Dissertations in Forestry and Natural Sciences

Taneli Väisänen: Effects of Thermally Extracted Wood Distillates on
the Characteristics of Wood-Plastic Composites
directions, are used for compounding, mixing or reactive
polymer materials. L/D ratios for the twin-screw extruders vary
from 39 to 48. The advantages of the twin-screw extruders
include the self-cleaning and high mixing ability. However,
unlike the single-screw extruders, these systems cannot develop
a up high pressure in the melt pumping zone. In addition, the
twin-screw extruders are more expensive. (Schwendemann
2008, Stokke et al. 2014)
The barrels are divided into the sections heated with the
individual control units (Stokke et al. 2014). The temperature of
the barrel gradually increases from the rear to the front which
allows the material to melt gradually and to prevent thermal
degradation or overheating. Sometimes the friction and high
pressure in the barrel provide the required heat for the system,
and the heaters can be turned off.
Multi-layered WPC structures are produced by coextrusion.
This process utilizes multiple extruders (single or twin-screw)
to melt and deliver different types of materials to a single
extrusion die that will extrude the materials in the desired form
(Stokke et al. 2014). In addition to the reduced material and
production costs, coextrusion makes the properties of final
products highly controllable, which is a significant advantage
over other production technologies.
2.3.2 Injection molding
Injection molding is used for producing large quantities of WPC
pieces with complex geometries (Migneault et al. 2009). The
molding of WPCs begins by inserting the pelletized raw
material into the hopper, which feeds the material into the
heated barrel with a reciprocating screw (Figure 6). The majority
of the injection molding machines are equipped with single
screws. The increased thermal energy reduces the viscosity of
the material, allowing the screw to push the material forward.
The simultaneous mixing and homogenizing increase the
friction and heat within the barrel. The material is collected at
the front of the screw and then injected at high pressure and
velocity into the mold. The volume of the material that is used
44 Dissertations in Forestry and Natural Sciences No 222