Process and installation for the manufacture of pencils

In the course of the process of manufacturing pencils the polymer binding material is at first melted in a lead extruder (19) in order to manufacture the lead. To the melted polymer material is then added via a loading extruder (2) a mixture of additives in powder form such as colors, gliding and adhesive substances. After intensive mixing of the components in the lead extruder (1) a lead profile (13) is extruded which is cooled in a first cooling range (3) to the point where, upon entering the coating nozzle (6), it is not fully plastic anymore but also not completely cold. Through the coating extruder (4) and (5) the appropriate materials are introduced for the formation of the pencil coating. In the course of the subsequent second cooling range (8) the pencil profile is cooled down completely and after passing the withdrawal station (11) it is cut in a sawing station (12).

FIELD OF THE INVENTION 
The invention concerns a process for the manufacture of pencils with a firm 
lead and at least one coating whereby Polymer is be used as a binding 
material and the lead material and the coating material are extruded. The 
invention also concerns an installation to implement this process with a 
lead extruder and at least one coating extruder connected with the coating 
nozzle. 
BACKGROUND OF THE INVENTION 
Several requirements are made of pencils, whereby the characteristics 
essentially depend on the quality of the lead. A pencil must be, among 
other things, easily sharpened, which depends, on one hand, on how hard 
the coating is and, on the other hand, because of the impacting torsion, 
on how brittle the lead is; it also depends on the adhesion between lead 
and coating material. Variations in the thickness of the lead also tend to 
be of a disadvantage. In addition, the lead in the coating material should 
not be subjected to varying tensions or show fractures, which can easily 
occur during the manufacturing process. 
Pencils tend to be made increasingly of synthetics, whereby profiles are 
being extruded; they are subsequently cut to the length of a pencil. As 
binding material in the material of the lead and the coating various 
polymers are used, preferably SAN or ABS Polymers, to which, in turn, 
appropriate additional substances such as color, gliding and adhesive 
substances and possibly filling substances can be added. 
Even though the characteristics of the pencils are basically determined by 
the materials used, it has become clear that the manufacturing process 
also has a significant influence on the characteristics of a pencil. 
The DE-OS 20 30 578 shows a device to manufacture and/or coat skein 
profiles, in particular for pencils. According to an operational formula 
an already pre-fabricated skein profile such as a wire, a rope, a tube or 
a similar device is entered in the skein compression nozzle, where the 
coating of the skein profile takes place. This process is not suited for 
the manufacture of pencils because pre-fabricated lead breaks very easily 
and is difficult to handle. A further disadvantage is that the 
pre-fabricated lead also breaks easily when it is put into contact with 
the hot coating material, because the differences in temperature between 
the lead and the coating material inevitably cause tension in the lead 
material. 
DE-OS 20 30 578 also suggests that, immediately prior to the coating, the 
skein profile, which is to be coated, should be extruded. The nozzle is 
formed in a manner that it serves simultaneously as a feed into a second 
skein compression nozzle. This process, however, has the disadvantage that 
the lead is still plastic when the coating is being extruded on to it. If 
the compression of the coating material is very strong, a deformation of 
the lead occurs. This can lead not only to variations in thickness, but 
also to a complete failure to produce the lead. 
The DE-PS 38 27 968 shows a process where all lead components are at first 
ground to the size of grains of corn and subsequently mixed in a 
turbomixer. In the turbomixer, a set temperature control is required 
because the mixture has to be regranulated before it reaches the extruder. 
In the lead extruder, the mixture goes through a filling zone, a passage 
zone, a compression zone and a extruder-head zone, whereby the temperature 
increases up to the extrusion zone and then decreases somewhat at the 
extruder head. This process is complicated and not very flexible because 
of the preparation of the lead components when, for example, the hardness 
of the lead is to be changed. 
The DE-AS 22 03 635 shows a pencil with synthetic coating. The manufacture 
of the lead occurs in two operational stages with the interposition of a 
drying stage. In the first stage, for example, a mixer thoroughly mixes 
ABS-resin powder, graphite powder and calcium stearate. After drying, the 
mixture is extruded at about 170 degree Celsius with the use of an 
extruder and then, while still hot, is cut to create the materials for the 
pencil core. The coating is done the same way as the coating of wires. 
