In the previous article, we focused on the basic principles and overall planning of aluminum high pressure die casting mold design. From the application of Pascal’s principle to the selection of mold concepts, cavity number, die casting machine, parting line, and gating system, we explained how to form a reasonable mold design during the planning phase. These factors determine whether the mold can be manufactured and put into production.
Whether a die casting mold can operate stably in long-term production depends on more refined design details. Problems such as gas entrapment, unstable filling, difficult demolding, and premature mold damage often stem from auxiliary systems that are not easily detected in the early design stage. This article will continue to discuss the design of overflow and venting systems, ejection structures, thermal control, and core pulling mechanisms, and explain how these elements work together to ensure stable quality of aluminum high-pressure die castings and mold durability.
High-Pressure Die Casting Mold Design – Overflow Channel
The overflow channel is an auxiliary structure set at the end of the cavity filling in a high-pressure die casting mold. It is used to collect cold material, oxides, and excess molten metal during the filling process, and works with the venting system to expel air from the cavity, thereby improving filling quality and reducing casting defects.
Location of Overflow Channels
- Where there is core obstruction and two streams of molten metal meet;
- Where the mold temperature is low;
- Where venting is poor;
- Located at the last filling point of the molten metal;
- In dead corners that are unfavorable for molten metal filling.
There are no specific regulations for the capacity of the overflow channel, which can be determined according to the actual product situation. Therefore, it is important to make additions after trial molding and consider possible changes in location.
In addition, ejector pins should be installed at the overflow channel, and the slope of the overflow channel should be as large as possible. Generally, one overflow channel has one gate.
Die Casting Mold Design – Venting System
The venting system is a critical structure in high-pressure die casting molds, used to quickly expel air and gases from the cavity during the filling process, preventing gases from being trapped in the molten metal, thereby reducing defects such as porosity, pinholes, and incomplete filling, and ensuring the internal density and surface quality of the casting.
Function of Venting
① To expel gas from the mold cavity and prevent porosity in the casting;
② To reduce the gas pressure at the final filling point, facilitating the filling of molten metal.
Calculation of Venting Groove Cross-sectional Area
According to Bernoulli’s theorem, Q = V1A1 = V2A2, where (V1: gas velocity, taken as 300 m/s. A1: cross-sectional area of the venting groove. V2: plunger speed, design basis is 3 m/s. A2: plunger cross-sectional area)
Venting Methods:
① Natural Venting
a. Advantages: Simple structure;
b. Disadvantages: During the die-casting process, due to the constant precipitation of release agents, damage to the aluminum die-casting mold, or “settling” of the core part, the thickness of the venting groove may change, leading to a lack of long-term stability in the venting channel, either causing blockage and hindering smooth venting, or becoming too thick and causing flashing;
c. After leaving the mold cavity by 35mm, the depth of the venting groove can be increased to 0.3~0.4mm to improve its venting effect;
d. When increasing the venting groove area is necessary, it is advisable to increase the width and number of venting grooves, rather than excessively increasing their depth, to prevent flashing;
e. To more effectively prevent flashing, the section with a depth of 0.1~0.15t can be designed with a bend.
② Venting Block Venting
Advantages:
a. Large venting volume and good venting effect;
b. Concentrated venting, ensuring smooth venting channels during die-casting operations, reducing flashing caused by blockages;
c. A vacuum device can be connected to the venting block for vacuum die-casting.
Disadvantages:
a. Complex manufacturing;
b. If the overflow groove design is unreasonable, it can easily cause several airflows to collide or the aluminum material in front to block the airflow behind.
Design Considerations:
a. Avoid opposing flows in the overflow channels leading to the exhaust block;
b. The cross-sectional area of the overflow channel should increase from small to large (opposite to the sprue);
c. To prevent aluminum from sticking to the teeth, a high heat treatment hardness (HRC48) or surface nitriding treatment can be used. Water cooling channels should be provided for larger dimensions.
