JPH0343220A - Mold for injection molding - Google Patents

Abstract

PURPOSE:To manufacture a mold designed to keep welds generated on a molded product inconspicuous by providing weirs with the wall thickness thinner than the wall thickness of peripheral sections in the vicinity of gates. CONSTITUTION:A mold with a plurality of gates 3a-3d on the center of a conical molded product such as a shade for lighting device and the like are examples of category of a mold with a plurality of gates on one surface of a molded product. When injection molding is carried out by using said mold, resin flowed from gates 13a and 13b provided on one end surface is flowed in the mold in the fluid path, and as the fluid pattern at a section B where resin is joined is not a straight line but with the angle less than 180 deg., welds are generated at the section B. In order to keep the welds inconspicuous, it is necessary to have the fluid pattern of said joining section B as close as possible to the straight line and the position where resin pressure loss from a sprue end 11 is equal should be a straight line, or 180 deg.. In order to make the position where the resin pressure loss from the sprue end is equal the straight line, weirs having the wall thickness thinner than the wall thickness of peripheral sections in the vicinity of respective gates should be provided.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a mold for injection molding, and particularly to a mold designed so that welds that occur in molded products are not noticeable.

[Prior Art] Welds occur in molded products because, for some reason, a resin flow path splits into two or more into two or more and then merges again in a mold.

Examples of molds in which the resin flow path splits into two or more and then merge again include cases in which the resin flow path separates at a portion of the runner and merges in the molded product, and cases in which the resin flow path splits into two or more and then merges in the molded product. They may diverge or merge depending on the situation.

Conventionally, a common method for making welds less noticeable is to raise the temperature of the resins that collide at the weld as high as possible so that they mix well during molding. As a method for increasing the resin temperature, for example, there are means such as setting the molding temperature high or increasing the filling speed to increase the resin temperature by shear heat generation.

However, when the resin temperature is increased, silver tends to be generated due to decomposition of the resin and other gases, and this method has its limits.

[Problems to be Solved by the Invention] The present invention was conceived in view of the above problems, and in particular,
The object of the present invention is to provide a mold design method that makes welds that occur due to having a plurality of gates on one end surface of a molded product less noticeable.

[Means for Solving the Problems] That is, the present invention is characterized in that, in an injection mold having a plurality of gates, a weir having a wall thickness thinner than the wall thickness of the peripheral part is provided near the gate. This is an injection mold.

Hereinafter, the present invention will be explained using the drawings.

In the present invention, a typical example of a mold having a plurality of gates on one end surface of a molded product is a shade for lighting equipment (
A conical molded product with a plurality of gates 3a, 3b, 3c, and 3d provided in the center part as shown in Fig. 1), and a large flat plate such as an automobile side visor (Fig. 2).
Since the flow distance is long, a mold having a plurality of gates 38 to 3C on one end surface may be used.

The occurrence of welds when injection molding is performed using these molds will be explained using a flat plate mold with a width of 200 mm and a length of 70 mm shown in Fig. 3. From the two gates 13a and 13b provided on one end surface. The inflowing resin flows inside the mold in the flow pattern shown in Figure 4, but the flow pattern at section B where the resin merges is not a straight line but 180°.
Since they merge at the following angle, a weld occurs in part B.

Therefore, in order to make the weld part less noticeable, it is necessary to make the flow pattern of the resin in the merging part B as close to a straight line as possible. It turns out that it is good to have a straight line, that is, 180 degrees.

As a means to straighten the position where the loss of resin pressure from the sprue tip is equal, it is easier to achieve this by providing a weir with a thickness thinner than that of the surrounding area near each gate. I can do it.

In other words, by designing the resin pressure loss in each flow path from the sprue tip to the exit of the weir to be equalized by adjusting the wall thickness and width of the weir, the weld part of the molded product can be made less noticeable. I can do it.

Taking the simplified mold shown in FIG. 3 as an example, a method of designing a mold in which welds are not noticeable will be explained in detail with reference to FIG. A thin wall portion with a width of 70 mm and a length of 10 au is provided at a position 10 mm apart from the two gates 13a and 13b.

The thin part is divided into a thin part 15, 15' near the gate and thin parts 16a, 16a', 16b, and 16b' far from the gate, each having a different thickness.

The design of thin-walled parts must be designed individually because the pressure loss of the resin varies depending on the type of resin used, resin temperature, mold temperature, and filling time, but computer simulations have shown that the flow behavior of the resin during injection molding has been determined. It has become possible to easily design by analyzing.

