DE2152018C3 - Method and apparatus for making a synthetic thermoplastic polymeric foam - Google Patents

Description

The invention relates to the manufacture of a synthetic thermoplastic polymeric foam by extrusion molding of a molten polymer containing a blowing agent system in solution the preamble of claim 1

Schaumstotfe by extrusion of a synthetic thermoplastic polymers in which a two-component blowing agent system is dissolved. to manufacture. One component of the blowing agent is with the molten thermoplastic Extrusion temperature miscible, while the other a critical temperature below the extrusion temperature and therefore in the form of a gas under the conditions prevailing in the extruder which is present in the mixture of molten thermoplastic polymers and the first blowing agent is dissolved.

This has the disadvantage that, especially when using relatively large amounts of the first blowing agent, ie at least 20 percent by weight, based on the weight of the thermoplastic polymer, the blowing agent easily squirts out of the die, and that local irregularities are formed in the extrudate. In addition, the Eurudate can have undesirably coarse iles, and in serious cases interruptions of the Hxtrudat can occur. It is assumed that this is due to the tendency of both components of the blowing agent vstenis, which are not really mold-forming agents. For the formation of large tabs in the extruder cylinder, so that the formation of a hose-like duct is due to the mixture of thermoplastic polymers and the former The blowing agent component can hardly be avoided in addition to the extrusion stoppage. Furthermore, a process for the production of synthetic, thermoplastic foams is known in which a mixture of molten thermoplastic and friction agent is introduced into the mixture of molten thermoplastic and abrasive which is required in an extruder by a zone of high pressure and a low pressure A second blowing agent is introduced, the critical temperature of which is below the extrusion temperature, and this process uses a device consisting of a screw extruder with inlet openings for the liquid and t '; as well as facilities / ur Druckanwenumig when findingosures of these sigen and gaseous /ti.at/en. In the known method, the two propellants consist of a liquid component and a gaseous germ-forming agent or nucleating agent. Any measures for the automatic control of the nucleating agent diueks in the extrusion cylinder are not provided. This results check K-mi extrusion with high Treibmittelgell.ilt the disadvantage that light tubes or pockets 111 form nerhalb the mass of Themoplasten This gives rise to irregularities in the resuliie rendcn foam, which on mechanical properties of the product Cc adversely affect the object of the invention is the production of »-vn thetical. thermoplastic polymer foam by extrusion molding with a high proportion of blazing agent to produce a product with a uniform cell structure without the formation of undesirable tubes

45 or pockets.

The invention solves this problem through the im Characteristics of claim 1 specified dimension

The device for carrying out the Veridhren * obtained from a screw extruder with inlet sockets arranged on the upstream side of its melting zone for the first and second blowing agent.

... * _n .Ι ^ η., η .11 .. LintrittL ηΓ {ηιιηη fill "ij ^> jri.t *> RluHrnitt.-l is connected to a liquid metering device. You are characterized in that the inlet opening (8) connected to a fluid under constant pressure reservoir for the second blowing agent via a throttle point (9). Preferably, the throttle point is for the second blowing agent as a capillary tube S au'.gebildet _0.

By the method and the device of the invention it is achieved that with an increase in the flow rate of the second blowing agent, the pressure drop at the throttle point increases, so that in-6 _{S, as a} result of the constant pressure of the reservoir of the second blowing agent, the pressure of the second blowing agent at its point of entry into the extrusion cylinder. This leads to the fact that -

even if there is a hose from the second first Blowing agent forms - this under the action of the Strangprel3uruck.cs collapses. Also will as a result of the pressure regulation automatically occurring through the invention, the formation of such unwanted gas hoses largely avoided from the outset.

It is of course necessary that both blowing agents be injected with pressures which are above the

Be a stopper, but because of the constructive difficulties which are inherent in the manufacture of such devices, the use of the length of a Tube with a narrow bore, for example a capillary tube, preferred.

It is desirable to have both blowing agents thoroughly with the molten thermoplastic polymer to mix. In a narrow extruder, in some cases the normal extruder screw results in a different

Pressure within the extruder, and mixing was measured 10 times, but when used it was found that there was a tendency towards injecting the blowpipes with the extruder, especially when ejecting at these high pressures; . -o below your calculated ^Λ eating impact, it rotates

The extruder screw is not fast enough to produce a properly measured Mixing result, if not special mixing devices also in the screw

there are pockets of blowing agents,
second blowing agent through a Strör ·>
clamping device clamped through ^: -

within the device a pressure nail of at least 7 kp cm ^:. preferably 21-70 kp / ^cm:, results, and if not the only ".- blowing agents metered as a liquid wild. It is of course important that

construction are included.

