Common fluororubber production problems mainly include:
Roller sticking during mixing, white particles in the mixed rubber, rough surface of semi-finished products, easy tearing and shrinkage of products, serious mold pollution, traces on joints, lack of rubber, cracks on the surface, slow vulcanization speed, product deformation, unstable size, etc.
Main problems and solutions
1. Easy to stick to the roller during mixing
The main manifestation of easy sticking to the roller during mixing is that the rubber wraps the front and rear rollers at the same time during the mixing process, or the rubber sticks tightly to the rear roller. The former causes the powder to be easily pressed into sheets and fall off, resulting in uneven dispersion of the powder; the latter makes it impossible to turn the rubber, prolonging the mixing time and increasing the difficulty of mixing.
The main cause of rubber sticking to the roller is low Mooney or raw rubber with too much low molecular weight content. Generally speaking, the mixing process of raw rubber with a Mooney viscosity [ML (1+4), 121T] of about 70 is good. The Mooney of imported fluororubber raw rubber or premixed rubber is mostly below 50. which is also the main reason why imported rubber is generally easy to stick to the roller. On the other hand, molecular weight distribution also has a certain influence on the mixing process. Most of the current domestic rubbers have a wide molecular weight distribution. High molecular weight provides physical properties of the rubber, and low molecular weight provides processing performance. Once the content of low molecular weight rubber is too much, it will cause the rubber to stick to the roller.
The solution to this problem is to select raw rubber with a Mooney viscosity/ML (1+4. 121℃) of about 70. and at the same time select raw rubber with a narrow molecular weight distribution or a moderate low molecular weight content; add a small amount of processing aids, such as low molecular weight polyethylene, hard wax such as palm wax, fluorine wax or VPA No.2. VPA No.3. etc.
2. White particles in the mixed rubber
The white particles in the mixed rubber are mainly manifested as white particles or white spots on the surface of the mixed rubber. After cutting and mixing, the white particles or white spots on the cut surface are more obvious. On the one hand, this will affect the appearance of the product, causing white spots on the surface of the product; on the other hand, it will also have a certain impact on the performance of the vulcanized rubber, making the performance of the vulcanized rubber, such as hardness and tensile strength, uneven.
Such white particles or white spots are generally caused by the uneven dispersion of one or more materials in the compounding agent. In the formula of fluororubber, the compounding agents that are not easy to disperse mainly include acid absorbers such as Ca(OH)2 and MgO, as well as other compounding agents that have been exposed to moisture.
Measures to solve this problem include the selection of surface-treated acid absorbers Ca(OH)2 and MgO [such as Ca(OH)2 of NICC5000 and MgO of STARMAG:150]; the bags should be sealed in time after the compounding agents are used to avoid contact with air and prevent the compounding agents from being exposed to moisture.
3. Rough surface of semi-finished products
The rough surface of semi-finished products is mainly manifested in that the surface of the semi-finished products extruded by the rubber is not smooth, there are multiple cracks, and in severe cases, the surface of the rubber strip is severely cracked and curled. When this phenomenon occurs, the cracked part of the rubber strip is prone to water and other impurities, which will also cause trouble for gluing.
This situation is generally caused by several reasons: high Mooney viscosity of the rubber material, poor extrusion performance, too fast extrusion speed, and low screw extrusion pressure.
The solution is to use a raw rubber with a moderate Mooney ratio; fully remix the mixed rubber before feeding; add a small amount of processing aids to the formula to improve the extrusion process performance; adjust the extrusion speed; and use a high-power extruder.
4. The product is easy to tear
The product tearing is generally caused during two processing processes: tearing during trimming and tearing during demolding. The former is mainly caused by the waste edge being too thick; the latter is caused by many reasons, including the following aspects: the mold fit is too tight, the waste edge is stuck by the mold during demolding, and it is easy to tear between the product and the waste edge; the force is uneven during demolding, which is easy to cause stress concentration and damage the product; the cavity surface is rough or the rubber is easy to stick to the mold, making demolding difficult and easy to tear the product; the vulcanization temperature is too high, and the tearing performance of fluororubber at high temperature is poor; the cross-linking density is large, resulting in a decrease in elongation, and the vulcanized rubber is also easy to become brittle and easy to tear; the waste edge is too thick and the product is easy to tear during demolding.
The above problems can be solved from the following aspects: loose mold fit (the fit gap should not be too large to ensure that the mold fit surface does not shake>; use an automatic vulcanizer for production. If an ordinary manual vulcanizer is used, the mold should be pried evenly when demolding; the surface roughness of the cavity Ra≤0.8. and the rubber should be added with an appropriate amount of processing aids to improve its demoulding performance; properly control the vulcanization temperature, generally between 160℃-170℃; properly reduce the crosslinking density by reducing the amount of vulcanizer; control the thickness of the waste edge, and the quality of the semi-finished product is weighed as required. Generally, its quality is about 1.05 times that of the finished product.
5. Shrinkage cracking
Shrinkage cracking is the phenomenon that the rubber shrinks and causes tearing at the fitting part when the pressure of the rubber product is removed after vulcanization. This phenomenon is different from general tearing. Common tearing is mainly caused by improper operation or relatively complex product structure and poor thermal tearing performance of the material. It is generally relatively smooth and basically in a straight line. However, the shrinkage cracking part is generally irregular, the tearing part is not smooth, uneven, and has a curved shape.
