Winning with Foam Extrusion


For Retrofitting. The production costs ofsemi-finished extruded products are significantlyinfluenced by the cost of rawmaterials. This makes technologies veryinteresting that reduce raw materials consumption. By means of extrusionfoaming, savings on raw material costs can be achieved without having to accept compromises in product quality. Retrofit solutions for physical foaming with CO2 orN2 enable conventional extrusion lines to manufacture foamed products.    


Foamed semi-finished-products are used today in numerous areas of application. Classic applications utilize the specific properties of high-expansion foam to obtain, for instance, insulating effects, damping properties or a specified compressibility. In such applications, the physical property of foam is the main thing. However, foaming is also used in cases where the properties of a foamed product scarcely differ from those of nonfoamed products. In such cases, the aim is to reduce raw materials consumption while largely maintaining the same properties. These "heavy" foams are even used in cases where they are not even recognized as being such: For example,multilayer laminates with a foamed center layer are used in adhesive tapes. In the food industry, dairy products are sold in thermoformed cups with a foam core layer. Special electrical properties are obtained in cable applications by means of intermediate foamed layers. Multilayer technical components with a foamed core layer are increasingly present in infrastructure areas as pressureless sewage pipes as well as for sheet applications in building or container construction.


Different Technologies


Heavy foams are relatively high-densityfoamed articles that ultimately compete with non-foamed products for final applications. These applications have mainlybeen foamed using chemical foaming additives. The reason for this lies in therelatively simple application of chemicalfoaming additives. These additives are generally dosed gravimetrically into theraw materials as a masterbatch and develop their foaming action under the influenceof warmth. Besides the high costs ofthe additives among their disadvantages include the limited foaming power ofchemical additives and the retention ofchemical residues in the melt. Chemicaladditives are used in numerous high-densityproducts and achieve weight reductionsof 10 to 50 %.


High-expansion foams are mainlyproduced on special foam extruders. Systemsof this type fulfill a number of requirementsthat exceed the capabilities ofclassic extrusion. Besides the process-relatedspecifics of melt conditioning anddischarge shaping by the die, foam extrudersalso have to assume the tasks ofpreparing, injecting and homogenizing the blowing agent, as well as melt coolingand controlled expansion of the meltwith its charge of blowing agent. This requiresspecial machines with very long processing units compared to those ofconventional extrusion equipment. Thephysical foaming fluid used then is injectedinto the melt under high pressure,then homogenized and cooled in screw mixing and shearing zones. The specifiedgoal is a single-phase mixture of melt andblowing agent with a defined temperaturethat undergoes controlled expansiondue to pressure loss in the shaping die.As a consequence of the ban on CFCs,aliphatic hydrocarbons such as butaneand pentane have become established.Due to their low level of solubility, inertgases, such as CO2 and nitrogen placestrong demands on processing technology,but offer a number of advantagesover hydrocarbon-based versions.Among these are unlimited availabilityand low costs. Since it is obtained from the surrounding air,CO2 also serves ecopoliticalgoals, since it does not contributeto global warming. CO2 is noncombustible,odorless and, in process relevantconcentration, non-toxic. Thus atrend can be seen toward the use of atmosphericgases as blowing agents. Specifichardware improvements have madeit possible to utilize CO2 for manyfoamed products that have been manufacturedby classic hydrocarbons.


Chemical and Physical Blowing Agents


However, the low solubility of atmosphericgases has its advantages. Thus CO2 andnitrogen can also be used in very low concentrations for heavy foams that used tobe manufacturer exclusively using chemicalblowing agents. All in all, the numberof high-density foam applications is increasingthat are being foamed more forreason of cost pressures than out of technologicalnecessity. Process engineering solutions using atmospheric gases arethus becoming more important. Solutionsfor retrofitting conventional extrusionlines for physical foaming offer a lowriskpoint of entry into this processing technology. In this way, the processor is in a position to utilize existing extrusionlines for physically foamed products. Thehigh investment in a special foaming extruder becomes superfluous. In addition,the retrofitted extruder, similarly as with chemical foaming, can be used for bothfoamed and non-foamed products. Theprocessor can then organize his productioncapacities flexibly and take the firststep into the market for foamed products by combining lower capacities with conventionalproducts. For a direct comparisonof physically and chemically foamed products, the necessary investment costsfor physical foaming equipment are comparedwith the running costs of additives.As a rule, the bottom line shows a cost advantagefor physical foaming in a short time


Solutions for Retrofitted Extrusion


Retrofit solutions for foam extrusion usingCO2 and nitrogen appeal both toprocessors who would like to replace theirexisting chemical foaming process withphysical foaming, as well as to those whowish to substitute existing compact articleswith foamed products.