After the polymer feed and melt region into a screw section. where the polymer sleeve in a number of separate streams / that

the / wide blowing agent is dispersed and used in the melt before the 20 er'e blowing agent, while

Dissolve as fine vesicles dispersed in the. to the Ensuring the production of a uniform timeframe. It has been found that the use of a contributing to such a flow restriction To achieve dispersion

It is believed that the effect of an Extruci snail itself is not sufficient. for what is satisfactory Mixing the blowing agent with the molten plastic / u achieve during mixing system

Do not pump material sufficiently
There is little pressure ball along the extruder / v, linder This means that the low-viscosity hotmelt, which under pressure into the more viscous material in the Kx

it is divided. Preferably both blowing agents are used during the splitting of the streams engaged / t, with the second blowing agent either upstream or is injected downstream of the first blowing agent The distribution of the melt in separate Currents can / wake-up be achieved by provides a special grooved portion in the screw, which consists of a cylinder, the has some axial channels on its outer

systems instead of Schneeken / ti extruding 30 »jher surfaces are formed. It is preferred that the Do not pump material adequately and the cylinder has at least three channels formed in it and it was found that a cylinder with 16 Channels is particularly suitable. In this way the polymer stream is divided into several streams when

truder is ylmdereingespritzt / as / u inclines, the cylinder is mounted by _35, that it passes / u are formed with the material a gear, which rotates the extruder screw. The first blowing agent can be fully inserted / pressure along this passage, then through an opening in the cylinder head, the first blowing agent can be injected into a low-viscosity can in the region where the flow tube is. it is still stored at the temperatures lower, and in this way the pressure and pressure that prevail in the extruder is a melted thermoplastic polymer into a liquid. It can therefore be measured, i.e. H
a fixed amount can be introduced in the unit of time, for example by means of a positive displacement pump. For example, lines piston or a diaphragm pump, which will fen per stroke 45, which allow the dispersion in molecular a fixed amount. This use of such a fumigation device
constant speed. Iu, the delivery speed is not possible to increase and therefore

ι - j

divide and therefore subject to mixing at the point where the first blowing agent is injected so that the injected, low-viscosity superplasticizer immediately subjected to the first subdivision stage.

immediate subdivision contributes to this. that the formation of a sufficiently large bubble is avoided, which form a passage through the melt

half the inclination of forming large bottles of the 50 could. It is also preferred that the cylinder be at least is as long as the inner diameter of the extruder bore, and in particular is Lange <Jes cylinders ocs 3- to make Jes Exnudci diameter, since this dk required relatively large

to be measured. However, if (fas /weit·.· Biahmit- 55 Ben amounts of the first blowing agent enabled, in one tel to be incorporated by a flow restriction device single injection stage, whereby the need to create some injection ports for the first blowing agent with the loading

first blowing agent avoided. However, it is for gases which are gelled with constant pressure instead of constant volume, not in a comfortable way possible to enter the extruder in this way

provision of appropriate screw sections and

through, for example a constricted pipe, is injected, along which a substantial pressure drop
If air occurs, an increase in the flow of the injected material causes an increased 60 injection equipment is avoided. Pressure drop in the Strömungsbeschränkungseinrich- In one embodiment form of the invention, tung, whereby the actual pressure in the cylinder more mixing sections in the cylinder liner of the bushing is reduced, the injected Mate fcxtruders prevents at a location above the points addition vorrial thereto, by da Material through, seen where the blowing agents are injected. Wherever it is located in the extruder, a grooved section is used to form a ciang, and at least the first blowing agent is injected into willow areas section which both shears the mixture and

further subdivided and mixes the separate streams, into which the polymer stream is divided during its passage through the grooved section has been. The grooved sections and the turbine sections are constructed so that along the Extruder barrel liner over the grooved section and the Ti-.rbinenabschnitt away only a little Pressure drop takes place. It was found that the Blowing agents all the better within the thermoplastic polymeric material, the lower the pressure drop

The turbine section can be followed by a section. i: i what polymer blowing agent solution in another mixing section is subjected to high shear. This section is expedient a section in wclchcM of Polymerstroni is divided into several different streams, where in that these currents subject the polymer to high shear. For example, a cylindrical Formations with outlet grooves, which do not look over the entire length of the cylindrical Formation «- to extend. These grooves stand with Aus-Moßrillen in the cylindrical structure over bridge canals in connection, which have a shallower depth than the grooves, so that the polymer blowing agents Ι.οΜίημ a shear against the wall of the t \ - truder / cylinder bushing is subjected when the solution passes through this bridge canal. The grooves can also have constant 'fields and be giving away with idle rollers which are in the grooves and which by the rotation of the extruder screw be rotated with respect to the cylinder liner walls. That way, that gets into the grooves flowing polymers subject to high shear.