In Figure 5, Delpet 8ON (manufactured by Asahi Kasei Kogyo, methacrylic resin) was used, the resin temperature was 260°C, and the mold temperature was 60°C.
The mold is designed so that the weld is least noticeable when molding is performed with a filling time of 2.09 seconds.At this time, the thickness of the thin wall part 15.15' near the gate is 1.3 mm, and the thickness of the thin wall part 16a far from the gate is 1.3 mm. , 16a', 16b, and 16b' had a wall thickness of 2 mm.

Figure 6.7.8 shows the results of flow analysis using the mold designed in this way. Figure 6 shows the pressure at each part when filling is completed.Since the loss of resin pressure from the tip of the sprue to the outlet of the weir is equal for each channel, the resin pressure at the outlet of the weir is It can be seen that it becomes uniform.

As a result, as shown in FIG. 7, the flow pattern of the resin became a straight line after leaving the weir, and a molded product with almost no visible welds was obtained.

In addition, on the gate side before the resin flows into the weir, the seventh
As can be seen from the figure, weld C is recognized, but this 0
Since the resin temperature at that point was high as shown in FIG. 8, this was hardly a problem in actual molded products.

In the present invention, the weir is a part of the molded product near the downstream side of the gate, and can be of any shape as long as it increases the pressure loss by making the wall thinner than the surrounding part and adjusts the resin pressure. However, typical examples include section 9a,
It is as shown in Figure 9b.

In other words, Figure 9a has the shape shown in Figure 5, with the length of the weir section being the same, but the pressure loss being equalized by gradually increasing the wall thickness as the distance from the gate increases. , Figure 9b shows a shape in which the wall thickness of the weir part is the same, and the length is gradually reduced as the distance from the gate increases, so that the pressure loss is equalized. Based on the above-mentioned concept, weirs can be installed in shapes and locations that do not impair the aesthetic appearance of the area.

[Example] Next, to explain the present invention more specifically with reference to an example, the shade shown in Fig. 1 is molded with four gates from the center. ) a weld occurred. (The flow pattern and resin pressure in this mold are shown in Figures 10, 11, and 12) The top surface of this shade has a wall thickness of 3 mm and 12 heat vent holes 7, so the wall thickness is thick enough to eliminate welds. Weirs 18a and 18b with a length of 2ff1m and a length of 8mm were provided between the two heat vent holes 7 closest to the gate. (Fig. 13) Using this mold, molding was performed using Delpet 560F (manufactured by Asahi Kasei Industries, methacrylic resin) at a resin temperature of 250°C, a mold temperature of 60°C, and a filling time of 6.35 seconds, and almost no welds were noticeable. A molded product was obtained.

Through flow analysis, the flow pattern and resin pressure at this time are shown in Figures 14, 15, and 16.

As can be seen from Figures 14 and 15, since the resin pressure becomes constant from the top surface to the umbrella, the subsequent flow pattern is linear (Figure 16).1. 1: It is easy to understand that the weld becomes less noticeable.

[Effects of the Invention] As explained above, according to the mold design method of the present invention, in a mold in which a weld occurs due to a plurality of gates on one end surface of a molded product, there is a weld in the vicinity of each gate. By providing a weir with a wall thickness thinner than that of the weir, the resin pressure loss in each flow path from the tip of the sprue to the weir outlet is equalized, so welds that would otherwise occur are almost eliminated. It is possible to obtain inconspicuous molded products.

[Brief explanation of drawings]

Figure 1.2 shows a typical example of a specific mold to which the present invention is effective.
Figure 3 shows a simplified flat plate mold, Figure 4 shows the resin flow pattern in the mold shown in Figure 3, and Figure 5 shows a design modification of the mold shown in Figure 3 according to the present invention. With a mold that
Figure 6 shows the resin pressure diagram when methacrylic resin is molded with this mold, Figure 7 shows the flow pattern, and Figure 8 shows the resin temperature.
Shown in the figure. Figure 9 is an example of the shape of a weir, Figures 10 to 12 are part of the resin pressure diagram when the shade shown in Figure 1 is molded with methacrylic resin, as seen from the top, and Figure 10 is a side view. Fig. 11 shows the resin pressure diagram as seen from the outside, and Fig. 12 shows the flow pattern. Fig. 13 shows a mold of this shade whose design has been changed according to the present invention. Fig. 14 shows a part of the resin pressure diagram seen from the top, and Fig.
The flow pattern is shown in FIG. 16. 1.11... Sprue 2.12... Runner 3.13... Gate 4... Shade top surface 9... Shade cap lO... Side visor 14... Flat plates 15, 16 .18...Weir A, B, C...Weld part

Claims (1)

[Claims] (1) In an injection mold having multiple gates,
A mold for injection molding, characterized in that a weir is provided near the gate with a wall thickness thinner than that of a peripheral portion thereof.