Wires, however, are pre-fabricated and are rolled off a spool and then led 
to an extruder. This means that the wire, which in this case would 
correspond to the lead, is totally cooled off and consequently fully 
hardened. It is also mentioned that after the extrusion the lead is 
introduced continuously in a crosshead nozzle for coating purposes. 
Because the lead is entered into the coating nozzle right away, it is 
still plastic and can be shaped. 
The DE-AS 28 03 206 shows a foaming mixture, particularly for the coating 
of the lead. The mixture comes from a skein press with a transverse nozzle 
in which simultaneously and continuously the synthetic lead is measured 
and pressed out through another skein press. The emerging six-edged 
profile is calibrated and subsequently cut at the appropriate classic 
pencil length. The surface of the pencils manufactured in this manner can 
be improved with a layer of varnish or a triple nozzle is used and the 
surface is covered with a thin decorative layer made of stiff synthetics. 
In the course of this well-known process the plastic lead material is 
coated, which results in considerable disadvantages because of variations 
in thickness. 
SUMMARY OF THE INVENTION 
The task of the invention is therefore a process and an installation which 
permits a flexible manufacture of pencils of different levels of hardness 
with less work, whereby the manufactured pencils should show a better 
quality: namely, the lead should not break easily and the pencil should be 
easily sharpened. This task is accomplished in a process according to the 
present invention. 
The invention derives from the recognition that a greater flexibility will 
be achieved when the polymer binding material and the other components of 
the lead material are prepared and introduced into the extruder 
separately. The hardness of the lead depends on the additional substances, 
such as colors, gliding and adhesive materials as well as suitable filling 
materials. They are materials in powder form which are mixed and prepared 
independent of the polymer binding materials. If the hardness of the lead 
or maybe the color should be changed, the mixture of the additives can be 
changed independently of the utilized polymer binding material. Several 
mixtures can be prepared as well, which can be added to the already melted 
polymer binding materials according to preference. 
Because the polymer binding material is already in a melted state when the 
powder additives are introduced, a relatively fast mixing of all 
components occurs. After the mixing, the lead material is extruded in the 
form of a lead profile which is subsequently cooled off. It is important 
that the cooling does not go beyond the point where the lead profile is 
not plastic anymore. This provides the advantage that the lead profile 
cannot be shaped upon extrusion of the coating and no variations in the 
thickness can occur. Also, the temperature is still so high that the 
difference to the temperature of the extruded coating materials cannot 
cause any tension in the lead profile. Particularly, when the same 
polymers or similar polymers are used as binding materials for the lead 
material as well as the coating material, whereby the polymers show 
approximately the same melting range, the difference in temperature 
amounts to only a few degrees Celsius or multiple tens of degrees Celsius, 
so that the advantages of a lead that is not plastic anymore can be 
combined with the advantages of a lead at a higher temperature, only after 
coating and completion of the pencil profile does the pencil profile 
completely cool off. 
The melting of the binding material and the mixing of the additives in 
powder form take place in an extruder which is described within the 
context of the installation of the manufacture of pencils. 
The temperature of the melted polymer substance is increased continually 
until the mixture in powder form is added, because a low viscosity of the 
melted polymer substance facilitates the mixing of the additives in powder 
form. After adding the additive, the temperature is somewhat lowered and 
subsequently raised to the level of the extrusion temperature, whereby the 
lowering of the temperature goes only to the point at which the binding 
material remains melted. The temperature of the melted polymer substance 
may not be raised to the level that would result in a depolymerization. It 
has been shown to be an advantage when the mixture of the lead materials, 
after introduction of the additives, undergoes not only at least one 
mixing process but at least also one kneading cycle. It is preferable that 
the mixture be subjected to two or three consecutive mixing and kneading 
cycles, whereby the subsequent mixing and kneading cycle takes place at 
increasingly higher temperatures. 
To avoid bubbles in the lead, which would reduce the stability of the lead, 
a degasification takes place in the course of the introduction of the 
additives. Preferably after the last mixing and kneading cycle or during 
the last mixing and kneading cycle, an degasification of the lead 
materials should take place. 
Depending on the desired pencil type and the required colorization, two or 
more coatings can be added to the lead profile. Preferably two coatings 
are extruded simultaneously onto the lead profile whereby the outer 
coating is formed as a thin colored layer. 
