Control of plastic injection

Apparatus for injecting thermoplastic material including means to form a tubular stream of plastic with a central stream containing a foaming agent.

BACKGROUND OF THE INVENTION 
In the fabrication from thermoplastic materials of massive, thick-walled 
molded parts with a smooth surface, it is known, in order to prevent of 
cave-in spots, which occur during cooling of an injected compact plastic 
mass, to provide the inside of the molded part with a plastic mass, 
containing a foaming agent. By means of this compact mass, the outer 
skinforming plastic mass is pressed constantly against the inside surface 
of the mold during the cooling procedure. 
It is known, for example, from German patent No. 1,778,457 that one can 
inject into a mold, a charge of non-foaming mass which does not fill the 
mold and which is a compact thermoplastic mass; then, before hardening of 
the center, one can inject a second charge of thermoplastic containing the 
foaming agent into the center of the first charge. In that way, the mass 
of the second charge (containing the foaming agent) presses equally on all 
sides towards the inside of the mold. Thereafter, a small amount of 
plastic mass may be injected into the mold. 
By extending this procedure, it has been made known through German petty 
patent No. 2 241 002 to inject simultaneously a charge with and without 
the mass containing the foaming agent after the injection of the first 
charge to form a smooth surface for the molded part. The plastic mass 
containing the foaming agent flows into the mold through a central jet, 
and the compact material forming the smooth outer surface enters the mold 
through a ring jet surrounding the center jet. 
The apparatus for executing this procedure is normally equipped with two 
separate extruders. In one of the extruders prepares the plastic 
containing the foaming agent and in the other extruder is prepared the 
dense plastic mass. The liquid forming the gaseous foaming agent is 
already mixed into the plastic mass within the extruder. The use of two 
extruders makes the apparatus quite complicated and expensive. 
When a dense plastic mass and a mass containing the foaming agent is 
injected into the mold one after the other, then it is possible to use 
only the extruder which delivers dense plastic mass. Shortly after the 
start of the injection procedure, a predetermined amount of dense plastic 
is already within the mold cavity. The foaming agent is added to the 
stream of compact flowing mass in the portion between the extruder and the 
mold cavity. The disadvantage exists, however, that the total 
cross-section of the flow canal is filled with the plastic mass containing 
the foaming agent and, during flushing with compact plastic, the plastic 
containing foaming agent is not completely removed. This will influence 
the surface quality of the next molded part. In addition, a thorough 
mixing of the compact plastic mass with the foaming agent becomes 
difficult and an exact metering of the plastic mass containing the foaming 
agent is not possible. 
It is, therefore, an outstanding object of the invention to provide 
apparatus for the discontinuous fabrication of the molded parts made from 
thermoplastics, which parts have a smooth surface and a porous core, 
whereby the use of only one extruder allows the simultaneous injection of 
dense plastic and plastic containing forming agent, thus permitting a good 
mixing as well as exact metering of the plastic containing the foaming 
agent. 
With this and other objects in view, as will be apparent to those skilled 
in the art, the invention resides in the combination of parts set forth in 
the specification and covered by the claims appended hereto. 
SUMMARY OF THE INVENTION 
In general, the apparatus constructed in accordance with the present 
invention makes it possible to sub-divide a compact plastic stream coming 
from the extruder, without special redirection into a core stream and at 
least into one tubular stream, then to mix one inner stream with a foaming 
agent and, thereafter, to inject these streams simultaneously into the 
mold. With this method there is always a stream carrying foaming agent 
surrounded by a dense plastic stream. According to demand, it is now 
possible before and/or after the simultaneous injection phase to guide 
only a small amount of the dense plastic mass into the molding tool. 
Through this procedure, molded parts which have a satisfactory smooth 
outer surface and a porous inside core are produced by smallest machine 
technical usage.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows apparatus for the discontinuous fabrication of molded parts 
made from thermoplastic plastic, each part having a smooth outer surface 
and a porous core. The apparatus consists mainly of a plastification unit 
1, an intermediate storage container 2 and an injection head 3 attachable 
to the mold not shown. The plastification unit 1 is shown in this example 
as a conveyor extruder, in which raw thermoplastic material, coming from 
the hopper 4 as granulers, transferred into the plastic state and is 
guided through the conduit 5 to the intermediate storage container 2. The 
intermediate storage container 2 normally consists of a piston-cylinder 
unit wherein the push-out piston 6 is retracted by the pressure of the 
entering plastic mass. As soon as a predetermined amount of the 
plasticized plastic mass (which amount is adjustable in a known manner by 
means of the piston rod 7 of the intermediate storage 2 control system) is 
inside the intermediate storage container 2, a displacement drive 8 
connected to the piston rod 7 is switched on and guides the plasticized 
mass to the injection head 3. The injection head 3 may be connected 
directly to the intermediate storage container 2 or (as indicated in the 
drawings) connected to it through a conduit 9. The injection head consists 
largely of a cylindrical housing 10 which is equipped with a jet tip on 
the end opposite the attachment to the intermediate storage 2. The jet tip 
is rounded at the exit opening 11. The housing 10 has a central bore 12 
which is cylindrical for most of its length and on its jet tip end is 
reduced at the exit opening 11. Inside the cylindrical bore 12 is arranged 
a tubular shaft 13 whose outer diameter is smaller than the diameter of 
the bore 12, so that a canal 14 is created between the housing 10 and the 
shaft 13, the canal being of ring-shaped cross-section. The shaft 13 is 
maintained inside the bore 12 by at least one web 15. The shaft 13 has a 
cylindrical bore 16 through most of its length, which serves to carry a 
closure pin 17. The closure pin 17 is displacable axially within the bore 
16 of shaft 13. For this purpose, an opening 18 is formed in the housing 
10 that continues through the web 15 and the shaft 13 and is lined up with 
a linkage lever 19 which extends through it. This lever 19 is supported on 
an axis 20 fixedly carried outside of the housing 10 and the lever is 
connected at the other end to a controllable adjustable drive 21, such as 
a piston-cylinder unit. 
