Device for the melting and measured discharge of thermoplastic adhesive

In a device for the melting and measured discharge of a thermoplastic adhesive, a sealing sleeve conveys the solid thermoplastic adhesive into the inlet end of a melting chamber. The sealing sleeve is formed of a temperature resistant, poor heat conductive material with good sliding properties, such as polytetrafluoroethylene. Further, the sealing sleeve is provided with slots extending in the axial direction of the sleeve from the end more remote from the melting chamber toward the melting chamber.

SUMMARY OF THE INVENTION 
The present invention is directed to a device for the melting and measured 
discharge of thermoplastic adhesive and, more particularly, to a sealing 
sleeve at the inlet end of a melting chamber in the device. 
Thermoplastic adhesives are being used to an increasing extent for both 
industrial and commercial purposes, because of their advantages, such as 
short curing time, the fact that the output can be adequately measured, 
and such adhesives are free of solvents which could, during processing, 
generate explosive vapors or vapors harmful to health. Devices used for 
processing such adhesives have undergone continued development and have 
been improved continuously. 
A significant problem experienced in the use of such thermoplastic 
materials involves sealing of the inlet into the melting chamber. To 
prevent the liquified adhesive material within the melting chamber from 
flowing out of its rearward end, it has been conventional to provide a 
sealing sleeve at the inlet into the melting chamber. The sealing sleeve 
should be adjustable to the varying transverse cross section of the body 
of thermoplastic adhesive admitted into the melting chamber. In the past, 
it has been usual to utilize elastic materials for the sealing sleeve, 
such as rubber-like silicon elastomers. When such sealing sleeves are new 
they afford a good sealing effect. Since the sealing sleeve leads into the 
melting chamber which is exposed to a relatively high temperature, the 
sealing sleeve is also heated to a high temperature during operation. Such 
heating, however, is detrimental to the elastic properties of the sealing 
sleeve. Accordingly, the sealing sleeve tends to become hardened and 
brittle after a short period of operation so that it can no longer 
effectively fulfill its intended use. Another disadvantage to the use of 
rubber-like materials for the sealing sleeve is that the thermoplastic 
adhesive tends to adhere to the sleeve material and the movement of the 
body of thermoplastic material into the melting chamber is impaired. 
To avoid the disadvantages experienced in the past, different materials 
have been used for the sealing sleeve. One particularly suitable material 
has been polytetrafluoroethylene (PTFE) known more familiarly by its 
tradename "TEFLON". Such material has a low heat conductive character and 
good sliding properties or, in other words, it has a low tendency to 
adhere to the solid thermoplastic material. A disadvantage of such 
material, however, involves the fact that its elasticity is too low. 
Therefore, it is the primary object of the present invention to provide a 
sealing sleeve formed of a material which is heat-resistant yet is 
sufficiently elastic. 
In accordance with the present invention, the sealing sleeve is formed of a 
temperature-resistant, poor heat-conducting material having good sliding 
properties. Further, the sleeve is provided with slots which extend over a 
portion of the axially extending length of the sleeve with the slots 
extending from the inlet end of the sleeve, that is, the end more remote 
from the inlet end, into the melting chamber. 
Because of the axially extending slots provided in the sealing sleeve in 
accordance with the present invention, at least the inlet portion of the 
sleeve is divided into individual, resilient lugs. These individual lugs 
which are resilient independently of one another, are capable of adjusting 
to the cross-sectional shape of the solid thermoplastic material entering 
the inlet end of the melting chamber. It is not unusual when the 
cross-sectional shape of the solid thermoplastic material is not round. 
The above-mentioned "TEFLON" (PTFE) appears to be especially suitable as 
the material used for the sealing sleeve. Other materials with an 
appropriate coating can also be used. The resilience of the individual 
lugs can be optimized by the thickness of the material of the sealing 
sleeve and also by the number and length of the individual longitudinal 
slots formed between the lugs. 
While certain materials are deformable, they are not sufficiently elastic. 
Therefore, in sealing sleeves formed of such material, it is advantageous 
if the individual lugs are pressed inwardly against the surface of the 
body of thermoplastic adhesive. This inward pressing action can be 
provided by a spring element encircling and pressing the sealing sleeve 
inwardly against the thermoplastic material. In one embodiment, the spring 
element can be formed as an annular spring, that is, a tension spring 
laterally encircling the sealing sleeve. One advantage of such an annular 
spring is its heat resistance. Another feature of such an arrangement is 
that the assembly and, if necessary, the disassembly of such a spring ring 
is particularly simple. It is also possible to use rubber rings, such as 
O-rings, as the spring elements with such rings being placed in an 
appropriate groove in the outer periphery of the sealing sleeve. Further, 
it is also possible to use compression springs located on the periphery of 
the sealing sleeve for pressing the individual lugs inwardly. 
By forming axially extending slots in the rearward portion of the sealing 
sleeves, openings are formed through which the liquefied thermoplastic 
adhesive can flow in the rearward direction. Accordingly, to prevent any 
rearward flow of the thermoplastic material and, at the same time, to 
assure the radial resilience of the individual lugs, it is advantageous if 
the spring element is in the form of a sheath-like member of an elastic 
rubber material. Since the sheath-like member does not contact the heated 
thermoplastic adhesive or only comes into contact with it to an 
insignificant extent, silicon rubber has proved to be useful in forming 
such sheath-like members. Silicon rubber has excellent elastic properties. 
In addition to silicon rubber, however, other types of rubber or elastic 
plastics materials can also be utilized. 
In still another embodiment, the spring element can be formed as an axially 
slotted bushing with the slots arranged in angularly offset relation to 
the slots in the sealing sleeve. Such a bushing can be formed of the same 
material as the sealing sleeve itself. Accordingly, contact of the two 
members does not create any problems. If necessary, for reasons of service 
life, the slotted bushing can be formed of glass fiber-reinforced 
thermoplastics material or of metal. The sealing sleeve separating the 
melting chamber and the spring element, prevents any excessive heating if 
the bushing is formed of metal. By angularly offsetting the axially 
extending slots in the sealing sleeve and the enclosing bushing, the slots 
do not open into one another, accordingly, the liquefied or melted 
thermoplastic adhesive cannot flow rearwardly out through the slots. Since 
the axially extending slots are covered, the slots can be made relatively 
wide so that no expensive special tools or methods are needed for forming 
them. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its use, reference 
should be had to the accompanying drawings and descriptive matter in which 
there are illustrated and described preferred embodiments of the invention 
.

