Injection molding is one of the most important processes in plastic processing, but we still get frustrated by "defective products" in our daily work? Have you ever experienced production phenomenon of "defective products starting out inexplicably, then suddenly working again"? In fact, vast majority of injection molding defects are related to improper process parameter settings. Continuous learning and mastering scientific process technology can not only significantly improve yield rate, but also form foundation for achieving efficient, energy-saving, and intelligent manufacturing.

Before pressing start button on injection molding machine, thorough preparation is half battle. This mainly includes following four aspects:
1. Plastic Color Matching: For products requiring precise color matching, two methods are usually used. First, use color masterbatch, mixing it with raw materials at a ratio of 1%-5%. Second, mix raw materials with white oil (dispersant) and 1%-5% colorant. Coloring thermosetting plastics is relatively simple; just mix in pigment.
2. Plastic Drying: This is a crucial step! Materials such as PA, PC, and ABS are prone to moisture absorption; excessive moisture can cause silver streaks and bubbles in product. It is essential to thoroughly dry product according to its material properties, ensuring moisture content is within allowable molding range (e.g., PC requires less than 0.02%).

 

3. Insert Preheating: When product contains metal inserts, stress cracks are easily generated around inserts due to large difference in shrinkage rates between plastic and metal. For rigid materials such as PC and PS, or large inserts, preheating (usually 110-130℃) is indispensable. Purpose of preheating is to reduce temperature difference between insert and melt, reduce thermal stress, and prevent rapid cooling of melt upon contact with cold insert, which can lead to shrinkage marks and insufficient bonding strength.
4. Release Agent Selection: For complex structural products, appropriate use of a release agent can ensure smooth demolding. However, caution is advised; excessive use may affect appearance of product or subsequent coating.

Process parameters are direct instructions for controlling molding process. Understanding meaning and function of each parameter is cornerstone of accurate machine tuning.
Injection Pressure: Key to overcoming melt flow resistance and providing filling speed. Pressure decreases progressively from nozzle to end of mold cavity, potentially resulting in a loss of up to 80%. It needs to be set appropriately based on runner length and structure.

 

Holding Pressure and Switching: Used to compact melt and compensate for shrinkage after filling. Holding pressure is generally around 50% to 80% of maximum injection pressure. Setting holding pressure switching point is crucial—switching too early leads to insufficient shrinkage compensation and shrinkage; switching too late may cause a sharp increase in pressure, resulting in flash or even mold damage. Optimizing holding pressure curve can effectively control product shrinkage and internal stress.

 

Back Pressure: Resistance encountered when screw rotates backward. Appropriately increasing back pressure can compact melt, expel gas, improve plasticizing and color mixing effects. Different materials exhibit vastly different sensitivities to back pressure: Heat-sensitive / low-viscosity materials such as PVC, POM, and PA require back pressure control between 0.5-3 MPa; exceeding 3 MPa can easily lead to decomposition. Medium-viscosity materials like PP and ABS only require 3-8 MPa, while only high-viscosity materials such as PC and glass fiber reinforced materials require 8-15 MPa. Excessive back pressure increases shear heat, potentially causing material decomposition. Glass fiber reinforced materials require higher back pressure (10-15 MPa), while transparent parts require precise back pressure control (8-12 MPa or lower) to prevent yellowing.
Clamping force: This ensures resistance to melt expansion forces. Using an oversized clamping force on a small mold wastes energy, while using an undersized clamping force results in insufficient pressure and speed. Based on formula Clamping Force = Cavity Pressure * Projected Area * Safety Factor, scientifically select a clamping machine with an appropriate tonnage. However, it's important to note that pressure of melt decreases significantly after passing through runner. Actual clamping force required must also consider influence of mold runner structure and number of cavities. Multi-cavity molds and long runner molds require appropriately increased clamping forces.

 

Temperature Control:
Barrel Temperature: Affects plasticization quality and melt flowability. Segmented control is necessary, requiring precise temperature control of feeding section, compression section, and metering section. Nozzle temperature should be slightly lower than the highest barrel temperature; otherwise, melt drooling may occur.
Mold Temperature: Directly affects cooling rate, crystallinity, appearance, and dimensional stability. High mold temperatures generally improve appearance and dimensional accuracy, especially for crystalline plastics (such as PA and POM), but extend production cycle. A balance needs to be struck between appearance and production efficiency.
Speed and Time:
Injection Speed: High-speed injection reduces flow resistance and improves weld line strength, but easily produces jetting marks and trapped air; low-speed injection facilitates venting and reduces flash, but may cause flow marks and incomplete filling.
Injection/Holding/Cooling Time: Injection time is much shorter than cooling time (approximately 10%-15% of cycle). Holding time is limited to time required for gate solidification, while cooling time is designed to ensure product is fully cured and demolded. It typically accounts for 70%-80% of the entire molding cycle and is crucial for optimizing efficiency.

