In the aluminum extrusion production process, there are some little grits attaching on aluminum profile surface, which are touchable and bad influence the beauty of aluminum profile surface processed by anodizing, electrophoretic coating and powder coating, resulting in the reduction of manufacturing efficiency and output of finished rate. Meanwhile, it is difficult to find out the attached grits unless we observing and touching carefully. These grits are not easy to be gotten rid of in the production process of electrophoresis and powder coating, which will destroy the beauty and cause wastes of aluminum profiles. Therefore, we need to keep on observing and summarizing the causes and take actions in time in order to decrease and avoid these imperfections of aluminum extrusion. 

Extrusion die, aluminum billet and extrusion process are three factors to cause grits, which also can reflect the standard level of operation. Based on the practice of production, we have to analyze and summarize the experience so that we can decrease and avoid imperfections---attached grits, sticky aluminum, surface roughness, scratches, tear, which will greatly increase the yield and manufacturing efficiency. Now, let’s explain the factors and pertinent solutions in detail.

1. Symptom of attached grits and influence on quality

Ways of aluminum profile surface treatment are various. Besides anodizing, kinds of electrophoretic coating, fluorine-carbon coating and wood-grain transferring treatment appear on the aluminum extrusion market in succession. The attached grits existing will not influenced too much by normal anodizing process, but seriously influenced by other treatment process.

  Most of grits can be easy to find out through careful observation or shaking the aluminum profiles in the aluminum extrusion process. In the hot saw cutting and loading the aluminum profile, the majority of attached grits can be wiped out by wind- blowing or scrubbing, but leaving some of them attached on aluminum profile surface for the reason of static. After aging process, these grits absorb on aluminum profile even tighter. Therefore, when preprocessing of surface treatment, some grits can be removed but leaving pockmarks, while some can’t then form bulges owing to the concentration of tank liquor, which is a common problem in production of electrophoresis and powder coating.

2. Factors of attached grits 

2.1 Influence from aluminum extrusion dies—Key Factor

In the aluminum extrusion process, elastic deformation happens to aluminum extrusion dies under the condition of high temperature and high pressure. At the beginning, dies working area is parallel to the direction of extrusion. After the extrusion force, it turns into trumpet-like, the cutting edge aluminum contact the aluminum profiles to form sticky aluminium, similar to tumor on lathe tool. In the process of forming sticky aluminium, grits are brought out and stick on the aluminum profiles, causing the phenomenon of attached grits. With sticky aluminium getting larger, the die will spring back instantaneously, causing bite marks. Once sticky aluminium are too much to be pulled out, they won’t fall off when the die springs back, causing aluminum profiles surface roughness, bright extrusion lines, profiles tear and extrusion die block. Most of aluminum extrusion factories use the flat dies. Impurities of aluminum extrusion billets pile up in the dead zone of flat dies where metal flows without aluminum billet surface peeling. As the amount of aluminum billets is increasing, impurities in dead zone are also changing, some of which are brought out by flowing aluminum, piling up in deformed working area; some absorbed on aluminum profile surface then form grits. 

2.2 Influence from aluminum extrusion process

The technical parameter of aluminum extrusion is important to cause grits. According to on-site inspection experience, the higher temperature and faster speed of aluminum extrusion is, the more attached grits are. Once the temperature is too high and the speed is too fast, the aluminum profiles will flow faster, enhancing the degree of deformation of extrusion die. Then once the aluminum flows faster, the resistance of deformation is relatively weaker, easily causing sticky aluminium. As for the great coefficient of aluminum extrusion, the higher the metal’s resistance to deformation is, the bigger the dead zone is, which enhances the probability to cause sticky aluminium and it will increase the cause rate of attached grits. What’s more, the difference of temperature between aluminum extrusion billet and extrusion die can be a factor to cause sticky aluminium, even block the die. Last but not the least, the extrusion dies surface roughness and hardness of working area is one of factors to cause sticky aluminum and form of attached grits.

2.3 Influence from aluminium billet quality 

Quality of aluminum extrusion billet is an essential factor to surface of aluminium aluminum profiles and extrusion aluminum. The most common imperfections of aluminum billet are slag inclusion, loosen, coarse grain, segregation and bright grain. Slag inclusion, which is from aluminum melting slag, oxide skin and something else. They usually present irregular black holes on the low magnification specimen, sinking into the matrix, and soft organizations with different colors and shapelessness, which damages the continuity of aluminum billet. When the aluminum billet in the extrusion process, slags are easily separated from the matrix and stick on the entry end through the die working area, so that they become sticky aluminium, which finally form attached grits brought out by flowing aluminum profiles.

Loosen is microscopic and macroscopic dispersive shrinkage cavities which appears on grain boundary and dendritic crystal network. It presents black pinholes on the low magnification specimen, in dendritic crystal network presents triangle hole, dis-compact of cut-off face organization and seriously loosen will be with pores and slags, it will be difficulty to weld with metals in the aluminum extrusion process, so that form sticky aluminum. 

Coarse grits form easily on aluminum billet at over temperature of melting aluminum or casting aluminum. In the aluminum billet casting process, it is easy to get the grain structure organization and inter-granular cracks which will damage the consistency of metals, so in the aluminum extrusion process, it also will be stick aluminum. Segregation means fusible educts that coagulated from exuded components on the surface of aluminum billets. In the aluminum extrusion production line, they gather in the dead zone, some of them are out by extruded or pulled out by flowing aluminum profiles with the amount of aluminum billets increasing, then form attached grits. All of these imperfections have one thing in common that they can’t well welded with matrix, leading to the discontinuity of flowing matrix, so that it is an important factor cased the attached grits. 

3. Solutions to decrease amount of attached grits

3.1 Notice for design, manufacture and usage of extrusion dies

(1) Design of extrusion dies should meet the requirements of calculating rigidity and intensity so that reduce the elastic deformation rate of dies in the extrusion condition. When designing the extrusion die working area, the length, shape of run-out, the die neck and welding space should be their optimum value under consideration. In order to reduce the extrusion pressure, larger value is used within bounds when selecting coefficients of porthole die and diversion die, etc.

(2) Reduce die making errors and avoid sharp corners in the dies manufacturing. It is necessary for flowing metals to smoothly transit through the extrusion dies surface and reduce the zones of sags and crests for the purpose of ensuring the deviation consistency of extrusion dies holes.

(3) Installation and selected suitable die backers and supports is necessary in the usage of extrusion dies.

(4) To avoid sticky aluminium, extrusion dies should be nitriding treatment before extrusion.

(5) When fix and correct the extrusion dies, Take care of extrusion dies parallel, surface roughness and matching size of combination dies and tighten the bolts.

3.2 Choose suitable extrusion process parameter

   Choose the best extrusion temperature, aluminum billet heating temperature, extrusion dies temperature, extrusion speed and extrusion pressure according to extrusion process parameter, cross-section of aluminum profile, aluminum alloy, dies situation and aluminum extrusion press running situation. And keep adjusting these parameters in the process of aluminum extrusion manufacture. Points above are main solutions for decreasing amount of attahed grits.

3.3 Enhance quality of aluminum billet.

   In the aluminum billet melting and casting production process, adopt grain refining technology to reduce the imperfections of slag inclusion, loosen, coarse grain, segregation and bright grain. Homogenization treatment processing to aluminum billets are effective ways to decrease absorbed grits. Furthermore, reinforce the analyzation on low magnification organizations of aluminum billet and use less ones with imperfections under the quality control of aluminum billet, which can be achieve the same effect of avoiding the attached grits..