Note: Descriptions are shown in the official language in which they were submitted.
W~ 91/18734 P~T/EP91/00817
2~3~
Process and Device for the Production of Plastic Mouldin~s
~escription
The invention relates to a process and a device for producing plastic mouldings, in
which at least two highly liquid reactants are mixed to form a reaction mixture
incorporating air, and the reaction mixture is injected through a gate into a cavity of
a mould, displacing the air in the latter and causing a build-up of the internal pressure
in it, and hardens by chemica! reaction in the cavity ~o form a moulding.
The process used is the so-called RIM process (RIM = Reaction Injection Moulding),
used to produce mouldings from microcellular polyurethanes, polyolefins and
polyamides, as well as other polymerized and unpolymerized thermoplastic and
thermosetting substances, which may be strengthened by means of fillers such as
fibre glass (RRIM = Reinforced Reaction Injection Moulding). An important advantage
of these plastics is the fac~ that the reactants, such as isocyanate and polyol, have,
as compared with thermoplastics, a very low coefficient of viscosity of an order of
magnitude from 0.1 to 1.5 Pa s, such that a high flow path/wall thickness ratio can
be obtained. Injection time. in the case of high-speed polyurethane systems, is
generally only 0.5 to 1.5 s at an injection pressure of 100 to 200 bar. Due to the high
reaction speed, mould release times are ~Iso relatively short, so that a high throughput
rate can be expected. (Reference to known technology: "Kunststoffe im Fahrzeugbau
- Technik und Wirtschaftlichkeit" [Plastics in Automobile Manufacturing - Technical
Aspects and Profitability], VDI Verlag Dusseldorf 1998, pp. 197-244).
To date, a particular problem associated with the RIM process has been that of the
open pores, which appear with increased frequency in components with a complex
geometrical shape and/or in certain flow and mould-filling processes particularly in
regions remote from the injection gate. This is particularly disadvantageous when an
additional coat of lacquer is applied, if the air-filled pores open to the exterior are
W(~ 91/18734 PCT/EP91/~0817
- 2 -
2~3~
sprayed with lacquer. When the lacquer is heated and the material hardens from the
outside inwards, pressure can build up to such an extent that it causes an eruption
through the coat of lacquer. This results in crater-shaped openings the size of a pin's
head on the outside layer of lacquer, which are not aesthetically acceptable. In order
to avoid this drawback wipe-on fillers have been used, which close the pores in the
surface of the workpiece before the application oF the lacquer. However~ problems
may, for example, arise during degreasing, as the wipe-on filler may be washed away.
Furthermore, it has been shown that in finished mouldings there can be differences in
density of up to 10% between the regions close to the injection gate and those that
are remote from it. This in turn leads to an undesirable variation in the mechanical
properties of the moulding.
The object of the invention is therefore to improve the aforementioned type of process
in such a way that, by relatively simple means, even in the case of large and complex
mouldings, the mechanical properties of components at the points indicated can be
positively affected.
In order to achieve this objective, the characteristics listed in Claim 1 are proposed.
Additional advantageous refiflements and improvements of the invention will be found
in the related claims.
The invention is based on the known fact that generally, in the RIMIRRIM technique,
and particularly in the PUR-RIM/RRIM technique, the internal pressure in the mould
decreases from approximately 30 bar at the gate to 10 bar in regions away from the
gate. The variation in density along the length of the component is a consequence of
the decrease in the internal pressure in the mould along the length of the cavity.
Therefore, if the internal pressure in the mould can be increased, particularly in areas
away from the gate, it is to be expected that the density and, hence, the mer;hanical
properties of the material will be improved. This however presupposes ~hat the
reaction mixture, because of its air inclusions, can be compressed while it is still in a
,: , ,- ,....
Wf )1/18734 PCT/EP91/00817
~3~
fluid or gel-like state. On the other hand, it is necessary to delay raising the pressure
until the viscosity of the reaction mixture, during the extremely quick reaction process,
has reached the point where the material, while subject to the pressure, can still be
displaced into adjacent regions in the desired manner, without the occurrence of holes
penetrating the entire ~hickness. This condition is met by filling the cavity with an
initial flow, with the result that the reaction mixture, during the shot, adheres to the
walls of the cavity and first reacts there, so that a more solid channel is formed for
the more fluid reaction mixture that flows in afterwards. Because of these dynamic
processes, the material, surprisingly, Teacts from the outside inwards and not, as in
a stationary process, from the inside outwards, so that at least around the time of the
end of the injec~ion a lower mass viscosity and, therefore, a higher compressibility
prevails inside the moulding, especially in those regions which are remote from the
gate.
Gonsequently it is proposed, in accordance with the invention, that around the time
of the end of the injection (shortly before until shortly after~ the internal pressure in
the mould in at least one region filled with reaction mixture should be raised and that
at the same time the reaction mixture in a transi~tional, gel-like state should be
compacted, thereby compressing the air inclusions. It is preferable that this increase
in pressure should occur in an area away from the gate, in order to counteract the
decrease in pressure which accompanies the filling process. The increase in pre~sure
can be achieved to the extent that a portion of the air included in the reaction mixture
is released and the microcellular structure is thereby further compacted. In order to
obtain reproducible results, the increase in pressure is initiated by a trigger signal
which can be synchronized with the start or finish of the injection.
