Control of Part Warpage

    The fireman side of the trailing truck was set up in the 3D printer aligned a bit more angled than the engineer side previously built.  The photo above shows significant separation of the support material holding up the frame above the raft at the bottom.  Because of the separation, the frame was significantly warped.  This was noticed part way through the 3D printer run and the machine was stopped before completing the part.

    The separation is believed to occur due to the extensive shrinkage of the just applied ABS material to the cooler material below.  The shrinkage applies force to the structure in a manner similar to a bowstring.  When a number of layers accumulate their total force, the support structure separates due to the pulling forces above.  The bottom most layers are quite strongly adhered to the build platform and remain fairly rigid.

    After removal of the support material the bend is quite apparent.  The center of the part is the bottom portion of the journal where the wheel bearing is located.  The vertical rib of the "I" beam sections to the left and right of the journal was just beginning, it too will be printed bent due to the curvature of the support material.

    The partially complete engineer frame at left is considerably more bent than the completed engineer side truck frame at right.  The main reason for the bending is that the support structure consists of long straight ribs in parallel which collectively apply a common bending force leading to the eventual separation.

    The Axon conversion software supplied with the printer permits a two material object to be built, with or without support.  The design consists of three files, one for each material and a third with both combined to provide alignment data.  The above design 3D illustration is the first attempt to use this method where the support will be a series of small tubes attached to the bottom of the part without support in this case.  The green tubes will act as supports for the grey part.  The part shown is the bottom closure of the trailing truck journal which will be glued in place.  The tubes will be cut or pulled away after printing.  The tube material will be green ABS while the dark grey part will be black ABS.  The intersection of the two materials is defined as well as the flat bottom of the green tubes that will attach to the raft.  The three files will be built from this one common file by deleting the green from the black part and deleting the black from the green support tubes to form the two material files.  The design file is where the part and support are aligned.  This method has not been used yet, but promises to allow custom designed supports which may eliminate the combined stress buildup and resultant support separation.