Injection molding is core process of plastic product manufacturing. Its quality is affected by multiple dimensions such as product design, mold manufacturing, molding process, material properties, and equipment maintenance. Mastering systematic knowledge from operation to defect resolution is the key to achieving efficient and stable production.

Essence of injection molding is plastic melting → high-pressure injection into mold → cooling and solidification → mold opening and product ejection.
All production, machine adjustment, and defect problems cannot escape five core elements of machine, mold, material, process, and people.
Injection molding follows principle of "source control - system optimization," requiring collaborative optimization in all aspects of product design, mold design, molding process, and production management.
1. Product Design: Rationally plan wall thickness, draft angle, corner radius, and rib layout to avoid inherent defects. For example, to prevent shrinkage, control ratio of rib thickness to main wall thickness (≤0.6T for non-crystalline materials, ≤0.5T for crystalline materials); to prevent short shots, ensure minimum wall thickness and consider material flowability.
2. Mold Design: Focus on three major systems: venting, cooling, and runners. Optimized venting can solve problems such as weld lines, silver streaks, bubbles, and scorching; rationally designing runner dimensions and gate positions can improve shrinkage, short shots, and flow marks; a uniform cooling system helps reduce shrinkage and sticking.
3. Molding Process (Machine Adjustment): Core lies in matching parameters such as temperature (material temperature, mold temperature), pressure (injection pressure, holding pressure), speed (injection speed), and time (injection time, holding time) to achieve "full filling, good compaction, and uniform cooling" of material.
4. Materials and Equipment: Ensure materials are fully dry and clean, and regularly maintain equipment to maintain stability.

 

1. Core Structure of an Injection Molding Machine
- Injection System: Responsible for plastic melting, injection, and material storage.
- Mold Closure System: Completes mold closing, opening, and locking actions.
- Control System: Sets process parameters and operates equipment.
- Heating and Cooling System: Maintains barrel, nozzle, and mold within a suitable temperature range to prevent over-decomposition or insufficient cooling of plastic.
- Safety Devices: Safety doors and emergency stop buttons are bottom line for operation.
Core structure is based on five major systems: injection, mold closing, hydraulic, electrical control, and temperature control.
2. Standard Operating Procedure
Start machine and preheat barrel → Adjust mold and confirm clamping force → Set temperature, pressure, and speed → Manual test injection → Semi-automatic production → Inspect product and make fine adjustments → Stop machine, clean material, and close mold.

1. Three-stage injection molding method
Stage 1: Slow injection to prevent spray marks and flow marks;
Stage 2: Rapid filling to completely fill cavity;
Stage 3: Holding pressure to prevent shrinkage and flash.
Typical division and function of three-stage injection:
Stage 1 (Low-speed injection stage)
Location: Starts from end of melt flow, typically covering runner and gate area.
Purpose: To smoothly establish melt pressure, preventing "jetting" or gate scorching; especially suitable for pin-point gates or heat-sensitive materials.
Speed setting: Lower (e.g., 10–30%) to ensure melt slowly pushes away cold sprue.
Stage 2 (High-speed filling stage)
Location: Starts after gate, filling main body of cavity (approximately 50%–95%).
Purpose: To quickly fill most of cavity, shortening molding cycle and reducing melt front-end cooling.
Speed Setting: Higher (e.g., 60%–90%), but needs adjustment based on wall thickness and venting conditions to avoid air trapping or scorching.
Stage 3 (Buffer Pressure Holding Switching Stage)
Position: Near full mold (95%–99%), used for precise control of filling end.
Purpose: To reduce speed, facilitate venting, reduce shear heat, and prepare for V/P switching (injection to pressure holding).
Speed Setting: Further reduce (e.g., 20%–40%) to prevent overfilling and flash.
⚠️ Key Note: Injection points (i.e., end points of each stage) should be set based on melt stroke, typically in descending order of "First Stage > Second Stage > Third Stage".
2. Core Points of V-P Switching
A critical adjustment point. Switching too early can lead to material shortages and shrinkage, while switching too late can result in flash and high product stress.
Purpose: To avoid flash, internal stress, or trapped air caused by high-speed injection when cavity is nearly full, instead compensate for melt cooling and shrinkage through holding pressure.
Key Parameter: Switching position (V/P switching point) is typically set at position where screw stroke corresponds to 95%–98% of mold cavity volume being filled.
Three Main Methods of V-P Switching

