Analysis of Design Process of Injection Mould

Ⅰ. Composition of injection mold gating system

Ordinary runner system, also known as runner system, or gating system, is the necessary passage for molten plastic to travel from the nozzle of the injection machine to the cavity. The runner system includes a main runner, a branch runner and a gate.

1. Main street

Also known as the main runner, sprue or vertical runner, it refers to the runner from the part where the nozzle of the injection machine is in contact with the main runner bushing of the injection mold to the runner. This part is the first part the molten plastic flows through after entering the mold.

2. Shunt channel

Also known as runner or secondary runner. With the design of the injection mold, it can be further divided into the first runner and the second runner. The runner is the transition area between the main runner and the gate, which can make the flow direction of the molten plastic change smoothly; for multi-cavity molds, it has the function of evenly distributing the plastic to each cavity.

3. Gate

Also known as the feed port, it is the narrow opening between the runner and the mold cavity, and it is also the shortest and thinnest part. Its function is to use the compressed flow surface to accelerate the plastic, and the high shear rate can make the plastic flow well (due to the shear thinning characteristics of the plastic); the heating effect of viscous heating also has the effect of increasing the material temperature and reducing the viscosity. effect.

After the molding is completed, the gate is first cured and sealed to prevent the plastic from reflowing, and to prevent the mold cavity pressure from dropping too fast, causing the molded product to shrink and sag. After molding, it is easy to cut off to separate the runner system and the plastic parts.

Ⅱ. The basic principles of injection mold gating system design

1. Consideration of cavity layout

1) Try to adopt a balanced arrangement;

2) The layout of the cavity and the opening of the gate should be symmetrical to prevent the uneven force of the mold from causing eccentric load and the problem of mold overflow;

3) The cavity arrangement should be as compact as possible to reduce the size of the injection mold.

2. Consideration of flow guidance

1) It can smoothly guide the molten plastic to fill the mold cavity without generating eddy current, and it can be exhausted smoothly;

2) Try to avoid the frontal impact of plastic melt on cores and metal inserts with smaller diameters to prevent displacement or deformation of the cores.

3. Consideration of heat dissipation and pressure drop of injection mold

1) The smaller the heat loss and pressure drop, the better;

2) The process should be short;

3) The cross-sectional area of the runner should be large enough;

4) Try to avoid the bending of the runner and the sudden change of the flow direction (change the direction at the arc angle);

5) The surface roughness should be low when the runner is processed;

6) Multi-point pouring can reduce the pressure and required injection pressure, but there will be suture problems.

4. Consideration of flow balance

1) When filling a mold with multiple cavities, the runners should be balanced, and try to fill each cavity with plastic at the same time to ensure the consistency of the quality of the molded products in each cavity of the injection mold;

2) The shunt channel should be arranged in a natural balance arrangement as far as possible;

3) When the natural balance cannot be achieved, use the artificial balance method to balance the flow channel.

5. Consideration of waste

On the premise of smooth filling without affecting the flow and pressure loss, the volume of the flow channel (length or cross-sectional area) can be reduced to reduce the waste generation and recycling costs of the flow channel.

6. Consideration of cold material

Appropriate cold material wells and overflow grooves are designed on the runner system to supplement the cold plastic wave front in the initial stage of filling, so as to prevent the cold material from directly entering the cavity of the injection mold and affecting the filling quality.

7. Exhaust considerations

The plastic should be smoothly guided to fill the mold cavity, and the air in the mold cavity should be able to escape smoothly to avoid the problem of encapsulation burning.

8. Consideration of the quality of formed products

1) Avoid problems such as short shots, burrs, encapsulation, sutures, flow marks, jets, residual stress, warpage deformation, and mold core offset;

2) When the runner system has a long process or multi-point pouring, the problem of warpage and deformation of the finished product caused by unbalanced flow, insufficient pressure holding or uneven shrinkage should be prevented;

3) The appearance of the product is good, it is convenient to remove and trim the gate, and the gate mark does not damage the appearance and application of the plastic part.

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