Dalian injection molding is one of the commonly used processes in plastic product production, which involves injecting high-temperature molten plastic into a mold and cooling it to solidify and form. However, due to material shrinkage, uneven cooling, or improper process parameters, internal stresses are often generated inside the product. Internal stress not only affects the appearance and dimensional accuracy of the product, but may also reduce its mechanical properties and service life. Therefore, reducing internal stress is one of the key objectives of injection molding process optimization.

1、 Causes of internal stress

Internal stress is mainly divided into two categories:

Orientation stress: During the flow process of the melt, the polymer chains are stretched and arranged along the flow direction, and stress concentration occurs due to shrinkage differences during cooling.

Thermal stress: The stress generated by temperature difference or uneven shrinkage during plastic cooling, especially in areas with large differences in wall thickness.

Common triggers include:

The melt temperature is too high or too low;

Excessive injection pressure or holding pressure;

The cooling rate is too fast;

Unreasonable mold design (such as improper gate position).

2、 Specific measures to reduce internal stress

1. Material selection and pretreatment

Choosing low shrinkage materials: Crystalline plastics such as PP and PE typically have lower internal stresses than non crystalline materials such as PS and PC.

Adding modifiers: By reinforcing with fiberglass or blending with elastomers, the shrinkage rate is reduced and stress is dispersed.

Fully dry the raw materials: avoid water evaporation that may cause bubbles or local stress concentration (such as nylon needs to be dried at 80 ℃ for more than 4 hours).

2. Optimize process parameters

Melt temperature control:

Appropriately increasing the melt temperature (such as the recommended melt temperature for PC at 280-320 ℃) can reduce the viscosity of the melt and decrease the flow orientation stress, but it is necessary to avoid overheating and degradation.

Injection speed and pressure:

Adopting multi-stage injection: first filling the cavity at high speed, and then filling and shrinking at low speed to avoid shear overheating;

Reduce holding pressure: Excessive pressure during the holding stage can exacerbate molecular chain orientation. It is recommended to use 50% to 70% of the injection pressure.

Cooling rate adjustment:

Extend the cooling time or use a slow cooling process (such as controlling the mold temperature at 60-80 ℃) to avoid excessive temperature difference between the surface and the core;

The "step cooling" strategy can be used for thick walled products.

3. Mold design optimization

Gate design:

Increase the gate size or use fan-shaped gates to reduce flow shear stress;

The gate position should avoid the stress concentration area (such as the tooth root of the gear).

Flow channel and exhaust:

The flow channel should have a smooth transition to avoid turbulence caused by right angles in the melt;

Add exhaust slots (depth 0.02~0.05mm) to prevent trapped air from causing local stress.

Mold temperature control:

Use a mold temperature machine to maintain a uniform temperature, with a temperature difference controlled within ± 5 ℃.

4. Post processing technology

Annealing treatment:

Place the product in an environment below the hot deformation temperature of 10-20 ℃ (such as ABS annealed at 80 ℃ for 2 hours) to relax the molecular chains and release stress.

Humidity regulation:

For hygroscopic materials such as POM, internal stress can be balanced by water bath or steam treatment.

5. Structural design assistance

Avoid sudden changes in wall thickness: adopt gradient transition or reinforcement design to reduce shrinkage differences;

Rounding treatment: Change sharp edges to R-angles (radius ≥ 0.5 times wall thickness) to reduce stress concentration.

3、 Case analysis

A certain enterprise encountered cracking problems during the production of PC lampshades. After analysis, it was found that:

The melt temperature is too low (260 ℃), resulting in high flow stress;

The mold cools too quickly (water temperature is 20 ℃).

Improvement measures:

The melt temperature rises to 300 ℃, and the mold is kept at a constant temperature of 80 ℃;

The gate is changed to a fan-shaped shape and annealed at 120 ℃ for 1 hour.

Result: The internal stress decreased by 70%, and the cracking rate decreased from 15% to below 1%.

Reducing internal stress in injection molding requires a comprehensive consideration of various factors such as materials, processes, molds, and post-treatment. The core is to control temperature uniformity and reduce molecular orientation. By setting scientific parameters and refining design, product yield and durability can be improved. In the future, combining CAE simulations (such as Moldflow analysis) will further optimize stress prediction and process adjustment efficiency.