Injection mold processing plant How to solve various types of silver wire produced by PC

Tell me in detail how to solve the problem of various types of silver wire produced by PC during injection molding!

1. Hydrolyzed silver filament

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

It is accumulated in the molten resin, and when filling the mold, it rushes into the mold cavity along with the flow material, forming a hydrolysis thread. 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 entire surface of the plastic part in serious cases.

The basic method to eliminate hydrolyzed silver wire is to fully dry the particles, and the moisture 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 moisture content of particles should be measured before molding.

A simple inspection method can be used in production. Even if two pieces of small glass are used, a particle to be detected is sandwiched in the middle, and then placed on a 280~300 degree heating plate. When the particle is heated and gradually melted, it is stably pressurized on the glass to flatten the molten material, and observe whether there are bubbles in the material. If there are no bubbles, it is qualified for drying and can be used for molding processing, Otherwise, 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. The one-time feeding amount shall be controlled so as not to cause the drying granular materials to stay in the hopper for too long and absorb moisture. Generally, the dwell time should not exceed 30~60 minutes. At present, most of the heating and thermal insulation devices of the hopper are equipped with infrared bulbs on the upper part of the hopper. The power can be determined according to the area of the hopper. Generally, 25~30W heating power is required per square meter. In addition, steam jacket, resistance wire heating and other methods can also be used to dry raw materials.

2. Decompose silver filament

Decomposition silver wire refers to the silver wire generated on the surface of the plastic part due to the overheated decomposition of the resin during the molding process and the generation of carbon dioxide and other gases.

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 dead angle of accumulated material in the barrel or nozzle;

Third, the molten material stays in the barrel for too long; Fourthly, the molecular weight of the resin has been reduced, so the impact strength of the plastic parts will inevitably be reduced, and the texture will become brittle, which cannot 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 regularity, and sometimes it appears as a comet. In addition, the appearance of decomposed silver wire is often accompanied by the darkening of the color of the plastic parts and even brown spots, which is particularly obvious in the main stream, which can be used as the main basis for distinguishing decomposed silver wire.

For decomposition of silver wire, corresponding measures shall be taken according to the causes of decomposition. If the temperature of a section of the barrel is too high, the temperature of that section should be reduced; If there is a dead corner in the barrel or nozzle, the dead corner shall be removed and the dead corner shall be cleaned; If the residence time of molten materials 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 still fails, it should be considered to use a smaller capacity injection molding machine for processing.

3. Structural silver wire

Structural silver wire refers to the silver wire formed when the air in the mold cavity mixes with the molten material due to the unreasonable structural design of the plastic part, serious non-uniformity of wall thickness or sudden change of cross section, which makes the molten material expand or contract sharply during the mold filling process. This kind of silver wire mainly affects the appearance, but has little effect on the strength and impact toughness of plastic parts.

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

Therefore, faults such as depression and shrinkage often occur on the surface of the plastic part at the same time, and sometimes bubbles of different degrees even occur inside the plastic part.

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 be mixed with air when passing through the abrupt section, and silver wire will not be produced. However, reducing the injection speed may lead to under injection, which needs to be solved by adjusting the mold temperature, nozzle temperature and other process conditions. When the cross section changes greatly, the injection can be carried out at a higher injection speed, and it is suitable to increase the injection pressure so that the gas can be forced out from the parting surface.

If the fault cannot be eliminated by improving the injection speed or increasing the injection pressure, the plastic part structure and mold exhaust system should be improved.

4. Sprue silver wire

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

(1) The taper angle of the sprue is too large. If the taper angle of the main runner is too large, the flow material will leave the cone wall at the initial stage of injection, resulting in a gap. As the mold cavity is gradually filled, air will be mixed in the flow and enter the mold cavity, forming silver wire. The main feature of this type of silver wire is that its distribution is completely arranged along the injection direction.

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

(2) The gate design is unreasonable. If the sectional area of the gate is too small, the flowing material will form turbulence or injection when passing through the gate, which will mix the molten material with air, thus producing silver wire near the gate. The main feature of this kind of silver wire is that it is radially distributed along the injection direction with the gate as the center. The elimination method is to make the silver wire disappear by enlarging the gate or changing the sectional shape of the gate. If the injection speed is properly reduced while trimming the gate, it will be more effective.

(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 the partial blockage of the shunt channel or gate is similar to that of the silver wire produced by the too small gate. When identifying such 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 mold and increase the nozzle temperature.

5. Pulsed silver wire

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

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

The main feature of this kind of silver wire is that there is no certain regularity in the position and quantity of the silver wire, and sometimes it is accompanied by such faults as under injection, shrinkage and internal bubbles. However, the nature of the silver wire should be determined mainly according to whether the screw has pulse pulsation during the pre molding process and whether the mat is constant at the end of injection.

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

The causes and treatment methods are as follows:

(1) The temperature of the rear section of the barrel is too high, which makes the particles near the feed inlet stick together, causing 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, which causes excessive load on the pre molding motor and abnormal screw speed. The molding temperature should be properly increased to strengthen the plasticization.

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

(4) The hopper insulation device is improperly set or used. The infrared bulb should not be too close to the material particles or the baking time is too long, which will cause the material particles to stick together at high temperature and affect the blanking.

During normal production, pulse 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, when adjusting the temperature, check whether the instruments and circuits are abnormal.

6. Sealing Silver Silk

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

The fundamental way to eliminate such silver wires is to change the gate position and form, set up effective exhaust slots or modify the shape structure of the plastic parts, but this requires a major modification of the mold. In the actual production or mold trial process, the method of correcting the process conditions is often adopted, such as adjusting the injection pressure and injection speed, changing the temperature difference between the fixed mold and the moving mold, etc.