③ Vacuum System Exhaust
a. Advantages: The vacuum die casting system can effectively remove gas from the mold cavity and pressure chamber, significantly reducing or eliminating internal porosity in die castings;
b. Disadvantages: Requires high demands on the mold’s sealing performance;
c. Applications: High-density castings, castings with minimal internal porosity, and complex thin-walled parts;
d. Vacuum die casting system working process:
When the injection plunger has just moved past the pouring hole in the pressure chamber, the vacuum pump switch is activated, and vacuuming of the mold cavity and pressure chamber begins. The vacuuming process continues until the filling stage of the die casting process is completed, and the molten metal flows through the last filled part of the die casting mold into the vacuum valve. The valve then automatically closes, stopping the vacuuming process.
Many aluminum die casting defects stem from underestimating die casting mold design details in the early stages. A comprehensive mold design review before manufacturing can significantly reduce trial runs and production risks.
Raidy Mold provides engineering-driven aluminum high-pressure die casting (HPDC) mold design and manufacturing support to help achieve stable mass production, reduce defects, and extend mold life. If you require technical consultation or mold design optimization, please contact Raidy Mold to discuss your project needs.
Ejection Mechanism Composition
The ejection system is a critical mechanism in die casting molds. Its main function is to safely and stably eject the casting from the mold cavity after the casting is filled, cooled, and the mold is opened. This system typically consists of ejector pins, an ejector plate, and guiding and resetting structures. The system is driven by equipment power to move the ejector plate, causing the ejector pins to evenly push against the back of the casting, effectively preventing white marks, cracks, and preventing casting deformation or damage, while also improving production stability and mold lifespan. 9. Design Considerations for Ejection Mechanisms in Aluminum Die Casting Molds
① The size, quantity, and distribution of the ejector pins should ensure smooth and stable ejection of the casting, preventing damage and deformation of the die-cast part;
② Common forms: ejector pins (round, flat, irregular, stepped), ejector tubes, ejector plates;
③ Ejector pins should also be provided at the gate, runner, and vent areas;
④ Ejector pins should be made of SKD61 material, surface nitrided, with an internal hardness of HRC40-45 and a surface hardness of HV900;
⑤ The fit between the ejector pin hole and the ejector pin should be H7/e8 (ejector pin tolerance -0.01~-0.02, ejector pin hole tolerance +0.01~+0.02). The ejector pin is allowed to be slightly above or below the mold surface, with a height difference controlled within ±0.1mm (depending on whether the relevant surface is machined);
⑥ Special attention should be paid to the dimensional accuracy of the diameter and shoulder depth of the ejector pin hole;
⑦ Round ejector pins with irregular ends (e.g., hook-shaped) should consider adding an anti-rotation structure;
⑧ When the clamping force of the moving and fixed molds is considerable, pre-ejection should also be considered for the fixed mold.
Mold Temperature Control
High-pressure die casting mold temperature control refers to the technical means of precisely adjusting and maintaining the mold temperature through specific methods and equipment during the high-pressure die casting production process to ensure that the mold temperature is within the optimal working range. The core objective is to optimize the quality of die-cast parts, extend mold life, and improve production efficiency by controlling the mold temperature.
Negative Consequences of High Mold Temperature in Die Casting
- Casting shrinkage defects;
- Local burning/scoring;
- Mold flashing;
- Casting deformation;
- Casting dimensional changes;
- Mold cracking and aging.
Aluminum Die Casting Mold Temperature Distribution
As can be seen from the mold flow analysis results in the right figure, the areas with higher temperatures include: the sprue, the runner, the mold surface near the gate, and areas with thicker material. Therefore, the cooling water layout should be focused on these high-temperature areas. 10.3 Mold Preheating Temperature and Working Temperature
Mold preheating temperature: 130–180℃
Mold working temperature: 180–250℃
Aluminum liquid temperature during die casting: 620–710℃
Mold Temperature Control Methods
① Preheating – burning natural gas (coal gas), circulating hot oil, circulating hot water vapor;
② Cooling – blowing air, circulating cold water, circulating cold oil;
③ Cooling methods include:
a. Combination of horizontal and vertical cooling holes (direct cooling + spot cooling), generally applicable to molds;
b. Oil temperature control (mold temperature controller), applicable to large molds (such as stepped molds), thin-walled castings, magnesium alloys, etc.