It is preferable to raise the internal pressure in the mould abruptly, so that the jump
in pressure can exhibit a wave front the duration of which is appropriate to thereaction speed of the reaction mixture, the local strength of the mould wall and/or the
amount of material to be displaced.
W~' ~)1/18734 PCT/EP91/~0817
2 ~
In accordance with a preferred embodiment of the invention, the internal pressure in
the mould is increased by feeding in a pressure medium, for which purpose either a
pressure gas, preferably nitrogen, or a hydraulic fluid can be used.
The pressure medium is preferably injected in the space between the wall of the cavity
and the surface of the moulding, or into a reservoir hollowed out in the mould and
filled with the reaction mixture during the ir.jection process. This makes it possible to
inject the pressure medium into the interior of the reaction mixture, no matter whether
the interior of the latter is to be hollow or filled.
In general, it is also possible to use as a pressure medium an additional, preferably
similar, reaction mixture the mixing of which is delayed and which is injected into the
mould cavity at another gate which is remote from the main injection 0ate.
In accordance with another advantageous embodiment of the invention, the internal
pressure in the mould is raised by means of a flexible diaphragm, preferably made of
rubber, to which prèssure can be applied, and which acts upon the reaction mixture,
or a suitable plunger. Pressure gas or a hydraulic fluid may be applied to the
diaphragm or the plunger.
It has been shown that when polyurethane is used as a reactant plastic, it is
particularly advantageous if the increase in pressure is initiated 0.05 to 2 s, preferably
0.1 to 1 s, after the completion of the injection. The increase in pressure should then
be maintained for approximately 2 to 10 seconds, preferably 4 to 6 seconds; in order
to prevent the shot from passing through the ma~imum value, between 30 and 50 bar
in order of magnitude and preferably approximately 40 bar, should not be set abruptly,
but should rather be increased gradually over a period of 0.1 to 2 seconds.
If for the purpose of executing the process to which the invention refers use is made
of a device with a mixer head to which at least two hi~hly liquid reactants are
W~ ')1/1~734 P~T/~P91/00817
-5- 2~3~
admitted through the intake, a mixer head jet located at the outlet of the head and
controllable by a control device, and a mould cavity communicating with the jet to
which the highly liquid reaction mixture can be admitted through the jet under
pressure, it is proposed, in accordance with the invention, that in at least one region
of the mould remote from the gate a thrust generator shall be located, engaging with
or opening into the cavity and acting upon the reaction mixture in the cavity while the
pressure is raised locally. The operation of ~he thrust generator by the control device
can be synchronized with the operation of the mixer head.
The thrust generator preferably has a gas jet which can admit pressure gas; the gas
jet can take the form of a seat valve opening into the cavity and closed at the preset
tension, with a valve body that can be displaced by the effect of the pressure
differential between the pressure gas and the internal pressure in the cavity. It is
preferable for the gas jet's seat valve to be activated through a control line and to
admit pressure gas through a gas supply line. For this purpose the gas supply line
contains a gas supply valve designed as a directional valve, as well as a release valve
designed as a directional valve and attached to the jet side of the gas supply valve.
If the seat valve is pneumatically controlled, a pressure valve designed as a directional
valve can be located in the ~ontrol line, and a release valve acting as a directional
valve can be located on the jet side of the pressure valve. The directional valves, in
turn, take the form of solenoid valves electrically activated through the control device
and the sequential control of the activation of these valves can be synchronized with
the operation of the mixer head.
To prevent an abrupt rise in pressure, a proportional valve can be located in the gas
supply line and operated through a ramp generator so as to set a defined increase in
pressure that will be triggered when the gas supply line valve is subject to remote
control. The gas supply line can open directly into the cavity in the vicinity of the gas
jet outlet and extend as far as the boundary surface or engage with the savity. In
general, it is also possible to locate a flexible diaphragm between the gas jet and the
W~` 91/18734 PCT/EP9~ 0817
-6- 2~3~
cavity or to use the thrust generator as a plunger, particularly as a plunger with a
diaphragm.
Instead of the gas jet it is also possible to use as a thrust generator a fluid jet to which
hydraulic fluid can be admitted, for example, an additional mixer head jet. In order to
accommodate the reaction mixture to be displaced under pressure, a displacement
chamber can be hollowed out in the cavity to which pressure, particularly pressure
gas, can be applied by means of the thrust generator.
The steps to which the invention refers make it possible to improve the mechanical
properties of the moulding in the aforementioned regions, because of the increased
density of the material and the formation, where required, of a hollow space. Further,
any surface pores are smoothed out, the reaction mixture being pressed against the
ca~/ity wall as a result of the increased internal pressure in the mould. Finally, it is
possible to preven~ sink spots or marking of the surface due to an accumulation of
material, such as reinforcing ribs, when the internal pressure in the mould is purposely
increased in those regions where material accumulates.
Further details of the inventien vvill be described below by means of the embodiment
shown as an example in the drawing.
