Quality of injection molded products is determined by four continuous and interconnected stages in molding process—filling, holding pressure, cooling, and demolding. Each stage requires precise control; deviations in any stage will directly affect final product's appearance, dimensional accuracy, mechanical properties, and internal stress levels.

Filling stage is process from mold closure and screw advancement to cavity being filled with approximately 95%-99% melt. This is beginning of molding cycle, determining melt flow pattern, molecular orientation, and quality foundation for subsequent stages.
1. Mechanical and Thermodynamic Comparison of High-Speed and Low-Speed Filling:
· High-Speed Filling (Shear-Dominated Zone):
· Principle: High injection speed causes melt to experience a high shear rate, producing a significant "shear thinning" effect, resulting in a substantial decrease in apparent viscosity and enhanced fluidity.
• Advantages: Reduces flow resistance, easily fills thin-walled or complex flow regions; reduces melt front temperature difference, resulting in better surface gloss and relatively higher weld line strength; shortens filling time.
• Risks: May lead to trapped air and scorching due to untimely venting; excessive shear heat may cause material thermal degradation; requires high rigidity of molds and injection molding machines, and is prone to flash.
• Low-speed filling (heat conduction-dominated zone):
• Principle: Low shear rate, weak shear thinning effect, and relatively high melt viscosity. Heat is mainly dissipated to mold wall through heat conduction, resulting in rapid cooling of melt front.
• Advantages: Facilitates venting and reduces gas defects; smooth mold filling, reduces molecular orientation and internal stress.
• Risks: Prone to obvious weld lines and flow marks; excessively low melt front temperature may lead to incomplete filling (short shot); extended production cycle.
2. Analysis and Optimization of Weld Line Depth:
A weld line is a weak zone formed when leading edges of two melt flows meet.
• Formation Mechanism: Molecular chains at meeting point are not sufficiently intertwined, potentially resulting in a V-shaped notch. This area often traps gas or impurities.
• Factors Affecting Strength:
• Temperature: Higher melt temperatures at meeting point lead to stronger molecular mobility and better fusion. Temperature of weld line area can be increased by raising mold temperature, optimizing gate location, or using sequential valve gating technology.
• Pressure: Sufficient holding pressure can compact weld area.
• Material: Materials with good flowability and low viscosity exhibit better fusion.
• Venting: Good venting prevents bubbles or blackening at weld line.
• Optimization Strategies: Design should avoid weld lines forming in critical stress areas; in process, a multi-stage injection speed control ("slow-fast-slow") should be adopted, using high speed and high pressure in the area where weld line is expected to form to improve fusion quality.

Holding pressure is stage where pressure continues to be applied after cavity is filled to compensate for cooling and shrinkage of melt, until gate solidifies.
1. Core Functions of Holding Pressure:
Shrinkage Compensation: Replenishes volume of material lost due to cooling and shrinkage into cavity, preventing surface shrinkage and internal pinholes.
Density Enhancement: Compacts melt, increasing density of plastic, thereby improving dimensional stability, mechanical strength (especially impact strength), and barrier properties of product.
Dimensional Control: The most important process parameter affecting weight and dimensions of product (especially in thickness direction).
2. Precise Control of Holding Pressure and Time:
Holding Pressure Curve: Multi-stage holding pressure is typically used. Initial holding pressure is relatively high (generally 80%-100% of injection pressure) to quickly compensate for shrinkage; pressure is gradually reduced in later stages to prevent excessive internal stress or demolding difficulties caused by over-holding pressure.
Holding Pressure Time: Must continue until the gate is completely solidified (sealed). Too short a holding time will cause material backflow and shrinkage; too long a time will waste energy, increase cycle time, and may damage mold or product. Optimal holding time can be determined by weighing product: optimal point is when product weight no longer increases with increasing holding time.
3. Importance of Mold Cavity Pressure Monitoring:
Actual pressure inside mold cavity is the most direct reflection of holding pressure effect. By installing a mold cavity pressure sensor, it is possible to:
• Precisely control switching point from injection to holding pressure (V/P switching).
• Monitor and optimize holding pressure curve.
• Ensure consistency between production batches, which is a core means of achieving scientific injection molding and intelligent manufacturing.