The pencil profile is to be calibrated immediately after the extrusion of 
the coating materials. 
The installation for this process consists of a lead extruder with a first 
and a second loading station. At least one melting and transportation 
range exists between the first and the second loading station, and at 
least one mixing and kneading range exists behind the second loading 
station. Between the lead extruder and the coating nozzle a first cooling 
range and behind the coating nozzle a second cooling range is planned. 
The lead extruder is of modular construction so that the individual 
melting, transportation, mixing and kneading ranges can be matched with 
the respective materials. 
The second loading station is connected to a load extruder which, in turn, 
is fed by a dosage installation, preferably a graphimetric dosage 
installation. Through the second loading installation the additives in 
powder form are introduced to the polymer melting material which is 
located in the extruder. The temperature range in the one or more heating 
and transportation ranges between the first lading station and the second 
loading station was chosen so that the temperature continues to increase 
between the first loading station and the second loading station and so 
that a melting process with the desired viscosity takes place within the 
area of the second loading station. Preferably, a degasification station 
is part of the second loading station in order to remove, at an early 
stage of the preparation of the lead material, the gases which could lead 
to bubbles in the subsequently manufactured leads. In order to be certain 
that the all the gases in the lead material can escape, a second 
degasification station is foreseen for the area of the last mixing and 
kneading range. 
The cooling range can contain a variety of cooling temperatures. In the 
area of the cooling range also installations to lead the extruded lead 
profiles can be projected. Preferably, the cooling range remains simply a 
free space between the lead extruder and the coating nozzle. In this free 
space the extruded, free-hanging lead profile is being led to the nozzle, 
whereby it suffices if the air in this open space is at room temperature. 
Additional installations are not necessary if the length of the first 
cooling range is set in accordance with the withdrawal speed of the lead 
profile so that the desired temperature reduction of the lead profile can 
be achieved. 
Behind the coating nozzle a calibrating station exists, whereby preferably 
the calibrating station forms at least a part of the second cooling range.

DETAILED DESCRIPTION 
FIG. 1 shows the installation of the pencils from above. At the side of the 
lead extruder 1 a load extruder 2 for the transport of the additives is 
flanged for the purpose of the melting of the polymer. The lead profile 13 
that is extruded from the lead extruder runs through a cooling range 3 and 
enters the coating nozzle 6 where a first and a second coating extruder 
4,5 are connected. The coating extruder 4 leads the material for the lead 
coating to the coating nozzle 6. Simultaneously, the second coating 
extruder 5 introduces material for the application of an outer layer, 
preferably a colored layer, to the coating nozzle 6. Both coating 
materials are simultaneously extruded in the nozzle 6 onto the lead 
profile 13. The extrusion happens so that the pencil profile 14 is 
extruded immediately behind the coating nozzle 6 into a calibrating 
station 7, which forms a part of the second cooling range 8. The second 
cooling range 8 consists of two cooling vats 9 and 10 (shown in the 
enclosed reproduction), which are filled with a cooling liquid, for 
example with water. At the end of the second cooling vat 10 a water wiper 
is located and the pencil profile 14 arrives at a withdrawal station and 
after that at a sawing station 12, where a saw is set up to separate the 
individual pencils of the pencil profile 14. The lead extruder 1 as well 
as the coating extruder 4,5 and the withdrawal station 11 are connected 
with a joint switch station 30. 
FIG. 2 shows the frontal part of the installation from the side. The first 
cooling range 3 consists of an open space between the lead extruder 1 and 
the coating nozzle 6, where the extruded lead profile 13 is conducted 
freely to the coating nozzle 6. The extruded lead profile 13 shows such a 
great stability that it does not tear in the first cooling range 3. The 
first cooling range permits visual control of the extruded lead profile 
13. At the end of the first cooling range 3, the lead profile 13 has 
cooled off to a point that it is not plastic anymore when it is introduced 
to the coating nozzle. If the length of the first cooling range is 
properly adjusted to the withdrawal speed of the lead profile 13, it 
suffices also if the surrounding air in this open space of the cooling 
range 3 is at room temperature. 
