymer of another vinyl monomer unit copolymerizable therewith.

Other vinyl monomers copolymerizable with the above methyl methacrylate include methyl, ethyl and n-
Alkyl acrylate having an alkyl group having 1 to 18 carbon atoms such as propyl, n-butyl, 2-ethylhexyl, dodecyl, stearyl, and alkyl methacrylate having an alkyl group having 2 to 18 carbon atoms similar to the above-mentioned alkyl acrylate except methyl. And maleimide compounds such as methacrylic acid, phenyl methacrylate, styrene, α-methylstyrene, maleic anhydride, phenylmaleimide, and cyclohexylmaleimide.

In the injection molding method of the present invention, when a compression molding system using an existing injection molding machine is combined, a molding product with less molding distortion or no distortion can be obtained.

[0024]

EXAMPLES Examples and comparative examples of the present invention will be described below. In this example, Dynamerta M-200-DM manufactured by Meiki Seisakusho Co., Ltd. is used as an injection molding machine, and the mold is 120 ×.
With 300 × 3t rib, hot runner / nozzle diameter is 2.8mmφ hydraulic valve type gate with “side gate” (Mold Masters Co., Ltd.) 300
mm It evaluated using the side gate from the long side direction.

[Examples 1 and 2], [Comparative Examples 1 to 3] A methacrylic resin (trade name "Acrypet" VH, manufactured by Mitsubishi Rayon Co., Ltd.) was used as a molding material, and the above injection molding machine and mold were used. Using the following conditions and the following conditions, the cylinder temperature, the manifold temperature and the nozzle temperature were molded under the conditions shown in Table 1, the birefringence index near the gate of the molded product was measured, and polarization (observation of distortion by the polarizing plate) was performed. Was evaluated. The birefringence was expressed as the average value of three measurements on a circle with a radius of 20 mm centered on the molded product gate. The results are shown in Table 1. In addition, a polarized photograph near the gate of the molded product in each example is attached.

Cylinder temperature 240 ° C. to 280 ° C. Manifold temperature 240 ° C. to 280 ° C. Nozzle temperature 240 ° C. to 330 ° C. (hot runner) Mold temperature 60 ° C. (actual measurement; fixed side 76 ° C., movable side 85 ° C.) Injection speed 110 116 cm ³ / sec Pressure holding time 10 seconds (time when nozzle is open) Pressure holding Short shot pressure +21 kg / cm ² Molding cycle 50 seconds (Pressure holding time 10 seconds, Cooling time 25 seconds)

The nozzle temperature of the hot runner is 300 ° C.,
A high temperature setting of 330 ° C., a manifold temperature and a cylinder temperature of 280 ° C., a nozzle opening time (holding time) of 10 seconds was set, and the birefringence index near the gate was small. A low distortion molded product in which no polarized light was observed by observation was obtained.

On the other hand, the nozzle temperature, manifold temperature, and cylinder temperature, which are general molding conditions, are 240 ° C., 260 ° C., 280 ° C. even though the hot runner is used.
Set the nozzle open time (holding time) to 10
The birefringence in the vicinity of the gate of the molded product molded by setting the second and the polarized light observed by the polarizing plate were large, and the effect of the present invention was clearly remarkable.

[0029]

[Table 1]

[0030]

According to the present invention, a low-distortion methacrylic resin molded article suitable for a laser disk substrate using an optical signal required to have a low birefringence and a compact disk substrate can be obtained. Further, according to the present invention, a product having no dark field due to polarization can be obtained for applications such as a liquid crystal type display display plate, a polarizing optical filter, a cover and a name plate.

[Brief description of drawings]

FIG. 1 is a drawing-substitute polarized photograph in the vicinity of a gate of a molded article obtained in Example 1 of the present invention.

FIG. 2 is a drawing-substitute polarized photograph near a gate of a molded article obtained in Example 2 of the present invention.

FIG. 3 is a drawing-substitute polarized photograph near a gate of a molded article obtained in Comparative Example 1 of the present invention.

FIG. 4 is a drawing-substitute polarized photograph near a gate of a molded article obtained in Comparative Example 2 of the present invention.

FIG. 5 is a drawing-substitute polarized photograph near the gate of a molded product obtained in Comparative Example 3 of the present invention.

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[Procedure amendment]

[Submission date] December 27, 1995

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] All figures

[Correction method] Change

[Correction contents]

FIG.

[Fig. 2]

[Figure 3]

FIG. 4

[Figure 5]

Claims (1)

[Claims] 1. A nozzle opening / closing hot runner injection molding die is used, the manifold temperature is set to 300 ° C. or lower, the nozzle temperature is set in the range of 280 to 350 ° C., and the molten resin is injected and filled into the die 15 A method for low-distortion injection molding of methacrylic resin, characterized in that the nozzle is closed within seconds.