Six Key Points In The Design Of Aluminum Extrusion Die
1. Size analysis of aluminum extrusions
The size and deviation of the extrusion are determined by the mold, extrusion equipment and other relevant process factors. Among them, the influence of mold size change is great, and the reasons for the change of mold size are the elastic deformation of the mold, the temperature rise of the mold, the material of the mold and the manufacturing precision of the mold and the wear of the mold.
(1) Selection of tonnage of aluminum extrusion press
The extrusion ratio is a numerical value indicating the difficulty of the extrusion of the mold. Generally, the extrusion ratio is applicable between 10 and 150. If the extrusion ratio is lower than 10, the mechanical properties of the product are low; on the contrary, the extrusion ratio is too high, and the product is prone to defects such as surface roughness or angular deviation. Solid profiles are often recommended for extrusion ratios of around 30 and hollow profiles of around 45.
(2) Determination of the external dimensions
The outer dimensions of the extrusion die refer to the outer diameter and thickness of the die. The overall dimensions of the mold are determined by the size, weight and strength of the section of the profile.
2. Reasonable calculation of extrusion die size
When calculating the die hole size, the chemical composition of the extruded aluminum alloy, the shape of the product, the nominal size and tolerance, the extrusion temperature, and the linear expansion coefficient of the die material and the extruded alloy at this temperature are mainly considered. The characteristics of the geometric shape and its changes during stretch straightening, the size of the pressing force and the elastic deformation of the mold.
For profiles with large wall thickness differences, the thin-walled parts and the sharp-pointed corners that are difficult to form should be appropriately enlarged. For the die-holes of flat and wide-wall profiles and wall profiles with large aspect ratio, the size of the purlins can be designed according to the general profile, and the thickness of the webs, in addition to the factors listed in the formula, must be considered. Elastic deformation and plastic deformation and overall bending, far from the center of the extrusion tube and other factors. In addition, the extrusion speed, the presence or absence of the traction device, etc. also have a certain influence on the die hole size.
3. Reasonably adjust the flow speed of metal
The so-called reasonable adjustment is to ensure that each particle point on the section of the product should flow out of the die hole at the same speed under ideal conditions. Try to use a porous symmetrical arrangement. According to the shape of the profile, the difference in wall thickness of each part and the difference in circumference and the distance from the center of the extrusion tube, the sizing belts of unequal length are designed. In general, the thinner the wall thickness of a part of the profile, the larger the circumference, the more complex the shape, and the farther away from the center of the extrusion cylinder, the shorter the sizing belt here.
When the sizing belt is still difficult to control the flow rate, the shape is particularly complicated, the wall thickness is very thin, and the portion farther from the center can adopt the flow angle or the guide cone to accelerate the metal flow. Conversely, for those parts with much larger wall thickness or close to the center of the barrel, the obstruction angle should be used to supplement the obstruction to slow the flow rate here. In addition, it is also possible to adjust the flow rate of the metal by using a process balance hole, a process margin or using a front chamber mold, a flow guiding mold, changing the number, size, shape and position of the split holes.
4. Ensure sufficient mold strength
Mold strength is a very important issue in mold design due to the harsh working conditions of the mold during extrusion. In addition to the proper placement of the die holes, the selection of suitable mold materials, and the design of a reasonable mold structure and shape, it is also important to accurately calculate the extrusion force and check the allowable strength of each dangerous section.
At present, there are many formulas for calculating the pressing force, but the modified Birling formula still has engineering value. The upper limit solution of the pressing force also has a good applicability value. It is relatively simple to calculate the pressing force by the empirical coefficient method. As for the check of the strength of the mold, it should be carried out separately according to the type of the product, the structure of the mold, and the like. Generally, the planar mold only needs to check the shear strength and the bending strength.
Tongue and flat split molds need to be checked for shear, bending and compressive strength, and tensile strength is also required for the tongue and tip. An important fundamental problem in strength checking is the selection of a suitable strength theory formula and a relatively accurate allowable stress. In recent years, the finite element method can be used to analyze the stress and check strength of a particularly complicated mold.
5. Working belt width size
Determining the working belt of the split-combined mold is much more complicated than determining the half-mold working belt, not only considering the difference in wall thickness of the profile, the distance from the center, but also considering that the die hole is covered by the splitter bridge. The die hole under the shunt bridge must be thinned due to the difficulty of metal flow.
When determining the working belt, first find the place where the wall thickness of the profile under the shunt bridge is the thinnest, that is, the metal flow resistance is the largest. The minimum working zone here is twice the wall thickness, the wall thickness is thicker or the metal is easy to reach. Where the work belt should be properly considered thickening, generally in a certain proportional relationship, plus the correction value of easy flow.
6. Mold hole empty knife structure
The die hole empty cutter is the structure of the cantilever support at the outlet end of the die hole working belt. When the wall thickness of the profile is t≥2.0mm, a straight-blade structure with easy processing can be used; when t<2mm, or with a cantilever, a slanting knife can be used.