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CELLMOULD Physical foaming with many advantages

Cellmould isaprocessdeveloped byWITTMANN BATTENFELDformanufacturing structured foamparts by directgasinjectionintothemeltwithaphysicalfoaming agent.Structuredfoampartsarecharacterizedby acompactouterlayerandafoamedcore.WithCellmouldlightweighttechnologyextremelylightmolded parts can be produced with high rigidity and without sink marks.

WITTMANNBATTENFELDhas had many yearsof experience instructured foamtechnology and has numeroussystems inoperation.Direct gas injectioninto plasticmeltasaproprietary processhasbeen practicedfor20 years.

The aim of Cellmould light-weight technology isto produce both thick-walled and thin-walled structuredfoamparts with afine,evenfoam structure andreduced weight.

Advantages

(3) Weightreduction
(x) Elimination of sink marks
\gg Reduction of warpage
(3) Reduced parts costs
_{rm{\scriptsizecent}}rm{\scriptsizemu}_{rm{\scriptsizecent}} Lower tolerances
(3) Higher rigidity with the same weight
y Longer flow paths through reduced viscosity
_{\Rightarrow} New design options
y Lowerinternal mold pressure
\gg Lower clamping force requirements

Cellmouldisafoamingprocessinwhichapressurizedgasnormallynitrogen,isinjectedintotheplasti meltinsidethebarrelofaninjectionmolding machine and alsofinelydistributed.Duetothehighpressure ofupto330bar,the nitrogenissuppliedinliquid form and subsequentlydissolvesinthe plastic meltasasupercriticalliquid.Wheninjected intothe(vented)cavityofthemold,thepressureisreleased into the melt.The plastic/nitrogenblend separates again.Simultaneously,the supercritical liquidreturnstoitsgaseousstate,formingafinebubblestructurebased onitshomogeneous distribution.Its actualstructure depends onthe conditionsintheinjectionmolding processTheseinclude the meltviscosityoftheplasticmaterialtheinjectionspeed(thehigherthespeed,thefnerthefoam structure),andfinallythequantityof material(thelessplastic,themore spaceisavailableforthe formationofafoamstructure withmaterialreduction).Thelatteris achieved eitherbyappropriately lowerdosingforafixed cavityorbycompletelyfilling thecavity andsubsequentlyopening it bya pre-determined high-precision stroke.

Effectoftheflling time(injection time)onthefoamstructure ofPCparts: (A high injection speed leads to a short injection time.)

Outer layer: thin Cell structure:fine

》Filling time =0.6:

》Fillingtime =1.2 s

CELLMOULD Process technology options

》Standard foaming

In the standard process,nitrogen isinjected into the plasticmeltinaspecialplasticizing unitbehind a needle shut-off nozzle,then finely distributed and dissolved in the melt.Following injection into the temperature-controlled,non-pressurized mold cavity, the gasisreleased from the solution and forms fine bubbles.Some of the gas bubblesreachthe surface of the polymermelt and become visible intheform of striations on themoldedpart surface.Improvement of the surface ispossiblein combinationwith additional process modules.

》Variothermic foaming

Where foamed parts must have a high-quality surface, it is necessary to combine the standard process with additional process technology equipment around the mold.

For example:

reverse embossing technology (HiQ Opening),by which the mold is completely filled and allows foaming of the core only afterwithdrawal of a mold insert orbyhigh-precisionopening ofthe clamping unit.
-additionalvariothermicheating/coolingofthe mold cavityby WITTMANN temperature controllers with Variotherm devices (WITTMANN TEMPRO plus Vario).

》Co-injection foaming

ByinstallingCellmould technologyon one of thetwo plasticizing units of a co-injection molding machine, molded parts canbe produced with a light-weight foamed coremade ofeithervirgin orrecycledplastic material surrounded by a compact covering layer. The core and covering layer need not necessarily consist of the same material.

PLASTICIZING UNIT 20 years of in-house development

TheWITTMANN BATTENFELD specificfeature of Cellmould technologyisthe separation of the plasticizing sectionfrom the gas injectionsectionofthe25DCellmeltscrewbyanexchangeablebarrier segment of the screw.

The 2021 screw design,in particularthebarriersegment,is protected bypatent underEP3338992B1.With thisnewprotected design,the backpressure canbe adjusted overa larger process window.Added to this is an increase in plasticizing performance of up to 10%^{1)} ，

The general design of the screw in three parts consisting of a plasticizingsection,abarrierzoneandablendingzone enablesquickexchange of individualsectionswithouthaving toreplacethe entirescrewif there is a need for adjustment.

CELLMOULDSYSTEM Everything from a single source

Formanufacturing Cellmouldparts,everyWITTMANN BATTENFELD injection molding machine from the EcoPower, SmartPowerandMacroPowerseriescanbecombinedwiththe necessary additional equipment.

》Compressor units (DE)

Nitrogen is used as foaming agent.It can be supplied from commonly available pressurized bottlesorfrom the ambient air via a nitrogen generator(also available as an extension module for every compressor unit).In the compressor unit,the nitrogen is compressed to an operating pressure of up to330 bar and transported through pipelines to a gas flow regulator and from there to the gas injector on the barrel.

