1. Hydrolyzed silver wire

Hydrolyzed silver wire refers to the silver wire formed by the hydrolysis of plastic in the barrel. The reason is that the material particles have not been fully dried before forming, or the fully dried material particles stay in the hopper for too long, and there are no effective thermal insulation measures in the hopper, so that they re absorb moisture. The water content exceeds the specified requirements. This kind of material particles with ultra-high water content are vaporized under the action of high temperature in the barrel, It leads to the degradation of resin in the melting process and produces carbon dioxide gas.

It accumulates in the molten resin and rushes into the die cavity with the flowing material during mold filling to form hydrolyzed quick wire. As a result, the strength of plastic parts is significantly reduced and the texture becomes brittle. This kind of silver wire is often encountered and easy to identify. It is usually evenly distributed along the injection flow direction and densely distributed on the whole plastic part surface in severe cases.

The fundamental way to eliminate hydrolyzed silver wire is to fully dry the particles, and the water content of the particles used in molding must be controlled below 0.03%. Therefore, the drying process conditions of raw materials must be strictly controlled, and the water content of material particles should be measured before forming.

A simple inspection method can be adopted in production. Even if two pieces of small glass are used, one of the tested material particles is sandwiched in the middle, and then placed on a 280 ~ 300 degree heating plate. When the material particles are heated and gradually melted, pressurize the glass stably, flatten the molten material, and observe whether there are bubbles in the material. If there are no bubbles, it is qualified for molding and processing, otherwise it needs to continue drying.

If the silver wire is caused by the moisture absorption of dry particles in the hopper, the heating and insulation device shall be set or improved in the hopper according to the specific situation. Generally, it will not stay in the hopper for a long time, which will lead to the maximum drying time

It should be more than 30 ~ 60 minutes. At present, most of the heating and insulation devices of the hopper are equipped with infrared bulbs on the upper part of the hopper, and their power can be determined according to the area of the hopper. Generally, 25 ~ 30W heating power is required per square meter. In addition, the raw materials can also be dried by steam jacket, resistance wire heating and other methods.

2. Decomposed silver wire

Decomposed silver wire refers to the silver wire produced on the surface of plastic parts due to the overheated decomposition of resin and the generation of carbon dioxide and other gases during the molding process.

There are many specific reasons for overheating and decomposition of molten materials, but the common reasons are:

First, the barrel temperature is too high;

Second, there is a dead corner of accumulated material in the barrel or nozzle;

Third, the retention time of molten material in the barrel is too long; Fourth, the molecular weight of the resin has been reduced. Therefore, the impact strength of the plastic parts must be reduced and the texture becomes brittle, which can not meet the use requirements.

This kind of silver wire can generally be identified from its appearance. Its distribution on the surface of plastic parts has no certain law, and sometimes appears in the shape of comet. In addition, the appearance of decomposed silver wire is often accompanied by the darker color of plastic parts and even brown spots, which is particularly obvious on the mainstream, which can be used as the main basis for judging the decomposed silver wire.

For the decomposition of silver wire, corresponding measures shall be taken according to the causes of decomposition. If it is caused by the excessive temperature of a section of the barrel, the temperature of that section shall be reduced; If there is a dead corner of stagnant material in the barrel or nozzle, the stagnant part shall be cleaned and the dead corner shall be repaired; If the retention time of molten material in the barrel is too long, the molding cycle should be shortened as far as possible on the premise of ensuring the quality of plastic parts. If it is still ineffective, it should be considered to replace the injection molding machine with small capacity for processing.

3. Structural silver wire

Structural silver wire refers to the silver wire formed by the rapid expansion or contraction of the melt in the process of mold filling due to the unreasonable structural design of plastic parts, serious uneven wall thickness or sudden change of section, resulting in the air in the mold cavity mixing into the melt. This kind of silver wire mainly affects the appearance, but has little effect on the strength and impact toughness of plastic parts.

The characteristic of structural silver wire is that when the process conditions are fixed, the shape and position of silver wire distribution are also fixed, and it is generally distributed along the injection direction, and most of the positions occur after the sudden change of section. The structural silver wire is caused by sudden change of section and serious uneven wall thickness of plastic parts.

Therefore, there are often failures such as depression and shrinkage on the surface of plastic parts at the same time, and sometimes even bubbles of varying degrees are generated in the interior of plastic parts.

Structural silver wires can generally be eliminated by changing the injection speed. When the cross section of the plastic part changes little, the injection speed can be slowed down. When slow injection is adopted, the flowing material can fill the mold cavity smoothly, so that the flowing material will not mix with air when passing through the abrupt cross section and avoid silver wire. However, under injection may occur when the injection speed is reduced, which needs to be solved by adjusting mold temperature, nozzle temperature and other process conditions. When the cross-section changes greatly, a higher injection speed can be used for injection, and it is suitable to increase the injection pressure to force the gas to be discharged from the parting surface.