According to the process according to the invention, any synthetic thermoplastic polymer can be combined. Examples of thermoplastics that can be foamed include polymers and (polymers of ethylene (with low or high density) propylene, butene-1, 4 -Methyl-pentene-1, including the polymers of olefin with unsaturated acids, in which at least 10% of the carbonic acid groups are neutralized by metal ions, as described in the (British) patent publication HU I '' s. Polystyrene, styrene, maleic anhydride -Copolymeic, polyvinyl chloride, polyesters, polyamides, polyoxymethylenes and polycarbonates, Vis has found that because of their cheapness and ready availability, polyolefins and especially polyethylene can be used very widely. Mixtures of these polymers can also be foamed using the technique of the invention

The first and second blowing agents are selected to be relative to the synthetic thermoplastic Polymers inert under the conditions of the thin and the temperature of the pressure vessel are

f) The first blowing agent is preferably the synthetic one thermoplastic polymers are completely miscible and can be used at normal temperature and normal Pressure cmc be liquid or gas. The • she blowing agent must, however, at the pressure, which in ikr Repion predominates, in which the mixture extmeliert (usually atmospheric pressure). a Boiling point which is below the tempi r tlnf In ι ·! Im ι \ vi Κ ln-i that (icmisth is extruded.

' ί I »> In ι 11 mi- · ι .tlur St h.itiriifn i-ttifMlt I) .iv first blowing agent must. As tested above, for the thermoplastic to be a good solvent under the pressure and temperature conditions which prevail in the extruder, at least immediately before the point from which the mixture is extruded into the zone of lower pressure. In practice this means that the first blowing agent at the pressure prevailing in the high pressure zone should have a boiling point above the temperature at

ίο With regard to which the thermoplastic is extruded on this and in the face of other limitations it was concluded. that the most suitable substances for use as the first blowing agent liquids vind. whose Boiling points at atmospheric pressure higher than room temperature, i.e. 2 (> (and are at least 10 '(below the temperature at which the homogeneous mixture is extruded.

/ u examples of liquids, which as eleventh Blowing agents can be used, number hydrocarbons. Ether. Ketones and Halogenated Hydrocarbons The special liquid which for a given synthetic thermoplastic polymer is of course limited by their mishb; » Veit with the thermoplastic among the Conditions of the pressure vessel

In the case of polyethylene, it was found that a a very suitable first blowing agent is pentane. ι!; · Β but other useful first blowing agents 1 I. ^ Trichl-.ir-1.2.2-tritluoroethane. Hexane. Pctroather (boiling point 4 () bisii () (or Ml to SO C) and methylene chloride are.

The concentration of the first blowing agent should be 20 to 75 percent by weight of the polymer "preferably less than 50 percent by weight of the blowing agent is used based on the weight of the polymer. If a concentration of less than 20 ^f i is used, it is not possible to produce low density foamed articles, and if a concentration greater than 5 (1 './ is used, the product is costly to manufacture, both because of the increased expense of a solvent recovery step and because of it Because a larger volume of flow agent results in a larger extruder for the same output weight of foam - and furthermore there is a tendency for the cell walls in the foamed product to collapse, so that an unsatisfactory product is obtained. The amount of first blowing agent used is lying in

Excess over that which is necessary for foaming. In this way the blowing agent absorbs latent heat of vaporization from the thermoplastic polymer as it exits the extruder escapes, and so cools the foaming thermoplastic and hardens it in its foamed state. If lower amounts of the first blowing agent are used all blowing agent evaporates before the mass has solidified. This allows it the foam structure to disappear, for example by partial collapse, or modified to because inadequate cooling of the foamed mass due to the evaporation of blowing agents to cause the solidification of the mass is present, while there is still sufficient cias pressure through the blowing agent system is created to maintain the foam structure

