Precautions for mold design:
⑴ The shape and wall thickness of the plastic parts should be designed to facilitate the smooth flow of the material to fill the cavity, and try to avoid sharp corners and gaps.
⑵The draft angle should be large, 1°～2° for 15% glass fiber, 2°～3° for 30% glass fiber. When the demolding slope is not allowed, forced demolding should be avoided, and a horizontal parting structure should be adopted.
⑶ The cross section of the pouring system should be large, and the flow should be straight and short to facilitate uniform fiber dispersion.
⑷The design of the feed inlet should consider preventing insufficient filling, anisotropic deformation, uneven distribution of glass fibers, and easy to produce welding marks and other undesirable consequences. The feed port should be flakes, wide and thin, fan-shaped, ring-shaped and multi-point feed ports to make the material flow turbulent, and the glass fiber is evenly dispersed to reduce anisotropy. It is best not to use needle-shaped feed ports. The cross section of the mouth can be increased appropriately, and its length should be short.
⑸The mold core and cavity should have sufficient rigidity and strength.
⑹The mold should be hardened, polished, and wear-resistant steel should be selected, and the parts that are easy to wear should be easy to repair and replace.
⑺ The ejection should be even and strong, easy to replace and repair.
⑻The mold should be equipped with an exhaust overflow groove, and it should be located at a location prone to weld marks.
Mold temperature setting
⑴Mold temperature affects the molding cycle and molding quality. In actual operation, it is set from the lowest appropriate mold temperature of the material used, and then appropriately adjusted higher according to the quality condition.
⑵To be correct, the mold temperature refers to the temperature of the cavity surface when the molding is performed. In the mold design and the condition setting of the molding process, it is important not only to maintain a proper temperature, but also to make it evenly distributed .
⑶Uneven mold temperature distribution will lead to uneven shrinkage and internal stress, which makes the molding mouth easy to deform and warp
⑷The following effects can be obtained by increasing the mold temperature;
①Increase the crystallinity of the molded product and a more uniform structure.
②Make the molding shrinkage more fully and reduce the post-shrinkage.
③Improve the strength and heat resistance of molded products.
④Reduce residual internal stress, molecular alignment and deformation.
⑤Reduce the flow resistance during filling and reduce the pressure loss.
⑥Make the appearance of the molded product more shiny.
⑦Increase the chance of burrs in molded products.
⑧Increase the near gate part and reduce the chance of recession in the far gate part.
⑨Reduce the obvious degree of bonding line
⑩Increase the cooling time.
Metering and plasticization
⑴In the molding process, the control (metering) of the injection volume and the uniform melting (plasticization) of the plastic are performed by the plasticizing unit of the injection machine.
①Barrel Temperature
Although about 60-85% of the melting of the plastic is due to the heat generated by the rotation of the screw, the melting state of the plastic is still affected by the temperature of the heating cylinder, especially the temperature near the nozzle front area-when the front area temperature is too high The phenomenon of drooling and wire drawing when taking out the parts is easy to occur.
②screw speed
A. The melting of plastic is mainly caused by the heat generated by the rotation of the screw, so if the screw speed is too fast, the following effects will be caused:
a. Thermal decomposition of plastics.
b. Glass fiber (fiber-added plastic) is shortened.
c. The screw or heating cylinder wears faster.
B. Rotation speed setting can be measured by the size of its circumferential speed:
Circumferential speed = n (rotation speed) * d (diameter) * π (circularity)
Generally, for low-viscosity plastics with good thermal stability, the circumferential speed of the screw rod rotation can be set to about 1m/s, but for plastics with poor thermal stability, it should be as low as about 0.1.
C. In practical applications, we can reduce the screw speed as much as possible so that the rotating feed can be completed before the mold is opened.
③BACK PRESSURE
A. When the screw rotates and feeds, the pressure accumulated by the melt advancing to the front end of the screw is called back pressure. During injection molding, it can be adjusted by adjusting the return pressure of the injection hydraulic cylinder. The back pressure can be as follows effect:
a. Melting glue more evenly.
b. Toner and filler are more evenly dispersed.
c. Make the gas exit from the blanking port.
d. The metering of incoming materials is accurate.
B. The level of back pressure is determined by the viscosity of the plastic and its thermal stability. Too high back pressure will prolong the feeding time, and the increase in the rotational shear force will easily cause the plastic to overheat. Generally, 5-15kg/cm2 is appropriate.
④松退（SUCK BACK，DECOMPRESSION）
A. Before the start of the screw rotation and feeding, the screw should be properly retracted to reduce the melt pressure at the front end of the mold. This is called forward loosening. Its effect can prevent the pressure of the melt on the screw from the nozzle part. It is mostly used in hot runners. Mold forming.
B. After the screw is rotated and fed, the screw is properly retracted to reduce the melt pressure at the front of the screw. This is called back loosening, and its effect can prevent dripping of the nozzle.
C. The disadvantage is that it is easy to make the main channel (SPRUE) stick to the mold; and too much loosening can suck in air and cause air marks in the molded product.