Die casting, as a molding process, has its inherent drawbacks besides its high efficiency and high output. Under high speed and pressure, molten metal inevitably experiences problems such as air entrapment and shrinkage in hot spots. How to dialectically view this contradiction is similar to philosophy of life. As human beings, no one is perfect except God; we all have innate strengths and weaknesses, kindness and violence, self-discipline and decadence. What we can do is maximize our strengths and minimize our weaknesses. Coordination between various processes and links in die casting is like relationship between people—a balance of strengths and weaknesses, mutual support and cooperation to achieve the best point of understanding. All measures should be problem-oriented. I recently chatted with an industry leader who said: "Defects in die casting cannot be completely eliminated, but we need to use various means to make defects occur in the least important areas." Just like hemorrhoids are incurable, they're harmless on buttocks, but on the face—that's a serious problem. The saying may be crude, but principle is sound. This applies to interpersonal relationships and controlling defects in die casting. Just as in interpersonal relationships, we must accept each other's strengths while minimizing their weaknesses. Similarly, achieving optimal integration across all aspects of die casting is what we need to relentlessly pursue. When facing problems, we must learn to analyze them dialectically, grasp logic of problem-solving, untangle contradictions and entanglements between different stages, rather than focusing solely on mold design or other single factors. Only in this way can we adapt to changing circumstances and achieve harmonious relationships.
Leakage is a major pain point for all companies under current high standards. Today, we'll use a representative example—silicone oil fan clutch—to discuss our analytical logic for leakage problems. Silicone oil fans are commonly used in commercial vehicles and off-road vehicles. Using silicone oil as a medium, they utilize shear viscosity of silicone oil to transmit torque, driving fan to adjust cooling intensity of engine. Required filling amount of silicone oil is ±1g, and a leak test must be conducted at a pressure of 2.5 bar, so its airtightness requirements are self-evident.

Leakage points in silicone oil fans are mainly concentrated in the area around mounting threaded holes in heat sink fin region. Thermal bottleneck area between fins causes slow shrinkage and porosity in this area, forming a defect channel with internal cavity after machining, leading to leakage.

 

1. Characteristics of thermal bottleneck area: Narrow area where mounting hole meets heat sink fin is instantly heated upon contact with high-temperature molten metal. Mold in this area is under high-temperature conditions, causing molten metal to solidify slowly during solidification stage, resulting in shrinkage porosity.

 

2. This area is usually close to machined surface of internal cavity, and connecting area has a thick wall. After machining, defects in internal cavity are exposed, forming a channel with inner wall of outer hole or aforementioned thermal bottleneck area of mold, leading to leakage.

 

3. As shown in diagram above, due to product functional requirements, inner cavity often has an annular groove. Narrow size of this groove usually prevents pre-casting, resulting in a large machining allowance. This leads to defects after machining, which is entry point for leakage.

In this context, to effectively solve this leakage problem, I believe we should analyze each stage from source to final product formation:
1. Metal Melting Stage: Purity of molten metal is root cause of all problems. If molten metal is substandard, subsequent adjustments will not solve corresponding issues. We are no longer in the era of machine-side heating furnaces. To ensure uniform quality standards for molten aluminum, centralized aluminum supply is necessary. Before transferring aluminum, strict degassing and removal of impurities are required, and proportion of recycled material must be strictly controlled to ensure purity of molten aluminum.
2. Design of Gating System: Based on characteristics of this type of product, gating system should be arranged along direction of ribs to ensure filling state of molten aluminum and provide feeding channels for defective areas. However, gate placement should avoid areas with large leaks to prevent impurities and cold material accumulation.
3. Mold Design
1) High-pressure cooling points need to be designed for corresponding holes to ensure surface quality of hole walls. Regular maintenance of these high-pressure cooling points is necessary to prevent scale buildup and reduced cooling efficiency.
2) For weak areas on mold as shown above, where cooling cannot be implemented, high thermal conductivity materials can be used locally to replace ordinary H13 steel, if conditions permit, to increase heat dissipation efficiency in these areas.
3) Control machining allowance of internal cavity machining surfaces. For annular grooves, partial inlays can be used; the entire machining area can be made into an inlay, with centralized cooling or 3D printing to reduce distance between cooling channels and cavity surface, enhancing cooling effect. Ensure dense layer at source of internal leakage is at least 5mm larger than machining allowance.
4. Process Parameter Settings: Due to dense radiant heat dissipation fins of these products, high-speed filling of ingate often creates a very large and uncontrollable splashing turbulence. Therefore, pre-filling is often necessary to control stability of the molten aluminum at head.
5. Temperature Field Control: In addition to controlling cooling of key areas, maintaining balance of temperature field within mold body is crucial. High mold body temperatures indirectly affect heat transfer efficiency of local areas.

Compared to traditional automotive power systems and new energy motor housings, silicone oil fans are relatively simple.
Leakage is mainly due to improper defect control leading to penetration between machined surfaces. For large products such as cylinder blocks, gearboxes, and motor housings, in addition to leakage caused by penetration between machined surfaces, leakage is mainly caused by penetration between high-pressure oil channels, water channels, and hot spots such as mounting holes or weight reduction grooves. In addition to above countermeasures, it is usually necessary to add a local pressure boosting mechanism to machined threaded holes near high-pressure water/oil channels. However, logic for analyzing any type of leakage is same: from controlling source to arranging gating system and adjusting process, each issue needs to be addressed one by one. Usual order is to ensure absolute reliability of aluminum molten metal melting, sufficient process range adjustment, and stable mold temperature control, only then should modifications to mold and adjustments to gate be considered.