Process for manufacturing objects from fiber-reinforced thermoplasts

The invention pertains to processes for manufacturing objects and in particular moulded components from fiber-reinforced thermoplastic resins. The process involves blending a thermoplastic resin with reinforcing fibers, plasticating the blend with the addition of heat inside a screw-type extruder and extruding a plasticated mass for moulding. The thermoplastic resin is fed to the screw-type extruder in powder form and in a blend with the reinforcing fibers.

BACKGROUND OF THE INVENTION 
The invention pertains to a process for manufacturing objects and in 
particular moulded components from fiber-reinforced thermoplastic 
materials by blending a thermoplastic resin with reinforcing fibers, 
plasticating the blend inside a screw-type extruder with the addition of 
heat, and extruding a plastic mass for moulding. 
A process of this class is generally known. In this known process, a 
thermoplastic resin in granulate form, with particle size greater than 2 
mm is mixed with reinforcing fibers and blended in a screw-type extruder. 
The resulting mass is kneaded and plasticated with the addition of heat 
from the exterior of the extruder. It is difficult to introduce the entire 
quantity of heat required to plasticate the thermoplastic resin from the 
exterior of the screwtype extruder since the walls of the extruder housing 
would become too hot. In this scenario, the low-viscosity thermoplastic 
material would collect along the inside surface of the extruder, adversely 
affecting the consistency of the material. Due to the poor heat 
conductivity of plastics, plastication solely by externally introduced 
heat to the interior of the extruder would also increase the time to 
plasticate the blend. 
Consequently a combined heat generation technique is practiced. This 
technique involves a two step process. First, heat is introduced through 
the walls of the extruder housing into the interior of the extruder and to 
the thermoplastic resin mass located therein. Secondly, the action of the 
screw resulting from its rotation in the extruder housing introduces 
friction energy to the thermoplastic mass moved by the screw. The kinetic 
energy applied by the rotating screw to the mass is converted into thermal 
energy which can be used for additional heating of the blend. If the screw 
applies too much power to the blend, the reinforcing fibers blended with 
the thermoplastic will break and be shortened. The strength of the 
resulting product will therefore suffer. 
In order to avoid this detrimental damage to the fibers, an intermediate 
step in semi-finished product manufacturing is practiced. Thus, for 
example, glass mats are manufactured and impregnated with a thermoplastic 
matrix or glass fibers are coated in an extrusion process with 
thermoplastic melt and the mass is subsequently granulated. 
In such a process, it is possible to produce moulded parts with reinforcing 
fibers which are well maintained and well embedded but the additional 
effort required for manufacture of the intermediate product is 
considerable. 
Thus, the object of the present invention is to indicate a process for 
manufacturing objects from fiber-reinforced thermoplastic resins in the 
known class, in accordance with which the parts moulded from 
long-fiber-reinforced thermoplastic resins can be manufactured more 
simply. 
To attain this object, it is proposed that in the generic process the 
thermoplastic resin be in powder form, be blended with the reinforcing 
fibers and fed to the screw-type extruder. It is preferable that the 
thermoplastic powder be fed to the screw-type extruder at a mean particle 
size of less than 1 mm and more preferably less than 0.5 mm. 
The object is achieved with the measures in accordance with the invention. 
As a result of the fineness of the thermoplastic materials mixed with the 
reinforcing fibers and fed to the screw-type extruder, the plastication of 
the blend is achieved at lower levels of power being applied by the effect 
of the rotating screw on the thermoplastic blend located in the extruder, 
i.e. with less friction energy. The result is that the long reinforcing 
fibers in the resulting plasticated mass generally retain their length. 
Consequently, their reinforcing effect is generally unaffected. 
Due to the small grain size of the thermoplastic resin powder, early 
distribution with the reinforcing fibers is achieved so that a larger 
contact surface area is available between the fibers and the matrix from 
the very start of the heating process. In this way, improved fiber wetting 
with gentle heating is achieved from the commencement of the process. In 
addition, a large amount of friction energy is not required. Consequently, 
the fibers experience minimal damage. The parts thus manufactured achieve 
considerably better mechanical properties, and in particular demonstrate a 
generally high impact resistance. 
Glass fibers are most frequently used as the reinforcing fibers. 
Alternatively, other reinforcing fibers such as natural fibers, e.g. flax, 
mineral-based fibers or even synthetic fibers are used. Fibers can be fed 
to the screw-type extruder pre-blended with the thermoplastic powder. 
Alternatively, a metered feed of the components can be effected separately 
wherein blending takes place inside the extruder. 
In a preferred embodiment, the thermoplastic powder is a non-compounded raw 
polymer which is already obtained in the polymerization process at the 
appropriate fine grain size. Additives such as stabilizers, flame 
suppressants, and the like can be added in granulate or powder form, to a 
quantity of about 10 per cent by mass. 
The invention is elucidated in greater detail using the following example.

EXAMPLE 
In a screw-type extruder unit, a mixture comprising 67% by weight of 
polypropylene (PP) powder with grain size of about 200 .mu.m, 30% by 
weight of glass fibers (12 mm long, 17.mu.m diameter) along with two 
master batches (granulate size about 2.5 mm) in each case 1 and 2 per cent 
by weight for thermal stabilization and for coupling the fibers to the PP, 
respectively, are metered through a hopper. 
Operating at low back pressure, the material is plasticated inside the 
extruder and discharged as a metered shot. The shot is then transferred to 
a compression mould where it is worked to form a moulded part by way of 
compression moulding.