I. Open Mill Mixing Technology

The mixing process includes three aspects:

Three processes: Open mill mixing can be divided into three stages: ① wrapping, ② powder feeding, and ③ turning.

I. Wrapping When unsaturated rubber is processed on an open mill, changing the roller temperature will result in four different wrapping states. At lower temperatures, the material is harder and more elastic, and the rubber mainly slides at the piled-up rubber area, delaying the production process. At a suitable temperature, the rubber can normally wrap around the roller, with both plastic flow and appropriately high elastic deformation, which is beneficial for mixing operations. As the temperature increases, the fluidity increases, the intermolecular forces decrease, the strength decreases, and the rubber sheet cannot tightly wrap around the roller, resulting in slippage or breakage. At higher temperatures, the rubber is a viscoelastic fluid that wraps around the roller and produces plastic flow.

II. Powder Feeding After the rubber wraps around the roller, to allow the compounding agents to mix into the rubber as quickly as possible, an appropriate amount of piled-up rubber should be maintained on the roller gap. During powder feeding, when the rubber enters the upper layer of the piled-up rubber, it is congested and folded due to resistance, forming wrinkle grooves in front of the piled-up rubber, allowing the powder to enter these grooves and be brought into the interior of the piled-up rubber. If there is no piled-up rubber, the compounding agent only relies on the shear force between the rear roller and the rubber to rub the powder into the rubber, which does not allow the powder to penetrate deep into the rubber, affecting the mixing effect. Moreover, the powder that has not been rubbed into the rubber will be squeezed into thin sheets by the rear roller and the rubber and fall into the receiving tray. If it is a liquid compounding agent, it will stick to the rear roller or flow to the receiving tray, making subsequent mixing difficult. During powder feeding, the amount of piled-up rubber must be moderate. If there is no piled-up rubber or the amount of piled-up rubber is too small, on the one hand, the compounding agent only relies on the shear force between the rear roller and the rubber to rub into the rubber, which cannot penetrate deep into the rubber and affects the dispersion effect; on the other hand, the powdery compounding agent that has not been rubbed into the rubber will be squeezed into sheets by the rear roller and fall into the receiving tray. If it is a liquid compounding agent, it will stick to the rear roller or fall into the receiving tray, making mixing difficult. If there is too much piled-up rubber, some of the rubber will rotate and roll at the upper end of the roller gap, unable to enter the roller gap, making it difficult for the compounding agent to mix in.

III. Turning The third stage of mixing is turning. Due to the high viscosity of the rubber, during mixing, the rubber only produces circumferential flow in the direction of rotation of the open mill roller, without axial flow, and the rubber flowing in the circumferential direction is only laminar flow. Therefore, the rubber layer close to the front roller surface at approximately 1/3 of the rubber sheet thickness cannot flow and becomes a "dead layer" or "stagnant layer," as shown in the figure. In addition, the piled-up rubber at the top of the roller gap will also form a wedge-shaped "backflow area." The above reasons cause uneven dispersion of the compounding agent in the rubber.

Due to the high viscosity of the rubber, the fluidity of the rubber near the surface of the roller is poor, and the shear force promotes the flow of the rubber only as laminar flow. The compounding agent cannot enter the inner layer of the rubber sheet with a thickness of about 1/3 in contact with the roller surface, forming a "dead layer".

To compensate for the adverse effects of piled-up rubber on mixing, the process must be supplemented with cutting and turning to continuously bring the rubber in the "dead layer" to the top of the piled-up rubber and into the "active layer," so that the rubber on both sides is mixed evenly, to break the dead layer and backflow area, ensuring uniform mixing and uniform quality.

II. Internal Mixer Technology:

 

 

 

1. Uses of Internal Mixers:

Mainly used for the plastication and mixing of rubber, but also for the mixing of plastics, asphalt materials, roofing felt materials, and synthetic resin materials. It is one of the main rubber mixing equipment in rubber factories. Although the development of rubber mixing technology and equipment has been rapid abroad since the 1970s, such as using screw extruders to replace internal mixers and open mills for plastication and mixing, they still cannot replace internal mixers. The rubber mixing equipment in new modern factories is still mainly internal mixers, and the mixing method is still the two-stage mixing method.

