JPH0312990B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is aimed at obtaining quality such as predetermined dimensional accuracy in the product when producing a product using an injection molding machine such as a die-casting machine, and preventing damage to the mold. This invention relates to a mold preheating method for preheating the mold to prevent the mold from heating up, thereby making the mold temperature appropriate.

[Conventional technology]

In injection molding such as die casting, the temperature of the injected molten metal is usually several hundred degrees, and therefore the temperature of the mold is generally 150 degrees Celsius or higher, although it varies depending on the type of metal being injected, the size of the product, etc. It is considered optimal to perform injection molding at a temperature of 250°C.

During continuous injection, the mold absorbs heat from the molten metal during the chill time between the injection of the molten metal and the opening of the mold, and the cavity surface reaches a temperature of 500℃ to 600℃. When the mold is opened and the product is extruded, the cavity surface comes into contact with the atmosphere and heat is dissipated into the air. Furthermore, it is cooled to around 200℃ by applying a mold release agent and spraying cooling water, etc., so that the mold temperature is appropriate. Mold clamping and injection are repeated while maintaining the range.

In this way, the temperature of injection molding molds is approximately 200°C immediately before injection, and locally reaches over 500°C immediately after injection, and the shape and dimensions of the cavity are designed based on this high temperature assumption. Molds are manufactured according to the shape and dimensions.

For this reason, when restarting injection after suspending or stopping injection for a long time, the mold temperature will drop to near room temperature (atmospheric temperature), the dimensions of the cavity will change, and the split cavity part to be incorporated into the mold will Not only does the gap between the core and the core become wider, burrs are more likely to occur, and the product yield is lower, but also the mold is damaged due to thermal shock due to injecting high temperature molten metal into a low temperature mold. Various damages occur, such as cracks, hair racks, and the product clinging to the mold, damaging the core pin and ejector pin.

Therefore, when the mold temperature is low due to a long interruption or stoppage of injection, it is necessary to preheat the mold to 150° C. or higher using a low-pressure burner, an infrared heater, or the like.

[Problem that the invention seeks to solve]

As mentioned above, injecting molten metal into a cold mold has various negative effects on the mold, etc., so it was conventionally believed that it was necessary to avoid raising the mold temperature due to dumping, and a preheater such as an infrared heater was used. Although the mold is preheated by the preheater, it is necessary to heat the entire mold as uniformly and gradually as possible to avoid local heating, and the heating may vary depending on the shape of the mold and cavity. required skill.

The present invention is a heating method that does not require such skill, does not use a preheater, and can bring the temperature gradient of the mold to a natural mold temperature close to that of continuous injection. This made it possible to heat the mold by dumping.

[Means to solve the problem]

The molten metal is injected only at a low injection speed, and the chill time in the first injection is short, and the injections are repeated at a low injection speed so that the chill time is gradually lengthened, and after the required number of injections, the chill time is close to the normal chill time. This will preheat the mold.

[Effect]

Since the present invention is a mold preheating method in which the mold is heated by the molten metal injected into the cavity, it is a preheating method that can heat the mold with a temperature gradient close to that during normal continuous injection. The chill time, which is the time during which the molten metal injected into the cavity is cooled, is shorter than the normal chill time, so even if the mold temperature is low, the injected molten metal is prevented from cooling too much and shrinking too much. Therefore, the risk of clinging to the mold and damage to the ejector pin etc. is eliminated, and since the injection is performed only at a low injection speed, the impact on the mold during injection is reduced. In addition, it is possible to reduce the occurrence of burrs caused by excessive intrusion of molten metal into the gaps between the divided cavities and the core, thereby preventing the risk of adversely affecting the joints between the molds. .

〔Example〕

In the embodiment of the present invention, when injection is interrupted for a long time longer than a certain time, the initial chill time is shortened, and the chill time is sequentially lengthened after a predetermined number of times, thereby approaching the normal chill time, and In this method, the mold is preheated by injecting the molten metal only at a low injection speed, and the mold is heated by a slow injection speed, and the molten metal is heated during this injection. The injection speed is set to be low, and the chill time from the completion of injection to the opening of the mold in the first throwaway injection is made short, for example, 60% of that of normal injection.

Then, the number of throw-off shots is set to N times, and the chill time is lengthened each time until it approaches the chill time of normal injection.For example, after the end of five throw-off shots, the injection speed is increased to low and high speeds with the normal chill time. We will perform continuous injection, which is normal injection molding.

That is, in this embodiment, after stopping injection for one hour or more in the NC-controlled continuous injection molding apparatus, the following program was used to restart injection.

In this program, the number of discard shots N is set to 5.
Unless the operator specifically changes the settings, the basic number of times will remain 5 times, and if the chill time during normal injection is 25 seconds, the chill time for the first throw-off will be 60% of the length of normal injection. The chill time for the final 5th shot is set as 15 seconds, and the chill time for the 2nd, 3rd, and 4th throw is increased equally as shown in a of the attached drawing. 17.5 seconds, 20 seconds, and 22.5 seconds.

Furthermore, the injection speed in this abandonment shot is to be carried out only at a low injection speed.

Note that the chill time for the first shot is not limited to 60% of the chill time during normal injection; if the interruption time becomes longer, the chill time for the first throw may be further shortened. Increase in size or
If the cavity shape is complicated, the number of throws is not limited to five, but may be set to a larger number.

Furthermore, depending on the shape and capacity of the cavity, the increase in chill time is not limited to the case where the chill time is increased equally as described above (see attached drawing a), but also when the initial increase rate is lowered as shown in drawing b. Or, as shown in c in the drawing, the initial increase rate may be increased to allow the final injection to reach the chill time during normal injection.

〔Effect of the invention〕

As mentioned above, the present invention shortens the chill time and injects the molten metal at a low injection speed, so it eliminates negative effects such as harmful effects on the mold even if the mold is at a low temperature, and eliminates waste injection, which was previously prohibited. Since this is a mold preheating method that heats the mold by filling the cavity with molten metal, it is possible to obtain a mold temperature distribution close to the mold temperature gradient during continuous injection, which is normal operation. This makes it possible to improve the initial product yield when normal operation is resumed, and in many cases there is no need to change the number of throw-offs and chill time increase rate that have been set, unless the mold is replaced. When resuming injection after a long injection interruption, the mold can be preheated by the number of times according to a predetermined program and the degree of increase in chill time, so the mold can be easily reheated without any skill required. This is a method for preheating a mold in an injection molding apparatus, which has various advantages such as being able to heat the mold.

[Brief explanation of the drawing]

The accompanying drawings are diagrams showing an increase in chill time in throwaway shots according to the present invention.

Claims (1)

[Claims] 1 The molten metal is injected only at a low injection speed, and the chill time is shortened for the first injection, and injections are repeated only at a low injection speed while gradually increasing the chill time, and after a predetermined number of injections, injection close to the normal chill time is achieved. A method for preheating a mold in an injection molding apparatus, characterized by performing the following steps.