Indirect extrusion is an important process in plastics manufacturing. It is a fast and efficient way to produce complex shapes. However, it has some advantages and disadvantages. Here we will discuss its applications and drawbacks, along with the heat transfer coefficients and friction parameters. To better understand how it works, we will use an example of an aluminum extrusion, AlSi25Cu4Mg.
Indirect extrusion offers a number of advantages over direct extrusion, including higher consistency and decreased friction. This consistency translates into better dimensions, grain structure, and mechanical properties. However, the process does come with some trade-offs. The process requires the removal of as-cast surface layer, as well as dirt and oil. It also requires the die to be supported during the extrusion process, which reduces the profile size.
During the extrusion process, the die moves relative to the ram in either a vertical or horizontal configuration. The extruder is driven with either hydraulic or mechanical power. This process requires high force, and the material must be lubricated to avoid friction between the material and the chamber walls.
Compared to the direct extrusion process, indirect extrusion results in more consistent work, and a lower temperature variance along the entire length of the extruded profile. It also produces more consistent dimensions and grain structure, while improving mechanical properties. Despite these benefits, there are trade-offs with indirect extrusion. The first is that the die has to be supported during the extrusion process, which reduces the size of the profile.
Another drawback is that indirect extrusion does not have the same cross-sectional area as direct extrusion, which limits its applications. Other limitations include the sizing of the stem, which limits the cross-sectional area, and the presence of impurities and defects. Impurities are removed through wire brushed billets and chemical cleaning.
Friction parameters are important for determining the correct extrusion force for indirect extrusion processes. They affect the rate of flow of materials and increase the necessary extrusion force. The friction parameters can also affect the flow of lubricant. Friction parameters are based on the Hensel-Spittel equation.
The friction parameters of an indirect extrusion process are lower than for direct extrusion. The pressure of a billet at the front of the process is greatest. This is because the area of contact between the billet and the container decreases. This change in force leads to a deflection of the die. This deflection affects the dimensions from front to rear.
Heat transfer coefficients
The heat transfer coefficients for indirect extrusion process are calculated using a novel method. It combines a numerical modeling code with a fitting procedure to estimate the heat transfer coefficients. The numerical model code is based on the Finite Volume Method and can compute steady-state and discontinuous solutions. Besides, the fitting procedure introduces alternative algorithms.
In order to study the effect of the process parameters on the composite extrusion quality, multiple parameters were studied. Then, a central composite experimental design (CCED) was implemented and runs were evaluated in terms of four response variables: product quality, material properties, and process stability. The statistical analyses showed that the five investigated process parameters had significant effects on the product quality. The statistical model also predicted the optimum setting of the process parameters.
Hydrostatic extrusion as alternative to direct extrusion
One method to process hard-to-work materials is hydrostatic extrusion, which forces the billet through an opening by applying a pressurised fluid. This process has the advantage of producing high extrusion ratios while minimizing friction. In addition, it can process billets of different diameters with the same set-up. Moreover, this process is flexible and highly productive.
Hydrostatic extrusion can produce a variety of extrudates with different properties, which can be controlled by changing the speed, ratio and temperature. In addition, it is a virtually adiabatic process, which means that the temperature of the extrudate is directly proportional to the applied pressure. This process can achieve temperatures of up to 300°C.