Mixing rubber compounds is a multi-stage process that uses both open mills and internal mixers.
This process ensures that the rubber compound is ready for further processing or vulcanization.
5 Key Steps to Achieve a Homogeneous Rubber Compound
1. Preparation and Initial Mixing on Open Mills
Adjusting Roll Distance: The first step involves setting the roll distance of the open mill to 0.5mm.
This narrow gap helps in creating a thin layer of rubber for better mixing.
Inserting Rubber Compound: The rubber compound is then placed into the mill.
A triangular bag is formed to facilitate even distribution of the compound across the rolls.
Thin Passes: The compound is passed through the rolls five times.
This ensures thorough mixing and eliminates any air bubbles.
Adjusting Roll Distance Again: The roll distance is then increased to about 2.4mm.
This facilitates the addition of rubber rolls and ensures a smooth surface without bubbles.
2. Cooling and Labeling
Weighing and Cooling: After mixing, the total mass of the rubber compound is weighed.
It is placed on a clean, flat metal surface to cool to room temperature.
Labeling: A label is attached to the compound indicating the formula number and mixing date.
This is for traceability and future reference.
3. Detailed Mixing Process in Internal Mixers
Preparation of Raw Materials: Based on the capacity of the internal mixer and a filling factor of 0.6-0.7, the amount of each component is accurately weighed and arranged in order.
Preheating and Checking: The mixer is preheated, and checks are performed on air pressure, water pressure, voltage, and the functioning of the temperature measurement system, timing device, and power system.
Sequential Addition of Components: The mixing process begins with the addition of raw rubber, followed by small materials, carbon black or filler, and finally, liquid softeners.
Each addition is mixed for a specified duration to ensure uniform dispersion.
4. Function and Mechanism of Mixing Mills
Initial Insertion and Wrapping: The untreated rubber block is inserted into the nip between the rolls.
The rolls pull the block in and wrap it around the front roll.
Addition of Additives: Additives such as fillers and softeners are added sequentially in the roll nip.
This ensures they are properly mixed with the rubber.
Repetitive Mixing: The rubber roll is repeatedly inserted and wound up.
This ensures complete incorporation of additives into the rubber base material.
Final Shaping: Once the additives are uniformly dispersed, the rubber mixture is shaped and prepared for vulcanization.
5. Role of Mechanical Shear Forces
Shearing and Mixing: The primary mechanism of mixing in both open mills and internal mixers involves the application of mechanical shear forces.
These forces help in blending, softening, homogenizing, and dispersing the ingredients within the rubber compound.
Temperature Control: The process of mixing generates heat.
This is managed to prevent degradation of the rubber.
The internal mixer, due to its higher efficiency and greater shearing effect, handles this more effectively than the open mill.
By following these detailed steps and utilizing both open mills and internal mixers, the process of mixing rubber compounds ensures a high-quality, homogeneous blend ready for subsequent stages of rubber processing.
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