Injection molding is a technology that involves heating plastic particles from a solid state to a molten state, injecting them into a mold to cool and form a specific product shape. In other words, it's a process from solid to solid. These two solid states differ in their internal structure. To help injection molding engineers better understand changes in material's internal structure during injection molding, this article analyzes differences between these two "solid states" from a molecular structure perspective and explains their correlation with product quality.

 

Although both raw material particles and final product are macroscopically hard solids, their microscopic molecular chain arrangement, orientation, and crystallization state are vastly different. We can understand this using an analogy:

 

- Solid plastic particles: Like a bowl of loose, cold, disordered spaghetti.
- Molded product: Like a bowl of spaghetti that has been stirred, heated, cooled, and shaped, with noodles intertwined, partially oriented, and "frozen" in specific positions.
Specific scientific analysis is as follows:
1. Solid Plastic Particles (Raw Materials)
- Molecular Chain State: Highly relaxed, randomly coiled. Molecular chains vibrate around their equilibrium positions, entangled but in a stable state with the lowest energy.
- Orientation: Extremely low. The molecular chains are not aligned along a specific direction; they are isotropic.
- Crystallinity (for semi-crystalline plastics, such as PP, PA, PBT): A certain crystalline structure and grain size have been formed during resin production. These grains are usually small and unevenly distributed.
- Internal Stress: Essentially zero. Molecular chains are in a naturally relaxed state, without stress that has been frozen due to external forced deformation.
- Summary: Structure of raw material particles is "equilibrium," representing material's "factory setting" state.
2. Products After Cooling and Shaping
- Molecular Chain State: "Frozen" in a non-equilibrium state. During injection molding, molecular chains are stretched and extended under high temperature and shear, then fixed within a very short cooling time before they can return to a relaxed state.
- Orientation: A distinct orientation distribution exists.
- Surface/Shear Layer: Due to contact between melt and cold mold wall, extremely high shear rates are generated, causing molecular chains to be highly stretched and aligned along flow direction, rapidly freezing to form an "orientation skin."
- Core Layer: Lower shear forces and slower cooling rates allow more time for molecular chains to relax, resulting in lower orientation and a state closer to random curling.
- Crystallinity (for semi-crystalline plastics):
- Crystallinity Variation: Thermal history of injection molding process (melt temperature, holding pressure, cooling rate) can drastically alter crystallization behavior.
- Fast Cooling Rate (Surface Layer): Formation of small, imperfect crystals, or even amorphous regions.
- Slow Cooling Rate (Core): Sufficient time for growth into larger, more complete spherulites.
- Crystallization under Holding Pressure: Pressure increases crystallization temperature and rate, potentially forming crystal structures different from those under normal pressure (e.g., striations).
- Internal Stress:
- Orientation Stress: Originates from frozen, highly oriented molecular chains, which have a strong tendency to recover their coiled state.
- Thermal Stress/Volume Shrinkage Stress: Due to uneven cooling rates and shrinkage in different parts of product, as well as replenishing effect of holding pressure, internal stress is "locked in."
Final product's structure is "non-equilibrium," a result of a frozen "process history." It is a complex system with gradients in orientation, crystallization, and internal stress from surface to core.

"Factory settings" of raw material particles are cornerstone of product quality. If cornerstone is unstable, even the most perfect subsequent processes will struggle to produce top-quality products.
1. Molecular Weight and its Distribution
- Correlation:
- High molecular weight: Longer molecular chains, more entanglement points, and higher melt strength. This is beneficial for holding pressure transfer, reduces flash, and improves mechanical properties of product (toughness, creep resistance). However, it results in poor flowability and requires higher injection pressure and temperature.
- Wide molecular weight distribution: Low molecular weight fraction acts as an internal lubricant, improving flowability, but it can impair mechanical properties and thermal stability of product (due to easy degradation). High molecular weight fraction provides strength.
- Impact on quality:
- Batch-to-batch variation: Fluctuations in molecular weight of raw materials between different batches can lead to changes in flowability, shrinkage, and mechanical strength, rendering stable process parameters ineffective.
- Process window: Raw materials with a narrow molecular weight distribution have more predictable rheological behavior, a wider process window, and more stable production.
2. Initial crystallization morphology and purity (for semi-crystalline plastics)
- Correlation:
- "Fish eyes": Incompletely melted, highly crystalline particles in the raw material. These become stress concentration points in final product, leading to decreased mechanical properties and even cracking.
- Impurities: Dust, gel, or other polymer impurities in raw material can undergo heterogeneous nucleation, altering local crystallization behavior, forming defects, affecting product's appearance (bright spots, blemishes) and strength.
3. Water Absorption Rate (for hygroscopic plastics such as PA, PET, and PC)
- Relevance:
- Water molecules at high temperatures can hydrolyze ester or amide bonds, leading to molecular chain breakage and a sharp decrease in molecular weight.
- Impact on Quality:
- Products may become brittle, lack strength, develop silver streaks on the surface, or develop bubbles. This is one of the most common and fatal problems in injection molding, must be addressed through thorough pre-drying.
4. Additives and Blend Components
- Relevance:
- Glass Fiber: Its content, length, and interfacial bonding with matrix directly determine product's stiffness, strength, and warpage. A high fiber length retention rate in raw material results in better mechanical properties in final product.
- Plasticizers: Affect flexibility of molecular chains, thereby altering flowability, hardness, and low-temperature toughness.
- Nucleating Agents: Adding nucleating agents to raw materials can significantly refine grains in final product, improving rigidity, heat distortion temperature, and dimensional stability, while reducing warpage.
Significance for Process Engineers:
Essence of injection molding is controlling phase transition process of plastic from one solid state (relaxed, balanced particles) through melting, shearing, flow, and compaction, ultimately transforming it into another solid state (frozen, non-equilibrium product).
As a process engineer, all adjustments you make—temperature, pressure, speed, time—are physically about controlling movement, orientation, and relaxation of molecular chains, as well as morphology and kinetics of crystallization process.
1. Understand your "raw materials": Fully understand type (crystalline/amorphous), molecular weight, hygroscopicity, and additives of plastic particles you are using. This is starting point for developing your process.
2. Injection parameters and molecular chains: Injection speed applies shear and orientation to molecular chains; mold temperature provides time and conditions for relaxation and crystallization; holding pressure compensates for shrinkage and influences crystallization process.
3. Product quality is a "frozen history": Warpage, shrinkage, internal stress, and strength differences in product are all results of effects of set process parameters at molecular chain level. A product exhibiting stress cracking is a manifestation of "resistance" of molecular chains after being excessively stretched and frozen; a product lacking stiffness may be due to insufficient crystallinity caused by excessively rapid cooling.
4. Stability is ultimate goal: Understanding relationships at molecular level allows injection molding engineers to understand why minute parameter fluctuations (such as a 5℃ change in mold temperature) can ultimately lead to differences in size or appearance by affecting crystallization and relaxation.