The closure pin 17 has a tip 22 on the end facing the exit opening 11 which 
tip is guided in a reduced exit opening 23 of the shaft 13. In the 
position shown in FIG. 1, (the so-called "closure" position of the pin 17) 
the pin extends with its tip 22 over the shaft, so that the connection 
between the canal 14 and the exit opening 11 is interrupted. Furthermore, 
the closure pin 17 contains an axial canal 24 which ends some distance in 
front of the two ends of closure pin 17. On the other end of the canal 24 
facing the exit opening 11 are arranged several sloped overflow canals 25a 
which end (as shown in the "closure" position of the pin 17 in FIG. 1) on 
the wall of the exit opening 23 and is only free of the wall in the 
postion of the closure pin shown in FIG. 3. Similar overflow canals 25b 
are also located at the other end of canal 24, but have a slope opposite 
to that of canals 25a. The overflow canals 25b end (in the closure 
position of the pin 17) at the inside wall of the shaft 13. At this end, 
the closure pin 17 has two circumferential extrusions which engage 
corresponding expansions 28, 29 of the shaft 13, as shown in FIG. 1. In 
the vicinity of the expansion 28, an annular canal 30 is formed in the 
wall of the shaft 13 and is connected to a canal 31. The canal 31 ends in 
the opening 18 and is connected there with a line 32 which again is 
connected to a metering device (not shown here) for liquid and gaseous 
foaming agent. 
In the operation of the described apparatus, let us assume that the closure 
pin 17 is positioned as shown in FIG. 1 (based on a previous operating 
cycle). The canal 14 is filled with the dense plastic mass and the canal 
24 is filled with the plastic mass containing the foaming agent. As soon 
as the intermediate storage container 2 is filled with the required amount 
of plasticized plastic mass from the extruder for one filling of the mold, 
the adjusting drive 21 is placed under pressure before energizing the 
displacement drive 8 and the closure pin 17 is moved into the position 
shown in FIG. 2. With this position of the closure pin 17, the connection 
between the canal 14 and the exit 11 is opened, so that the dense plastic 
may be transported from the intermediate storage container 2 into the 
mold, not shown. In this position of the closure pin 17, the overflow 
canals 25a reach into a still-closed annular space 23 between the closure 
pin 17 and the shaft 13. At the same time, a connection is created through 
the overflow canals 25b from canal 24 to the ring-shaped expansion 28. 
This position of the closure pin 17 is maintained for only a short time, 
and only in case a determined compact mass is to be pre-injected into the 
molding tool. In the position of the closure pin 17 shown in FIG. 3, 
achieved by the adjusting drive 21, the annular space 33 towards exit 
opening 11 is opened (on the one hand) and (on the other hand) the 
extension 27 outside of the shaft 13 is located in such a way that the 
compact plastic mass can now reach into the centrally-located canal 24 of 
the closure pin 17 through the expansion 28, 29 and the overflow canals 
25b. As soon as this flow connection is made, foaming agent will be 
introduced into the flowing compact plastic mass by means of the line 32, 
the canal 31, and the annular canal 30. To achieve a good mixing of the 
foaming agent and the plastic mass, several commonly-known mixing plates 
34 are arranged within the canal 24. The plates are arranged spirally and 
are displaced by 90.degree. relative to each other. By means of the 
overflow canals 25a, the plastic mass containing the foaming agent now 
leaves the canal 24 and is surrounded in the area of the exit opening 11 
by a tubular dense plastic flow. 
As soon as sufficient amount of plastic containing foaming agent is inside 
the mold, the closure pin 17 is brought back into the position shown in 
FIG. 2 and the transmission of the plastic containing the foaming agent is 
ended. Now only the dense plastic is flowing into the mold until the 
intermediate storage container 2 is completely emptied. With the complete 
emptying of the intermediate storage container 2, the closure pin 17 is 
retracted to its starting position, as shown in FIG. 1. Thereafter, the 
described filling operation of the mold is repeated. 