The second component of the Blahmittcisystcms wiri-i .il- K'-rnbildiinpsmitt «· tnr who hurry, give way

caused by the evaporation of the first components will. The / wide blowing agent zeroed a critical one Temperature below the temperature in the extruder and is therefore gaseous when it is in injected into the polymer stream, but it should be dissolve in the molten polymer so that t a single liquid phase within the cylinder liner of the extruder yields the blowing agent is therefore normally free room temperature ci / i (u » although certain low-boiling F '.u compounds can be used, especially ir I all one High-quality polymers F.s is essential that the / wide healing agent in the mixture of thermoplastics and the first blowing agent under the pressure and [emperaturbedineunsjen. called which the thermoplastic we extruded »! a solubility of at least HUI weight percent hesit / en »oli. because otherwise im homogeneous, CiemiNth not ninr>: tchend / wide Blowing agent vnrhandc η is. around when exiting the Pressure vessel germs ture a great number very k! Tir · ' Bubbles / u form nucleation can then take place whose from the first blowing agent take place with the Result that a small number of very large pumice are formed and the product has little commercial usefulness l) a <· special used / wide blowing agent is of course by the nature of the Thermop'.jsfen dependent, but was found to be dali carbon dioxide nitrogen. Air (for polymers, which not subject to oxidation. Methane. Athan. Propane. Ethylene. Propylene. Hydrogen. Helium. argon and halogenated derivatives of methane and Athans. ζ B tetrafluorochloroathane. Examples of substances are which can be used In the case of polyethylene, it has been found that carbon dioxide or nitrogen are particularly useful, preferably in concentrations of at least!).! ^ Weight percent of the thermoplastic polymer

f. ». is preferred of the second gaseous Add blowing agents as much as possible, however not exceeding its melt solubility, which is usually quite low. <1 hour less than 10 percent by weight of the thermop'a synthetic polymers

The solubility of the / wide gaseous blowing agent is of course dependent on the nature of the / wide Blowing agent, the nature of the polymers, the temperature and the pressure in the pressure vessel and, to some extent, the amount and type of the first Blowing agent.

When considering the solubilities of inert gases, such as nitrogen, in polyolefins and first Carbohydrate blowing agents. proceed the following considerations. For first-time blowing agents that don't have any Hydrocarbons are and for other polymers than r <nyaicfmt, ίΗτηεπ s.se also as a guide.

Above the melting point of the polyolefin, the first blowing agent and the polyolefin are unlimited miscible. A given weight of the mixture of the first blowing agent and polyolefin has a somewhat greater solubility for the second blowing agent than the solubility of the same weight of the polyolefin alone is because the entropy of mixing is due to the presence of the low molecular weight material is increased. (Other minor changes occur because of the changed ratio of CH, - to CH, groups.)

In the region of interest, the solubility increases with pressure and in (iegensat / to what for solvents Po-Ivmcr mixtures has been found. have I andf> erg. WiIk and Huyett. J Applied Physics, vol 31 (14hO). Page 1137. that the solubility increases with temperature. This is in contrast to the usual experience of dissolving more permanent Gases in condensed phases.

Determine the general printing restrictions how much of the / wide gaseous blowing agent

to have been incorporated, and normally the amount of de ·· / wide blowing agent · * lies within the range from ι M to I.>',. preferably from 11. Z to I.W ", based on the weight of the thermoplastic polymer

In the case of the method according to the invention, the anti-sedative agent tur Jen Schaum a soluble gas and not, as in some systems previously used an 'ester particle in. the foam can be free of non-thermoplastic impurities generated

a <«earth and is very useful for electrical applications suitable where high dielectric strength and low loss angle are required It is aueti cheap when using readily available blowing agents will

J5 The resin, thermoplastic and blowing agent system can be extruded through any suitable die shape to produce the final product of the required thickness. for example pipe. Sheet, thread, or angled section./lighter, or it can be extruded with a crosshead shape which can be used to coat wire. Since it has been found that the best foaming is produced when the distance, practice ; the pressure from the interior of the vessel / atmosphere falls as short as possible. it is preferred to use a mold with a short bridge.

The technique according to the invention can be used may be required to make foamed extrudates of any kind To create shape or size For example, the technique can be applied to wire to coat or to make foamed sheets or films. It has been found that this technique is particularly useful in making foamed films. The polymer is called a blowing agent mixture is extruded as a thin sheet which is then stretched to form film. The thin extruded Sheet can be flat or tubular, and if it is tubular. so it can be expedient according to the known blue extrusion technology are stretched. Foamed films produced in this manner, in particular made of polyolefins, are used as wall covering materials particularly useful.

