What are advantages and disadvantages of nylon reinforced with glass fiber? What problems are prone to occur in injection molding process?

 

1. After adding glass fiber to nylon, mechanical properties, heat resistance, dimensional stability and aging resistance of modified nylon are significantly improved, and fatigue resistance is 2.5 times that of unenhanced nylon.
2. Due to addition of glass fiber, mutual movement between polymer chains of plastic is limited. Therefore, shrinkage rate of reinforced plastic is greatly reduced, that is, shrinkage of product is much better than before glass fiber is not added, and rigidity is also greatly improved.
3. Glass fiber reinforced nylon has a high softening point, low friction coefficient, wear resistance, self-lubrication, shock absorption, noise reduction, electrical insulation, oil resistance, weak acid resistance, alkali resistance and general solvents, self-extinguishing, no toxic, odorless, good weather resistance.
4. After nylon is reinforced with fibers, water absorption rate of nylon chips can be reduced, so that it can work in high temperature and high humidity environments.

1. Decreased toughness and increased brittleness. This can be improved by adding tougheners.
2. Since glass fiber will be oriented along flow direction during injection molding process, mechanical properties and shrinkage rate will be enhanced in direction of orientation, resulting in deformation and warping of product.
3. During injection molding process, glass fiber enters surface of plastic product, making surface of product rough and full of spots, such as floating fibers, material flowers and other defects.
4. The greater proportion of glass fiber added, the greater wear on plasticizing components of injection molding machine, mainly wear of screw.
5. Liquidity will be reduced.

During injection molding process, there may be many defects in product due to poor raw material handling, unreasonable product or mold design, operators who have not mastered appropriate operating conditions for injection molding nylon technology, or mechanical reasons. The most likely to occur in production of glass fiber reinforced nylon is poor surface appearance, mainly for glass fiber exposure, burnt, flakes, dents, silver streaks, ripples, overflows, etc.

Fluidity of glass fiber is much worse than that of nylon, and flow of material in mold is from middle of interlayer to the front, and two sides are turned outward, so fluidity is good, it must run to the front, and fluidity is not good, it will stay on the surface of mold.
Solution to glass fiber exposure is as follows:
(1) Increase injection speed. After increasing speed, although flow speeds of glass fiber and nylon are different, compared with high-speed injection, proportion of relative speed difference is very small.
(2) Increase mold temperature. Purpose of increasing mold temperature is to reduce contact resistance between glass fiber and mold, to minimize speed difference between glass fiber and nylon, to make middle layer as thick as possible and shell layers on both sides as thin as possible when material flows.
(3) Reduce temperature of metering chamber and reduce amount of sol. This is to minimize possibility of nylon and glass fiber separation.
(4) Wipe clean nylon raw material to be processed, remove residual grease or release agent on the surface.
(5) Spray or wipe nylon plus glass fiber treatment agent on material to be treated to a dry film of 5-10 microns, and let it stand for 5-10 minutes.
(6) It is better to force-dry painted workpiece at 80℃ for 30 minutes.

During filling process, in addition to original air in mold cavity, there is also volatile gas formed during molding process. During high-speed injection, gas is compressed to generate high temperature, which may easily cause local carbonization or scorching of part.
Generally take following measures:
Adjust feeding speed, reduce mold temperature, dry modified nylon raw material before use or apply non-drying modified nylon material.

Material flower is caused by glass fiber clumping. If you need to obtain high-quality glass fiber reinforced nylon, it is recommended to use a twin-screw extruder to granulate. For choice of glass fiber, nylon-specific reinforced glass fiber can be selected, and coupling agent or compatibilizer can be added at the same time. Amount of graft type compatibilizer added is about 3%. You can also add about 0.6% silicone series additives, surface effect will be greatly improved.

Possible causes of dents and their solutions are as follows:
(1) If filling material is insufficient or amount of material is not enough, material should be increased or gate of discharge port should be opened;
(2) Position of gate is improper or gate is asymmetrical, which should be adjusted according to specific situation;
(3) Runners and gates are insufficient or too small, and runners should be increased and gate size should be increased;
(4) Wall thickness of product is uneven, and mold should be modified or injection pressure should be increased.

Silver streak defects such as metal bright spots appear on the surface of part added with glass fiber, especially near material flow area of raised part and joint line where fluid meets again.
Causes and solutions of silver streaks are as follows:
(1) Injection speed is too low, injection speed should be increased, and multi-stage injection should be considered (slow first and then fast);
(2) Mold temperature is too low, mold temperature should be increased;
(3) Temperature of melt is too low, temperature of barrel should be increased, and back pressure of screw should be increased;
(4) Temperature of molten material changes greatly, so back pressure of screw should be increased, speed of screw should be reduced, and a longer barrel should be used to shorten stroke.

Possible causes of ripple and their solutions are as follows:
(1) Gate is small, resulting in injection of rubber in mold cavity, size of gate should be modified or injection pressure should be reduced;
(2) Flow channel is tortuous, narrow, poor in roughness, and supply of rubber is insufficient. Flow channel should be modified and its roughness should be reduced to make supply of rubber sufficient;
(3) Section thickness of workpiece is uneven, area is large, and shape is complex. Workpiece should be set to make thickness of section uniform, or protrusions and raised lines on workpiece should be removed;
(4) Mold cooling system is improper or mold temperature is low, cooling system should be adjusted or mold temperature should be increased;
(5) If melt temperature, mold or nozzle temperature is low, temperature of melt cylinder and nozzle should be increased;
(6) If injection pressure and speed are set too small, corresponding parameter settings should be increased.

Thin flash may appear along parting line or on sealing surface of mold. In most cases, flashing occurs because mold clamping force of machine is not enough during injection and pressure holding process to lock and seal mold along parting line.
Possible measures are as follows:
(1) Increase clamping force;
(2) Reduce injection speed and adopt multi-stage injection (fast-slow);
(3) Pressurize and switch earlier;
(4) Reduce barrel temperature;
(5) Reduce mold wall temperature;
(6) Reduce holding pressure.