Archives

Category Archive for: 'troubleshooting'

Injection moulding process variables and their causes     To sum up the discussion of the causes and cures of faults in injection moulded components, Table should be consulted. Here, the main faults are listed on the left. In the second column is a generalized cause,and possible remedies are listed on the right,together with the …

Warping Mouldings warping after removal from the mould are very common and are always the result of frozen-in stress; differential shrinkage or,with filled materials,filler orientation. This is nearly always a processing problem,but it can also be caused by a machine failure or incorrect component design. Anything that prevents injection at the correct rate will contribute …

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 pattern of lines,roughly in a half-circle round the gate. The reason for splay marking is …

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 front of material as it enters the mould,or sometimes within the mould itself. Volatiles can …

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 volume there will be more shrinkage; in other words, thick sections shrink more than thin …

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 a melt viscosity for the material,will probably be a cause of flashing and that it …

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, it will flow in an ever-increasing semi-radial pattern. It will always flow more rapidly along …

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 fault. Taking these in order: Material at fault. Manufacturers are very careful about the quality …

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 the atmosphere or for a seemingly bland material such as Vaseline petroleum jelly. However,both are …

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 very high temperature as it becomes compressed by the material entering the mould. It reacts …