DE2406569B2 - METHOD FOR EXTRUDING OR INJECTION MOLDING THERMOPLASTIC PLASTICS AND DEVICE FOR CARRYING OUT THE METHOD - Google Patents

Description

The invention relates to a method for extrusion or injection molding of thermoplastics Plastics in which the plastic passes through at least one elongated melting zone external heat supply and frictional heat brought into the molten state and then accordingly the progressive melting under shear stress in a radial to the melting zone offset axially parallel discharge zone is pressed, the plastic at the transition from each melting zone is degassed in the discharge zone.

The invention also relates to an apparatus for carrying out such a method an extruder housing and at least two pistons or rotatable pistons arranged in separate spaces Screws, one of which is used as a discharge area and at least one as a melting area.

A method and an apparatus of this type are known (US Pat. No. 2,968,836). In this well-known In the case, the melting space and discharge space merge into one another on an elongated section, what was achieved in that at least two screws axially and radially offset from one another are provided are, of which one of the other hands over the material essentially over the cross-sectional width.

Since the entire mass to be melted is not suddenly and at the same time in the desired plastic State, taking into account the trouble-free transition from the melting room to the '' Discharge space melted and still granular or not completely melted material together in promoted the discharge space. Many gas inclusions remain in the material, so that none are sufficient Degassing is possible.

According to another known case, a single screw is provided in the melting chamber, which the material to be melted through nozzles at the end of the screw cylinder into a coaxial discharge space presses. In this case, the entire material becomes the exit end over the entire length of the melting chamber promoted at the end of the cylinder. Since on the way through the melting room, not a sudden, but a gradual melting of the material takes place, is melted and not yet melted Well pressed together to the end of the cylinder. As a result, from a certain zone to the molten good is unnecessarily heated and treated, while on the other hand the molten Gut still contains inclusions of unmelted or insufficiently plasticized Guts, whereby the melted Well, unnecessarily further heated and possibly overheated, while the not yet melted Well, because of poor heat transfer, it is only melted slowly further.

The object of the invention is to provide a method and a device for carrying out the method in accordance with the type described at the outset, in which only homogeneous material enters the discharge space is promoted, whereby a favorable degassing possibility is obtained.

This object is achieved in terms of the method according to the invention in that the Plastic with a strong constriction of each melting zone along an elongated transition section is pressed into the discharge zone. The constriction makes the material strong Subjected to shear stress, with essentially only the already melted, homogeneous Material passes from the melting zone into the discharge zone. It becomes the melted one Good immediately after reaching the sufficiently viscous state separated from the not yet melted good, d. H. there is a gradual transition corresponding to the progressive melting. With That is what is included in consideration of the relaxation of the material when it enters the discharge zone Gas free so that it can be effectively withdrawn.

The method according to the invention can be used in a device of the type described in the introduction the way to realize that the transition from each melting chamber to the discharge chamber by axially extending Slots is formed through which the material is forced and thereby a shear stress is subjected.

A device is known (DT-PS 1207 074), in which the melted material immediately after reaching the sufficiently viscous state of the still not melted material is separated. In this known case, a single screw is provided for this purpose, which is partially single-threaded, partially double-threaded, so that the melting chamber and discharge chamber are nested in one another are, with the

the first becomes steadily smaller and the second steadily larger. In this embodiment, it is not possible that from Melting chamber to degas the material passing over into the discharge chamber immediately when it passes over.

It is particularly advantageous if the melting zone is concentric to the discharge zone or if the melting zones are concentric Circle around the longitudinal axis of the discharge zone, as this ensures an even supply of material around the discharge zone and thus an even filling and a better throughput is possible.

It is known to arrange the melting zone and discharge zone concentrically to one another (US Pat 3289250).

The invention is illustrated below with reference to schematic drawings of an exemplary embodiment explained in more detail.

Fig. 1 is a horizontal sectional view taken along the line YY in Fig. 2A;

Fig. 2 is a vertical sectional view taken along line XX in Fig. 2A;

Figs. 2A and 2B are vertical sectional views taken along lines AA and BB, respectively, in Fig. 2;

FIG. 3 shows, in an enlarged illustration, a detail from FIG. 1; FIG.

FIG. 4 shows a section corresponding to FIG. 1 in a highly schematic form.

In Fig. 1, the right outlined part labeled MA represents a drive of an extruder E for thermoplastics. The extruder housing C is denoted, which has a central discharge space SAi with rotatable screw OE and several on a concentric circle 15 around the longitudinal axis of the discharge chamber has evenly distributed melting chambers SAe , each of which is penetrated by its own screw OCF ₁ , OCF ₂ , OCF ₃ , OCF ₁₁ , OCF ₅ , OCF _b . In simplified terms, it is assumed that all the melting spaces form a common concentric melting space around the discharge space, which are defined by an outer wall Ce and an inner wall Ci .