The sole diagram shows the layout of a foam-producing unit for reactant plastics L/'
~ ....
where pressure is applied in a region remote from the gate.
The mould 10 of the foam-producing unit has two halves 12 and 14, between which
a longitudinal cavity 16 is hollowed out for the production of a thin-walled workpiece.
The mould receives under pressure via a mixer head 18 through an injection line 20
and the gate 22 a highly liquid reaction mixture composed of at least two reactants
fed separately into the mixer head. The air which is initially in the cavity is displaced
through a vent line located in the vicinity of the end 24 of the mould 10 that is remote
W" 91/18734 PCT/EP91/0~817
2~3~
from the gate. Because of the flow of fluid from the end 22 adjacent to the gate to
the end 24 away from the gate, there is a decrease in pressure within ~he cavitywhich can cause a decrease in density and consequent variations in th0 mechanical
properties, as well as increased surface porosity. To prevent this, a device ~6 has
been installed in the region of the mould 10 which is away from the gate, which can
be used to raise the internal pressure locally within the cavity 16.
The device 26 has a gas jet 28 which contains a gas-activated seat valve 30 at its
outlet end 30. The seat valve 30 opens into a displacement chamber 32 which can
receive the reaction mb(ture; this chamber is part of the cavity 16 and is filled with
reaction mixture at each shot. The seat valve of the gas jet is pneunnatically activated
through a control line 34 and receives pressure gas (nitrogen, for example), from a
pressure cylinder 38, through a gas supply line 36.
Pressure is applied or released at the gas jet 28 by means of four electrically-activated
solenoid valves 40,42,44,46 acting as a pair of bi-directional valves and installed
respectively in the control line 34 and the gas supply line 36. The directional
valves 40 through 46 can be controlled by means of a control device 48 which maytake the form of a safety cor~trol; the valve switch settings are effected through six
timing relays. The control of the sequence is started through the control line 50 [TR-
SIC] by the signai "mixer head jet open" from the foam-producing unit (injectionbegins) .
Following a pause, which should at least equal the shot time, the valves 40 in the
control line 34 and 44 in the gas supply line 36 open, the seat valve 30 in the gas jet
48 [TR-SIC] is opened and thereupon pressure gas at a pressure of approximately 40
to 50 bar is admitted to the reaction mixture in the mould 10.
W~ 734 - 8 - PC~ g~7
When the gas has been supplied (the process can last a few seconds), first the
valve 40 is shut, then the line 34 is depressurized through the valve 42. The gas
supply line 36 must remain under pressure, so as to prevent the reaction mixture from
penetrating the gas jet 28. Once the line 34 has been depressurized, the gas supply
valve 44 is closed and the gas release valve 46 is opened in order to vent the gas
supply line 36. When the valve 46 is closed, the valve control is returned to its initial
setting. It is possible to install a proportional vaive (not illustrated) in the gas supply
line, which can be operated through a ramp generator to control the wave front of the
pressure.
Example
For testing purposes, a foam-producing unit with a mould for the production of
mouldings with the following dimensions was used:
Length approx. 1220 mm
Height approx. 230 mm
Wall thickness approx. 3.5 mm
The gas jet was installed in the upper portion 14 of the mould in a region remote from
the gate and opened into a displacement chamber 32 with a volume of approx. 29
cm3.
Polyurethane was used as the reactant plastic; its companion reactants polyol and
isocyanate were mixed in a ratio of 100: 47.8 (volumetric) in a transverse thrust
mixer head 18.
Wl~ 91/18734 PC2T/E~e~i~7
The other parameters of the process were as follows:
Fibreglass content : 1 5 /O
Gas content : 60%
Temperature of material : 40C
Temperature oF mould : 60C (upper and lower halves)
Pressure in components : 200 bar
Dosage volume : 1.1 L
Shot time ' : 0.77 s
Time of retention in mould : 30 s
Time of starting gas supply : 0.9 to 2 s following injection
Duration of gas supply : 5 s
Gas supply pressure : 40 bar
The volume displaced by the pressure gas was approximately 20 cm3.
The material properties of the moulding obtained both with and without the application
of pressure are shown in the following table:
_ _
With internal gas pressure Without internal gas
. pressure
_ . _ . _
Density 1.31 g/cm3 1.2 g/cm3
E module 1600 N/mm~ 950 N/mm2
The invention may be summarized as follows: It relates to a process for the production
of plastic mouldings from a reaction foam such as polyurethane, in which at leas~ two
reactants in a highly liquid state are mixed together with the inclusion of finely
distributed air and the resulting reaction mlxture is injected throuyh a ~ate into an
assembled cavity in a mould displacing the air in the latter and causing the build-up
of an internal pressure within the mould which decreases as the distance from the
,: '
Wf~ 91/1~734 ~)~17
- 10 -
gate increases and hardens by chemicai reaction to form a moulding. To improve the
mechanical properties of components and smooth the surface pores the internal
pressure in the mould is raised around the time of the end of the injection in at least
one region, preferably remote from the gate, which is already filled with reaction
mixture, and the reaction mixture, which is already in a gel-like transition state, is
compacted, while the air inclusions are compressed and /or released.