⚠️ Note: Currently, mainstream recommendation is to use position switching or cavity pressure switching (advanced solution). The latter uses in-mold sensors to monitor cavity pressure in real time, achieving more stable V-P switching, but requires modification of mold and equipment.
3. Five Key Elements of Injection Molding Process
Pressure (MPa) / Speed (%) / Stroke (mm) / Time (S) / Temperature (℃)
Balance of material temperature and mold temperature, injection speed, injection and holding pressure determines product quality.
4. Practical Injection Molding Tips
Powerful screw movement, balanced injection position. Control injection speed well, back pressure helps with color mixing. Mold temperature and holding pressure control dimensions, slow down for air bubbles and bleeds. First stage: medium speed through sprue, second stage: slow speed through gate, third stage: fast speed filling cavity, fourth stage: slow speed venting.
Slow → Fast → Slow, smooth molding; low temperature and low pressure reduce flash, high temperature and high pressure produce fuller products.
Mold temperature and holding pressure determine dimensions, three speed stages control appearance; powerful screw movement, stable back pressure eliminates color mixing; medium speed through sprue, slow speed through gate; fast filling of mold cavity, slow venting at the end.
Pressure and Speed Rules: Insufficient pressure results in product defects; too slow a speed leads to long production cycles. Excessive pressure causes flash; too fast a speed makes it difficult for air to escape.
Temperature and Time Rules: Low material temperature results in poor plasticization; high temperature leads to color changes. Cooling time depends on product; too short a time leads to deformation and difficulty in mass production.
Position Rules: Precise mold opening and closing positions are crucial; ejection position is also critical. Inaccurate injection position results in product defects; careful adjustment is essential.
Adjust melt parameters first; review melt quality. Remember to raise mold temperature first; do not adjust machine if mold temperature is not high enough. Carefully locate each injection stage; the first stage cuts material head. The second stage depends on the needs; the third stage is just right. Control speed for product appearance; abnormalities occur near injection point. Adjust the first stage speed gradually; the second stage speed depends on needs. The third stage speed should be reduced; it is very effective in preventing flash. Injection pressure should be increased. Holding pressure parameters are the most important. Dimensional deformation depends entirely on them. Number of holding pressure stages depends on needs. A large holding pressure in the first stage easily leads to mold saturation at product end. Deformation often occurs towards front mold. A second holding pressure stage depends on situation. Product deformation tends towards rear mold, requiring a larger holding pressure in the second stage. Adjust time last, and be able to read process curve. Focus on direction first and adjust slowly, don't adjust process parameters randomly. A stable process is essential, and injection residue should be monitored frequently. Equipment functioning properly is crucial; efficient production leads to a happy and productive workforce.

 

Basic Components: (1) Front mold (mother mold) (fixed mold), (2) Rear mold (male mold) (moving mold), (3) Inserts, (4) Sliders, (5) Ejectors, (6) Ejector pins, (7) Gate (sprue)
- Parting surface: Interface between mold's opening and closing, determining product's appearance and demolding.
- Gate and runner: Channels through which plastic enters mold, affecting filling efficiency.
- Cooling water system: Determines product's cooling rate, deformation, and production cycle.
- Ejection system: Ejector pins, angled ejectors, etc., responsible for smooth demolding.
- Venting structure: Key to solving air bubbles, burning, and material shortages.
Simply put: Mold determines whether it can be made; injection molding machine determines how well it can be made.

1. Two Main Classifications of Plastics
- Thermoplastics: Meltable upon heating, reusable (PP, ABS, PC, PA, POM, etc., most commonly used in injection molding)
- Thermosetting Plastics: Cannot be remelted after curing upon heating (Bakelite, epoxy resin)
2. Characteristics of Commonly Used Materials
- PP: Low cost, good toughness, easy to mold
- ABS: High hardness, good gloss, versatile
- PC: Transparent, high strength, high temperature resistance
- PA (Nylon): Wear-resistant, high strength, must be thoroughly dried
- POM (Polyoxymethylene): Ultra-wear-resistant, self-lubricating, high temperature resistant but easily decomposes
Hygroscopic materials (PA, PC) must be dried; Heat-sensitive materials (POM, PVC) require strict temperature control; Crystallized materials have large shrinkage, amorphous materials have stable dimensions.

For any product issues, check in this order:
1. Material: Is it dry? Is temperature correct?
2. Mold: Are mold temperature, venting, and demolding functions normal?
3. Machine: Are pressure, speed, and switching points reasonable?
4. Process: Are holding pressure and cooling time sufficient?
5. Structure: Is product uneven in thickness or has sharp corners?

1. Insufficient Material (Incomplete Injection): Increase material temperature, increase injection volume, and speed up injection.
2. Flash (Burs): Reduce pressure and temperature, increase clamping force, and repair mold.
3. Shrinkage (Dents): Increase holding pressure, extend holding time, and reduce material temperature.
4. Air Bubbles/Pores: Thoroughly dry material, slow down injection, and improve venting.
5. Flow Marks/Waves: Increase mold and material temperature, and stabilize injection speed.
6. Silver Lines/Air Lines: Dry the material, reduce temperature, and optimize mold venting.
7. Whitening/Cracking: Extend cooling time, adjust ejection balance, prevent sticking.
8. Warpage: Adjust mold temperature, extend cooling, optimize holding pressure.
9. Black lines/Scorching: Lower material temperature, increase venting, slow down injection.
10. Sticking: Lower mold temperature, repair and polish mold, use a small amount of release agent.

1. System Thinking: When defects occur, systematically check product design, mold condition, process parameters, material condition, and equipment performance, rather than adjusting individual parameters.
2. Parameter Interaction: When adjusting process parameters, pay attention to their mutual influence. For example, increasing material temperature may require corresponding adjustments to cooling time and injection pressure.
3. Mold Maintenance: Regularly inspect and maintain mold's venting channels, cooling channels, gates, and cavity surfaces; this is foundation for preventing various defects.
4. Recording and Analysis: Record detailed parameters for each machine adjustment and corresponding product quality to establish an experience database for rapid diagnosis and optimization. By understanding above-mentioned overall framework and defect resolution logic, a rapid understanding and practical foundation for injection molding processes can be established, thereby enabling more effective production operations and quality problem identification.