Discharge space and melting space run parallel, with the melting space ending in front of the discharge space, i.e. not extending over the entire length of the extruder E , which - apart from the longitudinal extension of the drive MA - extends from the inlet end 11 for the mass fed in from the funnel T to the outlet end 12 for the extruded mass is sufficient. The melting area SAe goes up to position 14.

Any number of extrusion screws can be present in the discharge space SAi. In the embodiment according to FIGS. 2A and 2B, however, only a single screw OE is provided, which preferably extends along the entire length of the extruder housing C. Melting space and discharge space are connected to one another in the end area of the melting space over a longer section 17, 18 through narrow slots PF .

It was found that after the plastic to be processed has been filled in at the feed end BC and the extruder housing C is appropriately heated, a concentrically increasing melting takes place along the melting chamber SAe and a concentric transfer along this section indicated at 17, 18 in the inner wall Ci of the housing is possible is. provided that the slots are so that the plastic can pass over as soon as it has reached the correct melted state. This is why the arrangement and dimensions of the braces PF are particularly important. They are provided in the area of the screws OCF ′ and the beginning of the screw OE and must have the length of the section 17, 18 so that the plastic can be transferred from the melting space SAe to the discharge space SAi as soon as it is melted. The breadth and the depth

■ "the slot PF must also have such dimensions that a considerable shear stress is exerted on the melted plastic mass, which passes to the screw OE , which the plastic to the extrusion mouthpiece at the outlet

! '■ brings to the end of 12, where it is carried out without parts that have not been completely melted. The longitudinal extent of the slots PF, ie the section between 17 and 18, is between 5% and 60%, preferably between 15% and 35% of the total

- ¹ Ii length of the snail OCF. The circumferential opening of the slots is within a small percentage range in relation to the entire circumferential extent of the inner wall Ci.

Preferably the end of all screws is OCF

Ji provided with a torpedo TOCF . These only have the task of the melted plastic located at the ends of the screws OCF via the channels 20 and 20 '(worked out in the knee ME of the wall Ce ) to the discharge space SAi and therefore to the

to pass into the second section of the screw Oe.

As noted above, MA indicates schematically the drive of the conveyor and serving as Aufschmelzorgane screw OCF _{1 6} and as

i'i extrusion organs serving screw OE. a shaft 31 driven by a motor (not shown) drives a gear wheel 32, which in turn moves a gear wheel 33 which operates the wheels 34 and 34 ', each of which is associated with a worm OCF. In

mi Fig. 1 drive the two wheels 34 and 34 'which the screw ₅ and OCF ₂ OCF adjunctive shafts 35 and 36.

Each of these shafts is supported on thrust bearings 37. A shaft 40 is driven by a motor, not shown

-n (which may, however, be the same as that which drives the shaft 31) moves and drives the shaft 41 which is connected to the central worm OE . The shaft 40 is supported on thrust bearings 42.

At this point it should be emphasized that the z. B.

> (i could be missing the part of the screw OE lying between points 50 and 51. However, it is advantageous to have a screw OE available that extends from the outlet end 12 to the inlet end 11 so as to cover the part between the points 50- and

- ,; 51 to be used essentially for the transmission of motion (in the section between points 50, 51 the screw can easily be missing the thread).

Fig. 4 serves to illustrate the movement

wi path of the plastic coming from the funnel T. From the entrance of the funnel T , the plastic penetrates via the feed end BC only into the anteroom SA of the extruder housing C , as indicated by the arrows / ('.

hi The screws OCF protruding into the anteroom SA cause the plastic to advance, which is caused by the heating of the extruder housing C and the pressure exerted by the screws OCF.

gradually melts. At 18, the slots / ¹ F, which extend to 17 and are arranged in such a way that the complete melting area of the plastic begins in 18, which, according to the arrows pf, now enters the discharge space SAi and therefore onto the central screw OE , during the Plastic itself gradually melts. The last part of the plastic reaches the screw OE according to the arrows fr via the channels 20 and 20 '(FIG. 1). All of the completely melted plastic transferred to the screw OE is discharged from the screw at the outlet end 12, according to arrow fe .