Classification:

(1) According to the shape of the rotor cross-section: elliptical rotor internal mixer, cylindrical rotor internal mixer, triangular rotor internal mixer

(2) According to the working principle: tangential rotor internal mixer, intermeshing rotor internal mixer

(3) According to the rotor speed and its changes: low-speed, medium-speed, high-speed, single-speed, double-speed, and variable-speed internal mixers

(4) According to the relative rotor speed: asynchronous and synchronous rotor internal mixers

(5) According to the structure of the mixing chamber: ordinary type and tilting internal mixers

(6) According to the relative gap between the rotors: fixed gap and adjustable gap internal mixers

2. Specifications and Technical Characteristics of Internal Mixers:

1. Specifications

In the past, the working capacity of the mixing chamber and the speed of the main rotor were used; now, the total capacity of the mixing chamber/the speed of the main rotor is used.

Domestic internal mixer specification notation:

XM-250/20

X represents rubber, M represents internal mixer, 250 represents the total capacity of the internal mixer, 20 represents the rotor speed

X(S)M-75/35x70

X represents rubber, S represents plastic, M represents internal mixer, 75 represents the total capacity of the internal mixer, double speed (35 and 70 rpm)

3. Overall Structure and Function of Each Part of the Internal Mixer:

(1) Mixing Part: The mixing part mainly consists of rotors, a mixing chamber, and a sealing device.

(2) Feeding Part: It mainly consists of a feeding chamber, a hopper-shaped feeding port, and a tilting door (feeding door) 11. The main function of this part is for feeding and momentary material storage.

(3) Pressing Part: It mainly consists of an upper ram 9 and a cylinder 14 that drives the upper ram to move up and down. Its main function is to apply a certain pressure to the rubber material, accelerate the mixing process, and improve the mixing effect.

(4) Discharge device section: Mainly composed of the lower ram 3 and the lower ram locking mechanism 2 installed below the mixing chamber. Its main function is to discharge the rubber after the rubber is finished, that is, to discharge. The lower ram can be cooled by circulating cooling water, and the ' ' shaped surface of the lower ram in contact with the material should be overlaid with wear-resistant alloy to increase its wear resistance.

(5) Transmission device section: Mainly composed of motor 22, elastic coupling 21, reducer 20 and toothed coupling 19, etc. Installed on the transmission base, its function: transmits power, allowing the rotor to overcome the working resistance and rotate, thereby completing the rubber mixing operation.

(6) Base: Mainly composed of the machine base, some are divided into the main machine base and the transmission base. Its function: for the use of the internal mixer, installing the main machine and transmission system components on it.

(7) Heating and cooling system: Mainly composed of pipes and distributors, etc., so that cooling water or steam can circulate in the mixing chamber, rotor, and the cavities of the upper and lower rams, etc., to control the temperature of the rubber. The heating and cooling system of the internal mixer imported from abroad is equipped with a temperature control device and uses constant temperature water heating and cooling. Function: Control the temperature of the rubber during the rubber mixing process according to the process requirements.

(8) Hydraulic system: Mainly composed of a double-vane pump 15, a rotary oil cylinder 17, a reciprocating oil cylinder 16, pipes, and an oil tank, etc. It is the power supply part of the discharging mechanism. Used to control the opening and closing of the lower ram and the lower ram locking mechanism.

(9) Pneumatic system: Mainly composed of a cylinder 14, a piston 13, a cylinder for the charging door, a pneumatic valve, pipes, and compressed air, etc. It is the power supply part of the charging and pressing mechanism. Used to control the lifting, pressing, and opening and closing of the upper ram and the rotary door.

(10) Electrical control system: Mainly composed of a control box, an operating console, and various electrical instruments. It is the operation center of the entire machine.

(11) Lubrication system: Mainly composed of an oil pump, a distributor, and pipes. The purpose is to reduce friction in various rotating parts (such as rotating shafts, bearings, and the friction surfaces of the sealing rings of the sealing device). Increase service life, inject lubricating oil into these friction surfaces. Function: Inject lubricating oil into each rotating part to reduce friction between moving parts and extend their service life.

4. Mixing process of rubber in the mixing chamber: Raw rubber and compounding agents are added from the hopper, first falling into the mouths of two relatively rotating rotors. Under the pressure and friction of the upper ram, they are brought into the gap between the two rotors, subjected to a certain degree of kneading, and then the lower ram's sharp edges separate the rubber, entering the gap between the rotor and the mixing chamber wall. After undergoing strong shearing and kneading here, the broken two streams of rubber meet again at the mouths of the two rotors, and then enter the gap between the two rotors, and so on.