Other uses of the foamed thermoplastics produced according to the invention include the use as very light packaging, the use in the production of cushion foams, for example in the upholstery for seats or mattresses, as heat or sound insulators, electrical insulation means, for example when covering wires, and as a capacitor or wave lead dielectric. in the production of non-woven materials, for example leather or, after punch perforation, felt, after compression of the platform, for example between rolls under a pressure of 7 to 700 kg cm ^:. as writing material, and as decorative ribbons. Braids and threads, which have been processed to create decorative frames

(09;

ίο

be able

The invention is illustrated in the drawings explained

Fig. I is a longitudinal view of a ·· I eiK de ·. Extruder, which also cuts the screw lengthways indicates:

Fig. 2 is an Otierschniti of the grooved section of the worm, along I inie 1Ι-Π in Fig. 3 is an isometric ski // e de-Mwchab through the axial grooves 5 of the grooved section ¹ · 4 combine again.

In order to further improve the dispersion of the blowing agent in the thctin · ¹ -plastic polymer, the solution is then sheared through a small section of IK. This section is also shown in Figure 4 and consists of a cylinder 19 which is provided at the inlet end with a number of grooves 2lt. These grooves are blind .1 Ii sii extended .: ""> ieh

Sections «· of the worm, which directly sulden 10 not over the entire 1 eye of the ignition switch IS. sun-

grooved section follows, with the sketch / c .lie \ nordnung from / ancestors in the first two / of the mixing section shows, and fig. 4 is an isometric sketch of Je- Sdierabderii You are standing with AusI.iHrillcn 21 over Bruckerka n.ile 22 in \ erbinihing. woKi die Hnn.ki.nk.in.ik one smaller Ί icle than the grooves 2 (1 and 21 possess.-n the The solution is thus divided into a number through the grooves 20

"Section of the snail, furthermore unrestricted. Does the 15 streams divided and wildly a": i "heren Scherunj:

suspected mixed

The extruder has a cylinder liner 1 .int. 'n which a single screw 2 rotatably brought in is. The thermoplastic polymer is liem f xtru- <Jer fed through a hopper (not shown) and is in a conventional compression section the screw is compressed and melted. The cylinder liner 1 is along this compression section by means of conventional (not indicated) Heating elements heated from the outside in order to protect the Schmcl The end of de-KompressKinsdbschnitJes is in Fig. 1 with the Be / iujs. * ahl 3 referred to. That now in molten State of the thermoplastic polymers is subjected when they are from grooves 20 in iiie Groove 21 via channel 22 between the base dt> channel 22 and the cylinder liner I of the extruder goes.

2f after stalking section 18. belinded another rurhinenmis h.ibschmti 23 one .ihnli Chen design as in German rtirbinenmischabsehnitt 10 l'n.-part the 1 solution a good mixing if the S -ome. which of the - \ iislal> nllen 21 desSchci.ib average of 18 lines. saw combine again

Downstream of the lurhitu nmisehabschniues 2Λ is a conventional screw section 24 for measuring and extruding the 1 iwuni; duul ¹ through a Mutrizenlorm (not shown). .1

forced by ilen knmpp.ssionsabschnitt 3, the solution is rather in a region of lower pressure

to be subdivided into a number of streams by means of the next section 4 of the screw. This section is grooved, having a series of axial grooves 5, and it is rigidly attached to the screw so that it rotates with it. The grooved section is in Fig. ¹ ZZ shown in cross section. The first blowing agent is fed from a reservoir (not shown) via a measuring pump 7 to an opening 6 in the extruder cylinder! Downstream of the inlet / outlet, the blowing agent causing the thermoplastic to foam and solidify illustrated

Jangs which a pressure drop of at least ~ "kp» m ^: occurs when the gaseous second blowing agent is injected.

example

A foamed polyethylene film ν is made from a Extruder de. Kind of extruded. as he did in the / eiihnun gen is illustrated. where the extruder has a / \ -

Opening 6 there is an injection opening 8. through 40 liner bushing diameter knives of 12.1 cm and one soft through the gaseous supply v (··. snail length of 54 ^ cm, and where the a constant pressure / supply screw (not shown) with a speed of I ' with a pressure control valve (not shown) between min. rotates.

