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Before informing about the mixers relevant to the production of SMC and BMC compounds, let us mention briefly about SMC and BMC compounds and their production processes.
SMC Compound
SMC (Sheet Moulding Compound) is a sheet-shaped material and SMC machine is used to combine resin including fillers and chopped glass strands in order to produce the finished SMC compound.
Describing the production of sheet-shaped SMC material as a two-stage process would not be wrong. The main raw material resin and the other auxiliary materials are mixed by a high-speed mixer machine and semi-manufactured SMC compound is produced at the first stage of the process.
Glass strands and semi- manufactured SMC compound are combined by using SMC machine at the second stage of the process.
SMC Compound Production Flow Chart
BMC Compound
BMC (Bulk Moulding Compound) is a dough-formed material and zet mixing machine (also called as Sigma Mixer) is used to combine resin including fillers and chopped glass strands in order to produce the finished BMC compound.
Like SMC compound, BMC compound production process consists of a two-stage process at both of which two different mixer machines are used, which are high speed mixer at the first stage and zet mixing machine (sigma mixer) at the second stage.
Although the internal processes of the manufacturers may differ from each other, the general application performed is as below. Resin and the other raw materials except some raw materials such as calcite or glass strands are added into the mixing vessel of high-speed mixer and get them mixed until getting a homogeneous mixture of the semi-manufactured BMC compound.
After production of the semi-manufactured BMC compound, this semi-product is taken to the second-stage of the process.
A horizontal mixer with a set of kneading blades commonly called as zet mixing machine in Turkey and mostly known as Sigma Mixer in the world is used at the second stage of the BMC compound production process.
At the second stage of the process, the semi-manufactured BMC compound and the other raw materials (glass strands, calcite and any other raw materials depending on product) are added into the mixer vessel respectively. The kneading cycle is continued until getting a homogeneous BMC compound.
BMC Compound Production Flow Chart
Mixer Machines Used for The Productions of SMC and BMC Compounds
The below mixer machines are commonly used to produce these compounds. Please note that these two mixers are mostly used mixer machines in the composite industry.
1. High Speed Mixer Machine (Dissolver)
2. Zet Mixing Machine (Sigma Mixer)
1. The High-Speed Mixer Machine (Dissolver)
The high-speed mixer with sawtooth impeller on the mixing shaft also known as Dissolver is commonly used for dispersion applications. Unlike standard high-speed mixers, this dispersion mixer needs to have some additional features in order to be used for the production of semi-manufactured SMC and BMC compounds.
The same high-speed mixer is used to produce both semi-manufactured SMC and BMC compounds. Due to the viscosities of both semi-manufactured SMC and BMC compounds at the first stages of their processes, the below optional features should be added into the high speed mixer configuration:
• More powerful main electrical motor option,
• Scarper arm option.
Although both semi-manufactured SMC and BMC compounds do not have very high viscosities, a more powerful motor should be added into the machine configuration in order to mix the material without difficulty. A scraper arm helps to exchange materials from different part of the vessel by providing additional agitation.
Thanks to constantly feeding the high-speed agitator (dissolver disk) with product, it helps get a homogeneous product in a shorter time compared to not having this option. The addition of scraper arm further improves product homogeneity and heat transfer efficiency.
2. Zet Mixing Machine
(Sigma Mixer)
Zet mixing machine falls into the horizontal mixer group and is used to mix liquid-liquid or liquid-solid mixtures like the vertical mixers such as high-speed mixers. Thanks to providing high torque capability, zet mixing machine may sometimes be only option in order to produce the products with very high viscosities.
All double arm zet mixing machine is equipped with two kneading blades with Z-shaped placed in a U-shaped horizontal trough. The mixing principle of zet mixing machine is based on the shearing action between the wall of trough and the blades and tearing action of material.
The zet mixing machine includes two blades that rotate toward each other at differential speeds. Each blade moves the material in opposite directions thereby providing excellent cross mixing of all raw materials.
The blades pass the trough walls and each other at close clearances. This close clearance action provides a shearing and tearing actions that are beneficial to the size reduction of solids.
Any zet mixing machine needs to have the below qualifications in order to perform a good mixing performance:
1. The kneading blades should pass the vessel wall and each other at close clearance so that the zet mixing machine can produce very high shearing action to reduce any large particles. Depending on the capacity of the mixer, the close clearance should be from 2 to 5 mm,
2. There are two kinds of blade actions are available, which are tangential or overlapping. In the tangential design, the blades rotate towards each other at differential speed, one of the blades rotates faster than the other blade.
In the overlapping design, the relative position of two blades is unchanged, which means both blades rotate at the same speed. The most important point here is that the zet mixing machine with tangential blade action should be chosen for higher viscosities such as adhesives, rubber compounds.
3. Combined Planetary Mixer Machine
Depending on the viscosities, both the design and features of the vertical mixers have to be changed. When a batch becomes too viscous, high speed mixer with a fixed axis of rotation will not finally produce sufficient flow, resulting in weak product homogeneity.
Mixer should be selected from the left to the right with the rising viscosities.
Due to having two features, combined planetary mixer machine distinguishes from the other vertical mixers. The first and the most important feature is that the two planetary shafts with angled blades rotate on their own axes while they orbit on a common axis.
Because of the advancing blades through the batch, the blades continuously move through the batch irrespective of product flow. The second feature is to have a high-speed shaft which is mandatory for some formulations. High speed shaft with sawtooth impeller is very effective at powder wet-out and performing dispersion.
In addition to these two shafts, the combined planetary mixer has also another shaft called as scraper arm which helps agitation process by removing material from the wall to the center.
Only combined planetary mixer machine can be chosen instead of zet mixing machine as an alternative. We should keep in mind that zet mixing machine is going to be the only option to produce the products with very high viscosities.
Because the combined planetary mixer machine fails to produce adequate flow inside the tank after a certain high viscosity value.
There are four shafts in the combined planetary mixer as follows:
1.One high speed shaft with sawtooth impeller in the center,
2.Two planetary shafts with angled blades on the both sides,
3.Scraper arm to scrape the wall of vessel and try to push the unmixed product into the center of the vessel. Also, scraper arm, turning in the opposite direction for mixing shafts, that provides the product to be homogenous in a much shorter time.
T. Emrah Sözer
Production Director
Sözer Makina Sanayi ve Ticaret Ltd. Şti.
