Abstract: Die-casting mold is main process equipment for die-casting production. Whether production process is stable and whether quality of castings is guaranteed depends to a large extent on rationality of mold structure and advancement of technology. In die-casting production, die-casting mold is basis for correctly selecting and adjusting process parameters. Die-casting molds have a very close and special relationship with die-casting technology and production operations that restrict and influence each other. Design of die-casting mold is essentially a comprehensive reflection of various factors that may occur during production process.
Therefore, when designing die-casting mold, it is necessary to comprehensively analyze structure of part, establish a correct pouring system, and reasonable gate size. During use process, management of mold must be careful and maintenance must be troublesome. Only in this way can die-casting mold serve stably.

 

Stability of die-casting mold during production process refers to ensuring a higher yield rate and a lower mold failure rate within service life of die-casting mold, including: mold design, maintenance during use, etc. .

2.1.1 On premise of ensuring quality and safe production of castings, a reasonable, advanced and simple structure should be adopted to reduce operating procedures so that action can be accurate and reliable, components have good rigidity, wearing parts can be easily replaced and repaired, and there are It is beneficial to extend service life of mold.
2.1.2 Various parts of die-casting mold should be made of appropriate materials and matching accuracy should be reasonable.
2.1.3 According to technical characteristics of die-casting machine, installation dimensions should be accurately selected so that connection and installation of die-casting mold and die-casting machine is convenient, accurate and reliable, technical functions and production capacity of die-casting machine can be fully utilized.
2.1.4 Parts of die-casting mold should be standardized, universal and serialized to shorten design, manufacturing and maintenance cycles.

When opening mold, casting can be kept out of fixed mold following direction of movable mold, so that casting remains in movable mold; in order to facilitate removal of casting from movable mold, parting surface should be taken on the largest cross-section of casting. It is conducive to reasonable layout of pouring system and overflow system.

Gating system is not only a channel for liquid metal to fill die-casting mold, but also regulates factors such as flow rate and pressure transmission of melt, exhaust conditions, and thermal stability of die-casting mold. Therefore, when designing gating system, structural characteristics, technical requirements, alloy types and characteristics of casting must be analyzed, type and characteristics of die-casting machine must also be considered, so that a reasonable gating system can be designed.

Mold should be equipped with an overflow groove and exhaust channel with sufficient overflow range, which is very important to ensure product quality. People often overlook phenomenon that overflow channel is blocked prematurely by incoming molten metal. A reasonable structure should be adopted so that molten metal flows into deeper part of overflow tank first to ensure that exhaust hole remains open for the longest time. In addition, overflow tank should be equipped with a ejector rod to remove metal from overflow tank.

Temperature control of mold is relatively high, which is beneficial to filling of liquid aluminum in mold and forming of castings. However, when mold temperature approaches 300℃, obvious aluminum sticking will occur, making surface of casting rough or lacking material, requiring frequent grinding of mold. At the same time, mold is opened before casting is fully cooled within predetermined time. Due to insufficient cooling, demoulding gap between casting and mold is not fully formed, coupled with phenomenon of aluminum sticking, mold core and casting pin will be bent or broken due to excessive stress during demoulding. Too low mold temperature control will affect fluidity of molten aluminum and cause cold shut in casting. We hope that temperature of each part of mold will not differ too much, otherwise uneven thermal expansion and contraction of mold will cause cracks in mold core, and tiny cracks will continue to expand due to stress concentration and infiltration of aluminum liquid, causing module to partially collapse, directly affecting service life of mold. In production practice, we have encountered scale due to poor cooling water quality, which caused blockage of some cooling pipes. As a result, temperature of movable core on the side of movable mold was too high (surface temperature was close to 300℃), causing severe aluminum sticking to the surface of mold cavity. Mold had to be polished almost every shift, and cracks at corners of mold core increased significantly. Through measures to clear pipes and improve water quality, situation has improved significantly.

First, establish a mold file, that is, establish a complete set of usage records for each set of molds when they enter factory. This is an important basis to ensure future care and maintenance. Each item must be detailed and clear, including daily production models, etc. As a mold manager, after mold enters factory, structural accessories of each part of mold must be recorded in detail in mold file, vulnerable parts in mold must be listed as needed, and accessories such as ejector pins, cores, etc. must be prepared in advance to set a minimum limit for vulnerable spare parts, so that production will not be delayed due to insufficient preparation. Because there are many such lessons in the company, only by being prepared can you be safe. If production is delayed because they do not prepare spare parts, cost to die-casting company will be huge. Time, manpower, electricity (or liquefied gas) for holding furnace, etc. are not small figures. The most important thing is that production is delayed and delivery is delayed, and the loss will be even greater!

