After PA is modified with glass fiber (GF), its strength, hardness, fatigue resistance, dimensional stability, creep resistance, etc. are greatly improved. The dispersion and bond strength of glass fiber in PA resin matrix have great influence on product performance. In the actual production process, glass fiber reinforced PA injection molding products often have various defects. Xiao Bian now analyzes the injection molding process of glass fiber reinforced PA and the causes and solutions of defects.
First, glass fiber reinforced nylon injection molding process After the determination of plastic raw materials, injection molding machines and molds, the selection and control of injection molding process parameters is the key to ensuring the quality of the parts. The complete injection molding process, in order of its sequence, should include preparation before molding, injection process, and post-treatment of the parts.
1. Preparation before molding
Preparation before molding In order to make the injection process go smoothly and ensure the quality of the plastic parts, some necessary preparations should be made before molding.
1) Confirmation of raw material properties
The performance and quality of plastic raw materials will directly affect the quality of plastic parts.
2) Preheating and drying of raw materials
During the plastic molding process, the residual moisture in the raw material is vaporized into water vapor, which remains in the interior or surface of the part, forming defects such as silver wire, streaks, bubbles, and pitting. Moreover, moisture and other volatile low molecular compounds also catalyze in a high heat and high pressure processing environment, causing cross-linking or degradation of PA, affecting surface quality, and also seriously degrading performance. Commonly used drying methods mainly include hot air circulation drying, vacuum drying, infrared drying and the like.
2. Injection process
The injection process generally involves several steps of feeding, plasticizing, injecting, cooling, and demolding.
1) Feeding
Since injection molding is a batch process, quantitative (constant volume) feeding is required to ensure stable operation and uniform plasticization of plastic.
2) Plasticization
The process in which the added plastic is heated in the barrel, converted from a solid particle into a viscous flow state and having good plasticity is called plasticization.
3) Injection
Regardless of the type of injection molding machine used, the injection molding process can be divided into several stages, such as filling, holding, and backflow.
4) Cooling after the gate is frozen
When the melt of the gate system has been frozen, it is no longer necessary to continue to hold the pressure, so the plunger or screw can be returned, the pressure of the plastic in the cylinder can be removed, and new material can be added, while cooling water, oil or air is introduced. The cooling medium is used to further cool the mold. This stage is called cooling after the gate is frozen.
5) Demoulding
The part can be opened by cooling to a certain temperature, and the part is pushed out of the mold by the pushing mechanism.
3. Post processing of parts
Post-treatment refers to a process for further stabilizing the performance or improving the performance of the article obtained by injection molding, and generally includes heat treatment, humidity control treatment, post-processing, and the like.
Second, defects in processing
In the process of injection molding, the product may be defective due to poor raw material processing, unreasonable product or mold design, operator's lack of proper process conditions or mechanical reasons. The most prone to the appearance of glass fiber reinforced PA is the poor appearance of the surface, mainly glass fiber exposed, charred, material, dent, silver, ripple, overflow and so on.
1. Glass fiber exposed
The fluidity of the glass fiber is much worse than that of the PA, and the flow of the material in the mold flows forward from the middle of the interlayer, and the two sides flow outward, so the fluidity is sure to run to the front, and the fluidity If it is not good, it will stay on the surface of the mold. Generally take the following measures to reduce the proportion of exposed fiber:
1) Increase the speed of the shot. After increasing the speed, although the flow speed of the glass fiber and the PA is different, the ratio of the relative speed difference is small compared to the high-speed shot.
2) Increase the mold temperature. Increasing the mold temperature is to reduce the contact resistance between the glass fiber and the mold, so that the speed difference between the glass fiber and the PA is as small as possible, and the intermediate layer when the material flows is as thick as possible, and the shell layers on both sides are as thin as possible.
3) Reduce the temperature of the metering chamber and reduce the amount of sol. This minimizes the possibility of separating the PA from the glass.
2. Charred
In the filling process, in addition to the original air, the mold cavity also has a volatilized gas formed during the molding process. When the high-speed injection is performed, the gas is compressed to generate a high temperature, which is liable to cause local carbonization or charring of the workpiece. The causes of charring are as follows:
1) The mold is poorly exhausted;
2) The molding temperature is too high;
3) The injection speed is too high;
4) The injection pressure is too high;
5) The moisture content of the raw material is too high.
In the production, corresponding improvement measures should be taken according to specific reasons.
3. Flower
The flower is made because GF is a group. If you need to obtain high-quality GF-enhanced PA, it is recommended to use granulation in a twin-screw extruder. For the selection of GF, you can use PA-specific GF, add coupling agent or compatibilizer, and graft type of compatibilizer. The amount is around 3%. It is also possible to add about 0.6% of silicone series additives, and the surface effect will be greatly improved.
Dent
The possible causes of dents and their solutions are as follows:
1) If the filling material is insufficient or the feeding amount is insufficient, the feeding or opening of the lower opening gate should be increased;
2) The gate is not in the proper position or the gate is asymmetrical, and should be adjusted according to the specific conditions;
3) If the runners and gates are insufficient or too small, the runners should be increased and the gate size should be increased;
4) The wall thickness of the product is not uniform, and the mold should be modified or the injection pressure should be increased.
5. Glass fiber silver
Silver surface defects such as metallic bright spots appear on the surface portion of the glass fiber-added part, especially in the vicinity of the convex portion flow region and the bonding line where the fluid rejoins. The causes and solutions for causing silver streaks are as follows:
1) The injection speed is too low, the injection speed should be increased, and multi-stage injection (first slow-fast) should be considered;
2) The mold temperature is too low, the mold temperature should be increased;
3) The temperature of the melt is too low, the temperature of the cylinder should be increased, and the back pressure of the screw should be increased;
4) The melt temperature changes greatly, the screw back pressure should be increased, the screw speed should be reduced, and a longer barrel should be used to shorten the stroke.
6. Ripple
The possible causes of ripples and their solutions are as follows:
1) The gate is small, causing the rubber to spray in the cavity, and the gate size should be modified or the injection pressure should be reduced;
2) The flow path is tortuous, narrow, and the roughness is poor. The supply of rubber is insufficient. The flow path should be modified and the roughness should be reduced to make the rubber supply sufficient;
3) The thickness of the cut surface of the workpiece is not uniform, the area is large, and the shape is complex. The workpiece should be set to make the thickness of the cut surface uniform, or the protruding disc and the raised line on the part should be removed;
4) Improper mold cooling system or low mold temperature, the cooling system should be adjusted or the mold temperature should be increased;
5) The temperature of the melt, the temperature of the mold or the nozzle is low, and the temperature of the melt cylinder and the temperature of the nozzle should be increased;
6) The injection pressure and speed setting are too small, and the corresponding parameter setting value should be increased.
7. Overflow
A thin flash edge will appear along the parting line or on the mold sealing surface. In most cases, the overflow occurs because the mold clamping force of the machine is insufficient during the injection and holding process, and the mold cannot be locked and sealed along the parting line. If there is a high pressure in the cavity, the deformation of the mold may cause overflow. At high molding temperatures and injection speeds, the melt still flows sufficiently at the end of the runner and creates a spill if the mold is not locked. The measures that can be taken are as follows:
1) increase the clamping force;
2) reduce the injection speed, using multi-stage injection (fast-slow);
3) early pressure holding switch;
4) lower the barrel temperature;
5) reduce the mold wall temperature;
6) Reduce the holding pressure.
Third, the conclusion
Process design plays a key role in correctly setting the processing technology, reducing production costs, and improving product quality. Through the selection and control of reasonable process conditions, the defects of GF-enhanced PA products can be greatly reduced.
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