Cooling is process by which melt solidifies from a molten state within mold cavity to achieve sufficient rigidity for ejection, typically accounting for more than 70% of the entire cycle time.
1. Scientific Design Principles of Cooling Systems:
Uniformity Principle: Cooling water path layout must follow "shape-following cooling" concept, meaning water path should follow cavity contour and ensure consistent cooling speed throughout cavity. This is the most important measure to prevent warping.
High Efficiency Principle: Under permissible processing conditions, use as many large-diameter cooling pipes as possible, with fewer series connections and more parallel connections. Cooling water should achieve turbulent flow (Reynolds number Re > 4000) to maximize heat exchange efficiency.
Zoning Control: For large molds or temperature-sensitive areas, implement independent cooling circuit control to precisely adjust mold temperature in different areas.
2. Calculation of Cooling Time and Influencing Factors:
Cooling time tc can be estimated using a simplified formula:

 

Where h is maximum wall thickness of product, and a is thermal diffusivity of plastic.
Plastic Properties: Crystalline materials such as PE and PP have latent heat of crystallization that needs to be released, so their cooling time is usually longer than that of non-crystalline materials such as PS and ABS.
Mold Material: Mold materials with excellent thermal conductivity, such as beryllium copper and aluminum alloys, can significantly shorten cooling time.
Cooling Medium: Using a mold temperature controller to control water temperature, or using chilled water, oil, or other media, can adapt to process requirements of different materials.

Demolding is process of removing product from mold cavity. Improper design can lead to defects such as whitening, deformation, and tearing.
1. Core Considerations for Demolding System Design:
Demolding Force Calculation: Demolding force must overcome clamping force, vacuum suction force, and friction force generated by shrinkage of product on core. Clamping force is directly related to shrinkage rate, contact area, mold temperature difference, and draft angle.
Ejection Balance: Ejector pin (or ejector rod) layout must be uniform and symmetrical to ensure that product is subjected to balanced forces during ejection, preventing excessive local stress. For deep-cavity, thin-walled parts, using a stripper plate (push plate) for ejection can make force-bearing surface larger and more uniform.
Ejection Timing: Part must be cooled below its heat distortion temperature and have sufficient "ejection rigidity" before ejection. This can be determined by a mold temperature sensor or time control.
2. Advanced Demolding Technology:
Gas-Assisted Ejection: Micro-vents are created in deep cavities or areas where ejector pins are difficult to install. Compressed air is injected during ejection to help part detach from core.
Undercut Treatment: For structures with undercuts, lateral core-pulling mechanisms such as sliders and angled ejectors must be designed. Their movement sequence, guiding accuracy, rigidity directly affect mold life and product quality.

Above four stages are controlled in conjunction with following core parameters:
1. Temperature System:
Barrel Temperature: Ensures sufficient and uniform plasticization, avoiding decomposition.
Mold Temperature: Core factor controlling cooling rate, crystallinity, dimensional stability, and surface quality. High mold temperatures improve filling, appearance, and weld lines, but increase cycle time; low mold temperatures shorten cycle time but may increase internal stress.
Melt temperature: Actual temperature entering mold cavity, a combined result of barrel temperature, back pressure, and screw speed.
2. Pressure System:
Injection pressure: Direct driving force to overcome flow resistance.
Holding pressure: Crucial for determining product density.
Back pressure: Improves melt uniformity and density, promotes venting and pigment dispersion, but excessive pressure increases shear heat and cycle time.
3. Time System:
Injection time: Inversely proportional to injection speed.
Holding time: Ensures gate sealing.
Cooling time: Determines production efficiency and product rigidity.
Switching points between stages: Such as transition from filling to holding pressure (V/P switching point), these are decisive moments determining final cavity volume and pressure, and must be precisely controlled.

High-quality injection molded products are culmination of scientific design (product and mold) and precise process control. Filling, holding pressure, cooling, and demolding must be viewed as a complete system engineering process. A deep understanding of their underlying physical and chemical mechanisms, along with use of modern technologies such as CAE (Cavity Flow Analysis) and cavity pressure sensing, is essential for precise and coordinated control of parameters at each stage. Only then can optimal balance between efficiency, cost, and quality be found.