 

When it comes to care and maintenance of molds, many die-casting operators think that it is job of mold repairman and has little to do with him. In fact, it is just opposite. Fate of all molds can be said to be in hands of die-casting operator. Therefore, following points should be paid attention to when using mold:
(1) Use of mold cooling system. When used correctly, mold cooling water not only extends service life of mold, but also improves production efficiency. In actual production, we often ignore its importance. Operators also want to save trouble. It is too troublesome to connect cooling water pipes. Some companies even do not need cooling water in order to save costs when customizing molds, which has caused serious consequences. Mold materials are generally made of special mold steel through various treatments. No matter how good mold steel is, there are limits to their use, such as temperature. When mold is in use, if mold temperature is too high, it is easy for cracks to appear on the surface of mold core early. In some molds, cracks will appear on a large area before even more than 2,000 molds. Even during production of mold, mold core changed color because mold temperature was too high. After measurement, it even reached more than 400 degrees. When such a temperature is quenched by release agent, it is easy for cracks to appear, and products produced are also prone to deformation, strain, mold sticking, etc. When using mold cooling water, use of release agent can be greatly reduced, so that operator will not use release agent to reduce temperature of mold. Advantage is that it can effectively extend life of mold, save die-casting cycle, improve product quality, reduce occurrence of mold sticking, strain and aluminum sticking, and reduce use of release agents. It can also reduce loss of ejector pins and cores caused by overheating of mold.
(2) Mold must be preheated during production process to prevent cracks from suddenly encountering hot molten metal in a cold mold. For more complex molds, a blowtorch or liquefied gas can be used if conditions are good. If you use a mold temperature controller, relatively simple molds can use slow injection preheating.
(3) Cleaning mold parting surface is very time-consuming and easy to ignore. A good mold will make work much easier. If quality of mold is not good, it is inevitable that there will be flash or dirt on parting surface of mold during production. Operator should clean these parts frequently and be equipped with a small spatula at all times. If flash is not removed in time, mold parting surface will easily collapse, causing aluminum to run off during production process. Once this happens, no matter how good a mold repair expert you are, possibility of complete repair is very small. It’s not that they don’t have ability. Consequences of aluminum leakage not only increase die-casting costs, waste aluminum, but also make product quality unstable, especially internal quality. It also the difficulty of determining process parameters, and pass rate will drop a lot. From safety considerations, probability of work-related injuries is increased. During shift change, operator should thoroughly clean mold parting surface with kerosene. This not only prevents mold from being squeezed, but also can open up exhaust grooves on mold that are blocked by residue of release agent or other dirt, which is conducive to discharge of gas in cavity during injection process and improves product quality. Mold surface must be cleaned after each shift to allow employees to develop a good habit.
(4) To complete a set of molds and a batch of tasks, after molds are removed, mold repair personnel must have a very detailed understanding of set of molds, such as how many molds have been produced so far and what faults have occurred, check final product to understand current mold status and whether mold's service life has expired.
(5) Mold repair personnel must master one principle during repair and maintenance process. They are absolutely not allowed to change size of mold without permission. If original size is changed, batch quality accidents will occur, and losses will be heavy.
(6) Carry out maintenance work in accordance with company's "Mold Repair and Maintenance Management Regulations". During maintenance process, mold repairmen lack awareness of tools they use. For example, they use relatively thick oil stones when polishing mold. Some even install a polishing machine with an impeller to polish mold core, which not only causes deep scratches everywhere on the surface of mold core, but also damages nitridation of surface of mold core, making die-casting impossible to use in next production. Either mold will stick or it will be strained. This is case in many cases. Mold production was good last time, but it will be impossible to work again this time. In addition to other external factors, this is a very important reason.
(7) Wearing parts such as ejector pins, cores, etc. should be carefully inspected to see if there are any bends, cracks, etc. If there are any, replace them in time. In many cases, core breaks before mold is re-produced before many molds are completed. Most of reasons are due to lack of careful inspection and problems not being discovered in advance, resulting in a huge waste of manpower and time.

 

(8) It is necessary to add that when polishing mold, polish it wherever there is sticky aluminum and carbon deposits to minimize wear and tear on mold caused by polishing.
(9) All moving parts of mold, joint parts, screws, etc. should be lubricated and rust-proofed.
(10) Mold management personnel supervise maintenance status at any time and pay attention to other management details. Keep maintenance and repair records for future reference.
Storage of molds should ensure consistency of ledgers, drawings, files, etc. Molds cannot be disassembled and stored to avoid loss of parts. Molds that have not been used for a long time should be rust-proofed regularly. New molds undergo stress relief treatment within specified mold times to extend their service life.
In process of mold maintenance and repair, some things are quite simple to say and everyone understands it, but it is difficult to actually do it. We need to implement it conscientiously and do job well.

From above analysis, improving stability of die-casting mold in production process lies in the entire process of mold development, use, management and maintenance. Using excellent mold materials and advanced manufacturing techniques while paying attention to every detail in production process and mold maintenance is extremely important for stability of mold in production.