Microscopic explanation of internal stress in injection molding

2025-09-04

Simply, internal stress is the mutually balanced stress that exists between molecules within a product when no external forces are acting on it. You can imagine it as a piece of plastic being pulled or squeezed from all directions by invisible hands.

From a microscopic perspective, this is mainly related to the "forced freezing" of polymer chains (the basic building blocks of plastic, like a mess of spaghetti) during the rapid cooling and molding process. Specifically, it can be divided into two categories:

1.Microscopic Essence: Two Types of Internal Stress

(1). Flow-induced or Molecular Orientation Stress

Microscopic process: During the Injection Molding process, molten plastic is injected into the mold cavity at high speed through narrow gates and runners under high pressure. This powerful shear force causes the originally tangled polymer chains to stretch and orient along the flow direction (like a chaotic crowd suddenly ordered to line up and run in one direction).

Freezing effect: When the melt contacts the cold mold wall, the surface material instantly cools and solidifies. This cooling rate is so rapid that the polymer chains that have just been stretched and oriented have no time to return to their original naturally curled and relaxed state and are "frozen" in the oriented state.

Stress Generation: Frozen molecular chains act like stretched springs, storing elastic potential energy and strongly craving to retract back into their curled state. However, they are firmly bound by the surrounding, similarly solidified molecular chains, preventing them from retracting. This creates inherent tensile stress in the direction of flow. This orientation effect becomes more pronounced with thicker products and greater differences in cooling rates between the surface and core.

Microscopic process: When plastic cools from a molten state (e.g., above 200°C) to room temperature, it experiences significant thermal contraction. However, this cooling of the product is uneven. Uneven shrinkage occurs when the surface of the product first contacts the mold, rapidly cooling and solidifying, maintaining a relatively constant volume. Meanwhile, the core material within the product continues to cool and contract.

Stress occurs when the core material wants to contract, but the solidified, hardened outer shell (the skin) acts like a "straitjacket," preventing it from doing so freely. The result is a core that is pulled by the skin and prevents it from fully contracting, thus being in tension; while the skin is squeezed by the core, thus being in compression. This is like a hot pie: the outer crust cools and hardens first, while the filling inside continues to cool and contract. Ultimately, the filling pulls the crust toward the center, causing cracks (if the stress is too great).

2. Problems caused by internal stress

(1). Warpage: After the product has been placed for a period of time, the frozen molecular chains inside will slowly relax and the stress will gradually be released, causing the product to twist, bend, and other shape changes.

(2). Cracking: Especially near the gate or in stress concentration areas (such as sharp corners), when the internal stress exceeds the strength limit of the material itself, the product will crack or even break.

(3). Dimensional Instability: Products with internal stresses will experience subtle changes in size over time, making them unable to meet high-precision assembly requirements.

(4). Decreased chemical resistance: Internal stress will cause "micro-gaps" inside the material. When it comes into contact with chemicals such as solvents and oils, "stress cracking" is likely to occur, accelerating damage.

(5). Impact on optical properties: For transparent products (such as polystyrene PS, polycarbonate PC), internal stress will cause birefringence and affect optical uniformity.

3. How to reduce internal stress (starting from the microscopic causes)

(1). Adjust process parameters: Increase mold temperature to slow cooling, giving the molecular chains more time to relax from their oriented state back to their naturally curled state. Reduce injection speed and pressure to reduce shear forces, thereby reducing the degree of molecular chain orientation. Increase melt temperature to improve melt fluidity, making it easier for molecular chains to move and unwind (but too high a temperature can cause degradation).

(2). Product design: Avoid sharp changes in wall thickness and use smooth transitions (such as arc transitions) to make the cooling rate as uniform as possible.

(3). Mold design: Optimize gate design and position to reduce flow resistance.

(4). Post-treatment: Annealing: The product is placed in an oven below its heat distortion temperature (usually 10-20°C lower) for a period of time, followed by slow cooling. This process is equivalent to "thawing" the frozen molecular chains, allowing them to gain enough energy to move and rearrange themselves to a lower-energy, more stable natural state, effectively eliminating internal stress.