The Swiss company Promix Solutionsspecializes in retrofit concepts for physicalfoaming and offers custom solutionsfor existing extrusion lines. Experiencewith numerous industrial projects shows that almost any conventional extrudercan be retrofitted as a foam extruder regardlessof its type or size. In order to beable to offer the optimum solution forany particular case of application, two aspects are of central importance: Aprocess-relevant evaluation of the existingplant technology and the specific requirementsof the foamed product. In additionto the physical properties derivedfrom its raw materials, foam density andcell size distribution, the surface quality of the foamed product is an importantquality feature in many instances. In suchcases it is important to consider the interactionof properties and the limits ofwhat is possible. Thus falling density isalmost always accompanied by a loss ofmechanical strength. Surface texture isalso a function of the degree of foamingwhereby individual characteristics can bespecifically influenced depending on thecase of application and process variants.Coextruded film and sheet with a foamcore layer and compact outer layer combinelow weight with good surface finish.


The technological processing requirementsderive directly from the requirementsfor the foamed product wherebyexisting technology determines the typeand scope of retrofit components. Promix Solutions offers two fundamentally differentconcepts that can be divided intostatic and dynamic solutions. Both conceptspursue the goal of integrating the    additional processing steps of blowingagent injection, homogenizing and coolingin the existing extrusion equipment. Which solution is suitable in a particularinstance depends on the concrete circumstancesof the extrusion equipment to beretrofitted.Thus, in addition to the existingfloor space, the extruder's pressure capacityand drive power as well as the meltcooling requirement are taken into consideration.


The Static Solution


In essence, Promix foam system's staticsolution combines a fluid injection nozzlefor the blowing agent with a downstreamstatic mixing zone. The system isinstalled downstream from the extruderbarrel immediately in front of the extrusiondie. This patented process has the advantagethat no alterations are requiredfor the plasticating unit of the extruder.All that is required for retrofitting is correspondingfloor space for installation aswell as reserve pressure from the extruder,since the static mixing zone is by naturea pressure consumer. The blowingagent is introduced under high pressureby a mass flow-controlled dosing station.It should be noted thereby that the quantityof blowing agent required by low-tomediumfoam levels is quite small, so thatprecise dosage has to be assured even atminiscule amounts as small as 1 g/min.Especially significant is the configurationof the static mixing zone. Laboratoryexperiments at Promix Solutions haveshown that the maximum blowing agentconcentration soluble at any given extruder throughput depends on the typeand configuration of the mixing zone.The influences of mixing technology onthe resulting foam can be illustrated bytaking the soluble quantity of blowing agent at a given point of operation asmeasure of mixture quality. Thus by optimizingthe mixing zone configurationat a given operating point, up to threetimes higher CO2 concentration could bedissolved in the melt than with a conventionalmixing zone. Process-relevant parameterssuch as pressure and temperatureas well as shear rate and the dwelltime of the polymer-blowing agent systemplay a large role in the formation ofa single-phase solution of polymer and foaming fluid.


With an optimum mixing zone, medium-to-high density microcellular foamscan be produced by the purely static processing variant at any throughput. If meltcooling is also required to further reducefoam density, the system can be combinedwith static melt coolers. An especially efficienttype of melt cooling can beachieved with the aid of a Promix bundleheat exchanger. The barrel section carryingthe melt is then fitted with a numberof tempered, bended pipes that functionsimultaneously as mixing components.The pipes are directly tempered by a heattransfer medium. The melt flows throughthe cooler and is intensively cooled bycontact with the pipes. At the same time,the special geometry of the pipe bundle sensures that the melt is constantly andthoroughly mixed radially. Despite thelarge heat exchange surface, the resultingempty volume of approx.75 % causes onlyslight pressure losses. The configuration of the melt cooler is determined byits cooling task, while taking the materialsand processing data of the particularapplication case into consideration. Bycombining these melt coolers with aretrofit solution, products can be producedwith foam densities that lie clearlybelow those of chemical foaming additives.


