In order to ensure continuous and stable quality of castings during die-casting production process, some factors that are easily overlooked in production are analyzed and corresponding measures are taken, such as reasonable application of cooling water in various links, impact of excessive resistance during injection on quality of castings, details of each of three elements (die casting alloys, die casting machine, mold) that need attention in production process.
Results show that finding corresponding solutions and countermeasures to avoid adverse effects of these problems on product quality, can better meet requirements for quality of castings in production process and ensure stability of production process, thereby improving qualification rate of products, reducing production costs, and improving competitiveness of enterprises.
Aluminum and magnesium alloy die-casting parts have been widely used in automobile engine and gearbox parts. With intensification of competition among major OEMs, quality requirements for die casting parts are getting higher and higher. Customers not only control qualified rate of delivery, but also conduct process audits of suppliers from time to time, so that they can enable suppliers to obtain a controlled and stable supply of castings for their needs.
Suppliers are also facing huge competitive pressures. With continuous increase of labor costs and raw material costs, realization of automated production has gradually become a development trend. Ensuring stability of production process, effectively controlling defective rate of castings, and reducing production costs have become compulsory courses for various manufacturers. However, while increasing investment in hardware facilities, if company's ability to manage product quality has not been improved simultaneously, even with advanced diecast equipment and excellent molds, qualification rate has not been effectively improved.
According to problems that occur in actual production process, this article starts with details that are easily overlooked, effectively controls common problems in production process to ensure that advanced production equipment, optimized production processes and well-designed molds can exert their better use.

Water, as an indispensable medium in diecast productions, plays a vital role in diecast equipment and molds. It is used to maintain heat balance between die casting machine and mold to achieve an ideal production state (oil temperature of die casting machine ≥55 ℃, mold temperature is controlled at 150-240 ℃). Oil temperature of die casting machine directly determines whether all aspects of performance of die casting machine can reach optimal design requirements. Thermal balance of mold itself directly affects internal and appearance quality of casting production process. In order to ensure stability of production process, we need to control following points.
(1) Use demineralized water to ensure clean water, remove calcium and magnesium particles from water to prevent scaling and blockage of die casting machine cooler and mold cooling water channels.
(2) Do a good job of filtering pipes after cooling water mains enter each die casting machine to ensure cleanness of water quality. Make a maintenance plan for heat exchanger of die-casting machine, periodically clean up scale, and clean pipe filter regularly (see figure 1).

 

(3) Regularly inspect and clean mold cooling system. For example, after completion of each batch or continuous production of more than 3,000 or 5,000 mold times, maintenance frequency and method of mold must be determined according to local water quality situation, size of mold, and cooling method.
(4) Due to different water quality in different parts of mold release agent, for products with high product appearance requirements, it is recommended to use purified water, which can greatly improve appearance quality of product, reduce production of carbon on mold surface area, and extend mold maintenance cycle.

Harm of water to die casting parts in diecast productions is first. A drop of water will instantly evaporate into gas after contacting high-temperature aluminum liquid, its volume change at this time is more than 1,000 times the former. Although a large amount of gas in melting cup and mold cavity is exhausted through mold exhaust groove during filling process, some still remains in cavity, and it is wrapped inside product to form air holes, which causes causes cold appearance of product and quality of internal air holes. Therefore, during casting process, there must be no water in any form during entire flow of alloy liquid. Let us analyze factors that generate water.
(1) Water is leaking from joint between injection head (hammer head) and connector, injection rod.
During injection process, it flows into melting cup (barrel) and is not completely taken away when injection rod is retracted, forming a layer of water film on the surface of melting cup. In next working cycle, it is brought into mold cavity. At this time, it should be considered whether sealing ring in connection process is suitable and whether raw material belt is effective (see Figure 2a).

 

(2) Causes of water leakage in gate sleeve: Water drips from joint surface between gate sleeve and flange of melting cup (barrel). This type of water leakage needs to check whether cooling system of fixed mold core has leaked. Cooling water pipe of sprue sleeve is not installed properly, water flowing out of installation hole enters melting cup, needs to be inspected and reinstalled; Cracks occurs in sprue sleeve, cooling water flows out of cracks into melting pot, and sprue sleeve is replaced with a new one (see Figure 2c).
(3) Mold cooling water pipe leaks and drips into cavity.
(4) After spraying, air is not completely blown, mold release agent mixed liquid remains in cavity, or there is water remaining in slot of core pulling slider, which is brought into cavity when inserting core, so it is necessary to find out reason, adjust spray blow position and time, improve local drainage of mold (see Figure 2b).
(5) Mold is cracked, cracked position penetrates cooling water path of mold, causing cooling water to seep into cavity from crack.
(6) During spraying process, release agent sprayed into melting cup (barrel) was not blown dry (see Figure 2a).