Mechanical action of rubber in the mixing chamber:

1. Kneading action between the outer surface of the rotor and the inner wall of the mixing chamber (especially obvious in elliptical rotor internal mixers)

2. Mixing, stirring, and pressing action between the two rotors (especially obvious in meshing internal mixers)

3. Diverting, shearing, and exchanging action of the upper and lower rams

4. Axial reciprocating cutting and kneading action of the rotor

Production capacity and filling factor:

G --- Production capacity, kg/hour;

V1 ---- Working capacity of the internal mixer, liters;

---- Specific gravity of the rubber, kg/liter;

t ---- Mixing time, minutes.

The working capacity V1 is calculated by the following formula: V1 = VβV ---- Total capacity of the mixing chamber (total capacity of the mixing chamber minus the volume occupied by the rotor)

β ---- Filling factor of the rubber

Methods to increase the pressure of the upper ram: (1) Increase the compressed air pressure; (2) Increase the diameter of the air duct; (3) Use hydraulic pressure instead of air pressure.

Determination of power consumption: N = 4•η•u2B/h

N ---- Power consumption per rotor unit;

η ---- Viscosity of the rubber;

u ---- Rotational linear velocity of the rotor ridge top;

B ---- Width of the rotor ridge top;

h ---- Gap between the rotor ridge top and the inner wall of the mixing chamber.

N = Cuk+1

k ---- Rubber characteristic coefficient, k < 1:

u ---- Rotational linear velocity of the rotor ridge top;

C ---- Coefficient.

Transmission type of internal mixer: A According to the number of internal mixers driven by the motor at the same time: multi-machine transmission, single-machine transmission. B According to the rotor speed of the internal mixer: single-speed, double-speed, variable-speed C According to the relative position of the motor and the internal mixer: left transmission, right transmission, middle transmission D According to the type of reduction mechanism: transmission with large driving gear, separate transmission without large driving gear, using a double output shaft reducer, higher requirements for the motor.

(1) The instantaneous overload coefficient should be greater than 2.48, with overload characteristics;

(2) Large starting torque;

(3) Can be reversed;

(4) Use a closed motor.

Rotor material: In the rubber mixing process, the rotor is subjected to friction, extrusion, corrosion, high temperature, and huge torque from the rubber, so it requires sufficient strength and rigidity, as well as surface wear resistance, chemical corrosion resistance, and good heat transfer performance. The rotor material is mostly cast steel (ZG45). The protruding ridge top is overlaid with a 5-8 mm thick layer of wear-resistant hard alloy, and the rest of the working surface should also be overlaid with a 2-3 mm thick layer of wear-resistant hard alloy.

Rotor cooling methods: (1) Spiral jacket type; (2) Spiral grooves on the inner surface of the rotor (GK series); (3) General forced cooling type

Mixing chamber: Function: It is the main working part of the mixing chamber, and it works together with the rotor to complete the kneading of the rubber material.

Material: A. The front wall is ZG45. To ensure its wear resistance, 2-4mm wear-resistant hard alloy can be welded on the inner wall of the mixing chamber. B. The side wall is a cast iron part

Mixing chamber structural forms: A. Front and rear combination type, composed of two front walls 14, 17, and two side walls. B. Top and bottom opening type, composed of upper and lower mixing chambers and upper and lower machine housings. C. Open and close type (wing type), composed of two openable front walls and two side walls. D. Inclined type E. Flip type

Common materials for mixing chambers: A. The front wall is ZG45. To ensure its wear resistance, 2-4mm wear-resistant hard alloy can be welded on the inner wall of the mixing chamber. B. The side wall is a cast iron part

Structure of the pressure applying part (upper top plug): Structure: This device is installed on the upper part of the mixing chamber.

① Cylinder - Cast iron part. To avoid collision between the piston and the cylinder cover during movement, buffer devices are installed at the top and bottom of the cylinder.

② Piston - Its sealing method uses rubber skin and cast iron piston rings.

③ Piston rod - It has certain strength and stability, and does not deform during the pressurization process.

④ Weight - That is, the upper top plug, which is the part that directly contacts and presses the rubber material.