》Gas flow regulator

The gas flow regulator is installed between the compressor unit and the gas injector.It serves to set the quantity of gas injected into the barreland the gaspressure measured on the injector.These parameters are set on the corresponding screen page of the machine's Unilog B8 control system.

x Gas injector

The gas injector is a device developed in-house by WITTMANN BATTENFELD.It is screwed into the barrel, and its function is to add liquefied nitrogen to the plastic melt.

》Increased injection speed

Since the formation of thecell structure canbe significantly influenced bytheinjectionspeed,a pressure accumulator to acceleratetheinjectionstrokeisanessentialpartof every Cellmould equipmentpackageformachineswithahydraulic drive system.In all-electric machines,aninjection drive with increased servo motor output functions as pressure accumulator.

》Cellmouldset-upviaB8controlsystem

WithintheB8machine controlsystem,aseparate screenpage is availablefor entering parametersettings forthe process. The controlsystem not only controls the gas supply,but also allprocess-relevant pressure/time parameters.

Alldatacanbe saved togetherwith the productinformation. It is alsopossible tomonitorquality-related trends of selected process parameters via the machine's control system.

Injection molding machine

MOLDED PARTS AND MOLD DESIGN Details for success

Foam injection-molded parts offermanyadvantages,but require speciallyadapted partdesignandan appropriatemold conceptfor optimal results.

》Adjust the part design

-The more even the wallthickness distribution,thebetterthefoamdistributionthat can be achieved.Therefore it is important to avoidlarge differences in wall thickness as far as possible.

-The overall wall thickness should be chosen tofill thecavitycompletelywiththelowest possible pressure.

-Local material accumulations are"hot spots"whichlead to secondaryfoaming during the cooling process after demolding. They should be avoided.

》Make allowances for lower shrinkage

The shrinkage rate of foamed parts is consider ably lower than that of compact parts.This must be taken into account in the detailed layout of both the molded part and the mold with:

-normally a minimum draft angle of 0.5 to 1 degree

-sand paper polishing in demolding direction, especially on ridges and screw bosses increasedstrengthof every screwboss base by reducing thelocal wall thickness to amaximum of 70 to90 per cent of the overall wall thickness and insertion of radi (min. 1\mathsf{mm} )atthetransitionpointbetween ridges and thebase surface

-increase of the draft angle for textured surfaces to at least 1.5 degrees

-avoidance of excessive ridge thicknesses of more than1.3 times the overall wall thickness, since these involve the risk of "hot spots", where secondary foaming may occur

》Optimize cold gating system

-Keep the length of runners to a minimum to minimize pressure loss
-Provide all branching points of the runner system with a radius of at least 1.5\mathsf{mm}
-Do not use tunnel gating (filling pressure required too high)

》Use exclusively needle shut-off hotrunners

-As the plastic/nitrogen blend must bekept underpressurepriortoinjection,onlyahot runnersystem withaneedle shut-off device may be used.

The needle shut-off valve must be able to withstand a pressure of at least 250 to 300 bar.

-Effective heat insulation must be provided betweenthehotrunnervalvesandthe cavity of themoldtoprevent theformation of "hot spots",where secondary foaming may occur.

》Provide efficient cavity venting

To enable relativelyfast injection of thelowviscosityplastic/nitrogenblend,the airpresent inside the cavitymust be driven out as quickly as possible.This can be effected by:

-Provision of venting ducts as large and as numerous as possible, starting from the flow lines and/orthe confluxlines of the melt -Separate venting of blind holes,ridges and thin spots,forexample via pin inserts with a vacuum system

CELLMOULD Application examples

》Mounting plate -Minimization of warpage -Material: PP with 30% glass fiber

》Motorcycleseat - Weight reduction -Material:PA6+GF

》Wheels -Insert technology -Material:PP

PRESSUREANDGASGENERATION Powerful, for one or several machines

DE pressure generator

The pressure generators compress the nitrogen to a suffcient pressure for all Cellmould applications.Depending on its capacity,one unit can supplyeitherone orseveralmachines.With thehelpofamodern control system,thepressure generators canbeoperated byremote controlviathecorporate network.Withitsmodulardesign,everypressure generator canbe retrofitted with a nitrogen generator at any time.

Advantages

y Retrofit with a nitrogen generator possible at any time \gg Moderncontrol systemwithremotecontrolfunction (3) Small footprint
\gg Compact, sound-insulated unit

SE Nitrogen generator

With thenitrogen generatorextensionmodulesoftheSEseries,every pressuregeneratorcanbeconvertedintoaself-sufficientappliance. Special fiter membranes are used to extract the nitrogenfrom the ambient air.The extension modules are adapted tofit the matching pressure generators of theDEseries andcan easilyberetrofitted at any time.

Advantages

y Independent operation,nologistic expense to replace nitrogen bottles
_{\gg} Nitrogen purity of at least 98%
》 One control system forthe entire equipment
》 Compact, sound-insulated unit

WITTMANNBATTENFELDGmbH WienerNeustadterStrasse81 2542 Kottingbrunn|Austria Tel.: +43 2252 404-0 info@wittmann-group.com www.wittmann-group.com