What do we need to pay attention to in the design of injection molds?

2021-03-23 12:46:23 admin

What do we need to pay attention to in the design of injection molds?

1. Injection mold measurement and plasticization

In the molding process, the control (metering) of the injection volume and the uniform melting (plasticization) of the plastic are performed by the plasticization mechanism of the injection machine.

1. Heating cylinder 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 front of the nozzle-the temperature of the front is over When it is high, it is easy to cause dripping and wire drawing when taking out the parts.

2. Screw speed

(1) 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:

① Thermal decomposition of plastic;

②The glass fiber (fiber-added plastic) is shortened;

③The screw or heating cylinder wears faster.

(2) The speed setting can be measured by the circumference speed:

Circumferential speed = n (rotation speed) * d (diameter) * π (circumference rate). 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.

(3) 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.

3. Back pressure

(1) 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:

① Melting glue more evenly;

② Toner and filler are more evenly dispersed;

③ Make the gas exit from the blanking port;

④The metering of incoming materials is accurate;

(2) The level of back pressure is determined by the viscosity of the plastic and its thermal stability. Too high back pressure prolongs the feeding time, and the increase in the rotational shear force can easily cause the plastic to overheat. Generally, 5--15kg/cm2 is appropriate.

4. Loosen

(1) 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, and its effect can prevent the pressure of the melt from the nozzle on the screw. It is mostly used for heat flow. Road mold forming.

(2) After the screw is rotated and fed, the screw is properly retracted to reduce the melt pressure at the front end of the screw. This is called back loosening, and its effect can prevent dripping of the nozzle.

(3) 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.

2. Precautions for injection mold design

1. The ejection should be even and strong, easy to replace and repair.

2. The mold core and cavity should have sufficient rigidity and strength.

3. The mold should be hardened, polished, and wear-resistant steel should be selected, and the easily worn parts should be easy to repair and replace.

4. The mold should be equipped with an exhaust overflow groove, and it should be located at a location prone to weld marks.

5. The cross section of the pouring system should be large, and the flow should be straight and short to facilitate uniform fiber dispersion.

6. 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.

7. The 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.

8. The design of the feed inlet should consider preventing insufficient filling, anisotropic deformation, uneven distribution of glass fibers, and easy to produce weld 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.

Third, the setting of the mold temperature of the injection mold

1. 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 to a higher level according to the quality condition.

2. Correctly speaking, mold temperature refers to the temperature of the surface of the mold cavity when forming is performed. In the condition setting of mold design and forming engineering, it is important not only to maintain a proper temperature, but also to make it uniform. distributed.

3. Uneven mold temperature distribution will cause uneven shrinkage and internal stress, which will make the molding mouth easy to deform and warp.

4. Increase the mold temperature to obtain the following effects:

(1) Increase the crystallinity of the molded product and a more uniform structure;

(2) Make the molding shrinkage more fully and reduce the post-shrinkage;

(3) Improve the strength and heat resistance of molded products;

(4) Reduce internal stress residue, molecular alignment and deformation;

(5) Reduce the flow resistance during filling and reduce the pressure loss;

(6) Make the appearance of the molded product more shiny;

(7) Increase the chance of burrs in molded products;

(8) Increase the location near the gate and reduce the chance of recession at the far gate;

(9) Reduce the obvious degree of bonding line;

(10) Increase the cooling time.