Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Forming plural articles
Reexamination Certificate
2001-06-05
2003-05-06
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Forming plural articles
C264S328800, C425S561000, C425S572000, C425S588000
Reexamination Certificate
active
06558603
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of and apparatus for producing injection-molded parts reinforced with long fibers.
It is generally known to make structural parts, such as thermoplasts reinforced with glass mats (GMT), from long fiber reinforced thermoplastic material in such a way that semi-finished products in the form of glass mats impregnated with melt are heated and shaped by a subsequent compression process. During the compression process, the mold cavity will be completely filled by the flow of the semi-finished product. The structural part is removed from the open press after a cool-down period for subsequent transfer to a post-treatment station. This type of manufacturing process is rather complicated in view of the need for successive processing steps and the required treatment stations.
It is also known to make structural parts of fiber-reinforced thermoplastics by processing plastic granules with randomly oriented fibers in a conventional screw-plunger injection molding machine. This process permits introduction of fibers of a length of only 0.2 to 0.4 mm into the structural part, so that the strength of the structural part can be improved only to a limited extent.
It is further known to make structural parts from long fiber reinforced thermoplastic granules. The granules involved here are so-called long fiber pellets which are produced by impregnating parallel glass fiber strands with a thermoplastic melt during advance through an impregnator head. The strand issuing from the impregnator head is subsequently cut to a defined pellet length between 10 and 50 mm by a pelletizer. During processing of the long fiber pellets in a screw-plunger injection molding machine, the relatively long fibers are, however, exposed in the screw cylinder of the plastifying unit to substantial shearing stress so that the major part of the fairly long fibers is sheared off or breaks. The strength of the finished injection-molded product can thus be improved also only to a limited extent.
It is also known from the Handbook, VDI-Verlag, Fortschrittsberichte VDI, Dipl.-Ing. Frank Truckenmüller, “Direktverarbeitung von Endlosfasern auf Spritzgie&bgr;machinen” [
Direct Treatment of Continuous Fibers with Injection Molding Machines]
, Series 3 Verfahrentechnik, No. 444, Apr. 1996, to introduce in a screw-plunger injection molding machine continuous fiber strands directly into the screw cylinder of the plastifying unit. Also in this case, the fiber strands are exposed during advance through the plastifying unit to substantial shearing stress so that the major fraction of the fibers is split in lengths that are too short.
German Pat. No. 195 23 490 discloses a two-shaft extruder with screw shafts running in same direction having a zone in which endless fibers are introduced into the screw cylinder at low pressure. Hereby, the endless fiber strands are thoroughly mixed with the melt in a continuous operation to a homogenous and gentle mixture which issues as mass strand at low pressure from the two-shaft extruder. This type of apparatus is known as compounder and effects an optimum mixture in a continuous operation. As the fiber-melt-mixture is produced continuously, this apparatus can be used for producing the long fiber granulate or long fiber pellets or for depositing the fiber-melt mixture on open mold halves via a slot die. After the fiber-melt mixture has been essentially applied on a surface of the mold half, both mold halves are pressed together and the structural part is completely shaped by a flowing process. While this process yields an optimum mixture of long fibers and melt, the further working steps of flat application of the fiber-melt-mixture upon a mold half and compressing are relatively time consuming operations to make thermoplastic parts reinforced with long fibers.
German Pat. No. DE 195 38 255 describes an extruder with two screws running in same direction for producing a continuous stream of a plastic melt which is laden with long reinforcement fibers and transferred by a subsequent injection unit, comprised of a cylinder and a piston, into the mold cavity of an injection mold at a pressure profile required for the molding process. In this apparatus, the continuous melt stream is initially conducted intermittently into an intermediate reservoir and from there into the cylinder space of the injection unit. Before entering the mold cavity, the melt stream laden with long reinforcement fibers is transported twice into the cylinder volume and out again. By reversing the flow direction twice, a great number of long fiber portions break.
FIG. 1
shows a schematic, partially sectional, illustration of a conventional arrangement for making injection-molded parts reinforced with long fibers, including a compounder
1
which has a housing
2
for accommodating two screws
3
,
4
running in same direction and meshing with one another. The screws
3
,
4
are operated continuously by a rotary drive
5
having a motor M. Arranged on the housing
2
is a feed hopper
6
for supply of thermoplastic granulate and a feed device
7
for introduction of a continuous long fiber strand
9
reeled off a roving
8
. The long fiber strand
9
may, for example, be made of glass fiber. Basically any type of reinforcement is suitable, such as, e.g. carbon fibers, Kevlar fibers or natural fibers like hemp fibers.
The compounder
1
has an anterior screw chamber
10
which is fluidly connected via two branches
11
,
12
to two pressure generating and discharge assemblies
13
,
14
, whereby a flow control valve
15
regulates the flow from the anterior screw chamber
10
in alternating sequence to the two pressure generating and discharge assemblies
13
,
14
. As shown in
FIG. 1
by the partially sectional illustration of the lower one of the pressure generating and discharge assemblies
13
,
14
, each of the assemblies
13
,
14
includes an injection plunger
16
and a hydraulic operating plunger
17
which is acted upon alternatingly via the hydraulic ports
18
,
19
by a pressure fluid conveyed by a, not shown, pump for providing a predetermined pressure profile. Each of the pressure generating and discharge assemblies
13
,
14
has an anterior cylinder space
20
which is fluidly connected via an injection nozzle
21
and a sprue channel
22
with a mold cavity
23
of an injection mold. The injection mold has a fixed mold mounting plate
26
for carrying a mold half
25
, and a moving mold mounting plate for carrying a mold half
24
, with guide rods
28
extending between the mounting plates
26
,
27
. The mold mounting plate
27
is moved to the opening and closing positions by a hydraulic unit
30
, which is shown only partially.
At operation, the compounder
1
produces a continuous mass stream of thermoplastic melt which is permeated in a homogenous manner by a continuous fiber strand from the long fiber strand
9
drawn in via the feed device
7
. The mass stream flows, controlled by the flow control valve
15
, into the cylinder space
20
of the assembly
13
. After the cylinder space
20
is completely filled with long fiber interspersed melt, the valve
15
is switched over to cut the connection between the anterior chamber
10
of the compounder
1
and the cylinder space
20
of the assembly
13
and to connect the anterior chamber
10
of the compounder
1
with the cylinder space
20
of the assembly
14
. Subsequently, the injection plunger
16
is operated to inject the melt via the injection nozzle
21
and the sprue channel
22
into the cavity
23
between the closed mold halves
24
,
25
. The predetermined pressure profile in the cavity
23
for producing an injection-molded part
29
is adjusted trough appropriate admission of hydraulic pressure fluid to move the operating plunger
17
.
It would be desirable and advantageous to provide an improved method and apparatus to obviate prior art shortcomings.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, an apparatus for producing thermoplastic i
Klotz Bernd
Wobbe Hans
Zwiesele Jochen
Feiereisen Henry M.
Heitbrink Jill L.
Krauss-Maffei Kunststofftechnik GmbH
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