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In the process of plastic injection molding, the design of the gate size is a crucial step, directly affecting flow efficiency, pressure transfer, cooling time, and even the product's appearance and internal quality. Although the gate is only a small part of the mold system, it is the entrance for molten plastic to flow into the mold cavity, bearing the challenge of high pressure and high speed flow, and is a vital position affecting the quality of molding.
I. Relationship between gate size and volumetric flow rate
The design of the gate size is mainly determined by the volumetric flow rate (Q). When the gate size is larger, the plastic can enter the mold cavity more smoothly, the filling resistance will be reduced, which will help the pressure be effectively transmitted during the holding pressure stage, and it is also less likely to cause problems such as short shot and insufficient flow.
However, a larger gate also means more material, resulting in longer holding and cooling times, which prolongs the overall molding cycle and reduces production efficiency. Conversely, if the gate is designed to be too small, the following problems are likely to occur:
- If the flow resistance is too high, the molten adhesive may not be able to fill the mold cavity smoothly.
- Premature cooling and solidification of the gate prevents the holding pressure from being fully transmitted to the product, resulting in uneven shrinkage or porosity.
- High shear rates generate shear heat, which may lead to material degradation or high-temperature cracking, and leave flow marks on the product surface.
- The gate is fragile and can easily break during demolding.
Therefore, the design of the gate size needs to strike a balance between molding efficiency and product quality.
II. Relationship between gate size and shear rate
As molten plastic passes through the gate, it experiences a shear rate, which is a very critical parameter. An excessively high shear rate will cause the molten plastic to generate a large amount of shear heat, which may lead to thermal cracking or material degradation if the material has poor heat resistance.
The magnitude of the shear rate is controlled by the volumetric flow rate Q and the gate geometry. Common estimation formulas are as follows:
For round gates:
Where r is the radius (cm) of the circular gate.
For rectangular gates:
Where w is the gate width (cm) and t is the gate thickness (cm)
These formulas can be used to quickly estimate the shear rate at the gate. Although they are not as accurate as CAE mold flow analysis, they can help designers grasp the direction in the early stages.
For example, if a rectangular gate is used and the thickness is increased, the shear rate will decrease inversely proportional to the square of the thickness, effectively reducing the risk of heat accumulation caused by high-speed flow.
III. How to calculate volumetric flow rate (Q)
Volumetric flow rate Q refers to the volume of molten plastic entering the mold cavity per unit time, and its calculation formula is as follows:
Q = V / t
- V: Cavity volume (cm³)
- t: Time required for filling (s)
For products with multiple gates, the cavity volume for each gate needs to be calculated separately. If multiple injection speeds are used, it is recommended to use CAE mold flow analysis to dynamically evaluate the shear rate and obtain a more realistic design basis.
IV. Maximum permissible shear rate for different materials
Different plastic materials have different maximum tolerable shear rates. This is related to factors such as the material's molecular structure, main chain flexibility, and branching structure. Below are Bayer Materials' recommended maximum shear rate values for several common plastics:
| Material type | Recommended maximum shear rate (1/s) |
|---|---|
| ABS | Approximately 20,000 |
| PC | Approximately 15,000 |
| PA | Approximately 30,000 |
| POM | Approximately 15,000 |
| PBT | Approximately 20,000 |
If the actual shear rate at the gate exceeds this recommended value, it may lead to problems such as material decomposition, bubble formation, decreased mechanical properties, and even abnormal appearance.
V. Case Study: Example of Shear Rate Estimation
Suppose you have the following conditions:
| Parameter Items | numerical values |
|---|---|
| Cavity volume (V) | 40 cm³ |
| Filling time (t) | 2 seconds |
| Gate shape | rectangle |
| Gate width (w) | 0.3 cm |
| Gate thickness (t) | 0.2 cm |
Step 1: Calculate the flow rate Q
Step 2: Substitute into the rectangular gate shear rate formula
Result evaluation:
If PC or PBT materials are used, such shear rates are within the permissible range (< 15,000~20,000 1/s); however, for some easily deteriorating materials such as PVC, the rates may be too high.
The design of the gate size involves many variables, including injection speed, cavity volume, material properties, and holding time. Appropriate size design can not only improve product quality but also shorten cycle time and save costs. It is recommended to incorporate CAE analysis tools in the early stages of product development, combining theoretical calculations with material data to find the best balance between efficiency and quality.
