The thermosetting reinforced plastic is composed of a resin, a reinforcing material, an auxiliary agent and the like. Among them, the resin is used as a binder, which requires good fluidity, suitable curing speed, few by-products, easy adjustment of viscosity and good mutual solubility, and needs to meet the requirements of plastic parts and forming.
The reinforcing material acts as a skeleton, and its variety and specifications are mainly made of glass fiber, generally 60% and length 15-20 mm. Adjuvants include viscosity-adjusting diluents (to improve the bonding of glass fibers and resins), glass fiber surface treatment agents to adjust the resin-fiber interface state, to improve fluidity, reduce shrinkage, improve gloss and resistance. Fillers and coloring pigments for grinding and the like. Due to the choice of resin, glass fiber 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 ratio, etc., their performance is also different.
(1) Process characteristics
1. The fluidity of the fluidity enhancing material is inferior to that of the general pressed plastic. When the fluidity is too large, the resin loss and the accumulation of the glass fiber are easily generated. When too small, the forming pressure and temperature will increase significantly. There are many factors affecting liquidity. To assess the fluidity of a certain material, it must be analyzed according to the composition.
2. The shrinkage-enhancing plastic shrinks less than the conventional pressed plastic, and it mainly consists of heat shrinkage and chemical structural shrinkage. The factors affecting shrinkage are first of all plastic varieties. Generally, the phenolic material is larger than the epoxy, epoxy phenolic, unsaturated polyester and the like, and the unsaturated polyester material has the smallest shrinkage. Other factors affecting shrinkage are the shape and thickness of the plastic part, and the thick wall shrinks greatly. The filler and glass fiber contained in the plastic have small shrinkage, the large volatile content shrinks, the forming pressure is large, and the loading amount is large. The shrinkage is small, the shrinkage of the hot mold release is larger than that of the cold mold release, and the shrinkage is insufficient and the shrinkage is large. When the pressurization timing and the molding temperature are appropriate, the shrinkage is small when the curing is sufficient and uniform. The shrinkage of different parts of the same plastic part is also different, especially for thin-walled plastic parts. Generally, the shrinkage rate is 0-0.3%, while 0.1-0.2% is mostly, and the shrinkage size is also related to the mold structure. In general, the shrinkage should be considered comprehensively.
3. The compression ratio enhances the specific volume and the compression ratio is larger than that of the general pressed plastic, and the premix is ​​larger. Therefore, it is necessary to take a larger loading chamber in the mold design, and it is also difficult to load the mold, especially premixing. The material is more inconvenient, but the compression ratio can be significantly reduced if the preform preforming process is employed.
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)
Where a - the amount of charge (g);
V - the volume of the plastic part (cm 3);
G - the specific gravity of the plastic used (g / cm 3);
3~5% ——The material is compensated according to the loss amount of hair, burr and so on. (2) The shape simplification calculation method simplifies the complex shape plastic part into a plurality of simple shapes, and simultaneously changes the size according to the simplified shape, as shown in Fig. 1-1.
(3) Specific gravity comparison method When a plastic part of a metal or other material is imitation, the ratio of the specific gravity and weight of the original part to the specific gravity of the reinforced plastic selected may be obtained.
(4) The injection type comparison method is performed by injecting a casting material such as a resin or a paraffin into a mold cavity, and then determining the amount of the material by the specific gravity comparison method.
4. The material state reinforcing material can be divided into the following three states according to the manner 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, and poor quality uniformity. Difficulties and poor working conditions.
It is 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 the premix, prevent the material "knot" from causing a rapid drop in fluidity. The material has poor mutual solubility, and the resin and the glass fiber are easily accumulated in the head.
(2) The prepreg is formed by dipping the whole bundle of glass fiber into the resin, drying and cutting. The fluidity is worse than that of the premixed material, the compatibility between the bundles is poor, the specific volume is small, the glass fiber strength loss is small, the material quality uniformity is good, and the reasonable auxiliary material is easily pressed according to the shape of the plastic part during the mold clamping, and is suitable for pressing. High-strength plastic with complex shapes.
(3) The felt is a felt-like material obtained by uniformly coating a chopped fiber on a glass cloth and impregnating the resin, and the performance is between the above two. It is suitable for pressing thin-walled large-sized plastic parts with simple shape and little change in thickness.
5. Hardening speed and storage reinforced plastics can be divided into fast and slow speed according to their hardening speed. The quick material curing is fast, and the charging mold temperature is high, which is suitable for compressing small plastic parts and common raw materials in mass production. The slow 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 too fast, the hardening is uneven, and the filling is poor. Too slow to reduce production efficiency. Therefore, the design of the mold should be known in advance.
All materials have their own storage period and storage conditions. Anyone who has exceeded the conditions or has poor storage conditions will cause the plastic to deteriorate and affect the fluidity and quality of the plastic parts. Therefore, attention should be paid to the test and production.
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