1. Temperature – Lifeline of Plastics
Barrel Temperature: Divided into 3-5 sections (rear section, middle section, front section, nozzle). Temperature setting needs to be determined according to processing window of specific material, gradually increasing from the feed port (rear section) to nozzle (front section). For example, barrel temperature of ABS is usually set between 180 and 230℃. Incorrect settings can lead to uneven plasticization, degradation, scorching, or cold material.

 

Mold Temperature: Mold temperature directly affects melt flowability, cooling rate, product surface finish, internal stress, crystallinity, and dimensional stability within mold cavity. High mold temperature results in good fluidity and high surface gloss, but longer cooling time and potentially increased shrinkage; low mold temperature leads to rapid cooling and a short cycle time, but may cause flow marks, weld lines, and internal stress.
Nozzle temperature: Nozzle temperature is usually set slightly lower than or equal to the front section of barrel to prevent drooling or cold material clogging gate.
Hydraulic Oil Temperature: A stable oil temperature (typically within range of 20-65℃) is a prerequisite for ensuring stable pressure and flow in hydraulic system, directly affecting repeatability accuracy of various injection molding machine actions.
Ambient and Drying Temperature: Ambient temperature affects machine heat dissipation and operator comfort; drying temperature is a key pretreatment indicator that must be achieved before molding hygroscopic plastics (such as PC, PA, PET).
2 Pressure – Driving Force of Melt
Injection Pressure: Provides main power for melt to fill mold cavity. Its magnitude depends on product wall thickness, flow path length, mold structure complexity, and plastic viscosity. Initial setting can be approximately 60% of machine's maximum pressure.
Holding Pressure: Used after injection stage to compensate for volume shrinkage of plastic due to cooling, preventing defects such as shrinkage and depressions in product. Holding pressure is usually lower than injection pressure.
Back Pressure: Reverse resistance acting on screw head when screw rotates and retracts to store material. Appropriate back pressure (typically not exceeding 20% of injection pressure) can improve density and uniformity of melt, expel gas, but excessive back pressure will generate too much shear heat, which may lead to plastic degradation and prolong holding time.
Clamping pressure/clamping force: Ensures that parting surface is not stretched open when mold is subjected to high-pressure injection, preventing flash (burrs). Setting must be greater than product of injection pressure and projected area of product and runner on parting surface.
Ejection pressure: Force that drives ejector system to eject product from mold cavity. Proper control can prevent ejection whitening, ejection through, or ejection deformation.
3. Speed – Determinant of Fill Pattern
Injection speed: Determines how quickly melt fills mold cavity. High-speed injection can improve filling of thin-walled parts, reduce weld lines, and improve surface gloss; however, it may lead to trapped gas, scorching, or excessive shear heat. Low-speed injection is beneficial for venting and reduces jetting marks. Modern injection molding machines typically use multi-stage injection speed control, set in segments according to mold cavity geometry.
Screw speed: Affects plasticizing capacity (amount of molten material per unit time) and plasticizing quality. Higher rotation speed results in stronger plasticizing capacity but also higher shear heat, potentially leading to plastic degradation, especially for heat-sensitive plastics (such as PVC) or reinforced plastics (such as long glass fiber).
Mold opening and closing speed: Affects production cycle and mold protection. A "slow-fast-slow" principle is generally adopted, meaning low speed at contact and separation, and high speed in the middle process, to protect mold and shorten cycle time.
Ejection speed: Excessive ejection speed can easily lead to product deformation or whitening; it needs to be set according to product structure and strength.
4. Position – Precisely controlled scale position
Metering stroke/storage position: Distance screw retracts determines amount of material injected in next cycle. Set amount of material should ensure complete filling of mold cavity with sufficient buffer.
Injection/holding pressure switching position: This is screw position point for switching from high-speed injection stage to holding pressure stage. Precise setting of this position is crucial for controlling final weight and dimensions of product and is core of precision injection molding. Switch typically occurs when mold cavity is approximately 95%-98% filled.
Mold Opening Position: Determines the total distance moving platen retracts, ensuring sufficient space for product and robot to remove smoothly.
Ejection Stroke: Distance ejector pins push forward, ensuring product is completely detached from mold core. For products with special structures such as undercuts or submerged gates, ejection stroke needs to be precisely calculated.
5. Time – Commander of Process Rhythm
Injection Time: Time taken from start of injection to switching to holding pressure. Determined by injection speed and stroke.
Holding Pressure Time: Duration of applying holding pressure, designed to continuously replenish material before gate solidifies, compensating for shrinkage. Insufficient holding pressure time leads to shrinkage, while excessive holding pressure may generate internal stress or cause sticking at gate.
Cooling Time: Time from end of holding pressure to start of mold opening. This is main stage of product cooling and solidification within mold cavity, typically occupying the longest part of the entire molding cycle (approximately 70-80%), preventing product deformation and ensuring dimensional stability.
Cycle Cycle: The total time required to complete one full injection molding process (mold closing - injection - holding pressure - cooling - mold opening - ejection).

 

6. Understanding Four Major Systems of an Injection Molding Machine
Injection System: Responsible for plasticizing and injecting plastic; core components are screw and barrel.
Mold Closing System: Responsible for opening, closing, and locking mold, providing sufficient clamping force; core components are toggle mechanism or direct-pressure cylinder.
Hydraulic Transmission System: Provides power and control for all movements.
Electrical Control System: Controls program, temperature, pressure, and other parameters of the entire injection molding process.
7. Screw Structure and Functional Segments
Conveying/Feeding Section: Screw channel is relatively deep, mainly responsible for compacting and conveying solid particles.
Compression/Plasticizing Section: Screw channel depth gradually decreases, where plastic melts, is pressurized, and vents air.
Metering/Melting Section: Screw channel is the shallowest, primarily responsible for homogenizing melt, establishing pressure, and precise metering.
8. Basic Classification and Properties of Plastic Materials
Thermoplastic vs. Thermosetting: Injection molding mainly handles thermoplastic plastics that can be repeatedly heated and melted.
Crystallized vs. Amorphous: Crystalline plastics (such as PA, POM, PP, PE) have a defined melting point, orderly molecular arrangement upon cooling, large shrinkage, and mold temperature control significantly affects their crystallinity. Amorphous plastics (such as PC, ABS, PS) do not have a defined melting point, only a softening temperature range, and smaller shrinkage.
Common Engineering Plastics: Polyesters such as PC (polycarbonate), ABS (acrylonitrile-butadiene-styrene), PA (polyamide), and POM (polyoxymethylene) each have their unique molding temperatures, water absorption, shrinkage rates, and application areas.
9. Generation and Detection of Internal Stress
Internal stress is a hidden killer affecting long-term performance of products (such as cracking and deformation).
Its main causes are orientation stress (molecular chains being stretched in flow direction) and temperature stress (uneven cooling).
Detection methods typically include polarimetry, solvent immersion (chemical method), and impact testing. Annealing can effectively eliminate or reduce internal stress.
10. Function of Check Valve (Obstacle Ring)
Check valve is located at screw head. Its function is to close during injection to prevent high-pressure melt from flowing back, ensuring accurate and stable injection volume; and to open during material storage, allowing melt to flow smoothly to screw front. Wear or failure of check valve is one of main causes of unstable injection volume and product weight variations.