The Stock Blender is an add-on accessory mounted on top of an open two-roll mill. It aids in rubber blending by pulling the mix compound from the front roll of the mill and guiding it to the nip gap between the two mill rolls.
Structurally, this machine assembly consists of a blender roll, a carriage unit with guide rolls, pressure roll and rear roll along with required automation.
The Blender Roll is a hollow roll with water passage for internal cooling. The surface of this roll is hard chrome plated, mounted on antifriction bearings at both ends and operates on a variable speed motor. The VFD allows you to tune for every compound or recipe, thus enabling the speed of the stock blender to follow mill speed. This feature is crucial to ensure that compound does not sack-in or tear-off when taken up and through the stock blender.
On the assembly frame is also a hollow pressure-roll mounted on antifriction bearings and has a swivel operating arrangement. Pneumatic cylinders actuate this movement. The Carriage with screw and guide rollers is equipped with two limit switches at either end of travel. These limit switches signal the carriage traverse motor to reverse at extreme ends during to & fro travel, while guiding flow of rubber to mill nip via blender roll.
A stock blender helps in homogeneous mixing and repeatability of the mixing operations. Many companies use them on the open mill (also known as dump mill) below the internal mixer. After masterbatch mixed in an internal mixer, curatives are added and final homogenization done on these mills. Because the mill helps incorporate undispersed curative or fillers that might have fallen off the ram or drop-door when the batch was dropped. In a different scenario, if you need to drop the mix batch at a higher dump temperature, you could use the stock blender to take some heat out of the batch and then add the curatives on the mill. Reduction in temperature is obtained by allowing the batch from the mill roll airborne to pass over the water-cooled blender roll before it returns back to the nip gap. When these mill rolls (centrally cored or peripherally drilled design) are properly cooled, the stock-blender allows you to reduce the batch’s temperature continuously for lower viscosity compounds.
Stock Blenders are also used on pre-warming mills before feeding the rubber sheet to a Calender.
A Stock blender enhances efficiency and productivity through reduced operator fatigue. Cutting down hot rubber many times, rolling the sheets and feeding them back into the nip gap of the mill rolls is a physically demanding task for the operator. Stock blenders eliminate this completely and improve your batch to batch consistency. The operator also benefits from a safe work-environment.
The operators, though need to be trained well. For an open mixing mill the stock blender can be used only when the batch has achieved a certain stage of homogeneity. When a batch from the internal mixer is dropped on an open mill with stock blender, the operator must pass the complete batch through the nip before diverting the batch to the stock blender. Else, there is a risk that only a part of the batch turns round the stock blender. A big lump,which is neither cooled nor mixed, would continue to ride and turn on top of the nip. This lump does not pass the nip until the operator evacuates the compound to second mill or batch off.
The stock blender designs vary among manufacturers. Hence, not paying attention to the design features can result in owner’s or operator’s nightmare. For example designs with ledges behind the rolls can cause compound to accumulate and these must be cleared out regularly to prevent contamination. Similarly, a ball screw with tube casing for carriage movement is superior to a conventional screw design and eliminates contamination from lubrication of the screw. Compact designs and increased level of automation with programmable carriage traverse movements make the stock blenders a joy to work with a high level of safety.
Summarizing, buying a good robust stock blender increases the mixing capacity of an open mill, improves the batch cooling, reduce mixing time and improves dispersion. With a trained operator, you can extract the best returns out of your investment in this rubber mixing machinery accessory.
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