 

Injection head (hammer head) and melting cup (barrel) are important parts of diecast productions. They play a key role in injection speed and pressure of die casting process. For example, common failure of diecast productions is stuck injection head(Hammer head). After injection head (hammer head) fly backwards after injection and injection completed, semi-circular aluminum skin can often be taken out from melting cup (barrel). All kinds of phenomena such as impact, stuttering are factors that affect instability of casting, so we need to perform life statistics and management on it, perform maintenance and inspection in stages during production process (see Figure 3).
(1) Establish files of injection head (hammer head), melting cup (barrel) and gate sleeve. File contains serial number, production mode and measurement size data (such as position of melting cup at 3-9 points and 6-12 points, purpose is to check whether there is wear and tear, whether it is within acceptable range), periodically replace it according to degree of wear, or by collect data, it is possible to determine production model for forced replacement.
(2) Cooling problem of injection head (hammer head), gate sleeve must keep water flowing smoothly. During normal production, surface temperature of injection head (hammer head) is regularly detected, data is accumulated, then a graph is made, upper and lower limits are set for monitoring. A flow meter is set at inlet of injection head to monitor flow of water, monitor internal clogging of injection rod to achieve visual management. For completion of each batch of gate sleeve production, it is necessary to test gate sleeve to ensure smooth flow.

 

There are many suppliers of accessories related to injection head (hammer head), melting cup (barrel), and gate sleeve. It is recommended to choose same supplier for these three sets of accessories, so as to achieve the best cooperation among three, so that they can better display their longest optimal use time and reduce waste caused by poor cooperation in production. (See Figure 4)

Mold is one of three major elements of diecasting productions. Die casting process is essentially a space replacement process. Alloy liquid is filled into mold cavity, and gas in mold cavity is replaced. As long as there is replacement, there must be a discharge channel, that is, overflow system in mold design. Failure modes of overflow system mainly have following points.

1. Blockage of overflow system: ① Mold designed with a simple exhaust tank, exhaust tank is blocked by flash; during spraying process, mold release agent remains to block exhaust tank (Figure 5a); ② Mould designed for washing board exhaust structure. Aluminum slag remains at the end of exhaust sheet, blocking air outlet; sludge dirt blocking air outlet; resulting in replaced gas can not be smoothly discharged, and eventually become involved in product, forming internal defects (Figure 5b); ③ Vacuum valve type exhaust method: Exhaust valve is stuck during production process, vacuum machine cannot complete evacuation (in this case, vacuum machine will give an alarm) (see Figure 5c).

 

2. Flying aluminum is also a problem often encountered in diecast productions. During production process, die running material not only affects internal quality of casting, but also adds a source of danger and an unsafe factor. Therefore, it must be ensured that die cannot run away, especially for castings with dense requirements (see Figure 6).

 

(1) Feeding operation is carried out according to specified ratio. Generally, it is required that: aluminum ingot: reflow material = 7: 3 is appropriate.
(2) Clean up concentration furnace at least once / 12 hours.
(3) Molten aluminum placed in transfer package must be refined.
Temperature of aluminum liquid discharged from centralized dissolution furnace is controlled at 720-760 ℃, refining degassing temperature is 700-750 ℃, transfer package baking temperature is 800-850 ℃, so that refining slag can play the best use effect.

(1) Ensure that oil is clean. Hydraulic oil is blood of die casting machine. If blood is not good, entire equipment capacity will be affected and restricted, which will seriously affect injection performance of die casting machine. Equipment is unstable and impact on casting is fatal. Therefore, suction filter must be cleaned regularly (once / month); precision filter (paper / non-woven) should be replaced regularly (once / 3 month); precision filter at servo valve have at least two sets of filters all the ways. Hydraulic oil is filtered (once / year) or replaced (once / 3 years), which can minimize failure rate of same service valve to the greatest extent, extend service life, and guarantee performance indicators that equipment should have.
(2) Periodic detection and adjustment of coaxiality of shots. If coaxiality is not good, speed and pressure of shots will be affected. In short, diecast productions is a comprehensive factor, involving many factors, each of which may cause fatal defects to casting. Effective control and effective precautions can be taken for every factor that may affect quality. Only in this way can guarantee entire production process to be stable and controllable, quality of castings is stable, reduce generation of waste products, and improve qualification rate of castings.