Splay marking This fault，which appears on a great many mouldings，shows an area, generally near the gate, which has an uneven, lined appearance.It may be reminiscent of the grooves on a gramophone record or it may appear simply as an irregular…
Splash marks，mica marks，silvering This fairly common fault is easily recognized by the streaky，silver appearance of the moulding, nearly always radiating from the gate area. Cause of the splash marks The cause is the vaporization of volatile matter in the advancing…
Sink marks and voids Sink marks and voids are two manifestations of the same fault, and their complete eradication is extremely difficult. Diagnosis is easy. All plastics take up more volume when molten than when solid，and where there is more…
Short mouldings The causes of short mouldings are many and varied and are sometimes difficult to diagnose. It is，for example, perfectly easy to flash a mould and still obtain a short moulding. On the basis that the same property，too low…
Flow marks Material entering a mould always follows an almost identical path with successive mouldings. From a sprue gate into a flat plate it forms a spreading disc pattern. From a side gate of proper design into a flat plate,…
Flash The escape of a thin web of material at parting lines,around ejector pins or at other parts of the mould，can be caused in several ways. It could be the material，the machine, the mould, or the process that is at…
Cracking or Crazing This fault occurs only when there is residual stress in a moulding which may be affected by an aggressive environment. The term ‘aggressive，may be considered a misnomer for such environments as a low concentration of ozone in…
Burn marks What is Burn Marks If an area of a moulding becomes the oxidized, discolored, or blackened, the first thing to do is to examine the venting of the mould. Air trapped in the mould is heated to a…
Undercuts An undercut is a part of the mould where the molten plastic flows behind a retaining section thereby preventing the hardened plastic moulding from being easily ejected. Undercuts, in general, require moulds with moving parts to allow the product…
Internal stress and warping When thick sections are moulded，rapid changes in temperature can cause thermally induced stresses due to differential expansion. If surface layers cool faster than the interior through poor conduction, the contraction of the surface will be greater…