Studies have found that in order for glass fibers to function well in plastics to increase strength, the length of the glass fibers must be greater than its critical length. When a fiber-reinforced material having a fiber length less than this critical length receives a certain load, the fiber may be pulled out, and the reinforcing effect of the fiber cannot be fully exerted.
The size of the critical length has a great relationship with the specific plastic variety. As far as glass fiber reinforced polypropylene is concerned, the critical length of glass fiber is 3.1mm, but for the specially chemically modified polypropylene material, the critical length of glass fiber can reach 0.9mm.
In ordinary glass fiber reinforced polypropylene, the length of the glass fiber is far below the critical length, and the failure mode of the material is mainly that the fiber is pulled out and cannot meet the strength requirements. Therefore, in order to take advantage of the performance advantages of LFT, it is necessary to make the fiber retention length greater than the critical length.
At present, LFT is mainly formed into products by injection molding or compression molding, but the most widely used is injection molding. For injection molding, the screw, nozzle, mold runner, and gate of the injection molding machine should help reduce the damage to the glass fiber and maintain the fiber length in the long fiber reinforced product as much as possible.
The special screw for forming LFT is generally divided into three sections: feeding section, transition section and metering section. The root diameter of the feeding section and the metering section is constant, but the root diameter of the metering section is larger and the middle gradually transitions in the form of an involute curve. Generally follow the following principles: (1) Three-stage length distribution: 40% each in the feeding section and the transition section, and 20% in the metering section; (2) The compression ratio is 2: 1 ~ 3: 1; (3) The aspect ratio is 18 : 1 ~ 22: 1; (4) The screw groove depth in the feeding section is 7.5mm, and the measuring section is about 3.5mm.
For the design of the screw head, it is necessary to ensure that the melt forms a smooth and open flow. Any sharp corners or severe vortexes will cause fiber damage and should be avoided. Throttle check valves are also designed to ensure that the melt can flow freely, while ball valves that restrict flow are not recommended.
For the nozzle design, it is not recommended to use a full cone internal design, but to adopt an open universal design.
Common molds can be used for the mold, but the diameter of the main flow channel is recommended to be less than 5mm, and the gate is recommended to use a large rectangular gate with a cross-sectional area of more than 20mm2.
In fact, even if the LFT is formed by ordinary injection molding machines, excellent performance can be obtained
Please feel free to contact us if you have any problems.
Xiamen LFT Composite Plastic Co.,Ltd
Candy Hu
Email: candyhu@lfrtplastic.com
Tel : 0086-186-5906-9962 (wechat/whatsapp)
Skype:candy328520
Address : No. 27,Hongxi Road, Ma xiang town, Xiang 'an district, Xiamen(361000),China
