Injection Mold Design Expert

Mold design considerations:
⑴ The shape and wall thickness of the plastic part should be designed to facilitate the smooth flow of material to fill the cavity, and try to avoid sharp corners and gaps.
⑵ The demoulding slope should be large, 1°~2° for 15% glass fiber, and 2°~3° for 30% glass fiber. When the demoulding slope is not allowed, forced demoulding should be avoided, and a horizontal parting structure should be adopted.
⑶ The cross-section of the pouring system should be large, and the process should be straight and short to facilitate uniform dispersion of the fibers.
⑷ The design of the feed port should consider preventing insufficient filling, anisotropic deformation, uneven distribution of glass fibers, and easy generation of weld marks and other adverse consequences. The feed port should be thin, wide and thin, fan-shaped, annular and multi-point feed ports to make the material flow turbulent and the glass fibers evenly dispersed to reduce anisotropy. It is best not to use a needle-shaped feed port. The cross-section of the feed port can be appropriately increased and its length should be short.
⑸ The mold core and cavity should have sufficient rigidity and strength.
⑹ The mold should be hardened, polished, and made of wear-resistant steel. The easily worn parts should be easy to repair.
⑺ The ejection should be uniform and powerful, and easy to repair.
⑻ The mold should be equipped with an exhaust overflow groove, and it should be set at the part where weld marks are likely to occur.
Setting the mold temperature
⑴ The mold 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 adjusted appropriately according to the quality conditions.
⑵ Correctly speaking, mold temperature refers to the temperature of the mold cavity surface when molding is being carried out. In the mold design and the setting of molding conditions, it is important not only to maintain the appropriate temperature, but also to make it evenly distributed.
⑶ Uneven mold temperature distribution will lead to uneven shrinkage and internal stress, which makes the molding port prone to deformation and warping. Picture
⑷ Increasing the mold temperature can achieve the following effects;
① Increase the crystallinity and more uniform structure of the molded product.
② Make the molding shrinkage more sufficient and reduce the post-shrinkage.
③ Improve the strength and heat resistance of the molded product.
④ Reduce residual internal stress, molecular orientation and deformation.
⑤ Reduce flow resistance during filling and reduce pressure loss.
⑥ Make the appearance of the molded product more shiny.
⑦ Increase the chance of burrs on the molded product.
⑧ Increase the chance of depression near the gate and reduce the chance of depression far from the gate.
⑨ Reduce the degree of obvious bonding line
⑩ Increase cooling time.
Metering and plasticization
⑴ In the molding process, the control of injection volume (metering) and uniform melting (plasticization) of plastics are performed by the plasticizing unit of the injection machine.
① Barrel Temperature
Although about 60-85% of the melting of plastics is due to the heat energy generated by the rotation of the screw, the melting state of plastics is still affected by the temperature of the heating barrel, especially the temperature near the front area of ​​the nozzle-when the temperature of the front area is too high, it is easy to cause dripping and wire pulling when taking out the product.
② Screw speed
A. The melting of plastic is mainly due to the heat generated by the rotation of the screw. Therefore, if the screw speed is too fast, the following effects will occur:
a. Thermal decomposition of plastic.
b. Shortening of glass fiber (fiber-reinforced plastic).
c. Accelerated wear of the screw or heating tube.
B. The setting of the speed can be measured by the size of its circumferential speed:
Circumferential speed = n (speed) * d (diameter) * π (pi)
Usually, for plastics with low viscosity and good thermal stability, the circumferential speed of the screw rod 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 try to lower the screw speed so that the rotary feeding can be completed before the mold is opened.
③ Back pressure
A. When the screw rotates to feed, the pressure accumulated by the molten plastic pushed to the front end of the screw is called back pressure. During injection molding, it can be adjusted by adjusting the oil withdrawal pressure of the injection hydraulic cylinder. Back pressure can have the following effects:
a. The molten plastic is melted more evenly.
b. The colorant and filler are dispersed more evenly.
c. The gas is discharged from the blanking port.
d. The feed is accurately measured.
B. The level of back pressure is determined by the viscosity and thermal stability of the plastic. Too high back pressure prolongs the feeding time and easily causes the plastic to overheat due to the increase in the rotating shear force. Generally, 5-15kg/cm2 is appropriate.
④ Suck back (decompression)
A. Before the screw starts rotating and feeding, the screw can be properly withdrawn to reduce the pressure of the molten plastic at the front end of the mold. This is called front suck back. Its effect can prevent the pressure of the molten plastic at the nozzle from exerting pressure on the screw. It is mostly used for hot runner mold molding.
B. After the screw ends rotating and feeding, the screw can be properly withdrawn to reduce the pressure of the molten plastic at the front end of the screw. This is called back suck back. Its effect can prevent dripping at the nozzle.
C. The disadvantage is that it is easy to make the main flow channel (SPRUE) stick to the mold; and too much suck back can suck in air, causing air marks on the molded product.