Core Pulling System in Aluminum High-Pressure Die Casting Mold Design
The core pulling system in high-pressure die casting mold design is a key mechanism in die casting molds used to solve the problem of demolding grooves, holes, or complex structures in castings that are perpendicular to the mold opening direction.
Main Components of the Core Pulling System
① Forming elements—form the side holes, concave and convex surfaces, or curved surfaces of the casting. Such as cores, slider heads, etc.;
② Moving elements—connect and drive the core or slider head and move in the guide groove. Such as slider bases, etc.;
③ Transmission elements—drive the moving elements to perform core pulling and core insertion actions. Such as hydraulic cylinders, angled pins, etc.;
④ Locking elements—press the moving elements after mold closing to prevent them from retracting due to expansion force. Such as wedge blocks, etc.;
⑤ Guiding elements—ensure the normal sliding of the moving elements and reduce the wear of the moving elements. Such as wear plates, pressure strips, etc.
Common Core Pulling Forms
Hydraulic cylinder core pulling system and angled pin core pulling system
Definition of Core Pulling Force
Core pulling force refers to the force required to move the slider.
Composition of core pulling force:
① The clamping force generated by the metal condensation shrinkage on the core, and the resulting core pulling resistance;
② The sum of various resistances during the movement of the core pulling mechanism. The force required is greatest at the beginning of the movement, and less force is required during continued pulling. 11.4 Main Factors Affecting Core Pulling Force:
① Core size and molding depth are the main factors determining the magnitude of the core pulling force;
② The more complex the geometric shape of the molded part, and the more holes to be pulled out from the side of the casting, the greater the core pulling force required;
③ Increasing the draft angle of the molded part, improving the surface finish, and aligning the machining texture with the pulling direction can all reduce the core pulling force;
④ Mold temperature, the composition of the metal material used in die casting, the dwell time of the casting in the mold, and the spraying conditions all affect the magnitude of the core pulling force;
⑤ If the clearance of the moving parts of the core pulling mechanism is too small, the core pulling force needs to be increased, while if the clearance is too large, it is easy for molten metal to penetrate, greatly increasing the core pulling force.
Estimation of Core Pulling Force
Calculate using the following formula:
| F_pull = F_resistance – F_buoyancy = μ1Fcosα+μ2 mg－ Fsinα = ALP（μ1cosα－ sinα）+μ2 mg |
| Where: F_extraction — core extraction force (N); |
| F_resistance — core extraction resistance (N); |
| F_expansion — expansion force generated on the core surface (only the side surface generates this force in the solid state); |
| F — clamping force exerted on the core by the casting due to solidification shrinkage (N); |
| A — perimeter of the cross-section of the core forming part enclosed by the casting (mm); |
| L — length of the core forming part enclosed by the casting (mm); |
| P — clamping force per unit area. For Zn alloys, P = 6–8 MPa; for Al alloys |
| P = 10–12 MPa; for Cu alloys, P = 12–16 MPa (Note: 1 MPa = 1 N/mm²); |
| μ1 — friction coefficient between the die-casting alloy and the core, generally 0.2–0.25; |
| μ2 — friction coefficient between the slider and other workpieces, generally 0.1–0.2; |
| α— draft angle of the core forming part. |
Since the draft angles of the mold cavities are generally small, for example, when the angle is 3º, sin 3º = 0.05234,
cos 3º = 0.99863, so the formula can be simplified to: F_pull = μ1ALP + μ2 mg
A well-designed aluminum high-pressure die-casting mold is not defined solely by its drawings; more importantly, it must operate stably in actual production. Every detail of the mold design directly affects the casting quality and mold lifespan.
Raidy Mold manufacturer has over 30 years of experience in the field of aluminum high-pressure die-casting mold manufacturing, specializing in solving complex mold design challenges that standard designs cannot fully address. If you are looking for a reliable partner to design or optimize your aluminum die-casting molds, please contact Raidy Mold. We will provide you with professional engineering evaluation and customized mold solutions to meet your product and production needs.