DETAIL DESCRIPTION OF THE INVENTION 
The device for the melting and measured discharge of thermoplastic adhesive 
is shown in FIG. 1 and consists of a hand-gun shaped housing 1 including a 
dependently arranged handle 2. A trigger 3 is mounted in the handle and an 
electrical supply line 4 is connected into the bottom of the handle. A 
melting chamber 5 is located in the forward part of the housing 1, that is 
the left-hand part as viewed in FIG. 1 and the part of the chamber shown 
is illustrated in section. When the trigger 3 is squeezed, a body of solid 
thermoplastic adhesive 6 is advanced into the heating chamber, that is 
into the right-hand end of the melting chamber as viewed in FIG. 1 by 
means of a known transport mechanism, not shown. Within the melting 
chamber 5, the body of solid thermoplastic adhesive 6 is melted and 
converted into the liquid state. At its rear end, the melting chamber 5 is 
closed by the body of thermoplastic adhesive which is in the solid state. 
By advancing the body of adhesive 6 into the inlet end of the melting 
chamber 5, a portion of the adhesive previously introduced into the 
chamber and in the liquid state is displaced from the chamber and is 
discharged from its forward or left-hand end through a nozzle 7 in the 
housing 1. At the inlet end of the melting chamber 5, a sealing sleeve 8 
is positioned for preventing leakage losses out of the inlet end in the 
direction opposite to the normal flow of the thermoplastic adhesive 
through the melting chamber 5. The sealing sleeve 8 is formed of a 
temperature-resistant, poor heat-conducting material with good sliding 
properties. Good sliding properties means that the solid thermoplastic 
adhesive moves relative to the sealing sleeve without sticking to it. 
These characteristics of the sealing sleeve material can be provided, for 
example, by polytetrafluoroethylene (PTFE), commercially available under 
the name TEFLON. Such a material, however, is not sufficiently elastic for 
affording a good sealing effect, accordingly, the sealing sleeve 8 is 
provided with axially extending slots 8a extending from the rear or 
right-hand end of the sleeve toward its forward or left-hand end. Due to 
these axially extending slots 8a, the axially extending rear portion of 
the sealing sleeve 8 is divided into a number of individual, axially 
extending, resilient lugs or fingers. These axially extending slots 8a , 
however, give rise to the possibility that the liquid or melted 
thermoplastic adhesive within the melting chamber flows out through the 
sealing sleeve. To prevent such an occurrence, in accordance with the 
present invention, an axially extending sheath-like member 9 formed of a 
rubber-like material is slipped over the sealing sleeve 8. Accordingly, 
the sheath-like member 9 affords a closure for the radially outer sides of 
the axially extending slots 8a and, in addition, affords a clamping force 
biasing the individual lugs between the slots in the radially inward 
direction. Since the sheath-like member 9 does not directly contact the 
body of thermoplastic adhesive 6 or the melting chamber 5, it can be made 
of less heat-resistant materials, such as silicon rubber which has a high 
elasticity. Because the sheath-like member is slid directly over the 
sealing sleeve 8, replacement of the sheath-like member is particularly 
simple. 
In FIG. 2 another embodiment of the device, in accordance with the present 
invention, is illustrated. The melting chamber 5 and the sealing sleeve 8 
are formed in the same manner as in the embodiment of FIG. 1. Sealing 
sleeve 8 has axially extending slots 8a extending from the inlet or 
right-hand end of the sleeve toward the outlet or left-hand end into the 
melting chamber. Instead of a sheath-like member 9, a bushing 10 is 
slipped over the sealing sleeve 8 so that it laterally encircles the rear 
portion of the sleeve. Bushing 10 has a number of axially extending slots 
10a which extend from its rearward end for approximately the length of the 
slots 8a in the sealing sleeve. The slots 10a are offset in the 
circumferential direction relative to the axially extending slots 8a in 
the sealing sleeve 8. As a result, the resilient lugs or fingers located 
between the axially extending slots 8a and 10a cover the slots in the 
adjacent member. As a result, the thermoplastic adhesive 6 which has 
become melted in the melting chamber cannot flow out of the slots 8a in 
the sealing sleeve. The bushing 10 can be made of the same material as the 
sealing sleeve 8. Use of the same material for both parts prevents any 
problems caused by a reaction at the contacting surfaces of the two parts. 
When the lugs formed between the slots 10a have a sufficient initial 
stress, the bushing 10 is automatically held on the sealing sleeve 8. 
In the section through the sealing sleeve 8 and the bushing 10 shown in 
FIG. 3, it can be seen that the slots 10a of the bushing 10 are offset 
angularly relative to the axially extending slots 8a in the sealing sleeve 
8. This offset relation prevents the adhesive from flowing out through the 
slots 8a. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the inventive principles, it 
will be understood that the invention may be embodied otherwise without 
departing from such principles.