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Process Characteristics Of Thermosetting And Thermoplastic Reinforced Plastics

2008/8/11 9:43:00 41773

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In order to further improve the mechanical and electrical properties of thermosetting and thermoplastics.

Glass fiber filler (glass fiber) is often added to plastics, and it is used as a reinforcing material and new composite material is made of resin as binder. It is commonly called reinforced plastics (thermosetting plastic reinforced plastics, also known as glass fiber reinforced plastics).

Because of the different types, lengths and contents of glass fiber made of plastic formula, its manufacturability and use characteristics are also different.

This section mainly introduces thermosetting reinforced plastics for moulding and thermoplastic reinforced plastics for injection.

一、熱固性增強塑料

Thermosetting reinforced plastics consist of resins, reinforcing materials and auxiliaries.

Resin as binder, it requires good fluidity, suitable curing speed, less by-products, easy to adjust viscosity and good mutual solubility, and needs to meet plastic parts and forming requirements.

The reinforced material plays a skeleton role. Its varieties and specifications are mainly made of fiberglass, with a general content of 60% and a length of 15-20mm.

Auxiliaries include thinners for adjusting viscosity (used to improve the bonding between glass fiber and resin), glass fiber surface treatment agents for adjusting the interfacial state of resin fibers, filler and pigments for improving fluidity, reducing shrinkage, improving gloss and abrasion resistance, etc.

Due to the selection of resin, glass fiber specifications (length, diameter, alkali free or alkali, number, number, twist or twisting), surface treatment agent, glass fiber and resin mixing process (premixing or prepreg, plastic ratio and so on, the performance is also different).

(1) process characteristics

1. The fluidity of the reinforced material is worse than that of the ordinary plastic. When the flow is too high, the loss of resin and the accumulation of glass fiber are easy to occur.

The forming pressure and temperature will increase significantly if too small.

There are many factors that affect liquidity. To evaluate the liquidity of a certain material, we must make a detailed analysis according to its composition.

2. Shrinkage rate increases the shrinkage of plastics than that of ordinary plastics. It is mainly composed of thermal shrinkage and chemical structural shrinkage.

The factors affecting shrinkage are plastic varieties.

Generally phenolic resin is larger than epoxy, epoxy phenolic, unsaturated polyester and other materials, of which unsaturated polyester material shrinkage is the smallest.

Other factors that affect shrinkage are the shape and thickness of the plastic parts, the shrinkage of the thick wall, the reduction of the amount of filler and glass fiber in the plastic, the large amount of volatile matter, the large shrinkage, the large forming pressure, the large amount of loading, the smaller shrinkage, the larger shrinkage of the hot demoulding than the cold demoulding, the less shrinkage of the curing, and the reduction of the pressure when the pressure and the forming temperature are appropriate, and when the curing is full and uniform.

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 the majority. The shrinkage size is also related to the die structure. In conclusion, the shrinkage should be considered comprehensively.

3. The specific volume and compression ratio of the compression ratio are higher than that of the general pressure plastics, and the premix is larger. Therefore, the larger loading chamber is needed when the mold is designed, and the loading is also difficult to the mold, especially the premix is more inconvenient, but if the preform Technology is adopted, the compression ratio can be significantly reduced.

The load can be estimated in advance and adjusted after trial pressure.

There are four ways to estimate the loading capacity:

(1) the calculation method of loading can be calculated according to formula (1-3):

A = V * G (1+3-5%) (1-3)

Type A- loading (g) V- plastic volume (cm3) G- plastic proportion (g /cm3) 3-5%- material according to the loss value of hair, burr and other compensation values.

(2) shape simplification method simplifies the complex shape plastic parts into several simple shapes, and changes the sizes accordingly, and then calculates the shapes according to the simplified shapes.

(3) when the plastic parts are made according to the parts of metal or other materials in the specific gravity comparison method, the ratio of specific gravity and weight of the original parts to the proportion of the selected reinforced plastics can be obtained.

(4) the injection type comparison method is injected into the mold cavity after casting with resin or paraffin wax casting material, and then the loading amount is calculated according to the specific gravity comparison method.

4. Material state reinforcements can be divided into the following three states according to the way they are made of glass fiber and resin.

(1) premix is a mixture of 15-30mm long fiberglass and resin. It has a larger specific volume and better fluidity than prepreg. When forming, the fibers are easily damaged, the quality uniformity is poor, the loading is difficult, and the labor conditions are poor.

It is not suitable to suppress plastic parts requiring high strength when it is used to suppress small and medium sized, complex shaped plastics and mass production.

The use of premix should prevent material from dropping quickly.

The material is poor in solubility, and the resin and glass fibers are easy to separate.

(2) the prepreg is immersed in resin and dripped and cut short.

Its fluidity is worse than premix, the solubility of material is poor, the specific volume is small, the strength loss of glass fiber is small, and the material quality uniformity is good. When assembling, it is easy to make reasonable excipients according to the force state of plastic parts, and is suitable for pressing high strength plastics with complicated shapes.

(3) dip felt is a felt material made of chopped fibers evenly spread on glass cloth and impregnated with resin.

It is suitable to suppress thin-walled large plastic parts with simple shape and small thickness change.

5, hardening speed and storage properties of reinforced plastics according to their hardening speed can be divided into two types: fast and slow.

Fast material curing, high charging temperature, suitable for compression molding of small plastic parts and mass production of raw materials.

Slow material is suitable for pressing large plastic parts, with complicated shape or special performance requirements and small batch production. Slow speed material must be carefully chosen to increase heating speed, easy to occur internal stress, hardening and uneven filling.

Too slow reduces production efficiency.

So the mold design should know the requirement of the material in advance.

All materials have their permitted storage period and storage conditions.

Any overrun or poor storage condition will lead to deterioration of plastics, affecting fluidity and quality of plastic parts. Therefore, attention should be paid to mold testing and production.