Shrinkage and quality problems of rubber extruded semi-finished products
一． Factors affecting the extrusion process
1. Composition and properties of raw rubber
The main results are as follows:
(1) in the common rubber varieties, the shrinkage rate of SBR, CR,IIR is higher than that of NR and BR..
(2) adding reinforcing filler to reduce rubber content and elastic deformation. The size of carbon black particle size had no effect on the shrinkage of the compound, but the structure and the amount of carbon black had a significant effect on the shrinkage rate. The higher the structure of carbon black, the smaller the compression shrinkage rate of carbon black, and the smaller the shrinkage rate of anisotropic particles (such as clay, magnesium carbonate, etc.).
(3) adding reclaimed rubber to the compound can increase the fluidity and reduce the shrinkage.
(4) adding ointment softener to the compound can lubricate the product, reduce the shrinkage and make the product surface smooth.
(5) the compound has the advantages of large plasticity, easy flow and low shrinkage.
2. Pressure-out temperature
High extrusion temperature can increase the fluidity of the compound, reduce the expansion rate, and smooth surface.
3. Pressure out speed
The greater the pressure out speed, the greater the shrinkage rate. However, the size of the extrusion speed is affected by the following factors:
The results are as follows: (1) the higher the angle of the mouth cone, the higher the smoothness, the faster the extrusion speed, the more smooth the surface of the semi-finished product and the better the compactness.
(2) the plasticizing degree of the compound is small and the extrusion speed is slow.
(3) the variety of softener has an effect on the pressure out rate, the viscosity softener such as resin and asphalt will slow down the pressure out speed, stearic acid and wax can accelerate the pressure out speed.
(4) the different kinds of fillers have different effects on the pressure out speed, and the pressure out speed of soft carbon black is faster than that of hard carbon black.
(5) if the pressure-out temperature is raised, the speed of pressure-out can be increased, but attention should be paid to the prevention of coke burning.
4. Semi-finished product cooling
Uneven cooling causes the semi-finished products to shrink and deform inconsistently.
II. Quality problems and causes of extruded semi-finished products
The common quality problems in the stamping process are: the surface of semi-finished products is not smooth, burnt, bubbles or sponges are produced, thickness is uneven, specifications are inaccurate and stripes, cracks and so on.
1. Causes of non-smooth surface of semi-finished products
The temperature of the nose and mouth is low;
Too high glue supply temperature or die temperature too high to produce scorch;
The speed of traction transportation is slow;
Uneven heat melting of rubber or uneven mixing of return rubber;
Press out too fast, etc.
2. Main causes of burning
Improper mix of rubber, poor scorch resistance, short scorch time;
The head temperature is too high;
The flow glue mouth is too small, there is accumulative glue at the head or the die angle between the mouth type and the die head, which causes the rubber material not to flow;
Insufficient screw cooling;
The supply of glue is interrupted, forming an empty car.
3. Production of bubbles or sponges
Pressure out speed is too fast, so that the air in the rubber not and out;
Raw materials contain more moisture and volatile matter;
The head temperature is too high;
The supply of glue is insufficient.
4. Causes of uneven thickness
The core type deviates or the mouth type is incorrect, the mouth type plate is deformed;
Uneven control of pressure-out temperature;
Uneven heating of rubber, pressure out speed and traction speed of inappropriate coordination.
5. Causes of inaccurate specifications
(1) the asymmetry of thickness is mainly due to the misalignment of the core or the mouth.
(2) the thickness compound requirement, but the width is not enough or too big, the main reason is that the traction speed is too fast or too slow.
6. Causes of striped cracks
The compound contains impurities or self-sulfur colloidal particles;
Plastic material is too small;
If the pressure temperature is too high, the compound has scorching tendency;
The shape of mouth or core is not only rough;
Blocking of flow nozzle;
Different parts are subjected to different pressures.