The Dynamic Solution


The dynamic retrofitting variant combinesstatic mixing zones withdynamic components. Theadditional processing stepsrequired for foam extrusionare implementedto begin withdynamically in a processingsection downstreamfrom the conventionalplasticationcylinder. Depending onperipheral circumstances, theplastication cylinder is extendedto include a tempered section for injectingand dissolving the blowing agent. Ifthe plastication screw is suitable, it islengthened to include additional functionalelements. If not possible, the existingscrew is replaced by a longer, specificallyequipped foaming screw. A staticmixing zone adjoins this dynamic processingsection. Special injectors inject thefoaming fluid into the extended sectionof the cylinder. In this way, it is not necessaryto modify the existing cylinder.Retrofitting can thus be accomplished ina short time without long-lasting loss ofproduction. The static/dynamic cylindersection is equipped with heater bands anda cooling fan or fluid temperature regulation.This provides the defined temperaturecontrol required to influence themelt strength that is so important forfoaming. Static mixing elements keep themelt's texture and temperature perfectlyhomogeneous. The same mass-flow regulateddosing station is used for preparingthe fluid as in the static solution. Thesingle-phase solution of blowing fluidand melt is thus created by the dynamiccomponents in the lengthened screw andcomplemented by the static mixing zone.The advantage of the dynamic solution isthat it generates almost no additionalpressure loss. The special configurationof the dynamic mixing section mixes anddissolves the blowing fluid thoroughlyand efficiently.Cooling takes place via thecylinder surface whereby static melt coolerscan be included here, too.


If the extruder already has a processinglength suitable for foaming applications,the cylinder may not have to be extendedin certain circumstances. Staticmixers in front of the extrusion die ensurea texturally and thermally homogeneousmelt, a fundamental preconditionfor obtaining high-quality foamed products.


Industrial Applications


Retrofit concepts of this kind have foundtheir way into industrial production.Promix Solutions has retrofitted a largenumber of foaming systems for variousareas of application.Among these are extrusionlines with outputs as high as1,500 kg/h. Substitution of chemical additivesfor foam extrusion as well as theconversion of conventional extrusionlines for physical foaming was realized.One European manufacturer of compositePP sheet, for instance, has replaced itspreviously used chemical foamingprocess with a Promix retrofit system forfoaming by CO2. The co-extrusion linewith a total output of 750 kg/h previouslyproduced PP foamcore sheet with adensity of 600 to 650 kg/m3 and was recentlyequipped with retrofit concept forphysical foaming. Annual savingsof over 20 t of chemical masterbatch arepaying for the investment in plant technology within a few months.Since the line. Foaming system with melt cooler for foamed sheet extrusion was also equipped with a static melt cooller, sheet can be produced with markedlylower density than by a chemical foamingprocess.This not only opens upadditional savings potential on the materialsside, but also accesses applicationsfor the producer that require lower densityPP foam sheet.


One leading manufacturer of plasticpipe has used the process to realize productionof pressureless sewage pipe witha foamcore layer. To do so, they convertedtheir previous production process forsingle-layer, compact PP sewage pipe inorder to produce multilayer pipe with afoamcore layer. Their main extruder wasretrofitted with a Promix foam system.Moreover, a new multilayer pipe head andancillary extruder were installed for theinternal and external pipe layers.Despitethe corresponding investments in an ancillaryextruder,multilayer pipe head andfoam extrusion system, the investmentpaid for itself in short time thanks to a25-30 % reduction in raw materials consumption.Currently three pipe sizes upto DN400 are being produced by this concept.These products have passed all testsfor the relevant standards. Meanwhile,three extrusion lines have been retrofittedwith the Promix System.




The production of medium-to-high densityphysically blown foams is being successfullyimplemented with the aid ofretrofit solutions. However, when oneconsiders the multitude of possible applications,it becomes clear that the potentialfor microcellular foamed products isnowhere near exhausted.Since retrofit solutionsare by definition always combinedwith existing plant technology, great significanceadheres to the particular surroundingcircumstances right in the planningstage. In addition to relevant experience,this presumes a holistic approachto the extrusion process. Besides thechoice of raw material and additivizing,process control and the ability to judgethe suitability of the extrusion die, this involvesan understanding of the interactionamong the parameters. If all this isgiven appropriate consideration, the costsof foam extrusion can be significantly reducedand long-term market competitivenessstrengthened.    

27 December 2017 Wednesday
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