The main function of the pressurizing cylinder is to provide power for the up and down movement of the upper top plug and to provide pressure for the upper top plug to press the material.

Safety device for the upper top plug: To prevent the breakage of the weight connecting parts and the weight falling into the mixing chamber causing major mechanical accidents, certain safety measures must be adopted. ① Increase the size of the pin hole on the piston rod. ② Install a locking pin on the upper part of the mixing chamber. ③ Weld a stop block on the cooling water pipe.

Structural composition of the feeding part: This device is installed on the upper part of the mixing chamber and mainly consists of a hopper, a flap gate, a rear door, and a filler box.

Discharge part forms: (1) Sliding type (2) Swinging drop type (3) Flipping type

Sealing device forms of the internal mixer: 1. End face seal 2. Labyrinth seal 3. Reverse thread seal 4. Packing seal

(1) External pressure end face sealing device:

A. Classification: According to the pressing method, it is divided into bolt spring pressing type, fork spring pressing type, and fork hydraulic cylinder pressing type.

B. Structure and sealing principle:

a. Bolt spring pressing type: Under the action of the spring pressure mechanism, a certain pressure is generated on the contact surface of the rotating ring and the pressing ring to prevent material leakage. It is used for low-pressure, low-speed internal mixers. It has a simple structure, reliable sealing, and a long service life, but it requires good maintenance during use.

b. Fork hydraulic cylinder pressing type (FYH for short) Advantages: The pressure on each sealing ring working surface is uniform, the specific pressure of the sealing surface remains relatively stable, and it can be adjusted during operation, the sealing is reliable, and maintenance is convenient.

C. Fork spring pressing type Its structural form: Similar to the fork hydraulic cylinder pressing type, the cylinder in the figure is replaced with a spring. The structure is simple, the pressure is small, and it is suitable for small and medium-sized internal mixers and flipping internal mixers.

(2) The internal pressure end face contact automatic sealing device mainly relies on the pressure of the rubber material being extruded outward during mixing to keep the inner sealing ring pressed against the outer sealing ring to achieve a good sealing effect. This sealing device can automatically adjust the pressing degree of the inner sealing ring according to the pressure change of the rubber material, and the sealing effect is good. It is suitable for sealing devices on high-pressure, high-speed internal mixers.

(3) Reverse thread and end face composite automatic sealing device When the material leaks out of the mixing chamber, the reverse thread can return it to the mixing chamber as much as possible.

(4) Reverse thread labyrinth composite sealing device: This device has a simple structure and is easy to manufacture and maintain, but when the rotor sinks, the two labyrinth rings contact, and the labyrinth will wear out quickly. When the rotor bearing is a sliding bearing, this type of sealing device should not be used.

(5) External pressure labyrinth sealing device: Sealing principle: When the material in the mixing chamber leaks out, it needs to pass through a relatively long and narrow path to reach the contact surface of the inner and outer sealing rings, playing the role of a "labyrinth". This sealing device can meet the needs of high-pressure and high-speed mixing, but the structure is complex, and the installation, adjustment, and maintenance workload is large, such as the GK type internal mixer using this sealing device.

(6) Double reverse thread packing composite sealing device This sealing device has a simple structure, and the packing can be replaced by opening the pressure cover. However, the packing wears out severely and has a short lifespan, and needs to be replaced frequently. Only when the material is under medium pressure, the effect is better.

Auxiliary machine system on the internal mixer: Carbon black pneumatic conveying system; Carbon black weighing and feeding system; Oil conveying, storage, weighing, and oil injection system; Rubber material guiding, weighing, and feeding system; Computer intelligent control system.

Lower auxiliary machine system of the internal mixer: Mainly includes A. Pressing machine: Currently, commonly used pressing machines include two forms: open mill pressing or twin-screw extrusion pressing.

And B. Rubber sheet cooling unit: The rubber material after pressing must be cooled. The purpose of cooling is to reduce the temperature of the rubber sheet and apply a release agent to avoid mutual adhesion and self-sulfurization during storage. After the plasticated rubber or mixed rubber is sheeted by a pressing machine or an extrusion pressing machine, a soap solution release agent is sprayed or immersed, and then cooled, cut (or folded), and stacked for storage for use in the next process. Currently, available rubber sheet cooling unit devices include hanging type rubber sheet cooling devices. There are two forms: floor-standing or overhead.