In order to further improve the electromechanical properties of thermosets and thermoplastics, glass fiber fillers are often added to plastics as reinforcing materials and resins are used as binders to form new composite materials, which are commonly known as reinforced plastics (reinforced plastics for thermosetting plastics) FRP). Due to the variety, length and content of glass fiber in plastic formulation, its processability and use characteristics are also different. This section mainly introduces thermosetting reinforced plastic for molding and thermoplastic reinforced plastic for injection.
Thermoset reinforced plastic
The thermosetting reinforced plastic is composed of a resin, a reinforcing material, an auxiliary agent, etc. Among them, the resin as a binder requires good fluidity, a suitable curing speed, few by-products, easy viscosity adjustment, and good mutual solubility. Meet the requirements of plastic parts and forming. The reinforcing material plays the role of skeleton. The variety of specifications is mainly glass fiber, generally 60%, length is 15-20mm. The auxiliary agent includes viscosity adjusting diluent (to improve glass fiber and resin). Bonding), a glass fiber surface treatment agent used to adjust the resin-fiber interface state, used to improve fluidity, reduce shrinkage, improve gloss and abrasion resistance, etc., due to the use of resins, etc. Variety specifications (length, diameter, alkali-free or alkali-containing, count, number of strands, twisted or flawless), surface treatment agent, glass fiber and resin mixing process (premixed or prepreg, plastic) The performance is different when the ratio is different.)
Process characteristics
1. The fluidity of the fluidity enhancing material is worse than that of the general pressed plastic. When the fluidity is too large, the loss of resin and the accumulation of the glass fiber are easy to occur. When the temperature is too small, the forming pressure and temperature will be significantly improved. There are many factors affecting the liquidity. The fluidity of a certain material must be analyzed by composition. Factors affecting liquidity
2, shrinkage-reinforced plastic shrinkage rate is smaller than ordinary compression plastics, it mainly consists of heat shrinkage and chemical structure shrinkage. The factors affecting shrinkage are first of all plastic varieties. General phenolic materials than epoxy, epoxy phenolic, unsaturated polyester The material should be large, and the unsaturated polyester material has the smallest shrinkage. Other factors affecting the shrinkage are the shape and wall thickness of the plastic part, and the thick wall shrinks greatly. The filler and the glass fiber contained in the plastic have small shrinkage and volatile content. Large shrinkage is also large, the forming pressure is large, the shrinkage is small when the loading amount is large, the shrinkage of the hot demolding is larger than that of the cold demolding, the shrinkage is insufficient, the shrinkage is large, and when the pressing timing and the forming temperature are appropriate, the curing is sufficient and uniform. Small. The shrinkage of different parts of the same plastic parts is also different, especially for thin-walled plastic parts. The general shrinkage rate is 0-0.3%, while 0.1-0.2% is mostly, the shrinkage size is also related to the mold structure. In short, the choice of shrinkage should be considered comprehensively.
3, the compression ratio of the specific material, the compression ratio is larger than the general pressure plastic, the premix is larger, so the mold design needs to take a larger loading chamber, while loading the mold is also more difficult, especially Premixing is more inconvenient, but the compression ratio can be significantly reduced if the preform preforming process is used.
The amount of charge can generally be estimated in advance, and then adjusted after pressure test. The method of estimating the amount of charge can be as follows:
(1) The calculation method can be calculated according to formula (1-3):
A=V×G(1+3-5%)(1-3)
In the formula A-loading amount (g) V-plastic part volume (cm3) G-specific gravity of the plastic used (g/cm3) 3-5% - the material is compensated by the loss amount of the hair, burr and the like
(2) The shape simplification calculation method simplifies the complex shape plastic parts into a plurality of simple shapes, and simultaneously changes the size and then calculates the simplified shape.
(3) Specific gravity comparison method When the parts of metal or other materials are imitation plastic parts, the ratio of the specific gravity and weight of the original parts to the specific gravity of the reinforced plastics selected can be used to determine the loading amount.
(4) The injection type comparison method uses a casting material such as resin or paraffin to inject into the mold cavity, and then the content is calculated by the specific gravity comparison method.
4. The material state reinforcing material can be divided into the following three states according to the way in which the glass fiber and the resin are mixed to form a raw material.
(1) Premix is made by mixing glass fiber with 15-30mm length and resin. It has larger specific volume, better fluidity than prepreg, easy to damage fiber during forming, poor quality uniformity, and charging. Difficulties, poor working conditions. Suitable for pressing small and medium-sized, complex-shaped plastics and mass production, it is not suitable for pressing plastic parts requiring high strength. When using premix, prevent the material 'knot' to make the fluidity drop rapidly. Poor miscibility, resin and glass fiber easy to accumulate.
(2) The prepreg is obtained by immersing the whole bundle of glass fiber in the resin, drying and cutting. The fluidity is worse than that of the premix, the compatibility between the bundles is poor, the specific volume is small, the glass fiber strength loss is small, and the material quality is small. The uniformity is good, and it is easy to carry out reasonable auxiliary materials according to the shape of the plastic part when the mold is loaded, which is suitable for pressing high-strength plastic with complicated shape.
(3) The felt is a felt-like material obtained by uniformly coating the chopped fibers on a glass cloth and impregnating the resin, and the performance is between the above two. It is suitable for a thin-walled large-sized compact, small thickness change. Plastic parts.
5, hardening speed and storage reinforced plastic according to its hardening speed can be divided into fast and slow speed. Fast material curing fast, high filling mold temperature, suitable for compression molding small plastic parts and common raw materials for mass production. Slow The quick material is suitable for pressing large plastic parts. When the shape is complex or has special performance requirements and small batch production, the slow material must carefully select the heating rate. The internal stress is easy to occur, the hardening is uneven, and the filling is poor. Efficiency. Therefore, the design of the mold should be known in advance.
All kinds of materials have their own storage period and storage conditions. Any failure or over-storage conditions will lead to deterioration of plastics, affecting fluidity and quality of plastic parts, so attention should be paid to the test and production.