Nanjing GIANT Machinery Co.,Ltd.

Single screw extruder having side arm extruders for the introduction of additives are well known for organic rubbers and plastics. Mixing of a plurality of substances in a single screw extruder having satellite extruders for the high pressure introduction of components such as colorants and extenders which may not otherwise survive the mixing process because of otherwise damaging temperatures, pressures or mixing characteristics.

It will be clear to a man skilled in the art that the above prior art relates to sheet production extruder for organic polymers/rubbers as opposed to silicone rubber extruders because there are significant processing differences between the two. For the purpose of this invention it is to be understood that an organic rubber has a substantially carbon-carbon polymer backbone and a silicone rubber has a substantially siloxane polymer backbone. Reasons for the differences in extruder processing of organic rubbers and silicone rubbers include, for example, the fact that organic rubber has a significantly lower adhesion to the inner wall of an extruder barrel than silicone rubber and as such it is necessary to apply shear in the extruder to heat organic rubber for processing. Screws for organic rubber extruders may be provided with double flights having different flight pitches to force the rubber to flow over the flights. Alternatively pin extruders may be utilised which comprise pins in the barrel used to increase shear and output preventing organic rubber from progressing along the inner walls of the extruder barrel.

Adhesion of silicone rubber to the internal wall of an extruder barrel (wall adhesion) is significantly greater than with organic rubber equivalents and as such heat absorption and temperature increases occur due to the shearing forces on the silicone rubber between the screw and the inner wall of the extruder barrel. Typically silicone rubber extruders have a barrel cooling system. Silicone rubber extruders therefore require shorter screw lengths of 8 to 15 diameters (D) and utilize internal cooling systems for extruding silicone rubber.

In some instances it may be possible to process silicone rubber through twin screw extruder designed for organic rubber but due to their high shear design and, in most cases, the lack of an appropriate cooling system, there are significant limitations in particular the fact that very low screw speeds will be required for processing silicone rubber to avoid scorching problems in the extruder barrel. Furthermore, Silicone rubber extruders are much more difficult to clean than organic rubber extruders and extruded silicone rubbers require better compression than organic rubbers. Conventionally it is known to the skilled man in the art of silicone rubber extruders that single extruders are generally used only for transportational purposes as they provide poor mixing. Multiple screw systems (typically twin screw) are generally used where mixing is required for silicone rubber.