If the failure cannot be eliminated by improving the injection speed or increasing the injection pressure, the improvement of the plastic part structure and the improvement of the mold exhaust system should be considered.

4. Sprue silver wire

Sprue silver wire refers to the silver wire caused by unreasonable design of gating system or local blockage. The causes and treatment methods are as follows:

(1) The cone angle of the main flow channel is designed too large. If the cone angle of the sprue is too large, the flow material will leave the cone wall at the initial stage of injection, resulting in a gap. With the gradual filling of the mold cavity, the air will be mixed in the material flow and enter the mold cavity to form silver wire. The main feature of this kind of silver wire is that its distribution is completely arranged along the injection direction.

When troubleshooting this kind of fault, first eliminate the possibility of resin hydrolysis and decomposition through air injection, and then check whether the cone angle of the main channel is appropriate. If the angle a is greater than 10 degrees, the above phenomenon may occur. The angle A should be equal to 4 ~ 6 degrees. If the angle a is too small, it will cause difficulty in demoulding and poor material flow. Sometimes the sprue silver wire is eliminated by changing the injection speed, but the fundamental method is to trim or replace the sprue sleeve to reduce the cone angle of the sprue.

(2) The gate design is unreasonable. If the gate cross-sectional area is too small, the flow material will form turbulence or injection when passing through the gate, so that the melt is mixed with air, resulting in silver wire near the gate. The main feature of this kind of silver wire is that it is distributed radially along the injection direction with the gate as the center. The elimination method is to expand the gate or change the cross-sectional shape of the gate to make the silver wire disappear. It is easier to work if the injection speed is properly reduced while trimming the gate.

(3) The cold material at the nozzle mouth causes the shunt channel or local blockage of the gating system during the injection process. The mechanism of partial blockage of shunt channel or gate is similar to that of silver wire produced by too small gate. When identifying this kind of silver wire, the trace of cold material should be found in the pouring and injection system. The way to eliminate this kind of fault is to increase the cold cavity in the die and increase the nozzle temperature.

5. Pulse silver wire

The so-called pulse silver wire does not mean that the silver wire is in pulse shape, but refers to the pre plastic screw

During backward feeding, uneven jumping occurs, just like pulse. This phenomenon mainly reflects the abnormal blanking. At this time, air is easy to enter the barrel, and the gas enters the mold cavity with the molten material to form silver wire.

The main feature of this kind of silver wire is that the position and quantity of silver wire are irregular, sometimes accompanied by faults such as underinjection, shrinkage and internal bubbles. However, the properties of silver wire should be determined mainly according to whether the screw has pulse runout during the pre molding process and whether the material pad is constant at the end of injection.

The method to eliminate pulse silver wire is to take corresponding measures according to the causes of pulse phenomenon.

The causes and treatment methods are as follows:

(1) The temperature at the rear section of the barrel is too high, which makes the material particles near the feed inlet stick together, resulting in abnormal blanking. The temperature of the rear section of the barrel shall be properly reduced.

(2) The material temperature is too low and the resin plasticization is poor, resulting in excessive load of the pre molding motor and abnormal screw speed. The molding temperature should be properly increased to strengthen plasticization.

(3) Screw back pressure is too small, usually the back pressure should not be less than one tenth of the injection pressure.

(4) Improper setting or use of hopper insulation device. The infrared bulb shall not be too close to the material particles or the baking time is too long, which causes the material particles to bond into blocks at high temperature and affects the blanking.

In the normal production process, the pulse phenomenon sometimes occurs suddenly, which is often caused by the abnormal temperature of the rear section of the barrel due to the failure of individual automatic control instruments or the failure of the barrel heating device. Therefore, while adjusting the temperature, check whether the instruments and circuits are abnormal.

6. Sealed silver wire

Gas sealing silver wire refers to the silver wire formed when the gas cannot be eliminated in the process of mold filling. The main feature of this kind of silver wire is that it is accompanied by obvious welding marks, and the silver wire often appears near the welding marks, while there is no silver wire in other parts.

The fundamental way to eliminate this kind of silver wire is to change the position and form of gate, set up effective exhaust slot or modify the shape structure of plastic parts, but it needs to make great modifications to the mold. In the process of actual production or mold test, the method of correcting process conditions is often adopted, such as adjusting injection pressure and injection speed, changing the temperature difference between fixed mold and moving mold, etc.