Polymers exist in three thermodynamic states (glassy, elastic, and viscous flow) plus a special aggregation state: crystalline. For injection molding engineers, understanding position and transition conditions of these four states on temperature-deformation curve is fundamental to scientifically setting material temperature, mold temperature, pressure, and time.
Following explanation addresses these states from three perspectives: molecular motion mechanism, macroscopic characteristics, and guidance for injection molding processes.

 

1. Molecular Motion Mechanism
In this state, movement of molecular chain segments is "frozen," with only side groups, chain segments, and main chain bond angles of small units capable of elastic deformation. Molecular chains are in a "constricted" state, possessing high internal energy but insufficient energy to overcome intermolecular forces for slippage.
2. Macroscopic Characteristics
Material exhibits rigidity, hardness, and brittleness. Deformation under stress is minimal, conforming to Hooke's Law; however, once stress exceeds yield point, material will fracture brittlely.
3. Guidance on Injection Molding Process
Glassy state is final usable state of plastic after demolding, and it's a state that's undesirable during filling stage but must be quickly achieved during cooling stage.
- Filling Stage: When melt enters mold cavity, if it comes into contact with mold wall below glass transition temperature (T_g), surface will instantly freeze, forming a "skin." If this skin is too thick, it will hinder subsequent melt filling, leading to underfilling or flow marks.
- Cooling Stage: To shorten molding cycle, core temperature of product needs to be reduced below T_g as quickly as possible to allow product to set and have sufficient strength for ejection. For semi-crystalline plastics, cooling below T_g means solidification of amorphous region; for amorphous plastics, cooling below T_g means complete solidification.
- Internal Stress: If time it takes for different parts of product to pass through T_g is inconsistent during cooling process (i.e., uneven cooling), molecular chains are frozen in a non-equilibrium state, generating internal stress, leading to later cracking or warping.

Although you listed four states, glass transition temperature (T_g) is a critical point in process control and needs to be emphasized separately.
- Process Key Points: In injection molding, mold temperature is usually set near or below T_g.
- Mold temperature below T_g: Molding is fast, but may lead to high internal stress, obvious weld lines, and poor surface gloss due to rapid cooling.
- Mold temperature close to or above T_g (for high-gloss, high-crystallinity products): Molecular chains have sufficient time to relax, eliminate internal stress, and replicate mold surface structure, but molding cycle will be longer.

1. Molecular Motion Mechanism
When temperature rises above T_g, molecular chain segments gain enough energy to begin rotating and stretching, but the entire macromolecular chain still cannot undergo relative slippage. At this point, material exhibits rubber elasticity.
2. Macroscopic Characteristics
Material softens, and its deformation capacity increases dramatically (it can stretch several times its original length), but deformation is reversible (i.e., molecular chains will curl back up after external force disappears). In injection molding, this state is usually undesirable during filling process, except for rubber injection.
3. Guidance for Injection Molding Processes
- Solid Molding Processes: For blow molding and thermoforming (vacuum forming), material is formed in a highly elastic state.
- "Awkward Zone" in Injection Molding: During plasticizing process of injection screw, if barrel temperature only heats plastic to a highly elastic state but not to a viscous flow state, then when screw rotates, plastic will be like "chewing gum," unable to be smoothly conveyed forward, easily getting stuck in compression section, leading to poor plasticizing or screw slippage.
- Near-wall flow: During high-speed, high-pressure injection molding, melt near mold wall may temporarily be at edge of a highly elastic state due to shear heat. If molecular chain orientation is severe at this time and is not relaxed in time, frozen molecular orientation will form in product, leading to severe anisotropy and deterioration of mechanical properties.

