The Pre-plasticising and Injection Unit

Pre-plasticising is the term given to the conversion of uncured rubber to a hot, soft (relative), and homogeneous, plastic mass. This is achieved by the rotation of a metal screw within a heater-jacketed barrel. The relationship between the temperature of the pre-plasticised rubber and that of the jacketed barrel is dynamic, as the hot rubber exchanges frictional heat to the barrel walls or, depending on the heater setting, gains more heat. Rubber strip is fed to the throat, or inlet to the screw where it is sheared 

The relationship between these variables is complex and needs to be fully understood through use of factorial experiment design for each type of machine used in the factory. The unit must allow the rubber to be processed uniformly without scorch. There must be a smooth pathway for the rubber, free from dead zones where there is little or no rubber flow. Rubber that collects in these zones will soon cure to form hard nibs that will grow in size until they re-enter the material flow, impede the injection, and produce moulding faults. This problem exists in many existing machines where the first rubber to pass into the injection barrel is the last to be injected, and may be partially retained for many cycles. The design of the injection unit is therefore crucial to the operation of the machine. 

The way the rubber feeds into the screw has a significant influence on product quality. Designs that allow the rubber to be cut off in the feed zone (throat) by the screw flight cause extra work for the operator, and will give rise to an increase in screw speed as the starved screw empties itself of rubber. This results in a rapid build-up of scorched material in the unit and, if it is not immediately detected and corrected, there will be insufficient rubber in the components ( light parts ). A number of design features may be adopted to overcome rubber strip feed problems. Three solutions are regularly adopted, either alone or in combination  

The type of rubber to be used may require that special metals are selected for the construction of both screw and barrel. Halogen containing polymers can generate extremely corrosive breakdown products and their proposed use must certainly be discussed with the machinery manufacturer. 

Some compounds contain fillers such as silica and silicates that have a significantly abrasive effect on the screw and barrel. Special steels may be appropriate to minimise wear, and additionally, the amount of wear should be measured and recorded on a regular basis (at least annually). Changes in machine performance can then be avoided by planned replacement of eroded machine parts. If wear goes unchecked then the performance of the injection machine will change and affect product quality. The first sign of this could be an onset of moulding rejects. 

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