Conical Surface Mixer Testing

Experimental Setup and Procedures

The system was set up in a drill press to allow variable speeds and feed rates and to monitor the mixing (Figure 8-14). The mixing body was clamped to the work table and the mixer head shaft was tightened into the drill head chuck.

Three tests were undertaken as the testing method was refined further. Two fluids were used throughout these tests. The fluid used for the first test was a rice flour mixture, with liquid food dye to visualise the mixing mechanisms. This helped get a basic understanding of the mixing, but the extrusion results were somewhat poor. To examine the mixing and extrusion performance of the mixer with a fluid more like the actual food batter, the second and third tests used a xanthan and rice flour mixture.

1st_Test

Initially, to examine the way that surface mixing drags and pushes the fluid, a spindle speed of 70RPM was used. This speed allowed the movement of particles and dye paths to be noticed and observed quite clearly.

70RPM– visualisation

To improve the uniformity of the mixing, a cyclic axial movement in and out of the mixing cavity (pecking) of increasing depth meant that the dye (deposited in the middle) would get more evenly distributed throughout the whole recess.

The spindle speed was increased to examine the effects on the mixing. At 270RPM, the mixing happened more quickly and also caused some transfer of fluid from the tip of the cone to the base. However, when the speed was increased to 540RPM, the fluid was transferred from the tip of the cone to the base much faster and when it reached a certain point, was thrown of the mixer in a radial manner. This increase in transfer from the tip of the cone was assisted by the fluid clumping. The clumps would almost roll and ‘climb’ up the cone.

270-540RPM – increase mixing

2nd

During the second test, the xanthan and rice flour mixture was used, with a spindle speed of 270RPM. This test was mainly to examine how well the mixer could extrude the xanthan and rice flour mixture. It was also useful to improve the mixing technique, by getting a balance between pecking time and full mixing.

Test

3rd

The third test also used the xanthan and rice flour mixture with the aim of obtaining consistent results with a consistent method. It was decided that the photographs of the extrusions should be taken in the controlled environment of a light box. From previous testing it was concluded that 10 seconds of the pecking cycle and 10 seconds of full mixing should produce consistent results. An auto-pipette was used to deliver a relatively consistent

amount of mixture into the mixer. The high viscosity of the mixture meant that the auto- pipette was not 100% reliable, but by operating the auto-pipette in a consistent way, it was still able to deliver a consistent mass of mixture (6.5g). This was achieved by inserting the tip of the auto-pipette into the mixture for 10 seconds while it sucked up mixture.

Results

Uneven mixing/Stagnant region

Although about the top 80% of the cone created effective and homogeneous mixing, the bottom 20% and the extrusion hole were areas where mixing was ineffective. Due to the small surface area of the tip of the mixing head, there is little drag force exerted on the fluid and there is nothing driving food dye down to this region in the mixer. No matter how long, how fast or whether pecking was used, there would be some unmixed volume of fluid that would be extruded first. Figure 8-15 shows an extrusion sample, with highlighted sections showing where the image was analysed.

FIGURE 8-15–A)EXTRUSI ON FROM THE SECOND TEST OF THE CONICAL SURFACE MIXER WITH WHITE UNM IXED M IXTURE ON RIGHT (ARROW SHOWS DIRECTI ON OF EXTRUSI ON)B)HIGHLI GHT ED AREAS SHOW

REGIONS ANALYSED

Figure 8-16 shows that the colour values at the six selected positions are fairly consistent over the length of the extrusion, with only relatively small change from one end to the other. The G and B values do show a noticeable increase in the middle, but is quite a minor change in colour. Saturation is again the most important measure of mixedness and it can be see that the mixture only had a slightly higher saturation towards the end.

R, G, B, H, S & L vs. Extrusion Position 0 50 100 150 200 250 300 1 2 3 4 5 6 Extrusion Position Co lo ur U ni ts _R G B H S L

FIGURE 8-16-COLOUR VALUES AT EXTRUSI ON POSITIONS

Extrusion

The first testing showed that after extrusion there was a significant amount of residue left on the mixing head and mixing body. This may have been caused by the nature of the rice flour mixture, which allows the moisture to be forced away from the mixture leaving mainly starch remaining.

The second and third tests used the xanthan and rice flour mixture. Two noteworthy observations were made during these tests. Firstly, the food dye, which was again deposited on top of the plain mixture came into contact with the mixer head before the mixture did, which created a sort of lubrication layer between the mixer head and the plain mixture. This layer, which prevented the mixing head from mixing effectively, was overcome when a deeper plunge into the mixer body was made. Once this lubrication layer was overcome, the plain mixture and the food dye mixed effectively. The second observation, which was very promising was that the extrusion process seemed easier than during the first test and there was very little residue left on the mixer head and body, only residue due to overflow (Figure 8-17).

FIGURE 8-17–CONICAL MIXER BODY AND HE AD AFTER E XTRUSION (OVER FL OW CAUSED BY TOO M UCH

This would be due to the viscoplastic fluid properties of the xanthan solution, which allow the mixture to flow once a certain yield stress is reached. This testing helped to show what kind of materials may behave well when used in the CSM. For a material to behave well in the mixer, it must allow two processes to happen. Firstly, the material must allow the friction of the mixing head to distribute it throughout the mixing volume to facilitate mixing. Second, it must also allow the mixing head to force it out of the mixing body while leaving minimal residue. For the mixing process to occur, the material must remain in the mixing chamber while being mixed. This indicates the material must have a viscosity within a certain range. It must be viscous enough to not leak out of the mixing body, but not be too viscous to prevent extrusion. Lubrication is also essential to allow the mixture to slip out once the mixer head is forced into the mixer body. As xanthan is a lubricating agent with viscoelastic properties [104], it makes it very suitable for this application. More materials should be tested with the CSM to determine how versatile the mixer is and if there are any other suitable materials. This test also tested the functionality of the spring to apply appropriate compressive force on the cover. The spring applied sufficient force to contain the mixture, and allowed easy extrusion. As seen in Figure 8-17, some overflow was present, so the following tests targeted this problem. More testing with a more appropriate amount of mixture is required to confirm the effectiveness of the spring loaded cover.

The third test demonstrated the functionality of the spring loaded cover, but also showed that a more controlled method is required. This is because even with consistent timing, the height of the mixing head during pecking cycle and full mixing is controlled by hand and therefore inconsistent. This is evident when the mixture is extruded after mixing (Figure 8-18).

FIGURE 8-18-INCONSI STENT MIXING

Improvements

The residue problem can be dealt with in two ways. Either avoid changing colour between one voxel and the next (leaving the residue in place), or have a cleaning cycle between making each voxel. Using an elastomeric material for the mixing head and implementing nutating movement are two ideas that may improve mixing. These are explained in Section 5.2.2.