JPH04262853A - Manufacture of die for plastic forming - Google Patents

Abstract

PURPOSE:To prevent the generation of casting flaws such as pin hole, shrinkage hole, and to manufacture a die for plastic forming in low cost for a short term. CONSTITUTION:Molten zinc base alloy incorporating, by weight, 8-15% aluminum, 5-13% copper, 0.01-0.5% magnesium is poured into a mold, in which copper pipe is disposed and cooling water is made flow in this copper pipe, and cast to a shape having the similar shape as the finished die, and this casting is finished to a final shape in a postworking process.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a mold for plastic molding which has fewer casting defects such as pinholes and shrinkage cavities and which has excellent mechanical properties such as strength and hardness.

0002

BACKGROUND OF THE INVENTION Conventionally, S55C carbon steel for mechanical structures has been widely used as a material for plastic molding molds, particularly injection molding molds. This carbon steel for machine structures not only has good strength, weldability, graining workability, polishing workability, etc., but also excellent machinability, and is relatively inexpensive among mold materials. This is because it has the following characteristics. On the other hand, pre-hardened steel and the like have been used especially in molds that require a mirror finish.

[0003] In recent years, housings and constituent parts of various types of equipment such as office equipment, and automobile constituent parts have been made of plastic, and frequent model changes are being made to improve their performance and change their designs. . As a result, the life cycle of plastic molded products has become shorter, leading to high-mix, low-volume production. Therefore, there is an increasing demand for lower costs and shorter delivery times for molds for molding such plastic molded products.

[0004] By the way, when making a mold using forged steel such as carbon steel for machine structures, many machining steps are required, and this machining cost accounts for most of the mold production cost. Therefore, there was a problem in that it was not possible to reduce the price of the mold or shorten the delivery time. Therefore, by sand casting a zinc-based alloy with a low melting point and good workability, such as ZAS (trade name, manufactured by Mitsui Kinzoku Kogyo Co., Ltd.), for the prototype mold, it was formed into a shape that required as little machining as possible, such as cutting. A method has been proposed in which a metal mold is manufactured by shaping the mold and subjecting it to finishing processes such as copying and polishing.

[0005] According to this, compared to a mass-produced mold that is manufactured by cutting a large forged steel block, the machining speed is faster, which has the advantage of being cheaper and shortening the delivery time. have. However, the above zinc-based alloys have poor mechanical properties such as strength and hardness, so a considerable margin must be taken into consideration in the design.
The range of products that could be used was limited because mirror finishing was not possible. Furthermore, it is difficult to prevent the occurrence of casting defects such as pinholes and shrinkage cavities, and repairs such as welding are essential. However, as the cooling conditions change significantly near the weld, unevenness occurs in the structure, which can cause There were problems such as transfer to molded products.

[0006] Aluminum alloys and copper alloys are also used for the same purpose, but the former aluminum alloys have low hardness, resulting in poor specularity, easy casting defects and pinholes, and extremely poor weldability. Unfortunately, it was virtually impossible to repair casting defects once they had occurred or to repair molds caused by processing errors. The latter copper alloy has problems such as poor casting surface due to high casting temperature and poor electrical discharge machinability.

[0007]

[Problems to be Solved by the Invention] Among these materials for molds, zinc-based alloys, which have a low melting point, are easy to cast, and have excellent weldability, are being reconsidered, and various improvements in mechanical strength are being made. Attempts are being made. For example,
As disclosed in Japanese Patent Publication No. 42 and Japanese Patent Application Laid-open No. 62-287030, mechanical Trying to improve strength.

However, with this method, casting defects such as pinholes and shrinkage cavities are often not improved but rather increased. Further, Japanese Patent Application Laid-Open No. 2-187308 discloses that a mold is manufactured by machining a raw material block made of a zinc-based alloy. According to this method, although the processing speed is high, the number of machining steps increases, and the above-mentioned advantages (short delivery time, low price) of the casting mold are not fully utilized.

In view of the above-mentioned current situation, the present invention provides an inexpensive casting method that can prevent casting defects such as pinholes and shrinkage cavities even in cast products having complex shapes such as molds. The purpose is to manufacture molds for plastic molding.

0010

[Means for Solving the Problems] The method for manufacturing a plastic molding die of the present invention has a weight percentage of aluminum 8.
~15%, copper 5-13%, magnesium 0.01-0.
A zinc-based alloy containing 5% zinc-based alloy is poured into a mold in which a cooling pipe is installed and a cooling liquid is flowed through the cooling pipe, and the casting is cast into a shape close to the final mold shape. It is characterized in that it is finished into the final shape in a post-processing process.

[0011] In the zinc-based alloy used in this invention, the aluminum content is 8 to 15% by weight.
% because if it is less than 8%, sufficient mechanical strength or hardness cannot be obtained, and if it exceeds 15%, segregation of aluminum will occur. Copper content in weight percentage 5
The reason why it is set at ~13% is that if it is less than 5%, sufficient mechanical strength or hardness cannot be obtained, and if it exceeds 13%, it will lose its toughness and become prone to cracking during post-processing.

[0012] Furthermore, magnesium is necessary for suppressing intergranular corrosion, and its content is 0.01% by weight.
The reason why it is set at ~0.5% is that if it is less than 0.01%, the suppressing effect is not sufficient, and if it exceeds 0.5%, it becomes brittle. Furthermore, in addition to the above-mentioned components, the zinc-based alloy of the present invention contains one or more elements selected from yttium, beryllium, titanium, zirconium, chromium, nickel, cobalt, manganese, lanthanum-based elements, and silver. Preferably. This contributes to improving mechanical strength, suppressing the occurrence of structural defects, delaying dimensional changes due to aging, etc., and its content is as follows:
If the weight percentage is less than 0.01%, a sufficient effect will not be obtained, and if it exceeds 2%, the effect of containing more than that cannot be expected, and it will become brittle, so the total amount should be 0.01 to 2%. is preferred.

