Simple solution for expansion deformation in injection molds

This paper discusses the impact of poor exhaust and mold plate expansion deformation on products, proposing an idea to control the distribution of injection molds pressure by changing the number and distribution of gates. An example is provided to introduce a method of eliminating mold plate expansion deformation by presetting a compressed gas space.

In the production of injection molded products, it is common for molds to become deformed after prolonged use, resulting in defects such as flash and burrs, leading to non-conforming products. Typically, people resort to major repairs or scrapping the molds to address these issues. However, for products with lower dimensional requirements, such solutions are not cost-effective. This paper proposes a simple and feasible solution for a common mold expansion deformation scenario.

Impact of Poor Exhaust and Mold Plate Expansion Deformation on Products

Before the plastic melt fills the mold, the mold cavity is filled with air, and during injection, the plastic melt generates a large amount of gas. During the filling process, these gases are expelled from the mold cavity through the following routes:

Gaps between mold inserts and push rods;
The mold parting line;
Specially designed exhaust holes and exhaust slots.

When the mold is poorly exhausted, the air in the cavity is compressed as the plastic melt is continuously injected, increasing resistance against the melt flow. The more compressed the air, the greater the obstruction. As the plastic melt flows, energy loss causes the temperature to drop, reducing fluidity. The obstructing compressed air can lead to two main consequences:

The melt cannot break through the compressed air, stopping its advancement and causing short shots or burnt products;
The melt breaks through the compressed air but excessive pressure (especially in multi-point gate molds) causes mold expansion.
After prolonged use, molds (especially those with multi-point gates) are more prone to expansion at the central gate, directly subjected to the injection pressure from the injection machine’s screw, leading to product non-conformance.

Injection Molds Causes and Solutions for Mold Plate Expansion Deformation

2.1 Injection Molds Example

The example discussed is an abalone plate mold with an outer diameter of 500mm, uniformly distributed with hundreds of equal-diameter through-holes, as shown in Figure 1, and the mold gating system shown in Figure 2. Due to prolonged use (5 years) and high production volume (300,000 pieces), the center gate area of the 5-point gating system experienced mold expansion under injection pressure, causing flash in the through-holes, reducing the through-hole rate to 70% and severely affecting product functionality. The non-through-hole parts were concentrated around the central gate.

2.2 Cause Analysis

Different flow distances lead to uneven pressure distribution. The 5-point central gating system’s pressure drop ΔP is proportional to the flow distance LLL as per the formula:

ΔP=jL

where:

• ΔP is the pressure drop,
• j is a constant,
• L is the flow distance.

From this formula, it can be derived that the central gate pressure Pcenter is greater than the other branch gate pressures Pbranch, i.e Pcenter>Pbranch. Hence, excessive pressure at the central gate is the root cause of mold expansion. Hence, excessive pressure at the central gate is the root cause of mold expansion.

Figure 3. The 5-point central gating system

During the molding process of the 5-point gate mold, the central gate fills first and then expands outward. To completely fill the product, the central part must endure excessive packing pressure.

2.3 Solution to Uneven Pressure and New Problems

The simplest solution is to block the central gate. Figures 1 and 2 show that after blocking the central gate, the ΔP values at the four gates become consistent, eliminating uneven pressure. However, this leads to a new problem: burn marks easily form at the center point after molding, which is unacceptable. Thus, the fundamental issue remains unsolved, as shown in Figure 4. Analysis of a sample after modification revealed a burn mark of diameter f3 to f8 mm.

Figure4， The 5-point central gating system

2.4 Injection Molds Presetting Compressed Gas Space

Based on the experiments and analysis, the problem was resolved by presetting a compressed air space, as shown in Figure 5.

Aluminum core,
Preset compressed air space.

In the original central gate cavity, an aluminum core with a conical shape and half the original cavity length was used to seal the upper half of the central gate. The lower part was drilled and reamed to a
f6 mm straight hole. This design allows the melt to compress the residual gas into the preset compressed air space during injection, forming a conical protrusion of about 5mm without affecting the product appearance, as shown in Figure 5.

2.5 Principle Diagram of Preset Compressed Gas Space

Compressed gas,
Melt flow direction.

3，Injection Molds Conclusion

By applying the method of presetting a compressed gas space, the injection pressure at the mold cavity center is reduced, solving the mold expansion problem. This method rejuvenated the mold, allowing an additional production of 55,000 pieces in the same year.

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