By varying the opening stroke of the closure pin 17, the relationship of 
the dense plastic mass to the plastic mass containing the foaming agent 
may be changed without difficulty. In a variation of this design example, 
it is possible to displace the closure pin 17 during the start of mold 
filling operation immediately from the position of FIG. 1 to the position 
of FIG. 2; that is to say, no dense mass is pre-injected into the mold 
tool, but dense and plastic mass containing foaming agent flows 
simultaneously into the mold. Also, during the end of the mold filling 
operation, the closure pin 17 may be brought immediately from the position 
of FIG. 3 into the position of FIG. 1. Thereafter, no dense plastic mass 
may enter the mold, but only the so-called end mass. 
The injection head shown in FIG. 4 has also a housing 35 in which a shaft 
is kept in place by at least one web 36 in such a way that a canal 38 of 
ring-shaped cross-sectional area is created between the shaft 37 and the 
housing 35. The shaft also has a cylindrical bore 39 extending through the 
larger part of its length, which bore incorporates a closure pin 40. In 
contrast to the previously-described design, the closure pin does not 
completely fill the bore 39. The closure pin 40 acts in relationship to 
the bore 39 to obstruct only the inlet and outlet. In addition, the 
closure pin 40 also closes the exit opening 41 at the jet tip. Extending 
over a large area between the closure pin 40 and shaft 37, an annular 
space 42 is formed in this design example in which mixing plates are 
arranged. In this jet head 3a, by complete opening of closure pin 40, the 
dense plastic mass also flows into the annular space 42, where the plastic 
mass will be mixed with the foaming agent in the manner already explained. 
FIGS. 5 to 7 show another form of the injection head 3b, which also may be 
connected to the intermediate storage 2 through a conduit, as shown in 
FIG. 1. This injection head 3b has a housing 44 with a shaft 45, which in 
this design example is fabricated in one piece along with a connector web 
46. The outlet opening 47 is formed in a jet tip 48 which is mounted on 
the housing 44 and may be fastened with bolts, for example. Into the shaft 
45 is arranged a displaceable closure pin 49, which is generally similar 
to the closure pin 17 of FIGS. 1 to 3. In this case also, overflow canals 
50a and 50b are formed on both ends. In contrast to the design of the 
closure pin 17, the closure pin 49 has a bore 51 extending all the way 
through into which is inserted a sleeve 52. The sleeve 52 is closed on the 
end facing the outlet opening and overflow canals 53 arranged in this 
area, which canals are closed by the inside wall of the closure pin 49, as 
shown in FIG. 5. The sleeve 52 is fixedly mounted within the housing 44 
and is connected to the housing 44 by webs which are not shown. The 
postion of the sleeve 52 within housing 44 may be displaced in the axial 
direction, depending on the plastic material to influence the division of 
the plastic mass which is introduced. 
The description of operation of the apparatus shown in FIGS. 5 to 7 is 
based upon the fact that the closure pin 49 takes the position shown in 
FIG. 5. From a previous cycle, the annular space 54 surrounding the shaft 
45, as well as the bore 55 within the sleeve 52 is filled with the dense 
plastic mass. The annular space 56 within the closure pin 49 is filled 
with plastic mass containing the foaming agent. To introduce the injection 
operation, the closure pin 49 is pushed into the postion shown in FIG. 6, 
so that the compact mass now reaches the outlet opening 47 through the 
annular space 54, through the bore 55 within the sleeve 52, and through 
the overflow canals 53. This position of the closure pin 49 is selected in 
such a way that the dense plastic is pre-injected into the mold. 
Additionally, the enlargement 58 is connected with the overflow canals 
50b. 
In the position of the closure pin 49 shown in FIG. 7, the dense plastic 
may also reach the annular space 56. To this plastic mass, foaming agent 
is added as previously explained. In this annular space 56 mixing elements 
are also arranged. The plastic mass containing foaming agent may now reach 
the outlet opening 47 through the overflow canals 50a, as well as the 
ring-shaped recess 57 in the shaft 45. The plastic stream flowing through 
the outlet opening 47 now inherits a core and a outer ring from the dense 
plastic mass, as well as an intermediate ring of plastic mass containing 
foaming agent. Shortly before the end of the injection operation, the 
metering pin 49 may briefly take the position of FIG. 6, so that a small 
amount of dense plastic may be injected thereafter. 
In a variation of this design, it is possible to push the closure pin 49 
immediately from the postion of FIG. 5 to the position of FIG. 7. 
The design of FIGS. 5 to 7 indicates how the housing 44 and the injection 
tip 48 can be formed in two parts. A corresponding, but also simpler 
solution for the fabrication is indicated in FIGS. 1 to 4. 
In spite of the fact that the procedure of this invention has been 
explained in connection with the fabrication of molded parts by 
discontinous operation, it is also possible to operate continuously. The 
inside design of the injection heads may then be simplified. 
It is obvious that minor changes may be made in the form and construction 
of the invention without departing from the material spirit thereof. It is 
not, however, desired to confine the invention to the exact form herein 
shown and described, but it is desired to include all such as properly 
come within the scope claimed.