Surface roughness of die castings
The surface roughness of a die-casting is an important parameter that describes the microscopic geometry of its surface. It refers to the unevenness of the small spacing and tiny peaks and valleys on the surface of the die-casting. It directly affects the appearance quality, wear resistance, sealing, and the effect of subsequent surface treatment of the die-casting. The smaller the surface roughness value, the smoother the surface; conversely, the rougher the surface. In different application scenarios, the requirements for the surface roughness of die-castings vary greatly. For example, the sealing surface of an automobile engine requires a lower surface roughness to ensure sealing, while some structural parts have relatively low requirements for surface roughness. Therefore, reasonable control of the surface roughness of die-castings is of great significance for improving product quality and reducing production costs.
The surface quality of the mold cavity is the primary factor affecting the surface roughness of die-castings. The surface roughness of a die-casting is largely a replica of the surface roughness of the mold cavity. The smoother the mold cavity surface, the lower the surface roughness of the die-casting. Therefore, during the mold manufacturing process, the cavity surface requires fine machining and polishing. For die-castings with high requirements, the mold cavity undergoes multiple processes, including rough grinding, fine grinding, and polishing, to achieve a surface roughness within Ra0.02-0.04μm. For die-castings with general requirements, the cavity surface roughness can be controlled between Ra0.1-0.8μm. Furthermore, the hardness of the mold cavity surface also affects its wear resistance. A cavity surface with a higher hardness is less susceptible to wear during long-term production and can maintain a stable surface roughness, thus ensuring consistent surface quality of the die-casting. Therefore, mold cavities are typically manufactured from high-strength hot-working die steel and undergo appropriate heat treatment to improve its hardness and wear resistance.
Die-casting process parameters significantly impact the surface roughness of die-cast parts. Improper settings of parameters such as injection speed, injection pressure, mold temperature, and pouring temperature can increase surface roughness. If the injection speed is too low, the molten metal flows slowly as it fills the mold cavity, easily forming cold shuts or flow marks on the surface and increasing surface roughness. If the injection speed is too high, the molten metal will wash over the mold surface, potentially carrying away tiny particles on the surface. Turbulent flow can also cause scale to form, leading to surface roughness in the die-cast part. If the mold temperature is too low, the molten metal solidifies rapidly on the mold surface, resulting in an uneven surface and increased roughness. While excessively high mold temperatures facilitate molten metal flow and filling, they can also cause sticking and compromise surface quality. If the pouring temperature is too high, the molten metal easily oxidizes, producing slag that adheres to the casting surface and increases surface roughness. If the pouring temperature is too low, the molten metal has poor fluidity, resulting in incomplete filling and prone to surface defects. Therefore, die-casting process parameters must be optimized based on the casting material and structure to achieve low surface roughness.
The properties of the alloy material also affect the surface roughness of die-cast parts. Different die-casting alloys have different fluidity and solidification characteristics, which in turn affect surface roughness. Zinc alloys have good fluidity and solidify quickly. Under appropriate process conditions, they can replicate the surface quality of the mold cavity and achieve a lower surface roughness. Aluminum alloys have less fluidity, but they also have better strength and corrosion resistance. By optimizing process parameters, a relatively smooth surface can be achieved. Copper alloys have a higher melting point and relatively poor fluidity, resulting in die-cast parts with a generally greater surface roughness than zinc and aluminum alloys. Furthermore, the impurity content in the alloy also affects surface roughness. Excessive impurities can reduce the fluidity of the molten metal, forming granular protrusions on the surface during solidification, increasing the roughness. Therefore, during the smelting process, the alloy composition and impurity content must be strictly controlled to ensure the purity of the molten metal and lay the foundation for achieving good surface roughness.
Subsequent processing also plays a significant role in improving the surface roughness of die-cast parts. For die-cast parts with extremely high surface roughness requirements, die-casting alone cannot meet these requirements and requires subsequent surface treatment. Common surface treatment methods include mechanical polishing, electrolytic polishing, and chemical polishing. Mechanical polishing uses abrasive materials to cut and grind the casting surface, significantly reducing surface roughness and achieving a mirror-like finish. Electrolytic polishing utilizes electrochemical principles to dissolve tiny peaks and valleys on the casting surface, resulting in a smooth and even surface. Chemical polishing uses chemical reagents to corrode the casting surface, eliminating minor irregularities and improving surface roughness. Furthermore, surface treatment processes such as electroplating and painting also require a low surface roughness as a minimum. Otherwise, these irregularities will be magnified, affecting the appearance and adhesion of the coating. Therefore, selecting the appropriate subsequent processing method based on the die-cast part’s intended use can further optimize its surface roughness, enhancing product quality and added value.
