Apparatus for liquefying a thermoplastic plastic

An apparatus for liquefying a polyurethane based thermoplastic adhesive which includes a heatable sealed supply container having an upper portion for receiving a downwardly open transport container for the adhesive and a lower portion serving as a buffer volume. A base portion is arranged below the supply container and includes a collecting trough leading to an outlet passage. A cover is provided at the upper end of the supply container and a conduit supplies an inert gas to the interior of the supply container. The conduit also opens into the interior of the transport container. A mandrel having a tip is provided at the inner side of the cover and the tip penetrates the transport container when the cover is closed. The inert gas is provided through an outlet opening located in the side wall of the tip of the mandrel. The gas supply through the conduit is periodically interrupted by a magnetic valve operated by a timer which only opens the valve for short periods, which provides an appreciable gas saving.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an apparatus for liquefying a thermoplastic 
plastic, in particular an adhesive on the basis of polyurethane. 
2. Description of the Prior Art 
In such a liquefying apparatus as is known from DE-GM No. 86 00 021 (U.S. 
Pat. No. 4,724,983) the heatable sealed supply container comprises an 
upper portion for receiving a downwardly open transport container for the 
thermoplastic plastic and a lower portion serving as buffer volume. A 
heating means heats the thermoplastic plastic disposed in the transport 
container so that said plastic becomes liquid and drops down into a base 
portion which is disposed beneath the supply container and the transport 
container and receives a collecting trough for the liquefied thermoplastic 
plastic. At the upper end of the supply container a pivotally constructed 
cover is disposed which is provided at its inside with a mandrel; when the 
cover is closed the tip of the mandrel penetrates into the upwardly 
directed face of the transport container. 
An inert gas usually serving as protective gas is supplied to the interior 
of the supply container via a conduit. 
However, in tests with such a liquefying apparatus problems have been 
encountered related mainly to the supply of the gas, for example nitrogen, 
carbon dioxide or also compressed air in the case of less sensitive 
plastics. Firstly, there is the danger that the gas collects only in the 
base portion of the liquefying apparatus and in particular does not reach 
the thermoplastic plastic in the transport container so that in the latter 
undesirable reactions can occur. In spite of this screening action being 
not satisfactory, the gas consumption is very high because gas has to be 
continuously supplied to the interior of the supply container. 
SUMMARY OF THE INVENTION 
The invention is thus based on the problem of providing an apparatus for 
liquefying a thermoplastic, in particular an adhesive on the basis of 
polyurethane, of the type indicated in which the aforementioned 
disadvantages do not occur. In particular, a liquefying apparatus is to be 
proposed which makes possible a still better protection from undesirable 
reactions for highly sensitive highly viscous compositions readily 
reacting in the presence of air. 
The invention therefore proposes in an apparatus for liquefying a 
thermoplastic plastic, in particular an adhesive on the basis of 
polyurethane, comprising a heatable sealed supply container which 
comprises an upper portion for receiving a downwardly open transport 
container for the thermoplastic plastic and a lower portion serving as 
buffer volume, a heating means for heating the thermoplastic plastic 
disposed in the transport container, a base portion arranged beneath the 
supply container and receiving a collecting trough for the liquefied 
thermoplastic plastic, a cover at the upper end of the supply container 
and a conduit for the supply of an inert gas to the interior of the supply 
container, the improvement that the conduit comprises an opening leading 
to the transport container. 
Expedient embodiments are defined by the features of the subsidiary claims. 
The advantages achieved with the invention are based on the fact that the 
gas, possibly also compressed air, is conducted specifically directed into 
the interior of the downwardly open transport container so that firstly a 
complete enclosure of the thermoplastic plastic in the transport container 
by the gas results and secondly the flowing of the liquefied thermoplastic 
plastic out of the transport container is supported by the gas pressure. 
A particularly suitable "conduit" for supplying the gas has been found to 
be the mandrel which is disposed at the inner side of the pivotal cover 
and the tip of which penetrates into the upwardly directed face of the 
transport container when the cover is closed. If the tip of the mandrel is 
provided with an outlet opening for the gas and the mandrel is connected 
to the gas conduit the gas pressure acts on the upper side of the 
thermoplastic plastic in the transport container, i.e. when the 
thermoplastic plastic flows out downwardly it protects said upper side and 
also supports the downward movement of the thermoplastic plastic. 
