8 typical mixing errors
The hand-mixing of ready-mixes, which generally come in powder form, is often carried out with an electric hand-held mixer or even using a power drill for the drive unit, in combination with a paddle.
In many cases too little attention is given to the mixing process. However, mixing is a production process at the building site which has a vital influence on the results of subsequent work in terms of both quality and workability.
1. The mixing attachment is unsuitable for the material
Problem: Formation of lumps in cement-bonded, self-leveling fillers and leveling compounds. These are no longer broken up during mixing.
Cause: The wrong type of paddle was used for these low-viscosity materials.
Prevention: Powdery, cement-based fillers are quick to form lumps on contact with water. For these to be broken up as quickly and thoroughly as possible during mixing requires a paddle with a high shear force. The paddle must have upright mixing blades in order to effectively break up the lumps with turbulence and produce a homogeneous mix in the shortest possible time.
2. Mixed from the bottom up instead of top down
Problem: Unmixed residual material can be found on the base of tubs when screeds or fillers with a larger filler fraction are mixed. These can cause the characteristics of the material to change, since the mixing ratios between the components is not longer right.
Cause: The paddle used has the wrong mixing action. The heavy material is conveyed with great effort during mixing. The mixer forces itself out of the material.
Prevention: The mixing action of the paddle varies according to the viscosity of the material being mixed - either from the bottom up or vice versa. The heavier the material, or the more aggregates it contains, the better it is for the material to be conveyed from the base of the tub to the top. In this way the paddle has no difficulty reaching the base and therefore mixes all the material thoroughly. With a paddle which rotates anticlockwise, working from the top down, the base of the tub is not reached in many cases when used manually, because this involves too much physical strain for the operator. Moreover, this incorrect mixing action can also overload the drive machine.
3. Order for adding the materials is ignored
Problem: The mixed material is difficult to use for further work or is very lumpy. It requires a lot of reworking or the mixture is unsuitable for use.
Cause: The order in which the water and material has to be added was not followed.
Prevention: Cement-bonded materials in particular need to have the water poured into the tub first, with the dry material coming afterwards. Doing this in reverse order will often result in the powder not being completely wetted. This leads to the formation of lumps and adhesions which will no longer dissolve. The result is a much longer mixing time to at least achieve a reasonably homogeneous mixture. For screeds it is possible to wait until after the initial wetting stage before adding a certain amount of the water (e.g. 1/3 of the quantity) during mixing.
4. Mixing container is too small
Problem: Excessive splashing of the material out of the mixing tub during mixing. This makes a mess of the surrounding area and the operator’s clothing.
Cause: The size of the mixing tub was not appropriate for the mixing volume.
Prevention: The more appropriate the selected mixing tub for the mixing volume, the quicker an optimum mixture can be achieved and the cleaner the work gets done. The tub should be selected so that the paddle of the mixer is always completely covered by the material being mixed. If it projects out of the material, it will be flung up.
5. Incomplete mixing / mixing time not long enough
Problem: The product characteristics of the material do not develop to proper effect. This can lead to warranty claims and expensive reworking.
Cause: Non-observance of mixing times by the person carrying out the work or insufficiently thorough mixing of the mixing volume with the mixer.
Prevention: Many chemical building products require very specific characteristics. To ensure that these develop to full effect it is essential that the mixture is prepared precisely in accordance with the technical information issued by the manufacturer. This is the only way to achieve the required product quality. Inadequate adhesion, cavities or spalling are often the result of insufficiently thorough mixing. Some materials require a certain amount of time for maturing to be allowed after mixing.
6. Mixing without enough speed
Problem: The spreading characteristics of a 1-component or 2-component filler do not develop. This results in the need for extra manual work and in many cases the need for more material as well.
Cause: The mixer does not have enough speed. There is not enough power input to the material to bind the liquid components with the solid components.
Prevention: Depending on the material, the speed of the drive has to be selected in accordance with the specifications issued by the manufacturer of the material. The paddle diameter also has an influence on its circumferential speed. A large diameter achieves a higher power input and facilitates better material flow characteristics.
7. Unsuitable mixing tool
Problem: Reaction resin mortar or coatings harden too quickly or still have sticky areas left over after hardening. This can be interpreted as a defect, because it generally leads to consequential damages.
Cause: In this case the wrong mixing technology has been used. Two-component systems always require great care when it comes to dosing and mixing. The use of a power drill with a home improvement store paddle or even with a simple mixer is extremely risky. Optimum mixing results cannot be guaranteed with unprofessional tools.
Prevention: It is important to check whether the A- and B-components (= resin and hardener) each have to be premixed separately in a clean tub. This ensures that proportionally thorough mixing is already achieved as a start. When adding quartz sand, the use of a dual-spindle paddle or even a forced-action mixer is always strongly recommended for the mixing process. An intensive bond between the resin mix and the heavy sand can only be achieved by using these tools. This will also cut mixing times dramatically.
8. Cleaning negligence
Problem: Deposits such as dried-on residue on the mixing attachment become detached during the next mixing operation. Rather than dissolving they form undesired foreign bodies in the mixture, often leading to fatal consequential damages.
Cause: Due to pressure of time the proper cleaning of the paddle is often neglected at the building site. This results in heavy deposits which “grow” with every mixing operation.
Prevention: Collomix has developed the Mixer-Clean mobile cleaning system to enable paddles to be cleaned quickly and easily. The dirty paddle is placed in a tub with robust bristles and sufficient water, and the drive unit started up for a short duration. The bristles completely strip off the residue, getting the paddle completely clean and ready for the next professional job again in next-to-no-time.