The above-mentioned zinc-based alloy preferably contains all elements (inevitable impurities) that may be mixed in other than the above-mentioned elements. The allowable amount varies depending on the extent to which intergranular corrosion becomes a problem, but it is usually preferably within 0.1% by weight. Naturally, if intergranular corrosion becomes a serious problem, it is desirable to further suppress the incorporation of lead, cadmium, tin, etc.

[0014] The cooling pipe arranged inside the mold is preferably made of a material that is easy to bend and has good thermal conductivity, such as copper pipe, and is placed near the mold surface as long as the strength of the mold allows. It is preferable to arrange it so that This is because they are more effective as cooling holes in a mold during plastic molding. In addition, when copper is used for the cooling pipe material, there is an advantage that the copper is eluted during casting, and the mechanical strength such as strength and hardness of the alloy can be improved.

[0015] Furthermore, it is of course possible to use a feeder or a top heat method in combination during casting, thereby making it possible to more reliably suppress the occurrence of structural defects. In this invention, the means for finishing the casting into the final shape in the post-processing step is not particularly different from conventional methods, and includes means such as cutting, copying, and polishing. As a result, the mold is finished into a desired shape or mirror-finished, and the mold is completed.

[0016]

[Function] Since the present invention is as described above, the solidification rate of the zinc-based alloy is accelerated by the liquid flowing through the cooling pipe during casting, so that phase separation is less likely to occur and shrinkage cavities are less likely to occur. Furthermore, heat generation due to α transformation is suppressed at around 285° C., the crystallization rate is not delayed, and crystals do not become coarse.

[0017] Furthermore, since the cooling pipes are already embedded, the subsequent process of cutting cooling holes and the like in the mold is omitted.

[0018]

[Example] Next, an example of the present invention will be described. (Examples 1 to 15, Comparative Examples 1 to 17) A predetermined amount of zinc-based alloy (Examples 1 to 15, Comparative Examples 1 to 17) having the composition shown in Tables 1 and 2 was dissolved in a clay carbon crucible. death,
This was cast into a cavity mold for molding an automobile lens cover using a CO2 mold. After casting, dismantle the mold 15
A cavity for molding an automobile lens cover was obtained by finishing with 0-800 polishing paper.

Note that Examples 1 to 15, Comparative Examples 16 and 17
In Comparative Examples 1 to 15, no such cooling pipe was provided in the CO2 mold. For these cavity molds, the polished surface was observed under a microscope to determine the state of shrinkage cavities on the mold surface corresponding to the outer surface of the molded product, and the shrinkage cavities of 50 μm or more were counted, and the surface hardness of the parting line surface (
Brinell hardness) was measured. The results are also shown in Tables 1 and 2.

As is clear from Tables 1 and 2, in Comparative Examples 1 to 15 and 17, microscopic nests were generated, whereas in Examples 1 to 15, no microscopic nests were observed. Further, the Brinell hardness was 170 or more, and had sufficient hardness to be used in a mold. In addition, in Comparative Example 16, although no microporosity was observed, the hardness was extremely low.

[0021]

[Table 1]

[0022]

[Table 2]

(Example 16) A mold using the cavity mold of Example 1 shown in Table 1 was attached to a 140-ton injection molding machine, and molding was performed (injection pressure: 700 kg/cm2, plastic raw materials used; Polypropylene (manufactured by Mitsui Noblen Co., Ltd.; JHH-G). In this case, even after 50,000 shots of molding, no deformation or change in surface roughness occurred in the cavity mold. (Comparative Example 18) Repairing the microscopic cavities on the surface corresponding to the outer surface of the molded product of the cavity mold of Comparative Example 1 shown in Table 2, and installing the mold with this as the cavity mold into a 140 ton injection molding machine,
Molding was carried out in the same manner as in Example 16, but after 7,500 shots, the molded product had significant burrs, and the molding had to be discontinued.

[0024]

[Effect of the invention] Since the present invention is as described above,
During casting, the liquid flowing through the cooling pipe accelerates the solidification rate of the zinc-based alloy, making it difficult for phase separation to occur and shrinkage cavities to occur. Furthermore, heat generation due to α transformation is suppressed at around 285° C., the crystallization rate is not delayed, and crystals do not become coarse. Therefore, after casting, there is no need to repair shrinkage cavities, and mechanical strength, hardness, etc. are improved. Furthermore, since the cooling pipes are already embedded, the subsequent process of cutting cooling holes and the like is omitted.

[0025] Therefore, the mold manufacturing period is shortened, greatly contributing to lower prices, and a mold suitable for high-mix, low-volume production can be provided.

Claims (2)

[Claims] [Claim 1] 8-15% aluminum by weight percentage
, a zinc-based alloy containing 5 to 13% copper and 0.01 to 0.5% magnesium is poured into a mold with cooling pipes arranged inside the mold and a cooling liquid flowing through the cooling pipes. A method for producing a plastic mold, which comprises casting into a shape close to the shape of the mold, and finishing the cast into the final shape in a post-processing step. 2. The zinc-based alloy contains 8 to 15% aluminum, 5 to 13% copper, and 0.01% magnesium by weight percentage.
~0.5%, yttium, beryllium, titanium,
At least one selected from zirconium, chromium, nickel, cobalt, manganese, lanthanum-based elements, and silver.
2. The method of manufacturing a plastic mold according to claim 1, wherein the zinc content is 0.01 to 2% and the remainder is zinc.