The molding process

Raw rubber plasticizing is a process in which the raw rubber changes from a strong elastic state to a soft and convenient plastic state by means of mechanical stress, heat, oxygen or the addition of some chemical reagents.

The purpose of raw rubber plasticizing is to reduce its elasticity, increase plasticity, and obtain the appropriate fluidity, in order to meet the requirements of mixing, sub - derivatives, pressing, forming, vulcanization and glue manufacturing, sponge manufacturing and other processing processes.

It is very important for the processing and quality of rubber products to master the proper plasticizing degree. The plastic degree should be reduced as far as possible under the premise of meeting the processing requirements. With the appearance of constant viscosity rubber, low viscosity rubber, some rubber has no need for plasticizing and direct mixing.

In the rubber industry, the most commonly used plasticizing methods are mechanical plasticizing and chemical plasticizing. The main equipment used in mechanical plasticizing method is open rubber mixer, closed rubber mixer and screw plasticizer. Chemical plasticizing method is to add chemicals in the process of mechanical plasticizing to improve the effect of plasticizing.

Open machine plasticizing temperature is generally below 80℃, belongs to low temperature mechanical mixing method. The discharging temperature of the mixer and screw mixer is above 120℃, even as high as 160-180℃, which belongs to high temperature mechanical mixing.

Before mixing, raw rubber needs to be baked, cut, selected and broken before plasticizing.

Plasticizing characteristics of several kinds of rubber:

When natural rubber is plasticized with open mixer, the temperature of roller is 30-40℃, and the time is about 15-20min. When the temperature reaches more than 120℃, the time is about 3-5min.

Styrene butadiene rubber Mooney viscosity between 35-60, therefore, styrene butadiene rubber can not be plasticized, but after plasticizing can improve the dispersion of the machine with butadiene rubber has cold flow, lack of plasticizing effect. Butadiene rubber Mooney viscosity is low, can not be plasticized.

Chloroprene rubber plastic, plasticizing before the thin 3-5 times, thin temperature in 30-40℃.

The molecular main chain of ethylene propylene rubber is saturated structure, plasticizing is difficult to cause molecular cracking, so choose varieties with low Mooney viscosity without plasticizing.

Nitrile butadiene rubber has small plasticity, large toughness and large heat generation during plasticizing. Open smelting to use low temperature below 40℃, small roller distance, low capacity and section plasticizing, so that you can receive better results.

Mixing process

Mixing refers to the process in which all kinds of compounding agents are evenly mixed into raw rubber. Mixing quality is to further processing of rubber and the quality of the finished product has a decisive influence, even good sizing formula, mixing is bad, if also can appear uneven compound dispersed, rubber plastic through high or low, easy to scorch, spray frost, etc., make rolling, extruding, coating, and vulcanization process can not be normal, but also to cause a decline in product performance.

Mixing methods are usually divided into open mixer mixing and dense mixer mixing. Both of these methods are batch mixing, which is the most widely used method.

The mixing process of the mixer is divided into three stages, that is, rolling (softening stage by adding raw rubber), eating powder (mixing stage by adding powder) and turning over (the stage by evenly dispersing raw rubber and compound agent after eating powder).

The process conditions are different according to the type, use and performance of the rubber material. In mixing, attention should be paid to the amount of glue, feeding sequence, roller distance, roller temperature, mixing time, roller speed and speed ratio and other factors. Neither can the mixer fail, nor can he be overtried.

Mixer mixing is divided into three stages, namely wetting, dispersing and refining, and mixer mixing stone under high temperature and pressure. The operation method is generally divided into one section of mixing method and two section of mixing method.

One section of mixing method refers to the mixing of the mixer at a time, and then press the mixing glue method. He is suitable for all natural rubber or mixed with synthetic rubber is not more than 50% of the rubber, in a section of mixing operation, often using batch feeding method, in order to make the rubber material will not rise sharply, generally using slow mixer, can also use double speed mixer, adding sulfur temperature must be less than 100℃. The feeding sequence is rubber - small material - reinforcing agent - filling agent - oil softener - discharging - cooling - adding sulfur and super accelerator.

The two-stage mixing method refers to the method of mixing rubber made by mixing and pressing tablets in a mixer twice. This method is suitable for rubber material with more than 50% synthetic rubber content, which can avoid the missing point of long mixing time and high temperature of rubber material in a period of mixing process. The first stage of mixing and a mixing method, just do not add vulcanization and activity of the promoter, a section of mixing after the next piece of cooling, parked for a certain time, and then the second section of mixing. After mixing evenly, the vulcanizing agent is added to the tablet press, and the next piece is turned over after refining. The subsection mixing method has the advantages of shorter mixing time, lower mixing temperature, more uniform distribution of the compound and higher quality of the compound.