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Metal Mass Production – Processing Methods: A Guide to Filling and Injection Speeds

Introduction

In metal casting, the speed at which molten metal fills the mold is a key parameter determining the quality of the finished product. Filling speed not only affects production efficiency but also directly impacts whether defects such as porosity and cold shuts occur within the casting. Different casting methods utilize distinct methods and levels of filling speed control. This article will explain the filling characteristics of various methods.

Detailed Explanation of Filling Speeds for Each Method

Sand Casting & Shell Casting

These two processes are gravity-casting and operate without external pressure. The molten metal slowly fills the sand mold under its own weight, so there’s no concept of “injection speed.” While the filling process is relatively smooth, thin-walled structures can be difficult to form.

Gravity Casting & Low-Pressure Casting

Gravity casting is similar to sand casting, but low-pressure casting uses a slight air pressure to help the molten metal fill the mold. Generally, filling speeds are slow to medium, which helps expel air from the mold and produce dense castings.

High-Pressure Die Casting (Die Casting)
Die casting is a typical high-speed filling process. Molten metal is injected into the mold at extremely high speeds, with secondary injection speeds reaching 1–5 m/s. This high filling speed is key to die casting’s ability to efficiently produce thin-walled, complex parts, but it also easily entraps gases, forming pores.

Processes Not Applicable to the Concept of “Filling Speed”

Forging, sheet metal forming, and aluminum extrusion: These processes do not involve filling the mold with liquid metal. Instead, they deform the material under pressure (forging and sheet metal forming) or continuously extrude (aluminum extrusion) while in a solid state.

Metal Injection Molding (MIM): MIM uses a mixture of metal powder and a binder. The injection process is performed at low pressure, more similar to plastic injection molding, and does not involve high-speed liquid metal filling.

CNC machining: CNC machining removes material using cutting tools and does not involve liquid metal filling.

Key Concepts

Filling Speed: Refers to the speed at which molten metal flows within the mold cavity.
Injection Speed: Specifically refers to the speed at which the piston pushes the molten metal into the mold during high-pressure die casting.
A simple rule of thumb is: high filling speeds are beneficial for forming thin-walled, complex parts, but can easily generate turbulence and gas entrapment. Low filling speeds result in smoother flow, facilitate gas discharge, and improve the internal quality of the casting, but are detrimental to forming thin-walled parts.

Conclusion

Filling speed is a core parameter in the casting process. High-pressure die casting, leveraging its unparalleled high-speed filling, has achieved significant success in areas such as consumer electronics. Gravity casting and low-pressure casting, with their stable, low-speed filling, have gained a foothold in the manufacture of parts requiring higher internal density. Understanding these fundamental differences will help you select the most appropriate manufacturing solution for your product.