The Importance of Injection Speed of an Injection Molding Machine

Are you paying attention to the injection speed of the molding machine? Here’s everything you should know.

The Importance of Injection Speed of an Injection Molding Machine - KB Delta

Injection molding refers to the manufacturing process commonly employed for fabricating items such as automotive body parts, plastic trinkets, cell phone cases, containers, water bottles, etc. In essence, most of the plastic materials and items used extensively on a daily basis are injection molded.

The mold industry uses high-speed injection molding equipment in injection molding. During the injection process, the injection molding machine is expected to have a particular injection speed in order to make the melt fill out the cavity on time. This is in addition to the ample injection pressure that ensures the faster flow rate of the melt.

During production, a crucial factor that helps ensure product quality includes choosing a logical injection speed.

More often than not, the low-speed injection of molten material leads to a long filling time and produces products highly prone to defects like uneven density, weld lines, as well as large residual stress.

This is commonly noticeable for thin-walled products as well as crystalline plastics with narrow process temperature ranges.

Even though high-speed injection minimizes the temperature difference of the melt within the mold cavity, shortens the molding cycle, enhances the pressure transmission effect, and obtains precision products with low stress and uniform density, excessive injection speed will cause the melt to flow through the nozzle.

The frictional heat caused by this action may result in plastic discoloration and decomposition. It may also bring about several disadvantages, such as poor exhaust, gas inhalation, etc., which directly impact or affect the surface quality of the plastic product. However, the requirements for program control are still higher.

 

What is the Relationship Between Injection Speed, Injection Rate, And Injection Time?

The best way to highlight the relationship between these is to define each term. Let’s define each one:

 

  • Injection speed

This refers to the speed at which the plunger or screw moves during the injection. The faster the plunger goes, the shorter the time it takes to complete a single injection stroke, allowing for greater yield.

 

  • Injection rate

It refers to the volume flow rate that can be attained per time at the specific period of injection, such as the ratio of injection time and injection capacity.

 

  • Injection time

This refers to the shortest period utilized by the plunger or screw in order to complete the longest injection stroke. It can also be said to be the shortest period required for the complete injection of the material with volume and injection capacity.

It must be mentioned that injection speed or rate considerably determines the secondary heating of this plastic as it moves through the gate and right into the mold. But frictional heat is generated at the gate restriction as well as between the material against the walls of the solidified part and the flowing material at the center of the part.

The temperature will rise significantly at the gate, though this depends primarily on the injection speed as well as pressure. If the fill rate is too slow, the plastic material starts cooling even before the cavity gets filled up. By then, the pressure needed to fill the cavity skyrockets.

But if the injection rate is too low, it will inhibit the packing of the cavity since the material cools down during the filling phase, and the gate will freeze swiftly right after the mold is filled up. The result of this development is higher shrinkage.

However, considering the other extreme, if the cavity fills up too quickly, the considerable pressure drop at the gate becomes excessive. The pressure needed to fill that part goes up immediately.

This can increase shrinkage since the plastic’s temperature in the cavity is far more likely to be higher than the temperature at the optimal fill rate. The latter is usually found near the minimum filling pressure and depends on the size, geometry, as well as location of the gate.

 

Setting the Injection Speed of an Injection Machine

The injection speed of an injection machine is not a one-size-fits-all thing. It depends significantly on the mold, temperature, and material. The setup person is responsible for determining this for individual job runs.

However, here are some vital tips you will find useful for setting the injection rate of your injection molding machine:

 

  • The basic principle

Setting the injection rate of an injection molding machine has a basic principle: to reduce or increase it. According to the section size, plastic flow within the mold cavity forms.

It should also follow the sequence of slow-to-fast-to-slow and as fast as possible in order to pinpoint any appearance defects.

 

  • Quality assurance

High-speed mold filling, under the assumption of not engendering side effects, should be carried out as fast as possible.

This enables or ensures the welding appearance and strength quality of plastic items or pieces. Low pressure helps to minimize the plastic pieces’ internal stress and boosts their overall strength.

 

  • Avoid shrinkage defects

High-pressure slow feeding stabilizes the flow rate and lowers the shear rate while leaving the overall size of the plastic pieces unvaried in order to avoid any shrinkage defects.

The ‘pressure’ is the injection pressure set within the system, while the ‘rate’ is the injection rate set within the system.

 

  • Inactive ingredients

As you already know, some non-polar plastics don’t have active atomic groups. For plastics in this category, a significant increase in shear stress engenders a decrease in the viscosity of the melt.

This is of immense benefit to injection since injection rate can be employed to adjust and control the viscosity of the material. This makes it the third means right after pressure and temperature.

The injection speed of injection molding machines must have a particular adjustment range in order to comply with the processing prerequisites of different plastics as well as products with a wide range of structures.

The only way to successfully adjust the injection speed of the equipment is by controlling the flow of pressure oil direct into the injection cylinder. Moreover, multi-stage injection is presently employed for meeting the numerous requirements of product structure.

 

Conclusion

The injection speed of injection molding currently has an increasing trend. Increasing injection speed shortens the injection time, resulting in the production of high-quality products at significantly lower mold temperature while reducing the molding cycle.

High injection speed is a vital requirement for producing high-quality plastic products. This is quite true for injection molding for a wide variety of long-run and thin-walled products.

 

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