B.O.B. problem in extrusion processing

The vast majority of the “ Billet on Billet “ or “ Blok on Blok, “ ( B .O.B. for short ) problems are from the welding seam lines in the hollow profiles and the ends of the profile for very wide flat dies that use a spreader plate to provide sufficient feed to the outside details of the profile. Where the B.O.B problem happen?

1) The different areas of the profile where the BOB problem may occur. Within the wall thickness of a profile. Occurring laterally and seen as a slightly darker area inside the wall thickness. These areas are highlighted after cutting, polishing and chemical etching. On close inspection a very narrow white band can be seen on the outer edges of the wall. These areas are in direct contact with the bearing surface. This type of problem emanates from insufficient pressure within the die from a low extrusion ratio ( below 40:1 ) or from a low press specific pressure.

2) From the extremities of the over sized profile. This is quite often the case where profiles much larger than the container and the feed has to be artificially forced to flow to the extremities of the profile. The resultant low pressure and velocity of the alu inium and the diagonal direction of reduced flow can often give rise to problems similar in nature as those given above, namely, different colors within the aluminium. The banding is clearly seen following chemical etching. Within a wall thickness that is fed from indirect feed.These usually emanate from some internal features or wall thickness on the inside of a hollow profile. In such cases there will be a dramatic decrease in the pressure and rate of flow to such areas coupled with high coefficients of drag from the localised areas adjacent to the wall in question. The wall is very likely to be fed from opposite sides and the meeting point is called the “kissing bond” often lacks sufficient rate of flow and pressure to maintain a sound weld. This will manifest itself as a white line across the profile which may tend to widen towards the outside of the wall thickness. Something like a mirror image of a “V”.

3）From a weld seam line. These are directly related to the mechanical properties

obtained from within the dies welding chamber and the pressures and rates of flow

coupled with the bearing friction ( or lack of it ).