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C O M P O U N
D I N G ■
Kunststoffe international 9/2007
HEINI GRÜTTER RICO TRACHSEL
丝瓜面膜HANS-ULRICH SIEGENTHALER
T
he operating principle of the Buss Kneaders from Buss AG,Pratteln,Switzerland,has proven itlf and allows plastics to be compounded in an especially gentle manner.A hall-mark of this specific processing technique is the combination of longitudinal and radial mixing of the melt that results from the oscillating motion of the screw (e box on page 3).
The high-performance kneaders in the new MX Series (Fig.1)reprent an advanced version of the uni
versally applicable MKS Kneaders.The innovative four-flight screw technology [1,2] that was already ud successfully in the quantec Kneader developed for PVC compounding has been further refined and imple-mented throughout the new ries.As a result of the new con-cept,which is explained below,the MX Kneaders can be operat-ed at speeds up to 800 rpm.Conquently,the throughput is 2.5times higher than previously,although the machine size remains the same.At the same time,the MX Kneaders produce enhanced product quality,for instance,when processing halogen-free,flame-retardant polyolefin-bad cable better mechanical properties,more effective flame retardance and im-proved processing characteristics.The MX 105 that will be ex-hibited by Buss at the K 2007 (Booth no.A59 in Hall 16) has a
screw diameter of 105mm and achieves a throughput of 800 to 1,200kg/h for halogen-free,flame-retardant cable compounds.
Four-flight Screw and Larger Processing Chamber
A fundamentally new concept for the entire processing ction provides the basis for the improved performance and quality.The ratio of outside to inside (root) diameter of the screw has been incread.As a result,the MX Kneaders have a larger processing
Buss Kneader
for Cable Compounds
Compounding.With the MX Series, Buss will prent at the K 2007 a new genera-tion of its Buss Kneaders for compounding of temperature and shear nsitive compounds. Without any increa in size, they offer up to 2.5 times the throughput and require about 15 % less specific energy compared to the prent machine.
Fig. 1. Major parts of the high-performance MX Kneader Series: motor-gearbox unit, processing ction with feed locations, directly flange-mounted single-flight discharge screw with melt filtration a
nd pelletizing
Translated from Kunststoffe 9/2007, pp. 207–211
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© Carl Hanr Verlag, Munich Kunststoffe international 9/2007
chamber.The transition to four-flight screw technology,howev-er,is the major reason for the performance and quality improve-ment (title photo) [1].The ratio of stroke to outside diameter was also incread,permitting a high screw pitch.In addition,there are greater opportunities with regard to design and optimization than with the three-flight screw:Flights with longer flanks im-prove the conveying characteristics,and the flight geometry can be ud to influence the mixing action in specific ways in terms of both distributive and dispersive mixing.
Within the processing ction (Fig.2),four functional steps can be distinguished:
■Feeding of polymers,additives and fillers
■Melting with dissipative introduction of energy ■Addition of further fillers
■Homogenizing and conveying with little introduction of energy
For each of the steps,the interaction of kneading flight geom-etry and kneading pin was optimized with the aid of mathe-matical models and confirmed by means of trials.In this way,it was possible to lower the specific energy input overall – it is about 15% less in the MX Kneaders than with the previous de-sign – while simultaneously improving the mixing action.The new design of the cond feed ction,which is ud for fillers,makes it possible for the air entrained during infeed of material to escape largely via rear venting.The venting/degassing rves to remove volatiles and moisture.
手揉纸The improvements achieved with the modified geometry of the processing ction can be illustrated in a diagram showing the melt temperature and throughput versus the screw speed (Fig.3).In the MX kneader,the melt temperature increas with screw speed much more slowly (rate of about 3.5°C/100rpm) than in the MKS Kneader (rate in excess of 20°C/100rpm).Only by gain-ing control of the temperature ri in the processing ction has it become possible to increa the speed in the MX Kneader to as high as 800rpm and simultaneously maintain a temperature lim-it of 190°C when compounding halogen-free flame-retardant (HFFR) cable compounds.It is the higher speed that resu
lts in the increa in throughput – confirmed by trials - from 170kg/h in the MKS 70 with 70mm screw diameter to 525kg/h in the MX 77 with 77mm screw diameter.Taking the diameter into ac-count,the throughput increas by a factor of 2.5.
From Drive to Discharge面鱼子
In addition to the processing ction,Buss has also modernized all other asmblies in the MX Kneader and optimized them in terms of manufacturing and operating costs.
A new motor-gearbox unit consisting of four water-cooled asynchronous motors and a planetary gearbox was chon for the drive of the MX Kneader.The motors drive the planet gears which power the sun gear.The sun gear functions at the same time as a “longitudinal coupling”for the oscillating action that imparts the longitudinal motion to the screw.The new drive unit is more compact than the previous drive with its centrally positioned electric motor.Due to the water cooling,the drive is so quiet that the MX Kneader already complies today with stricter noi limits coming in the future.
Normally,the processing ction of an MX Kneader has an L/D ratio of 15:1 (L =length of processing ction,D =screw diameter) and is divided into three barrel gments.For espe-cially difficult-to-process compounds or when the compound-ing quality must satisfy exceptional requirements,the pr
ocess-ing ction can be lengthened by 7D to an L/D =22:1.
