Mixer Solutions for BMC and SMC Materials Production

Before delving into mixers related to SMC and BMC production, it would be useful to briefly discuss SMC and BMC materials and their production processes.

SMC Material

SMC (Sheet Molding Compound) is a material produced by combining chopped glass fiber with resin containing filler material in an SMC machine, resulting in a paste-like material. It would not be incorrect to compare the production of paste-form SMC material to a two-stage process. In the first stage of this process, resin as the main raw material and other auxiliary materials are mixed with the aid of a high-speed mixer, producing the SMC paste. In the second stage of the process, glass fiber is combined with this paste using an SMC machine.

BMC Material

BMC (Bulk Molding Compound) is a material produced by combining chopped glass fiber with resin containing filler material in a Z-blade mixer machine, resulting in a paste-like material. As with SMC material, BMC paste production consists of a two-stage process, and different mixer machines are used in each stage. In the first stage of the process, a high-speed mixer is used, and in the second stage, a Z-blade mixer (sigma mixer) is used. While companies' internal processes may differ, the general application is conducted as follows. In the first stage, resin and other raw materials are mixed homogeneously in a high-speed mixer to produce BMC pre-paste. The BMC pre-paste is then taken to the second stage of the process. In the second stage of the process, horizontal mixers known in our country as "Z-Blade Mixer" and referred to in literature as "Sigma Mixer" are used. In the second stage of the process, BMC pre-paste and other raw materials (glass fiber, calcite, and depending on the product, other raw materials) are fed sequentially into the mixer vessel. Mixing continues until a homogeneous BMC paste is obtained.

Mixers Used in SMC and BMC Material Production

The following mixers are the main mixers used to produce these materials. These two mixers are the most commonly used or preferred mixers in the composites sector. 1. High-Speed Mixer (Dissolver) 2. Z-Blade Mixer (Sigma Mixer)

1. High-Speed Mixer

High-speed mixers with a saw-tooth impeller on the mixing shaft, also known as dissolvers, are the most commonly used mixers in dispersion applications. The dispersion mixer to be used for SMC and BMC pre-paste production must have certain optional features unlike standard high-speed mixers. Due to the viscosity of SMC and BMC material pre-pastes, it would be beneficial for a mixer to have the following options: • A more powerful main electric motor • A scraper blade option Although SMC and BMC pre-pastes do not have extremely high viscosity values, a more powerful motor is necessary for the motor to mix the product without strain. The scraper blade provides additional mixing by assisting product movement from different areas of the vessel. Since the high-speed mixer continuously feeds the product, the scraper blade helps achieve a homogeneous mixture in a shorter time compared to a mixer without this feature. The scraper blade enables a more homogeneous product to be obtained and better heat transfer to be achieved.

2. Z-Blade Mixers (Sigma Mixers)

Z-blade mixers are part of the horizontal mixer group and are used to produce liquid-liquid or liquid-solid mixtures, similar to high-speed vertical mixers. Due to their ability to generate high torque, Z-blade mixers sometimes emerge as the only option for producing products with very high viscosity. All dual-blade Z-blade mixer machines have 2 "Z"-shaped kneading blades inside a "U"-shaped horizontal vessel. The mixing principle of Z-blade mixers is based on the material being squeezed between the vessel inner wall and the blades, and the product being mixed by tearing or shearing. The Z-blade mixer machine has 2 blades that rotate toward each other at different speeds. Each blade moves the material in the vessel in different directions, thus ensuring perfect cross-mixing of all material. The Z-blade mixer blades pass as close as possible to both each other and the vessel wall. This close movement creates both shearing and tearing action, thereby reducing the particle size of solids. For a Z-blade mixer to demonstrate good mixing performance, it must have at least the following characteristics: 1. The mixing blades must pass as close as possible to both each other and the vessel wall. Thus, high shearing can be created, reducing particle size. Depending on mixer capacity, the clearance to the vessel wall should be between 2-5 mm. 2. Two types of blade movement can be observed in Z-blade mixers: tangential or overlapping movements. In tangential rotation, the Z-blade mixer blades or paddles rotate toward each other at different speeds. One blade's speed is higher than the other. In overlapping rotation, the relative position of the blades remains constant. This means these blades rotate at the same speeds. The very important point here is that if a Z-blade mixer is to be used for a product with high viscosity such as adhesive or rubber, a Z-blade mixer with tangential blade movement characteristics must be selected. This is because tangential movement is required to produce high-viscosity products.

Alternative Mixers to Z-Blade Mixers:

Combined Planetary Kneader Machines. The design and characteristics of vertical mixers vary depending on viscosity. When viscosity increases significantly during production of a product, high-speed mixers with a fixed shaft cannot sufficiently mix the product in the mixing vessel, and this results in poor homogeneity of the product. With increasing viscosity, mixer selection shifts from left to right as follows: Combined planetary kneaders differ from other vertical mixers due to two characteristics. The first and most important characteristic is that they have two planetary shafts and angled blades on these shafts. These planetary shafts rotate both around their own axes and in a certain orbit within the vessel. Due to the orbital movement of the planetary shafts in the vessel, the planetary blades constantly move within the production charge. The second characteristic is that it has a high-speed shaft, and this characteristic is a mandatory feature for producing certain formulations. The high-speed shaft with a dispersing blade is very effective in powder wetting and dispersion operations. In addition to these shafts, the combined planetary kneader machine also has another shaft called the scraper blade that assists the mixing operation by moving the product along the vessel wall toward the center. Only the combined planetary kneader machine can be evaluated as an alternative to the Z-blade mixer machine. It should not be forgotten that for producing products with very high viscosity, sometimes the Z-blade mixer machine is the only option. This is because beyond a certain very high viscosity value, the combined planetary kneader machine will also fail to create adequate mixing in the mixing vessel. The combined planetary kneader machine has the following four mixing shafts: 1. A high-speed dispersing shaft in the center 2. Two planetary shafts on the right and left sides of the high-speed shaft, rotating both on their own axes and orbiting within the vessel 3. A scraper blade or shaft to scrape the vessel wall     T.Emrah Sözer Industrial Engineer and Production Director Sözer Makina Sanayi ve Tic. Ltd. Şti.     Mustafa Önder Sales and Marketing Coordinator Sözer Makina Sanayi ve Tic. Ltd. Şti.