To avoid clogging of the outlet opening for the gas by contact with the 
liquefied thermoplastic plastic the outlet opening is preferably formed at 
the side of the mandrel and provided with an additional bend. 
To reduce the gas consumption the guard gas, or the possibly used 
compressed air supplied, is introduced only periodically, i.e. a valve, in 
particular a magnetic valve, provided in the supply conduit for the gas is 
controlled for example via a programmable time switch so that the magnetic 
valve opens after every 20 seconds for one second; the amount of gas then 
emerging, which is very small compared with that in continuous operation, 
suffices to protect sensitive thermoplastic plastics; at the same time, 
compared with the hitherto usual continuous operation a saving of about 
90% gas is obtained. This saving cannot be achieved with conventional 
means, i.e. using sensitive adjusting elements for the gas supply, for 
example extremely fine regulation of the gas flow. 
A further advantage of this "clocked" gas supply is that the system is 
subjected to the protective gas in surges and this has a favourable effect 
on the liquefaction. 
Conveniently, provided in the flow direction of the gas between the 
magnetic valve is a pressure and/or flow regulator with appropriate meter 
so that extremely low rates of flow can also be set.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The liquefying apparatus generally denoted by the reference numeral 10 
comprises a base plate 11 which carries an electric motor 16, a switch 
cabinet 12 which accommodates the electrical switch elements for 
controlling the temperature at the various heating elements and is 
provided with ventilating slots 14, and on the right side of the 
illustration in the Figure the actual liquefying part. The electric motor 
16 drives via a coupling, in particular a magnetic coupling, or a gearing, 
generally a reduction gearing, which are designated by the reference 
numeral 18, a pump 20, generally a gear-type pump, for conveying the 
heating and thus liquefied polyurethane adhesive. 
The actual liquefying part is disposed in a heat-protection hood 38 which 
also screens the sensitive electronic components of the switching cabins 
12 and is sealed at its upper end by a pivotally mounted cover 40. For 
sealing the gap between the cover 40 and the lower region of the 
liquefying part seals 43 and 42 are provided. Moreover, a schematically 
indicated latch means 41 is provided which holds the cover 40 in its 
operating position shown in the Figure. 
The actual liquefying part comprises a block-shaped substructure 22 with 
heating bores 24 in which for example electrical resistance heaters are 
disposed which are insulated by means of chamotte tubes. 
At its upper end the block 22 comprises a recess 28 formed as collecting 
trough which is connected via a passage 29a to the pump 20. From the pump 
20 a further position 29b of the conveying passage leads to the outlet for 
the liquefied adhesive. 
By brackets 36 the actual supply container 30 is detachably secured to the 
block 22 and the walls of said container 30 are likewise provided with 
heating bores 32 in which for example electrical resistance heating 
elements insulated by means of chamotte tubes are disposed. 
Between the upper edge of the walls of the block 22 in the region of the 
recess 28 and the lower edge of the walls of the supply container 30 seals 
34 are provided, in particular insulations on the basis of mineral fibers, 
expediently asbestos. 
As is apparent from the Figure the two upper seals 43 and 42 bear on the 
outer wall of the upwardly open supply container 30. The supply container 
30 comprises two parts, that is a lower buffer volume 48 and an upper part 
which is separated from the lower part 48 by a narrow encircling web 31 
projecting inwardly from the inner wall thereof. Resting on this web is 
the lower edge of a downwardly open transport container 46 for the 
adhesive which is indicated in the Figure in dashed lines. 
The inner edge of the cover 40 is provided with a downwardly projecting 
pointed mandrel 44 whose tip penetrates into the upwardly directed bottom 
face of the transport container 46. 
A conduit 50 for an inert gas, in particular nitrogen or carbon dioxide, 
possibly also however compressed air, is connected to a flow control 52 
indicated only schematically in FIG. 1 and extends through beneath the 
block 22 and then laterally past the supply container 30 upwardly and 
opens into the cavity which is formed between the upper end of the supply 
container 30 and the cover 40. From the conduit 50 an extension 54 
branches off and is connected to the mandrel 44. Extending through the 
mandrel 44 is the passage 56 shown in FIG. 1, said passage emerging 
laterally at the tip of the mandrel 44. 