The first barrel ction is ud to feed in the polymers,addi-tives and,where necessary,a portion of the fillers.In the sub-quent melting region,it is possible to meter in liquid compo-nents (upling agents).The cond barrel ction with the previously mentioned rear venting is ud to add the remain-ing fillers to the now-molten material.The resultant cooling of the melt contributes significantly to keeping the stock temper-ature in the MX machines very low.This is followed by ho-mogenizing the melt without any significant ad-ditional energy input,and a cond venting/de-gassing port shortly before the end of the pro-cessing ction.
The energy required for melting and mixing is provided almost entirely by the screw as shear en-ergy.The temperatures of the barrel halves of the MX Kneader are controlled using water or oil.This heating acts primarily as a dynamic insulation and
rves to temperature-condition the inner surfaces of the barrel.This conditioning is necessary to en-sure the desired flow characteristics.
A single-flight discharge screw is flange-mounted directly to the MX kneader.It builds up
the pressure for melt filtration and pelletizing and is equipped with an additional venting/degassing port.A gear pump can also be attached for pres-sure build-up as an alternative discharge device for special requirements or applications.The optionally available automatic screen changer has been designed without any decrea in cross-ction ensuring that the downstream pel-Fig. 2. Flip-open processing c-tion of the MX 105high-performance Kneader, fitted with two vertical
material feed screws and a vac-uum connection
for degassing
Fig. 3. The plot of stock temperature and throughput versus screw speed shows clearly the巧的成语
improvements in the MX Kneader over the previous MKS Kneader
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Kunststoffe international 9/2007
letizing die is covered uniformly with melt.The pelletizer is de-signed for hot melt pelletizing,where the pellets drop directly in-to a fast-flowing water bath that also conveys the pellets away.It has also been possible to reduce the noi from the pelletizer in view of the forthcoming noi limits.The aerodynamically opti-mized cutting rotor as well as the new design for the pelletizer hood and the conversion from sheet steel to aluminum castings have been major contributors in this respect.
The new MX Series features a modular design for the Buss Kneader and the downstream modules such as discharge screw,screen changer and pelletizing unit.The can be shifted as nec-essary along two axes on a lin-ear system.This facilitates ac-cess to the individual modules色即使空
for cleaning and maintenance work and makes fast product changes possible.The dis-charge module can be either a single-flight screw or a melt pump.The PC control utilizes a Simatic S7 with Cimplicity vi-sualization on a Windows XP Pro platform,has an intuitive
operator interface and provides control and monitoring of the entire system,including data acquisition,recipe management etc.The control system can be expanded as necessary to meet customer requirements.
Field of Application for the MX Kneader
The improvements described above for the Buss Kneaders in the MX Series reprent decisive advances with regard to applications and economics.For the same size,they achieve 2.5 times greater output.At the same time,product quality increas:Halogen-free flame-retardant cable compounds posss better mechanical properties even at very high filler levels,exhibit better values for flame retardance and have better processing characteristics.The most important field of application for MX Kneaders is in the production of high-quality polyolefin-bad cable com-pounds.U of aluminum hydroxide as a halogen-free flame re-tardant is especially important in this regard.In the event of a
fire,this chemical releas water in an endothermic reaction at temperatures as low as approx.200°C.This cools the burning area and restricts the flow of oxygen.For effective flame retar-dance,filler levels of up to 65wt.% are common – while the ca-ble insulation must still exhibit good val-ues for strength and elasticity.The antic-ipated improvements in throughput and product quality have been confirmed in trials.Semiconducting cable compounds po a special challenge.They rve to smooth out spikes in field strength in the vicinity of the strands in medium- and high-voltage energy cables and provide ex-ternal shielding for the cables.T o fulfil this function,miconductive co
mpounds usually contain 30 to 40wt.% conductive carbon black.During compounding,the MX Kneader must disper carbon black
agglomerates,but not destroy the fine-grained structure of the conductive car-
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bon black in the process of incorporating it into the matrix resin.Here,too,the MX kneader met all expectations in production trials:The surface quality of the cable sheathing achieved with the new compounds is better than previously,with no loss in conductivity.This indicates that the carbon black is disperd without any damage to its structure.
Summary and Outlook
One of the principle applications for the new MX Series is for the production of halogen-free,flame-retardant cable com-pounds.For environmental and safety reasons,the compounds continue to gain in importance.With the new MX Kneaders,they can now be produced at considerably lower costs thanks to the 2.5 improvement in throughput.At the same time,the HFFR compounds exhibit better characteristics in terms of pro-cessing and in their application as cable sheathing.
Furthermore,trials at the Buss Test Centre have shown that the same improvements can also be achieved with mi-conducting compounds.It can be concluded from the re-sults that the MX Kneaders will also provide similar advances with other cable compounds,for instance,black master-batches.Further developments will certainly show the ad-vantages can be applied in other shear and temperature n-sitive applications.■
REFERENCES
1Franz, P .: High-output Compounding. Kunststoffe international 92 (2002) 2, docu-ment number PE102261
2Gotzmann, G.: Innovationspreis für Kneter. Kunststoffe 96 (2006) 5, pp. 24–25 THE AUTHORS
DIPL.-ING. HEINI GRÜTTER, born in 1944, is Senior
Process Engineer at Buss AG, Pratteln,
DIPL.-ING. RICO TRACHSEL, born in 1974, is Manag-er of Development at Buss AG, Pratteln, DIPL.-ING. HANS-ULRICH SIEGENTHALER, born in 1965, is Manager of Process Technology at Buss AG,Pratteln, Switzerland; hansulrich.
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