Plastic Injection molding process parameter 2:Speed

Injection mold process

In the injection molding process, speed is a very important parameter, which directly affects the quality and production efficiency of injection molding products. Here is some basic information about the speed of the injection molding process parameter:

1. The Speed At Which The Screw Turns

The rotary speed of the screw of the plastic injection molding machine (RPM) is the two edges of the knife. When the rotary speed is fast, the feeding speed is fast, which can shorten the plastic injection time and fill the plastic liquid into the mold cavity to make it full, but at the same time, the Shear force relative to the material will also increase. The ability of the material to melt uniformly (degree of mixing) also becomes poor.

For Resin with poor stability, it is easy to be destroyed by the heat generated by the larger shearing force; Resin with higher viscosity requires higher rotational torque to drive, so for these two kinds of materials (poor stability, high viscosity), generally recommended to use a lower screw speed.

For low viscosity, the good thermal stability of the material can be considered to take high speed to shorten the plastic forming time. Because of the higher speed, the shear force will produce friction and then generate heat energy, so it can achieve the benefit of saving electricity. Still, it should remind that this will increase the chance of uneven mixing. 

To Sum Up:

The rotation speed of the screw will directly affect the mixing degree of plastics in the spiral groove. 

The small screw groove is shallow; the absorption heat source is fast enough to soften the plastic in the compression section. The friction heat energy between the screw and tube is low, suitable for high-speed rotation, increased plasticizing ability. 

Large screw is not fast rotation so as not to plasticize unevenly and cause excessive friction heat. 

The plastic with high heat sensitivity will easily decompose the plastic if the screw speed is too large. 

Usually, each size of the screw has a certain speed range, and the general speed PC is 40 ~ 70, the speed is too low, can not melt plastic; Too high, and the plastic may burn. 

The current maximum surface speed of 1m/ SEC is limited, so shear-sensitive materials should be less than 0.5m/ SEC.

2. Injection Speed

blow molding Vs injection molding

The speed at which the screw will melt glue into the hole through the nozzle is called the Screw Advance speed. The injection speed that can be set on the forming machine is the filling speed. 

When the melt enters the cavity, the real flow rate of the melt in the cavity will change with the geometry of the cavity, even if the filling speed remains constant. 

When melting through a narrow passage, the flow velocity tends to become quite fast (just like the flow through the nozzle will grow fast), easily in the surface of the finished product produce the bad appearance, such as spray mark jet mark high-speed flow of the melt at the same time also can produce higher in narrow channels of shearing force. Thus local overheating may be caused by the burning of the bad.

On the contrary, when the melt passes through a wider path, the flow rate will become slower, and it is easier to produce a corrugated or dull surface appearance. 

Therefore, the filling speed setting should be adjusted according to the cross-sectional area covered by the front end of the melt flow and strive to have a stable speed and fast filling of the melt flow in the cavity.

Effects of high Screw Advance speed (shortened filling time) :

It can maintain high temperature, reduce the obvious Welding lines and increase the mechanical strength.

Can make the finished product surface becomes richer gloss.

Reduce the degree of coordination and increase the crystallinity of the molecular structure.

The soil temperature in the cavity is increased, and the uniformity of pressure is improved.

Reduce the need for clamping force.

Reduce the layers of skin that form during filling.

Effects when the initial stage of filling use lower filling speed:

Avoid deformation of weak cavity parts. 

Reduce the scorch marks, burning focus, and Jetting lines at the gate. 

At the end of filling, deceleration filling is used. 

It can make the timing of the pressure holding switch more accurate to control the filling amount and reduce the deformation. 

It can improve exhaust and reduce short shots or burning caused by poor exhaust lock. 

Can avoid the cavity pressure rise too high.

3. Pressure Maintenance Time

During injection molding, a certain amount of pressure needs to be maintained in order to fill every corner of the mold with plastic. Pressure maintenance time refers to maintaining a certain amount of pressure after the injection is completed.

If the pressure maintenance time is too short, it may lead to insufficient plastic flow, resulting in weld lines, shrinkage holes and other defects; If the pressure is maintained for too long, it will lead to plastic melting and overheating, resulting in bubbles and other defects.

4. Injection Molding Cooling time

Cooling time refers to the time for the plastic to cool and set after the injection is completed. If the cooling time is too short, it may lead to gelatinization, deformation and other defects on the product surface. If the cooling time is too long, it will lead to low production efficiency.

Conclusion

Speed is an important parameter in the injection molding process, which needs to be set reasonably according to the nature of the plastic, the structure of the mold, the requirements of the product and other factors to ensure the production of high-quality products.

 

Author

Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.

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