see the reservoir and the capillary tube 9 over The geometric dimensions of the screw

this capillary tube 9 is injected. The capillary 45 sections, numbered as in the drawings, are pipe acts aK flow limitation device. the following:

Section 3: auger section with a total length of about 330 cm. at which the screw depth decreases from 12 7 mm to 4.2 mm. In the grooved section 4, the thermodes are:
flast and the first and second blowing agent under BiI section 4: grooved section with a length of

a solution mixed together. This lo- 48 cm. consisting of 16 axial channels of 12. ⁷ »ung is then in a turbine mixing section 10 mm deep and 11.2 mm wide: printed, through the polymer, which with Section 10: distributing mixing section with a

lels of the compression section 3 of the screw 55 in length of 24 cm. consisting of a number of roles grooved section 4 is pressed. These mixing gates. soft are at an axial distance from each other. '' * "'"' · - <l where each rotor Hi has fins of a depth of 12.7

mm has:

Section 18: shear section with a length of

are brought so that these rotors are with the Oo 24 cm and with bridge channels 21, soft a depth Rotate the worm. As shown in FIG. 3, there are alternating 0.38 mm;

Section 23: Mixing section with a length of 24 cm. identical to section 10; and Section 24: normal screw section with a

Alignment with the space between the 65 ncr total length of 96.5 cm with a transition teeth 16 and 17 of the first rotor 11. This turbine section, which becomes a Me: section with a Nine section 10 gives a good mixing of the Lo screw depth of 4> rnm leads, solution if the currents after passing through the openings 6 and 8 are 60.3 mm

Section 10 is in the shape of a series of spaced apart of each other, toothed rotors Jl. 12, 13-14 before. soft on the snail

fnd rows of teeth staggered, being in the vigur only the first two toothed rotors 11, 12 are shown. The teeth 15 of the rotor 12 are axially in

Ii. ·· * INl min \ (ΐιιι beginning of the grooved section 4 removed

1-.iii mixture of ¹ Ki weight weight polyethylene lower! light unil ld parts by weight tilane dioxide pigment VMnI fed to the extruder at a speed of 45.4 kg per hour. Pentane, at a rate of 7> \ percent by weight of the material, is injected into ilen f-atruders through C) lung 6 and mixed into the molten ollyir.ergeniU. Nitrogen at a rate of (1.5 percent by weight of the (icsanituemisches. Is in the melt through the capillary tube *> through (inner

diameter 0.17 (S mm, long 122 cm) injected, with a pressure drop of 21 kg 'along the capillary cm occurs.

The temperature of the cylinder liner in the region of the screw section 2 is 140 to I fill "C and along the remaining length of the Zyiii.Jerhuchse held at 1 K) 'C. A die shape on the The discharge end of the extruder is kept at a temperature of 11) 5 C, and from this die shape

ίο a foamed polyethylene film is extruded. which one is free from splits and has a specific weight of 0.15.

For this purpose 3 sheets of drawings

Claims (1)

Patent claims: I. Method of making a * synthetic thermoplastic polymer foam by extrusion molding a molten, a Blowing agent system containing dissolved polymer with a screw extruder from a high pressure zone in a low pressure zone, wherein the blowing agent system, based on the weight of the Polymer, 20 to 75 percent by weight of a first, liquid metered blowing agent, which in the High pressure is completely miscible with the molten polymer and above the extrusion temperature boils and at the low pressure below the polymer temperature when entering the low pressure zone boils, and up to K) percent by weight contains a second blowing agent metered in in gaseous form, which at the extrusion temperature and the compression pressure in the mixture of polymer and first blowing agent / u at least * 0.01 percent by weight is soluble and its critical Ti .iperatur below the extrusion temperature is, characterized by, dal. ', the second blowing agent from a constant pressure Vorr..t through a Throttle point with a pressure drop of at least 7 kp cnv into that located in the extruder introduces molten polymer. IT. Device for carrying out the method according to claim 1. consisting of a screw extruder mii abst.omseiten their Melting zone arranged F-.intr. openings for the first and second blowing agent, of which the inflow opening for the first blowing agent with one Liquid dosing device is connected. characterized in that the inlet opening (8) for the second blowing agent via a throttle point (9) to a reservoir under constant pressure connected. 3. Device according to claim 2, characterized in that that the throttle point for the / wide blowing agent "■. Capillary tube is formed