Causes and Solutions of Pimple Defects in Aluminum Extruded Profiles

Causes and Solutions of Pimple Defects in Aluminum Extruded Profiles

Aluminum extruded materials, particularly aluminum profiles, frequently exhibit a type of surface imperfection known as "pitting" during the extrusion process. This defect, often called a "pockmarked surface" in manufacturing, appears as small nodules of different sizes. It also has trailing tails on the surface.

These adhere to the product's surface. In the production of large profiles, this problem is more likely to happen. This is due to factors like ingot structure, extrusion temperature, extrusion speed, and mold complexity.

Pimple defects can be reduced during the surface pretreatment process. This is especially true in the alkali etching stage, where most fine particles are removed. However, a few larger particles with strong adhesive properties may stay on the surface. This can affect the final product's look.

Minor pitting defects are usually okay in common building doors and windows. However, customers often reject these defects in industrial profiles. These profiles need both strength and a nice appearance. Decorative features are very important in these cases. This is especially true for pockmarked defects (black slag spots) that present an inconsistent background color.

Our factory has faced ongoing problems with rough surfaces on extruded profiles and particle defects. These issues are hard to fix, even after flat oxidation or electrophoresis treatment. This significantly affects the surface quality of our products and reduces the production yield.

To understand how these napping particles form, we studied their shape and composition. We looked at defect positions with different alloy compositions and extrusion processes. We will compare the defects and the matrix. Our goal is to find a good solution to fix the napping and particle defects.

To effectively address the issue of pitted defects in profile surfaces, it is essential to understand their formation mechanism. During the extrusion process, the primary cause of these defects on the extruded aluminum surface is the adhesion of aluminum to the working belt of the die. This phenomenon occurs due to the high extrusion temperatures, which can reach up to 450°C.

The combination of deformation heat and friction heat further elevates the metal temperature as it exits the die hole.

When the product comes out of the die hole, the high temperature causes aluminum to stick to the mold's working belt. This bonding process often involves repeated cycles of bonding, tearing, rebonding, and re-tearing, as the product continues to move forward.

This results in numerous small pits appearing on the product's surface, causing pitted defects on the extruded products. This bonding behavior is affected by several factors. These include the quality of the ingot and the surface condition of the mold's working belt. Other factors are the extrusion temperature, extrusion speed, degree of deformation, and the metal's resistance to deformation.

Test Materials and Methods

Early studies indicate that several factors can lead to rough particles on the surface. These factors include metal purity, mold condition, extrusion processes, and composition. In this study, two types of alloy rods—6005A and 6060—were used to extrude identical cross-sections. The shape and makeup of the nap particles were studied with a direct reading spectrometer and SEM. These were compared to the nearby normal matrix.

To clearly differentiate between the morphologies of nap and particle defects, the following definitions apply:

(1) Pulling defects, also known as pockmarks or burrs, represent a type of point defect. These defects manifest as irregular, tadpole-shaped or point-like scratch marks on the surface of a profile. Starting from scratch stripes, they gather into metal beans at the end of the scratches. Eventually, the defect comes off. Typically, the size of these naps ranges from 1 to 5 mm. Following oxidation treatment, they turn dark black, potentially impacting the visual appearance of the profile.

Surface particles, also called metal beans and adsorption particles, are small, hard, gray-black metals. They are found on the surface of aluminum alloy profiles. Their structure tends to be loose. There are two categories: those that can be removed by wiping and those that cannot. Generally, their size is under 0.5 mm, and they create a rough texture upon touch. These particles do not exhibit scratches in the preceding section and display minimal variation from the substrate after oxidation.