An extrusion die is rightly considered the heart of the press for the extrusion of aluminum. New dies or dies after their serious correction are almost always subjected to preliminary testing on the press.. This testing is called а die trial .
Die trial consists of extrusion of one or two billets the tested die. Then according to the profile samples, which came out of the matrix is judged:
- on the correspondence of the actual dimensions of the profile to the drawing;
- the tendency of the profile to a deviation from straightness and twisting.
During the trial of the new die need to be sure to follow, to samples from the front ends of the profiles are correctly marked and cut off. Similarly marked and cut pieces from the middle part of the molded profile length of about 300 mm (figure 1).
When cutting profiles of samples need to be very careful - especially for multi-channel dies - so you do not correct the wrong channel matrices.
Controlling the size and shape of the profile
First of all, at the front ends of the profiles evaluate the uniformity of metal outflow - determined, which of the profile surfaces tends to slow down or accelerate the flow of metal.
On samples from the middle part of the profile is checked by cross-sectional dimensions, including the thickness of all walls. Die correction begins with adjusting the channel width, which form the walls. Very often, after corrections for the wall thicknesses of the die for aluminum profiles aligned, and they go out of the die has no distortion and twisting.
If after the adjustment of the channel width of the walls the profile still "twists" or "leads off" , the routine proceeds to the adjustment of the die metal to equalize the currents in the light of the analysis of the flow front ends profiles.
Billet temperature and extrusion speed
When pressing dies, temperature of the billet and extrusion speed are very important parameters.. The die must work at the same billet temperature and the same extrusion speed, which were during its crimping.
Increasing the pressing speed results in an increase in temperature, profile which extends through die bearing. This leads to an acceleration of the flow in the critical section, where the current must be sustained to ensure the uniformity of flow in the whole profile. This most commonly occurs at extrusion through the hollow mandrel die to form, for example, screw holes.
Adjustment of the press
Another important factor, which must be taken into account when testing (pressing) the matrix, the adjustment of the press - the alignment of all elements of the press on one axis. Die, which has been adjusted for a well-calibrated press, It will not work properly on the press, that is poorly verified. In this case, it is necessary to alter the die, to compensate for deviation from the normal alignment press.
Container offset relative to the die
Figure 2A shows an array of, which has been adjusted for the press, who had a good alignment of the components, including the container relative to the die. aluminum flow through the die was uniform and balanced.
In Figure 2B, the same die was set at the same press, at the same temperature of the preform and with the same speed of extrusion, but this time the press had a good alignment of its components - the center of the container is displaced relative to the center of the diematrix. Therefore, for the metal, naturally, changed. In most of these cases required further adjustment die for, to make the metal flow at the exit from the die a uniform.
container offset to the left with respect to the die is the same shift workpiece . This gives the offset of the left leg profile to the center of the workpiece. center of the preform material always has a maximum speed. When there is such a way the metal flow, it is clear, that the die need to re-adjust, to compensate for unbalanced, which arises from the violation of reconciliation press.
therefore, if there is a need to adjust the die, which has been previously adjusted, it is necessary to ensure, that the press, where she worked, properly verified.
The temperature of the container
Another reason, why well-corrected die suddenly starts with pressing problems, It may be that, that on one of the container "burned" parties or just bad heats one of the heating coils. As a result, one of the sides of the container gets colder, than the other. This leads to, that the flow of metal on this side of the container will be slower, than at the hot side of.
Heating of the billet
Similar problems may be heated preform, especially with electrical heating: some heating elements may fail. Therefore, even, if the die is well adjusted, During metal workpiece therethrough will be uneven.
Multi channel die
All these above-mentioned problems of a good spotter "scrolls" in his head before taking a decision on the implementation of correction matrix. For multichannel (multipoint) matrices, these problems are much more complex., than for single-channel (one-point) matrices. For multi-channel dies is not a problem in balancing one part of the profile relative to the other, in the lining of metal flow through all channels, points.
Otherwise, press sections receive different length from one matrix, increasing the number of process waste. Moreover, This greatly complicates the work of stretching machine.
3A and 3B are illustrations show the same case, as in Figures 2A and 2B, but not even for a single channel, and for four-channel die. immediately obvious, that the problem is more complex, but the adjustment procedure is the same in both cases. these profiles, that have reached the outer edge of the blank, will flow slowly, than those, that are closer to the center of the workpiece. Therefore, slow points of the die need to accelerate, to compensate for the accelerated flow of points, which were closer to the center of the workpiece. This is done by adjusting the die.
Various working bearings of dies
Another problem, which causes uneven outflow of aluminum die, are the various types of working bearing s in its various channels. They can be parallel, expands and contracts.
Figure 4A shows a two-channel die which has one channel - №2 - arranged parallel bearings, and the other - №1 - with expanding bearings.
Because the two channels have a width 3,18 mm, to profiles, that will come out of these channels, I shall have the same flange thickness. However, bearings made parallel channel №2, and channel №1 - expanding. Therefore the profile of the channel will be much longer №1, than the profile of the channel №2. Profile of channel №1 will go faster, than channel №2, so as aluminum in this way is minimal resistance to movement through the expanding channel.
The same problem arises, if the working channel formed by parallel belts №2, and the belts of channel No. 1 - tapering (Figure 4B).
The difference is that, №2 that the channel is now more fast, although it is a parallel. The reason is, the channel tapers №1, which creates a greater resistance to metal flow. Therefore, the channel profile is longer №2, than channel №1.
First think, then adjust
The above examples show, why die corrector should check running web prior to press testing of dies. It is necessary, to make sure, that all workers bearings are parallel and flat - for as many, as much as possible. The control of these small details will save a lot of time to die corrector, It saves quite a few dies from overcorrection, and also will reduce the number of expensive press working time.
Luis Bello, Die Corrections for Changing Flow Characteristics, Aluminum Extrusion Technology Seminar, Chicago, 1977