Process parameters of friction materials (below)
3. Volatile content
Moisture and other volatile substances contained in compressed plastics are collectively referred to as volatiles, mainly water. The number of volatiles contained in 100 parts of plastic is called volatile content.
Whether the volatile matter content in the pressed plastic affects the hot pressing operation, the quality of the pressed product and the rate of defective products is an important factor. When the volatile matter content is too high, the fluidity of the pressed plastic is too large, and the overflow is too much during pressing, and the edge of the overflow is too long and thick, which not only wastes the pressed plastic, but also increases the loss of raw materials. What is more important is that under the high temperature of hot pressing, the steam pressure generated by the volatile matter will cause the pressed product to bubble, swell, crack, and delaminate to become a waste product, or produce surface ripples, warpage, and increased shrinkage. , The physical and mechanical properties of the product are affected.
Reasons for high volatile content include:
(1) Some raw materials in the components are damp, and the moisture content is high;
(2) The degree of drying of wet process pressed plastic is not enough;
(3) The polycondensation degree of the resin is not enough, and there are many low molecular substances that have not participated in the reaction;
(4) During the storage of the compressed plastic, it is damp due to the storage environment or weather.
4. Particle uniformity
For wet-process short-fiber pressed plastics, the blocks should be small and uniform, so that the dry humidity inside and outside is relatively consistent during heating and drying, the auxiliary materials of the mold cavity are easy to uniform, and the appearance and density of the pressed products are relatively uniform. If the uniformity of the block size is poor, The dryness of the surface and inside of the block is inconsistent: sometimes the inside of the block is properly dried but the outer skin is too dry; sometimes the outer skin is properly dried but the inside of the block is still relatively wet, which will affect the appearance and quality of the pressed product.
5. Specific volume
The specific volume of the pressed plastic indicates the area (cm3) occupied by 1g of the pressed plastic. The specific volume of the pressed plastic is closely related to the mold design. Compared with compressed plastics with large specific volume and small specific volume, under the same feeding weight, the volume of the mold feeding cavity is required to be larger. Therefore, when designing thermoforming molds and preforming molds, the feeding cavity should be increased. the height of.
The specific volume value of extrusion plastics is affected by factors such as material type, shape and particle size.
6. Compression rate
The compressibility refers to the ratio between the specific volume of the compressed plastic and its product or the specific gravity of the product and the compressed plastic.
7. Compactness
Powdery or loose compressed plastics can be pressed into compact, unbreakable blocks, sheets, strips or other shapes of cold blanks at room temperature and under a certain pressure. This property is called compacted plastics. sex or billet.