For crystalline plastics such as polyethylene (PE), polypropylene (PP), and polyamide (PA, nylon), crystalline state is aggregated state in solid state, and is a parallel dimension with glassy state, highly elastic state, and viscous flow state (crystallization affects physical state, not temperature state).
1. Molecular Motion Mechanism
Molecular chains are arranged in a regular pattern, forming a microstructure in which crystalline (ordered) and amorphous (disordered) regions coexist. The higher crystallinity, the more tightly molecular chains are arranged.
2. Macroscopic Characteristics
High crystallinity: High strength, high stiffness, good solvent resistance, high shrinkage, poor transparency, and poor impact toughness (typically).
Low crystallinity (quenched state): Good toughness, good transparency, and low shrinkage.
3. Guidance for Injection Molding Process
Controlling crystalline state is the most difficult and critical part of injection molding process, with mold temperature and cooling rate being core factors.
- Influence of Mold Temperature:
- Low mold temperature (rapid cooling): Melt cools rapidly, and molecular chains do not have time to arrange themselves neatly, resulting in low crystallinity and small crystal size. Products have good toughness, low shrinkage, and high transparency (e.g., transparent PP cups), but post-crystallization is significant, and product may experience secondary shrinkage or deformation after a few days, with a lower heat distortion temperature.
- High mold temperature (slow cooling): Molecular chains have sufficient time to move and arrange, forming large spherulites, resulting in high crystallinity. Products have good dimensional stability, high rigidity, good heat resistance, and high surface gloss, but shrinkage rate is high (prone to shrinkage marks), and molding cycle is long.
- Key Points of Injection Molding Process:
- Material Temperature: Crystalline plastics have a significant melting point T_m; material temperature must be ensured to be higher than melting point (entering viscous flow state) to destroy crystal nuclei. If material temperature is insufficient, unmelted crystals will become "seed crystals," leading to poor flowability during injection molding and presence of unmelted material in finished product.
- Holding pressure: During crystallization, significant volume shrinkage necessitates sufficient holding pressure to compensate for this shrinkage until gate freezes; otherwise, shrinkage cavities and sink marks are highly likely to occur.

 

1. Molecular Motion Mechanism
When temperature rises above viscous flow temperature T_f (for amorphous plastics or melting points T_m, and crystalline plastics), the entire molecular chain begins to undergo relative displacement. Intermolecular forces are overcome by thermal motion, and material exhibits viscous fluid characteristics.
2. Macroscopic Characteristics
Material possesses flowability and plasticity, and deformation is irreversible. This is a necessary state during injection molding filling stage.
3. Guidance for Injection Molding Process
Viscous flow state is "main battlefield" of injection molding process. Control objective is to obtain sufficient and stable flowability while ensuring no degradation.
- Temperature Window Control:
- Lower Limit: Must be higher than T_m or T_f; otherwise, flowability will be poor, filling resistance will be high, and equipment may even be damaged.
- Upper Limit: Lower than thermal decomposition temperature T_d. Process window is usually between T_m and T_d. For heat-sensitive plastics (such as polyvinyl chloride (PVC) and polyoxymethylene (POM)), window is very narrow; even slight temperature fluctuations can lead to material decomposition, yellowing, gas production (silver streaks), or mold corrosion.
- Viscosity and Shear Rate:
Plastic melts are non-Newtonian fluids and exhibit shear-thinning properties.
- Process Implications: When filling is difficult (e.g., thin-walled parts), prioritizing increasing injection speed (increasing shear rate) is often more effective in reducing viscosity than simply increasing temperature. This is because a high shear rate can break down molecular chain entanglement, significantly improving flowability.
- Viscous Flow Activation Energy:
- Temperature-sensitive plastics (e.g., polycarbonate (PC), polymethyl methacrylate (PMMA, acrylic)): Temperature has a significant impact on viscosity, making temperature adjustment suitable for controlling flowability.
- Temperature-insensitive plastics (e.g., PE, PP): Temperature has a smaller impact on viscosity; adjusting injection pressure and speed is more effective for controlling flow.

For injection molding engineers, following process monitoring awareness can be established in actual production:
1. Plasticizing Stage: In barrel, solid plastic (glassy/crystalline state) → heated, passing through T_g → high-elasticity state (compression stage) → continued heating to T_m or T_f → viscous flow state (metering stage). At this point, screw rotates and shears, and melt acquires flowability.
2. Filling Stage: Viscous flow melt is injected into mold cavity under high pressure and high speed. During this process, melt front contacts cold mold wall, and heat is rapidly dissipated.
- Surface layer: Instantly cooled to below T_g, freezing into a glassy skin (poor conductor, providing insulation).
- Intermediate layer: Remains in a viscous flow state and continues to flow forward.
3. Holding pressure stage: Melt is continuously forced into cavity in a viscous flow state to compensate for cooling shrinkage. For crystalline plastics, pressure at this stage determines final crystallinity and shrinkage rate.
4. Cooling stage: After gate freezes, internal melt is no longer replenished. The overall temperature of product decreases, from a viscous flow state → a highly elastic state (brief transition) → a glassy state. For crystalline plastics, crystallization occurs in this stage (from molten state to crystalline state), and cooling rate directly determines final properties of product.
Core logic of process control is to ensure that melt is in a uniform viscous flow state during filling, and to avoid generation of elastic turbulence in a highly elastic state by controlling shearing and cooling at the end of filling; during cooling stage, by precisely controlling crystallization rate (for crystalline plastics) or cooling rate (for amorphous plastics), product is finally shaped into a stable glassy state or a specific crystalline form, thereby balancing production efficiency and product quality (size, internal stress, mechanical properties).