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injection mold design points

composition of Runner system of injection mold

mold runner system

Runner system of injection mold is also named sprue system. it is the necessary passage for the melt material from the nozzle of injection molding machine to the mold cavity.The runner system includes primary runner,sub-runner,and gate.

  • Primary runner

It refers to the flow path from the part where the injection nozzle in touch with the primary runner bush of the mold, and the flow path to the sub-runner. This part is the first part of the molten plastic that flows through the mold.

  • Sub-runner

sub-runner is also named branch channel.it can be seperated as first runner and secondary runner according to the mold design.The sub-runner is a transition zone between the primary runner and the gate, which enables the flow direction of the molten plastic to be smoothly converted; for the multi-cavity mold, it has the function of evenly distributing the plastic to each cavity.

  • Gate

gate is also known as the feed port, it is a narrow port between the sub-runner and the mold cavity, and is also the thintest part of the shortest. Its function is to make the plastic accelerate by using the tightening flow surface, and the high shear rate can make the plastic flow good (due to the shear thinning characteristics of the plastic); the heating effect of the viscous heating also increases the temperature of the material and lowers the viscosity. effect.
After the molding is completed, the gate is first cured and sealed, which prevents the plastic from flowing back, and prevents the pressure of the cavity from dropping too fast, so that the molded product has the function of shrinking and dent. After molding, it is easy to cut off to separate the runner system and plastic parts.

  • cold slug

The purpose is to store the cold plastic wave front in the initial stage of the filling and filling, to prevent the cold material from directly entering the cavity, affecting the filling quality or blocking the gate. Cold wells are usually placed at the end of the primary runner. When the length of the sub-runner is long, a cold slug should be opened at the end.

Basic principle of injection mold design

  • mold cavity layout
  1. Try to use Balanced Layout;
  2. The arrangement of the cavity and the opening of the gate are symmetrical, so as to prevent the uneven force of the mold from being generated, and the problem of the die overflow occurs;
  3. The cavity arrangement is as compact as possible to reduce the size of the mold.
  • injection mold flow design
  1. It can smoothly guide the molten plastic to fill the cavity, without eddy current, and can be smoothly vented;
  2. Try to avoid the impact of the core melt and the metal insert on the front side of the plastic melt to prevent core displacement or deformation.
  • Heat loss and pressure drop considerations
  1. The smaller the heat loss and pressure drop, the better;
  2. The process should be short;
  3. The cross-sectional area of the flow channel should be large enough;
  4. Try to avoid the flow channel bending and suddenly change the flow direction (change the direction with the arc angle);
  5. The surface roughness should be low when the runner is processed;
  6. Multi-point pouring can reduce the pressure drop and the required injection pressure, but there will be suture problems.
  • Flow balance considerations
  1. When multi-cavity filling, the flow path should be balanced, and the plastic should be filled at the same time as much as possible to ensure the quality consistency of each cavity molding product;
  2. The shunt channel should be in a natural balanced manner
  3. When there is no natural balance, the artificial balance method is used to balance the flow path.
  • Waste material consideration

Under the premise of smooth filling without affecting flow and pressure loss, reduce the flow volume (length or cross-sectional area) to reduce the cost of flow path waste generation and recovery.

  • cold material consideration

The appropriate cold slug well and overflow trough are designed on the runner system to supplement the cold plastic wave front in the initial stage to prevent the cold material from directly entering the cavity and affect the filling quality.

  • air exhaust considerations of injection mold design

The plastic should be smoothly guided to fill the cavity and allow the air in the cavity to escape smoothly to avoid the problem of encapsulation and charring.

  • Molded product quality considerations
  1. Avoid problems such as short shots, burrs, encapsulation, sutures, flow marks, jets, residual stress, warpage, and mold offset;
  2. When the flow channel system is long or multi-growth, the problem of warpage of the finished product due to unbalanced flow, insufficient pressure holding or uneven shrinkage should be prevented;
  3. The appearance of the product is good, the removal of the trimming gate is convenient, and the Gate Mark does not damage the appearance and application of the plastic part.
  • Productivity considerations

Minimize the required post-processing, shorten the forming cycle and increase production efficiency.

  • Ejection point consideration

Appropriate ejection position should be considered to avoid deformation of the molded article.

  • Use of plastic considerations

Plastics with higher viscosity or shorter L/t avoid using flow channels that are too long or too small.

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