Some principles of injection mold casting system design

I. The composition of the pouring system

A common runner system, also known as a sprue system or pouring system, is the necessary passage of molten plastic from the nozzle of the injection molding machine to the mold cavity. The runner system includes Primary Runner, Sub-Runner and Gate, etc.

1. Main runner

Also called primary runner, Sprue or vertical runner, it is the runner from the part where the nozzle of the injection machine contacts the bushing of the main runner to the sub runner. This part is the first part through which the molten plastic flows after entering the mold.

2. Split runner

It is also called a sub-pouring channel or secondary pouring channel. Depending on the mold design, it can be further differentiated into First Runner and Secondary Runner. The runner is the transition area between the main runner and the gate, which can make the flow of molten plastic get a smooth transition; for multi-cavity molds, it also has the function of evenly distributing plastic to each cavity.

3. Gate

It is also called the inlet, which is the narrow opening between the manifold and the mold cavity and is also the shortest and thinnest part. Its function is to use the tight flow surface to accelerate the plastic, and the high shear rate can make the plastic flow well (due to the shear-thinning characteristics of plastic); the heating effect of viscous heating also has the effect of raising the material temperature and reducing the viscosity.

After molding, the gate is the first to be cured and sealed, which prevents the plastic from flowing back and prevents the mold cavity pressure from dropping too quickly, causing shrinkage and depression of the molded product. After molding, it is easy to cut off to separate the flow system and plastic parts.

4. Cold material well

Also called a cold cavity. The purpose is to store the cold plastic wavefront in the initial stage of filling to prevent the cold material from entering the mold cavity directly, which may affect the filling quality or block the gate. The cold material well is usually set at the end of the main flow channel, but when the length of the manifold is long, a cold material well should also be opened at the end.

II. Basic principles of pouring system design

1. Consideration of Cavity Layout

1) Balances Layout should be used as much as possible;

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

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

2. Consideration of flow guidance

1) Smoothly guide the molten plastic to fill the mold cavity, no vortex, and smooth exhaust;

2) Avoid the plastic melt to impact the small diameter core and metal insert as much as possible to prevent the core shift or deformation.

3. Consideration of heat loss and pressure drop

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

2) The flow should be short;

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

4) Avoid bending of the runner and sudden change of flow direction (change direction by arc angle);

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

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

4. Consideration of flow balance

1) When filing a mold with multiple cavities (Multi-Cavity), the flow channel should be balanced to make the plastic fill each cavity at the same time to ensure the consistency of the quality of the molded products;

2) Adopt Naturally-Balanced Layout for the manifold as much as possible;

3) When natural balancing is not possible, an artificial balancing method is used to balance the runners.

5. Consideration of waste

In the premise of smooth filling without affecting the flow and pressure loss, reduce the volume of the runner (length or cross-sectional area size) to reduce the runner waste generation and recycling costs.

6. Consideration of cold material

In the runner system design appropriate cold material well (Cold Slug Well), overflow groove, to fill the initial stage of filling colder plastic wavefront, to prevent the cold material directly into the mold cavity, affecting the quality of filling.

7. Consideration of exhaust

The plastic should be guided smoothly to fill the mold cavity, and the air in the mold cavity can escape smoothly to avoid the problem of scorching the package.

8. Consideration of molded product quality

1) To avoid the short shot, burr, seal, suture line, flow marks, spray, residual stress, warpage, mold kernel deflection, etc;

2) When the flow channel system is long or Multiple Gating, warpage and deformation of the finished product caused by unbalanced flow, insufficient pressure retention, or uneven shrinkage should be prevented;

3) Good product appearance, easy gate removal, and gate mark will not damage the appearance and application of plastic parts.

9. Consideration of production efficiency

To reduce the required post-processing as much as possible to shorten the molding cycle and improve productivity.

10. Consideration of ejection point

Consider the proper ejection position to avoid deformation of the molded part.

11. Consideration of plastics used

Avoid using too long or too small size runners for plastics with high viscosity or short L/t.