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Common Defects and Solutions for the Injection of Nylon Office Chair Bases
Injection defects of nylon base in processing
In the injection molding process, due to the following reasons, many defects often occur in the product.
- Poor handling of raw materials
- Unreasonable product or mold design
- Operators not mastering the appropriate process and operating conditions
- Mechanical reasons
The most likely defect in the production of glass fiber reinforced PA is poor surface appearance. They mainly are glass fiber exposed, burnt, dents, silver streaks, ripples, overflow, etc.
1. Exposed glass fiber
The fluidity of glass fiber relative to PA is much worse, and the flow of material in the mold is to flow forward from the middle of the interlayer and turn outward on both sides. Thus, the materials with good fluidity must run to the front, and the bad ones will stay on the mold surface. Generally, the following measures can be taken to reduce the proportion of exposed fiber:
Solutions for the exposed glass fiber of nylon bases
(1) Increase the injection speed.
After increasing the speed, though the different flow speed of glass fiber and PA, compared with high-speed injection, the ratio of the relative speed difference is very small.
(2) Increase the mold temperature.
It 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.
Besides, the middle layer when the material flows can be as thick as possible, and the shell layers on both sides are as thin as possible.
(3) Lower the temperature of the measuring room and reduce the amount of sol.
This is to minimize the possibility of separation of PA and glass fiber.
During the filling process, in addition to the original air in the mold cavity, there is also volatile gas formed during the molding process. The gas is compressed during high-speed injection to generate high temperatures. That may easily cause partial carbonization or scorching of the part. The causes of scorching are as follows:
Causes for the scorching of nylon bases
(1) Poor mold exhaust.
(2) The molding temperature is too high.
(3) The injection speed is too high.
(4) Injection pressure is too high.
(5) The moisture content in the raw material is too high.
Corresponding improvement measures should be taken according to specific reasons during production.
3. Silver streak
The silver streak is because of the GF group. If you need to obtain high-quality GF-reinforced PA, it is recommended to use twin-screw extruder to pelletize. For the choice of GF, you can choose PA-specific reinforced GF. Meanwhile, add coupling agent or compatibilizer, and the addition amount of graft type compatibilizer is about 3%. You can also add about 0.6% of silicone series additives, the surface effect will be greatly improved.
4. Sink marks
The possible causes of sink marks and their solutions are as follows:
(1) Insufficient filling or insufficient feeding amount. – Increase the feeding or open the feed gate gate.
(2) Improper gate position or asymmetric gate. – It should be adjusted according to specific conditions.
(3) Insufficient or too small runners and gates. – Increase runners and gate size.
(4) The wall thickness of the product is not uniform. – The mold should be modified or the injection pressure should be increased.
5. Glass fiber silver streak
The surface of the glass fiber-added parts has metal bright spots and other silver streak defects, especially the protruding part of the material flow area and the vicinity of the bonding line where the fluid meets again. The causes and solutions for silver streaks are as follows:
Causes and solutions for the silver streak of nylon bases
(1) The injection speed is too low.
- Then, increase the injection speed. What’s more, multi-stage injection should be considered (first slow-then fast).
(2) The mold temperature is too low
- Increase the mold temperature.
(3) The melt temperature is too low.
- Then, we need to increase the barrel temperature and the screw back pressure.
(4) The temperature of the molten material changes greatly.
- Increase the screw back pressure and reduce the screw speed. Besides, use a longer barrel to shorten the stroke.
The possible causes of ripples and their solutions are as follows:
1) The gate is small, causing the rubber to spray in the mold cavity.
- So we should modify the gate size or reduce the injection pressure.
2) The runner is tortuous, narrow, poor in roughness, and insufficient rubber supply.
- Then, modify the runner and reduce its roughness to make the rubber supply sufficient.
3) The thickness of the cut surface of the part is uneven, the area is large, and the shape is complicated.
- The part should be set to make the thickness of the cut surface consistent. Or the protrusions and raised lines on the part should be removed.
4) If the mold cooling system is improper or the mold temperature is low
- We should adjust the cooling system or increase the mold temperature.
5) If the temperature of the melt, mold or nozzle is low.
- Then, increase the temperature of the melt cylinder and nozzle.
6) The injection pressure and speed settings are too small
- Increase the corresponding parameter settings.
There will be thin flashes along the parting line or on the sealing surface of the mold. In most cases, the overflow occurs because the clamping force of the machine is not enough during the injection and pressure holding process. And the mold cannot be locked and sealed along the parting line. If there is a place in the cavity where the pressure is high, the deformation of the mold here may cause overmolding. Under the conditions of high molding temperature and injection speed, the melt can still flow fully at the end of the runner. But if the mold is not locked, it will produce edge overflow. The measures that can be taken are as follows:
Solutions for the overflow of nylon bases
1) Increase the clamping force.
2) Reduce the injection speed and adopt multi-stage injection (fast-slow).
3) Keep pressure switching earlier.
4) Lower the temperature of the barrel.
5) Reduce the mold wall temperature.
6) Reduce the holding pressure.
Note: The documentation is sourced from “Engineering Plastics Application” magazine collated and edited by the Aibang polymer team.
Process design plays a key role in correctly setting processing technology, reducing production costs, and improving product quality. Through the selection and control of reasonable process conditions, the defects of GF reinforced PA products can be greatly reduced.
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