As apparent in FIG. 2 the passage 56 first extends in the axial direction 
of the mandrel 44 and then bends at an angle of more than 90.degree. to 
the side, i.e. even somewhat against the flow direction rearwardly so that 
the gas emerging from the passage 56 in the mandrel 44 blows upwardly 
against the bottom of the transport container 46. 
The flow control 52 (FIG. 3) comprises a magnetic valve 58 which is 
controlled via a time switch 60. Said time switch 60 closes the magnetic 
valve 58 for example for 20 seconds, i.e. for this period of time 
interrupts the gas supply, and then opens said magnetic valve 58 for one 
second to permit the gas flow. Provided behind the magnetic valve 58, seen 
in the flow direction, is a fine adjustment valve 62 with extremely fine 
manometer 64 so that even extremely small rates of flow and/or pressures 
can be adjusted. 
When the operation is started the cover 40 is opened and a transport 
container 46 for the adhesive on the basis of polyurethane, which is open 
at one side, is introduced into the supply container 30 with the opening 
downwards so that the lower edge of the transport container 46 comes to 
rest on the encircling web 31. The adhesive on the basis of polyurethane 
is so viscous that it cannot flow out of the downwardly opened transport 
container 46. 
The cover 40 is then closed so that the tip of the mandrel 44 penetrates 
from above into the transport container 46. At the same time the gap 
between the cover 40 on the one hand and the heat-protection hood 38 or 
the supply container 30 on the other hand is sealed by means of the seals 
43, 42. Nitrogen for example is then supplied via the flow control 52 and 
the conduit 50 so that firstly the air is expelled from the supply 
container 30 and the collecting trough 28 and simultaneously nitrogen acts 
via the mandrel 44 and the passage 56 from above on the adhesive in the 
transport container 46, i.e. generates an additional downwardly directed 
force component. Thus, the adhesive can no longer come into direct contact 
with the ambient air. 
The two independently controllable heating devices for the block 22 on the 
one hand and the supply container 30 on the other are now switched on. As 
a result the adhesive in the transport container 46 liquefies and then 
flows out of said container 46 downwardly into the buffer volume 48. Said 
buffer volume 48 is so dimensioned that it suffices to accommodate an 
amount of adhesive corresponding to the largest transport container volume 
to be handled The liquefied adhesive then passes from the buffer volume 48 
over the recess 28 serving as collecting trough and the passage 29a to the 
pump 20 which supplies the liquefied adhesive via the passage 29b to the 
outlet. 
Further gas is supplied via the passage 56 in the mandrel 44 to shield the 
surface of the adhesive until complete discharge thereof from the 
transport container 46. The supply of the nitrogen is periodic by means of 
the flow control 52, that is with a cycle which can be set by means of the 
time switch 60. It is possible to take account here for example of the 
properties of the adhesive used on the basis of polyurethane and also of 
the volume and/or pressure of the nitrogen supplied. 
When the transport container 46 has been completely emptied the cover 40 is 
opened again, the empty transport container 46 is removed and a new full 
transport container 46 inserted. The processing can be carried out 
simultaneously because liquefied adhesive is still in the buffer volume 
48; at the top towards the ambient air this heated and thus particularly 
reactive adhesive is shielded by the nitrogen hood so that a continuous 
operation is possible without any impairment of the properties of the 
heated adhesive in the buffer volume 48. 
Since the heatable supply container 30 is secured detachably to the block 
22 with the collecting trough 28 it can easily adapt itself to different 
processing volumes and thus transport container volumes by for example 
replacing the hollow cylindrical or cuboid supply container 30 by another 
embodiment of corresponding volume. 
The temperatures in the block 22 on the one hand and the supply container 
30 on the other are kept to a settable desired constant value by means of 
the usual thermostat devices and corresponding temperature sensors, which 
are not shown. The corresponding electronic components are disposed in the 
switching cabinet 12.