Poor venting and mold plate deformation have an impact on the product. We proposed an idea to control the distribution of injection pressure by changing the number and distribution of gates, and introduced a method to eliminate mold plate deformation in injection molds by pre-setting a compressed gas space through an example.
In the production of injection-molded products, it is often encountered that after prolonged use, the mold plate deforms, causing defects such as flash and burrs on the product, leading to non-conformity. Typically, such issues are addressed through major repairs or scrapping. However, for products with less stringent dimensional requirements, this solution is not cost-effective. This article proposes a simple and practical solution for a common type of mold deformation known as mold plate expansion.
I. Impact of Poor Venting and Mold Plate Deformation on the Product
Before the plastic melt fills the mold cavity, the cavity is filled with air. During the injection process, the plastic melt also generates a large amount of gas. During the filling process, these gases are entirely expelled from the mold cavity. The pathways for gas expulsion are roughly as follows:
① Gaps between mold inserts and ejector pin gaps;
② The mold parting line;
③ Specially designed vent holes and vent slots.
When the mold has poor venting, as the plastic melt continuously fills the mold cavity, the gas inside the cavity is gradually compressed. The greater the degree of compression, the stronger the resistance to the melt's advance.
During the flow process, the plastic melt experiences energy loss, and its temperature decreases, resulting in reduced fluidity. Combined with the obstruction of compressed gas, the consequences are the following two aspects: First, the melt is insufficient to break through the blockage of compressed gas, forcing it to stop advancing, causing short shots or burning of the product. Second, the melt breaks through the blockage of compressed gas, but due to excessive pressure (especially in multi-point gate molds), mold expansion occurs.
After prolonged use, molds (especially multi-point gate molds) are most likely to experience expansion at the center gate, which is directly subjected to the injection pressure of the injection machine screw. This is one of the main factors leading to product non-conformity.
II. Causes and Countermeasures for Mold Deformation Due to Expansion
1. Mold Example
This example involves an abalone tray mold with an outer diameter of 500 mm, uniformly distributed with hundreds of small holes of equal diameter, all through-holes. The product shape is shown in Figure 1, and the mold gating system is shown in Figure 2.
Due to the mold's long service life (5 years) and high production volume (300,000 pieces), expansion occurred around the center gate of the 5-point gating system under injection pressure, causing flash on the through-holes of the product. This resulted in a through-hole rate of only 70%, severely affecting the product's functionality. The non-through-hole areas were concentrated around the center gate.
2. Cause Analysis
The flow distance ratio leads to uneven pressure distribution. Since the mold uses a central 5-point gate, according to the die formula:
ΔP = jL (1)
Where:
ΔP — Pressure drop in the die
j — Die constant
L — Die length
From Equation (1), it can be seen that the pressure drop at the gate is proportional to the flow distance. From this, it can be deduced that the pressure at the center gate during molding, P_center, is greater than the pressure at other runner gates, P_branch, i.e., P_center > P_branch. Therefore, it can be concluded that excessive pressure at the center gate is the root cause of mold expansion.
The molding process of the 5-point gate mold is shown in Figure 3. The center gate fills first, then expands outward. To ensure the product is fully filled, the center part of the product must withstand excessive packing pressure.
Figure 3: Molding Process of a 5-Point Gate Mold
3. Solutions to Avoid Uneven Pressure and Resulting Issues
The simplest solution to the above problem is to block the center gate. As shown in Figures 1 and 2, after blocking the center gate, the ΔP values at the four gates become consistent, and there is no longer uneven pressure distribution. However, a new issue arises: the product is prone to forming a burn mark at the center point after molding, which is unacceptable for the product. Clearly, the problem has not been fundamentally resolved, as shown in Figure 4. Accordingly, we conducted a sample test on the modified mold and found that the molded product left a burn mark with a diameter of 3–8 mm.
Figure 4: Molding Process of a 4-Point Gate Mold
4. Pre-Set Compressed Gas Space
Based on the above experiments and analysis, we adopted the method of pre-setting a compressed gas space to resolve the issue. The specific method is shown in Figure 5.
(a) (b)
Aluminum Core
Pre-Set Compressed Gas Space
Figure 5: Pre-Set Compressed Gas Space
a — Before Modification
b — After Modification
In the original center gate cavity, using the upper half's diameter and taper as a reference, a truncated cone-shaped aluminum core with a length half of the original cavity length was made to seal the upper half of the center gate. The lower half was drilled and reamed to a straight hole with a diameter of 6 mm.
In this way, during the injection process, the gas not fully expelled from the center area is compressed into the pre-set compressed gas space under the pressure of the melt. Even some of the melt at the junction at the bottom is pressed into it, forming a tapered protrusion about 5 mm in height, with a diameter similar to the breakpoint mark left by the original center gate. This does not affect the product's appearance, as shown in Figure 5.
5. Schematic Diagram of Using a Pre-Set Compressed Gas Space
Compressed Gas
Melt Flow Direction
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