1  What are“Absorbed particles” and their influence on aluminum profiles’ quality

 “Absorbed particles” have little influence on anodizing aluminum profiles, but they have strong impact to other surface treatment forms. During aluminum profile extruding production, the absorbed particles on the extruded profiles can be observed if carefully enough or can be felt with your hand moving on the profiles’ surface. After aging, these particles stick more firmly to the aluminum profiles’ surface. During the pre-handling process for aluminum profiles surface treatment, due to the density of bath solution, some of them can be removed, leaving tiny pit on the aluminum profile’s surface. Those that can not be removed will swell up. It usually happens in electrophoresis process and powder coating process, which do harm to the beauty of the aluminum profiles surface and also influence the products’ quality. 

2  Analysis on cause of absorbed particles on surface of extruded profiles

2.1  Influences on the mould

During the aluminum profiles’ extrusion, the mould works in high temperature and high pressure circumstances. Influenced by pressure and temperature, it will have elastic deformation. Mould working belt is originally parallel to the extrusion, but after pressurized, it is deformed into trumpet shape, only the cutting edge section contacts the extruded profile and adhesion aluminum is formed. During this process, the particles are brought out by extruded profiles continuously and stick to the surface of the aluminum profiles. Thus become “absorbed particles”.

With the adhesion aluminum keep accumulating, the mould has instant rebound and the bite mark defect comes into being. If adhesion aluminum accumulates too much and can not be pulled out by extruded profiles, the adhesion aluminum on the mould can not fall off when it has instant rebound. It will make the surface of the aluminum profile rough, have bright lines, profile crack or mould-stuck, etc.

Cast rod surface as well as inner impurity accumulates in the metal flow dead zone of the mould. With the pushing of the extruded cast rod and increasing of extrusion times, the impurity in the dead zone is changing too. Some of it is brought out by normally running metal, and accumulates in the work belt deformation space. Others are pulled off by aluminum profiles and become absorbed particles.

2.2  Influences on extrusion techniques

Aluminum profile extrusion technique parameters also play an important role in absorbed particles. After on-site observation, the higher the extrusion temperature and the faster it extrudes, the more the absorbed particles will be. Because high temperature and fast speed make the profile flows faster and the mould deforms more seriously, thus the adhesion aluminum phenomenon becomes more common.

2.3  Influences on the cast rod quality

After repeatedly analysis and comparison, we find that the cast rod quality is one of the main reasons for absorbed particles.

3  Measures to be taken to eliminate the absorbed particles on aluminum profile’s surface

3.1  Mould design and use

（1）The mould’s design must meet the demand of rigidity and strength, so that the mould’s elastic deformation can be decreased when pressurized. When deciding work belt, its length, empty knife form, mould neck as well as welding chamber forms all need to choose the best parameters. When choosing parameters for the mould’s deflector hole and porthole, it’s better to choose the largest figures within the allowable range so as to reduce the pressure. But the porthole can not be too big, otherwise impurities will be brought in when extruding.

（2）Strengthen the testing of mould. The requirements for the mould’s hardness must follow the company’s standards.

（3）When using the mould, appropriate mould pad or support cushion must be installed and used.

（4）The mould must get nitrogen treatment before getting onto the machine, so as to reduce the adhesion aluminum.

（5）When fixing and polishing the mould, pay attention to the parallel of work belt, smoothness of surface and the coordinating sizes of combination mould in assembling.

3.2  Adjust the aluminum profile extrusion technique parameters

3.2.1  According to extrusion parameters, profile section, mould situation and equipment conditions, choose the best extrusion temperature, cast rod heated temperature, mould temperature and extrusion speed, and keep adjusting these parameters during production so as to eliminate or reduce absorbed particles.  

3.2.2  Adjust the aluminum rod heated means. First increase the temperature to 520-560℃, then reduce to 450-480℃, keep the temperature stable, and this will remain the status of semi-uniformization, eliminate the casting stress, increase the aluminum rod’s plasticity, decrease the deformation resistance, improve the products’ structure and performance, and reduce the amount of absorbed particles.

3.3  Improve the cast rod’s quality

Use al-si master alloy that is produced in big furnace, prolong the temperature holding time for producing al-si master alloy, fully dissolve the silicon, and decrease the silicon content from former 15-20% to 10-12%. After this, the aluminum rod quality is improved. On the other hand, promote the analysis on cast rod, carry out the quality control and prevent the use of defected cast rod.

4  Conclusion

Influence factors for absorbed particles mainly include the mould, cast rod and aluminum profile extrusion techniques. We shall analyze the problems, sum up experience, reduce or avoid defects like absorbed particles, adhesion aluminum, rough surface, scratch, and profile tear, and improve the product quality and aluminum profile yield as well as production efficiency.

——Abstract from wechat Aluminum Profile Processing