This is - the continuation of the review of violations of technology, that may occur in the aluminum extrusion. The first four "bad" see. here.
№5 – The workpiece is poorly cut and dirty
To avoid air entrapment in the container and the formation of bubbles on the profiles, the workpiece from the post must be cut off as evenly as possible, special, when a two-piece composite workpiece is used. Blanks must always be clean, so their surface layers can get into the pressed product. Then a marriage in the production of profiles is inevitable.
№6 – Heating the workpiece without a gradient
Friction, including, on the belts of the matrix leads to, that heat is constantly generated in the workpiece, while she is "pushed" through the matrix. The so-called isothermal pressing is ideal., i.e. pressing, in which the alloy passes through the die at the same optimum temperature and maximum speed. To do this, the workpiece is heated to a temperature, which decreases from its front end to the back, so as, to compensate for this increase in the temperature of the workpiece from friction. Such heating is called gradient heating.. This requires special equipment, for example, induction heating. If such heating is performed incorrectly or not at all, then reaching the maximum pressing speed is not possible.
№7 – The press washer does not compress
When the press washer works normally, then a thin layer of aluminum always remains between it and the container wall. Its thickness must be the same. For soft alloy, for example, alloy 6060, gap, which this layer creates, is about 0,15 mm. If this gap is larger, alloy will flow out from under the washer in the initial punching of the workpiece through the die. If this gap is less, then this layer of aluminum will “scrape off” from the container wall and fall into the pressed product. This can lead to blistering., as well as the ingress of contaminants from this layer into the profiles, instead of press residue.
A good press washer should expand readily under the load of the ram, easy to separate from the workpiece at the end of the pressing cycle, instantly shrink when the ram moves backward without any "scraping" of aluminum from the container wall. It must be able to continue to fully compress even after a large number of pressing cycles..
№8 – The container is overheating
If the temperature sensors and heating elements are not located near the container sleeve, then the container may overheat. Nickel chrome heaters can reach temperatures 700-760 ° C. The container body is usually made of steel 4340, which can begin to release and soften at a temperature 540 ° C. If the container body loses some of its strength, This can lead to the formation of a widening inside the sleeve of the container - "belly". This will allow contamination from the outer layers of the workpiece to get into the pressed products.. The result will be increased profile rejection..
№9 – Top container temperature is higher
At the other end of the container, which is adjacent to the matrix, the temperature of the bushing at the top is usually much higher, than below. This is mainly due to the heat, which rises from inside the container. The resulting alloy, which enters the matrix from above is more "fluid", than that, which comes in from below. Usually consider, that every 5-6 ° C the difference between the top and bottom of the container gives 1 % difference in length of profiles. On a long table, if the top "point" of the matrix is not muted, this problem is very noticeable, especially when using a puller.
№10 – A lot of grease at the end of the workpiece
The last operation before feeding the workpiece into the container is its lubrication.. Ideally, the press washer should pass freely through the container sleeve and at the end of the pressing cycle, the press residue should fall out of the press. Unfortunately, not always everything goes so smoothly.
Too much lubrication has always been the bane of the pressmen. There was even, they say, such a saying “No lubrication, and wipe the excess ". Much time has passed since then and now there is complete clarity about the need and effective use of lubricants..
At the end of each pressing cycle, the press washer, which has long been permanently attached to the ram, should instantly and cleanly separate from the press residue, without pulling the extruded profile out of the matrix. And, without breaking the press washer itself. Sticking can be a serious problem here.. It is therefore important, so that the press washer, and the workpiece has been properly lubricated, to ensure their immediate and effortless separation.
Effective lubrication ensures instant and clean separation of the press washer from the press cake. It also ensures a clean separation of the cake from the knife of the cake separation mechanism.. it, in turn, gives a clean surface to the container sleeve end, which is important for its tight adhesion to the matrix during pressing.
Typical Press Washer Face Lubrication – this is the deposition of a layer of soot as a result of the combustion of acetylene. Currently, many press operators abroad are increasingly using powder or liquid boron nitride lubricant., which was specially designed for pressing non-ferrous metals.