Extrusion presses force the rubber compound through dies to form flat, tubular, or specially shaped strips. There are two types of extruders: Ram extruders and screw extruders.
Ram extruders: Limited to specialized machines, use pressure to force the rubber through the die.
Screw extruders: Most widely used. Continuously converts feed to a finished form, such as a rod, tube or profile. Feed can be in the form of strip or granules. Feed material is forced forward by the rotating screw.
As it moves forward, the feed is softened by the frictional heat developed through the shearing action of the screw, supplemented by the heat conducted from the barrel wall. By the time the feed reaches the end of the screw, it is in a viscous state that can be forced through an orifice, dyed, and formed into the desired shape.
Extrusion is used in the manufacture of rubber tubing, hose, inner tubes, and such stripping as channels for setting windows or sealing doors. Specially designed extrusion heads are used for such processes as coating tubular fabric in making pressure hose.
Among the processing problems are those related to output rate, dimensional stability, heat generation and rough extrudate.
Output Rate - Output can vary by a factor of two, depending upon screw design. High mastication screws give the lowest output. Temperature profile can affect output by as much as 20% or more. Output will decrease as head pressure increases.
Output is very dependent upon the properties of the compound being processed. Scorch time of the compound must be sufficient to permit extrusion at maximum screw speed.
Dimensional Stability of the extrudates is dependent upon many factors. Here are a few:
- Take Away - The strength of the rubber extrudate is relatively low as it exits the extruder.
- Screw Speed Control - Output is nearly directly proportional to screw speed. Since surges may not be evident with meters, installation of speed recorders is desirable.
- Feed - Output will change if feed to extruder fluctuates.
- Temperature Control - Each temperature control should be tuned to maintain uniformity of +1
- Compound - Mooney scorch should be determined for both the feed stock and the extrudate.
- Heat Generation - Temperature build-up in the extrudate can result in scorch. It is affected by extrusion speed, type of screw, head pressure and compound formulation.
- Rough Extrudate - Surface roughness of the extrudate not caused by scorch compound is either due to air entrapment or insufficient mastication. Both causes are related to screw design and speed.
Silicone, EPDM, Neoprene, Nitrile, Natural Rubber, SBR and more.