As can be seen from FIG. 2A, the section plane runs through the full area of the extruder housing C between two screws OCF, e.g. B. between OCF ₁ and OCF _h . This area is preferably traversed by the degassing channel 60 for the melted plastic, this channel extending as far as the opening DG for gas extraction. The channel 60 connects the annular discharge gray; SAi with the space outside the wall Ce of the housing C (see Fig. 2A). The plastic impinging on the screw OE and coming from the screws OCF ₁ ^ is completely melted and can therefore be degassed at the moment in which it emerges from the slots PF and before it is conveyed to the exit of the extruder. In Fig. 2, RE indicates the heating coils or resistors. FIG. 2B is a section similar to section 2A of FIG. 2, but taken in a plane which runs through the feed end BC . This FIG. 2B shows the end of feed BC, which only feeds the melting chamber SAe, ie the screws OCF. The holes indicated by 65 are used for the circulation of any thermally influencing agent.

FIG. 3 better illustrates the slots PF for the passage of the melted plastic, which gradually becomes larger as it _{advances along the screws OCF n} _ _m or OCF ₁ . _b in the section between 17 and 18 forms. The torpedo TOCF with its channel 20 is also shown enlarged.

The configuration that the machine has when the screw OE is used for injection molding is not very different from that shown in FIGS. In injection molding, the extruder screw is a rotating punch screw that is operated with a rotary motion and with a displacement along its longitudinal axis. The screw OE is provided with a conventional sealing valve on the head part, which prevents the backflow of the plastic during injection molding.

The OCF screws only have the task of conveying and melting the plastic, but they do not have the task of preparing the amount that can be discharged from the continuous casting area.

Differentiation of the screws OCF is only necessary if the specific weight is greatly reduced, that is, in the continuous casting of powdery instead of granular mass.

The fact that the plastic changes from the outer screws OCF to the inner screw OE during the gradual melting process always allows the resin to be degassed.

The speed or number of revolutions of the screw OCF can be changed as desired compared to that of the screw OE ■ '. The increased speed of rotation of the screw OCF does not give cause for concern that too much air or volatile substances could be absorbed, since the air in any case when the melted plastic passes from the melting chamber to the discharge chamber via the slots PF, ie during degassing, is completely withdrawn.

The transition of the melted plastic to the tendon OE prevents it from being processed further and therefore from being overheated. At the same time, this smaller, plastic-demanding processing has a positive effect on power consumption.

Since the plastic is melted from the screws OCF to at least one screw OE via -Ii passages with rather small dimensions (depending on the type of plastic, the cutting speeds are between 100 and 300 sec " ¹ ), the surface of the melted plastic exposed to degassing is much larger than y> in an extruder with a conventional degassing system.

The central screw OE can change the speed of rotation independently of the screws OCF . This gives the extruder considerable flexibility.

Examples

The average capacity of extruders with degassing and a diameter like that of

Γι different manufacturers of extruders on the World market can be specified, for example, as follows:

ABS and polystyrene (shockproof) 500-600 kg / h

Polyethylene 400-500 kg / h

in polypropylene 300-400 kg / h

The same conveying capacities can be achieved with an extruder according to the invention, which has six screws OCF with a diameter of D = 60 mm, which rotate at 120-150 rpm

• n fen, and with a central screw OE with a diameter of D = 120 mm, which rotates at 80-120 rpm.

An extruder according to the invention which has a cylinder which is only 300 mm longer than that

■ .ο in the above example and the one with six screws OCF with a diameter of D = 80 mm, which rotate at 120-150 rpm, and with a central screw OE with a diameter of D = 120 mm, which with 120 -150 rpm, rotates, is equipped,

v. has the following delivery rate

ABS and polystyrene (shockproof) 800-1000 kg / h

Polyethylene 600-800 kg / h

Polypropylene 500-600 kg / h

These conveying capacities are with an extruder

H) with conventional degassing with a diameter of D = 150 mm and a cylinder length, which is at least 4500 mm, is difficult to achieve.

For this purpose 2 sheets of drawings

Claims (4)

Patent claims: 1. Process for extrusion or injection molding of thermoplastics, in which the plastic in at least one elongated melting zone by external heat supply and Frictional heat is brought into the molten state and then according to the progressive melting under shear stress in a radial to the melting zone offset axially parallel discharge zone is pressed, the plastic at the transition from each Melting zone is degassed in the discharge zone, characterized in that the plastic with strong constriction of each melting zone along an elongated transition section is pressed into the discharge zone. 2. The method according to claim 1, characterized in that the melting zone is concentric to the discharge zone or that the melting zones are on a concentric circle are located around the longitudinal axis of the discharge zone. 3. Apparatus for performing the method according to claim 1 or 2, with an extruder housing and at least two pistons or rotatable pistons arranged in separate spaces Screws, one of which is used as a discharge area and at least one as a melting area, characterized in that the transition from each melting chamber to the discharge chamber is through axially extending slots is formed. 4. Apparatus according to claim 3, characterized in that the melting chamber is concentric is arranged to the discharge space or the melting spaces on a concentric circle are arranged around the longitudinal axis of the melting chamber.