FIG. 3 shows the lead extruder in a schematic manner. This lead extruder 1 
is a double worm extruder with various range sections and stations which 
can be put together in a modular manner. Next to the propulsion 17 is the 
first loading station 18 through which the polymer binding material is 
introduced to the lead extruder 1 via a first dosage installation 15, 
which can be a gravimetric dosage installation. In the area of the first 
loading station 18, a first melting and transportation range 20 is planned 
which heats the approaching polymer material beyond the melting 
temperature and transports it to a second melting and transportation range 
21, where a further melting of the polymer material takes place. 
Subsequently, the polymer material, heated to an high temperature, arrives 
at the second loading station 19 where the additives, such as graphite, 
adhesive and gliding substances as well as possible filling substances are 
introduced as a powder mixture from a gravimetric second dosage 
installation 16. In the area of the second loading station 19 also a 
heating and transportation range is planned, equipped with a first 
degasification station 22. In addition to the second loading station 19, a 
total of three mixing and kneading ranges 24, 25 and 26 exist whereby the 
third mixing and kneading range shows a second degasification station 23. 
In addition to the second degasification station 23, a transportation 
range 27 as well as the compression range 28 exist, which terminates at 
the extruder head 29 out of which the lead profile 13 is extruded. 
This installation can manufacture pencils which, for example, contain 
Polystyrol as binding material. The Polystyrol, which represents about 10 
to 40% of the weight of the lead material, is introduced in granular form 
to the first loading station where it is heated to approximately 240 
degrees Celsius on the way to the second loading station. The second 
loading station introduces a mixture of graphite in powder form, which 
represents 50 to 70% of the weight of the lead material, as well as 
stearic acid and calcium stearate with shares of 2 to 5% of the weight 
resp. 2 to 10% of the weight. The temperature in the subsequent mixing and 
kneading ranges 24 to 26 amounts to 190 to 200 degrees Celsius. This 
temperature is maintained in the area of the transportation range 27 as 
well the compression range 28 and the lead material on the extruder head 
29 is extruded at about this temperature. In the area of the first cooling 
range 3 the cooling down to about 190 degrees Celsius takes place, a 
temperature that permits a hardening of the lead material to a point where 
it cannot be shaped upon its introduction into the coating nozzle 6. 
Polystyrol can be used as coating material as well as binding material and 
represents a share of 60 to 70 percent of the weight. The coating material 
can continue to show LDPE filled with talcum as well as color batches and 
propellant of up to 30 to 40 percent of the weight. The outer layer, which 
is also extruded onto the pencil in the coating nozzle 6 can also show 
Polystyrol which contains a share of more than 50 percent of the weight. 
In addition, the material for the outer layer contains color batches so 
that the pencil can be finalized in color. 
The process evolves in a manner that the extrusion of the coating comes 
first and than the lead extrusion takes place. The beginning of the lead 
profile is introduced in the coating nozzle and grabbed and pulled along 
by the extruded coating material. 
A perfect example of the temperature ranges for the lead materials which 
show Polystyrol as binding material can be seen in the following table. 
TABLE 
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Centigrades 
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1. Melting and 140-180 
Transportation range (20) 
2. Melting and 160-220 
Transportation range (21) 
2. Loading station (19) 
140-240 
1. mixing and Kneading 
150-190 
range (24) 
2. Mixing and Kneading 
150-200 
range (25) 
3. Mixing and Kneading 
160-200 
range (26) 
Transportation range (27) 
150-200 
Compression range (28) 
150-200 
End of the 140-190 
1st Cooling range (3) 
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Reference List: 
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1 Lead Extruder 
2 Coating Extruder 
3 First Cooling Range 
4 First Coating Extruder 
5 Second Coating Extruder 
6 Coating Nozzle 
7 Calibrating Station 
8 Second Cooling Range 
9 First Cooling Vat 
10 Second Cooling Vat 
11 Withdrawal Station 
12 Sawing Station 
13 Lead Profile 
14 Pencil Profile 
15 First Dosage Installation 
16 Second Dosage Installation 
17 Propellant 
18 First Loading Station 
19 Second Loading Station 
20 First Melting and Transportation Range 
21 Second Melting and Transportation Range 
22 First Degasification Station 
23 Second Degasification Station 
24 First Mixing and Kneading Range 
25 Second Mixing and Kneading Range 
26 Third Mixing and Kneading Range 
27 Transportation Range 
28 Compression Range 
29 Extruder Head 
30 Switching Installation 
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