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High viscosity thermoplastic materials should be specified on the basis of the material’s behavior in a sigma mixer rather than the nominal volume of the vessel. The evidence gained from small scale trials must then be related to the production machine’s effective working volume, torque output, temperature control, vacuum application, contact surface material and discharge route. A 5 L mixing bowl may establish a mixing window but reproducing material movement at this high loading in production is a very different proposition.
Write the key specifications as testable criteria. List mass of batch, viscosity range, melting behavior, and discharge characteristics. Perform a witnessed test using the actual to be mixed compound. This way, selecting a ‘large’ mixer on volume alone is avoided, and later the compound overheats or is too large to pass the discharge screw, are avoided. Karvil Machinery can provide the necessary machine type and configuration.
· Size the machine from the effective working volume and batch mass and not from the total volume of the vessel.
· Document softening range, peak temp, change in viscosity and level of shear sensitivity.
· The blade drive, heating, vacuum and discharge are to be treated as inter-linked design variables.
· Use the lab trials to set up the mixing window and then test the production behavior in a material test or in a Factory Acceptance Test.
· Request documented information regarding contact surfaces, safety functions, control logic and cleaning access.
The temperature and shear rate behavior of this compound and how it behaves as a thermoplastic throughout its softening point and charging/discharging cycles.
Record these values from the formulation or a controlled test.
· Batch mass and bulk density: for conversion of recipe into working load.
· Softening point: in order to choose between an electric, thermal-oil, steam jacket and others.
· Viscosity curve: a single viscosity value is not sufficient when the compound undergoes changes of state upon heating.
· Shear sensitivity: Some dough types require intense folding and kneading while others even can suffer from local overheating.
· Volatile or solvent content: This parameter determines whether a vacuum system and/or explosion protection have to be included in the specification.
· Discharge condition: As material i.e. strand, slab, paste or even cooled mass.
The Sigma blades are working to fold and compress a solid like mass rather than just trying to mix a liquid. Torque will reduce as the mix softens but will then sharply increase as fillers, fibers or cold ingredients such as flour or margarine are added. The drive and discharge system should therefore be selected for the hardest working period of the mix rather than the average.
Scale-up works when the laboratory trial produces control limits rather than one successful batch. Track charging, heating, blade speed, vacuum timing, mixing time, and discharge so the production request can separate fixed targets from adjustable ones.
The 5L Explosion-Proof Vacuum Sigma Kneader with Screw Extrusion Discharge is an example for a development-oriented configuration. The total volume of this kneader is 5 L, the effective volume is 3.5 L. The kneader is designed for sigma-blade kneading and is equipped with electric heating, vacuum processing and a screw for the extrusion discharge. The page refers to SS304 stainless steel as material, with mirror-polished surfaces on all material-contacting surfaces. Also the components for the motor, screw motor, heating, vacuum pump and control cabinet are designed to be explosion-proof. The vacuum system is operational up to -0.09 MPa.
Use your lab machine to develop a process window and not to predict conversion by multiplying 5 L batches by some larger number. Record the speed of the blades, the heating up of the machine, vacuum development time, material temperature, end point, and discharge characteristics and motor load. Without this information, scale-up becomes a discussion of nominal capacity.
Space must be allowed for the product to circulate around the blade area, plus space for the discharge system to push the most resistant form of the compound. A previous article on testing a vacuum Sigma kneader before ordering applies the same test discipline to an RFQ.
The 3000L Explosion-Proof Sigma Mixer for High-Viscosity PIB is an example of a detailed production specification. The mixer has a total volume of 3,000L and a working volume of 2,100L. It is a 2 drive configuration with 2 x 75kW and blade speeds of 0-30rpm and 0-21rpm. It is also equipped with a 45kW screw-extrusion discharge and a 0-61rpm screw speed. Additional features are a 72kW thermal-oil heating, SS304L mirror-polished contact parts and a Siemens PLC with corresponding explosion-proofed control cabinet.
These settings are not standard. The relevant points for a specification are: useful volume, blade speed, discharge power, heat transfer, hazardous areas. Each of these points must be related to suitable material tests. Motor power alone is not enough to guarantee suitability.
Specification item | Laboratory reference | Production question | Why it affects the result |
Total and working volume | 5 L total, 3.5 L effective working volume | What batch mass fits the usable range? | A large vessel does not necessarily carry enough material to allow effective contact with the blades. |
Blade drive | Small-scale variable kneading load | What is the peak load during cold charging and softening? | Torque required for startup and for filler addition may be greater than the torque required for normal running. |
Temperature control | Electric heating on the lab configuration | Is thermal oil, steam, electric heating, or cooling required? | This needs to be packaged as a jacket that follows the formula, and does not create a hot wall or slow center. |
Vacuum | Up to -0.09 MPa listed for the lab unit | Skal man bruge vakuum før, mens eller efter man ælter? | Bubbles, volatiles, apparent volume of a compound all are affected by timing. |
Discharge | Screw extrusion discharge | Does the production screw move the final compound at the actual temperature? | A good kneader does not necessarily make a good outlet mixer. |
Contact surface | SS304, mirror-polished on the lab unit | Is SS304L, SS316, or another material justified? | Material compatibility and cleaning requirements must match the desired formulation. |
A larger motor does not compensate for poor filling, wrong blade profile or undersized discharge screw. Ask for the rated range, for overload protection, for the hardest-stage load and for independent blade speeds if you need different shear.
Most thermoplastic compounds require gentle softening rather than a sudden rise in wall temperature. Thus, while the surface is still fluid, the core can still be firm to increase local shear. Log material temperature where possible, jacket medium, inlet temperature and heating time. Rating of heater alone is not enough.
While Vacuum can remove air and volatile components from the foam, it will also change the foam volume, evaporation and blade load. Verify the vacuum level, type of pump, condensate protection, filter type and application point. The deepest available vacuum is not necessarily the best.
Two checks catch many scale-up problems.
First, compare material level at the minimum and maximum planned batch. Below the active blade zone, mixing may become unstable; too close to the cover or seals, expansion and carryover become risks. Ask for a working-volume drawing using actual bulk density.
Test discharge after end of batch. As thermoplastics cool off in a long outlet or one that is not heated, check screw speed, outlet configuration, residual material amount and cleaning access with a test material or a documented alternative material.
Also verify the protective architecture for hazardous formulations. Explosion-proof motor, heating, vacuum and control equipment must be suited for the area of application and comply with local electrical regulations. Reference to CE/ATEX support at the destination is not sufficient; rather, the required marking, documentation and inspection records must be specified in the RFQ. Application-specific information on equipment can be found in Karvil Machinery's application resources.
No. Total volume describes the vessel in question. The working volume on the other hand is the volume which is actually supposed to be filled with material. This can be checked by multiplying the material’s bulk density by the batch mass.
None. Vacuum is beneficial where air or volatiles are critical in the formulation. Test the effect of vacuum on foaming, evaporation, temperature and discharge prior to specifying.
A test record of charging order, blade speed, heating response, material temperature at end of test, end point, vacuum time, motor load and discharge would be useful information. A sample alone is weak evidence without the operating window of that sample.
