Techniques for Adjusting the Injection Speed

The Close Relationship Between Injection Speed and Product Quality Makes It a Key Parameter in Injection Molding.

By determining the start, middle, and end of the filling speed segments and achieving a smooth transition from one set point to another, stable melt surface velocity can be ensured to produce the desired molecular orientation and minimal internal stress.

Techniques for Adjusting the Injection Speed

Principles of Segmented Injection Speed

We recommend the following principles for stage injection speed:

  • The velocity of the fluid surface should be constant.
  • Fast injection should be used to prevent meltplastic freezing during the injection process.
  • Injection speed settings should pay attention onslowing down the speed at the gate while ensuring rapid filling in critical areas such as the runner.
  • Injection speed should ensure an immediate stop once the mold cavity is filled to prevent over-packing, flash, and residual stress.
  • The basis for setting stagespeeds must consider the mold’s geometry, other flow constraints, and unstable factors.

The setting of the injection speeds must base on a clear understanding of injection molding processes and material. Otherwise, product quality will be difficult to control.

Since it is challenging to measure the melt flow rate directly, it can be indirectly calculated by measuring the screw forward speed, or cavity pressure (ensuring that the check valve does not leak).

Material characteristics are very important because polymers may degrade differently due to stress.

Increasing the molding temperature may lead to severe oxidation and degradation of the chemical structure.

However, degradation caused by shear decreases because high temperatures reduce material viscosity, reducing shear stress.

Undoubtedly, stage injection speeds are helpful for molding materials that are heat-sensitive, such as PC, POM, UPVC, and their compounds.

The geometry of the mold also plays a crucial role

  • Thinner sections require the highest injection speed.
  • Thick-walled parts require a slow-fast-slow speed curve to avoid defects.
  • To ensure that part quality meets standards, the injection speed setting should maintain a constant melt front velocity. Melt flow velocity is highly significant as it affects the molecular orientation and surface condition of the part.
  • When the melt front reaches a junction in the structure, it should decelerate.
  • For complex molds with radial diffusion, the melt should be added evenly.
  • Long runners must be filled quickly to reduce cooling of the melt front. However, it is an exception when injecting high-viscosity materials like PC because excessively high speeds can carry cold material into the cavity through the gate.
Techniques for Adjusting the Injection Speed-2

Setting and Impact of Injection Speed

Adjusting the injection speed can help eliminate defects caused by a slowdown in flow at the gate location.

When the melt plastic passes through the nozzle and runner to reach the gate, the surface of the melt front may have already cooled and solidified. Or it may have stagnated due to a sudden narrowing of the runner until enough pressure is built up to push the melt through the gate, resulting in a pressure peak through the gate.

High pressure can damage the material and cause surface defects such as flow marks and gate burn, which can be overcome by slowing down just before the gate.

This deceleration can prevent excessive shear at the gate location, followed by an increase in injection speed back to the original value. Because precise control of the injection speed slowdown at the gate location is challenging, deceleration at the end of the runner is a better solution.

We can avoid or reduce defects such as flash, burning, and air entrapment by controlling the end-stage injection speed.

Deceleration at the end of filling can prevent over-packing of the cavity, avoid flash, and reduce residual stresses.

Air entrapment caused by poor venting at the end of the mold or filling issues can also be addressed by reducing the venting speed, especially at the end of the injection stage.

Short shots occur due to slow speeds at the gate or local flow obstructions caused by solidification of the melt, among other reasons. Speeding up the injection speed as it passes through the gate or when encountering local flow obstructions can solve this issue. Defects such as flow marks, gate burn, molecular breakage, delamination, and peeling on thermosensitive materials result from excessive shear as the melt passes through the gate.

Smooth parts depend on injection speed, with glass fiber-filled materials being particularly sensitive, especially nylon. Dark spots (ripple marks) result from flow instability caused by changes in viscosity. Twisted flow can lead to ripple marks or uneven haze, and the specific defect depends on the degree of flow instability.

When high-speed injection occurs as the melt passes through the gate, it results in high shear, causing thermal-sensitive plastics to burn. This burnt material can pass through the mold cavity and reach the flow front, appearing on the surface of the part.

To prevent jetting, the injection speed setting must ensure fast filling of the runner area, followed by slow injection through the gate. Identifying the point of this speed transition is the essence of the problem.

  • If it’s too early, the filling time will increase excessively
  • If it’s too late, excessive flow inertia will lead to jetting.

The trend of jetting becomes more pronounced as melt viscosity decreases and barrel temperatures increase.

Additionally, small gates that require high-speed, high-pressure injection are also a significant factor leading to flow defects.

Shrinkage can be improved by more effective pressure transmission and smaller pressure drops.

Low mold temperature and slow screw advancement significantly reduce flow length, and this must be compensated for by high injection speeds.

High-speed flow reduces heat loss, and the frictional heat generated due to high shear causes an increase in melt temperature, slowing down the thickening rate of the outer layer of the part. The cross-sectional area of the cavity must have sufficient thickness to avoid excessive pressure drops; otherwise, shrinkage will occur.

Most injection molding defects can be resolved by adjusting the injection speed, so the key to adjusting the injection process is to set the injection speed in its stages reasonably.

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