 

Screw-related parameters:
Screw speed: Affects plasticizing capacity and shear heat. Low speeds are recommended for heat-sensitive materials (such as PVC).
Anti-drooling amount (retraction): Prevents nozzle drooling;
Residual material amount (shield): Melt pre-retained at screw head, acting as a buffer and stabilizing injection volume;
For small-diameter screws (below φ20mm), retraction requires 2-3mm and residing material 3-5mm; for large-diameter screws (above φ50mm), retraction requires 6-12mm and residing material 8-15mm. Proper settings are essential to avoid inaccurate metering with large screws and air entrapment with small screws. In practical applications, impact of different screw types on parameters must also be considered.

Initial Settings (Safety First): Start with lower temperature, pressure, and speed, reasonable clamping force, a longer time to prevent damage to mold and equipment.
Plasticization and Mold Temperature: Set according to recommended values for material and check actual temperature.
Determine Injection Endpoint: Initially set to 2/3 of cavity volume as a safety buffer.
Screw Speed and Back Pressure: Set to minimum value that can complete plasticization without prolonging cycle.
Injection Pressure and Speed: Initially, injection pressure can be gradually increased from low to high, generally starting from 50%–70% of system pressure to observe flow.

 

Holding Pressure and Time: Holding time is determined based on product wall thickness, material shrinkage rate, and gate freezing time.
Cooling and Mold Opening Time: Cooling time is initially set roughly based on product wall thickness data; mold opening time is generally 2-5 seconds.
Gradual Filling: Increment injection volume gradually, using screw positions as unit, and observe filling status through short shots.
Switch to Automatic Mode: Ensure stable continuous process.
Optimize Mold Opening and Ejection: Use a "slow-fast-slow" mode to protect mold and ensure smooth ejection.
Complete Filling: Adjust injection endpoint to 99% full, fully utilizing injection speed.
Optimize Holding Pressure: Gradually increase holding pressure until the lowest effective pressure is found to eliminate shrinkage marks, thus reducing internal stress.
Find the Shortest Holding Time: Gradually shorten holding time until product quality begins to decline (shrinkage marks appear). This critical point is the shortest effective holding time.

 

Find the Shortest Cooling Time: To ensure no whitening after ejection, dimensional stability, and no deformation, gradually shorten cooling time until product deforms or shows whitening after demolding, thus determining limit cooling time.
Understanding Cavity Pressure Curve: A comprehensive analysis of stage characteristics, pressure distribution, process parameter correlations, and curve shape of cavity pressure curve is crucial to accurately identify melt flow state during injection molding.

 

Some products require post-processing after demolding to improve performance:
Annealing: Product is heated in a constant temperature oven or hot liquid for a period of time, followed by slow cooling. Purpose is to eliminate internal stress generated during molding, preventing cracking or deformation during future use. For non-crystalline plastics (PC, ABS, PMMA) and products with internal stress, processing temperature should be 10-20℃ higher than product's operating temperature and 10-15℃ lower than material's heat distortion temperature, with a processing time of 1-4 hours. For example, annealing temperature for PC products is 100-120℃, and processing time is 2-3 hours. After processing, product is slowly cooled to room temperature to avoid secondary internal stress generation. Specific figures can be further confirmed by referring to the latest TDS (Technical Data Sheet) from material supplier.
Humidity Conditioning: Specifically designed for nylon (PA6/66) products, this treatment aims to eliminate internal stress and stabilize dimensions. Treatment method involves hot water immersion at 80-90℃ for 2-4 hours until product reaches moisture equilibrium; or steam treatment at 100-120℃ for 1-2 hours. After treatment, product must be allowed to cool naturally to room temperature, with controlled humidity to prevent dimensional fluctuations.
Other Surface Treatments: Coated and electroplated products require surface degreasing and polishing to remove mold release agent residue and surface defects.

Mastering injection molding process technology is not about rote memorization. Its core lies in establishing following systematic thinking:
Cause-and-effect logic chain: Deeply understand how each process parameter (cause) affects melt state, flow, and cooling (process), ultimately determining product quality (effect).
Balance and Compromise: Process adjustments are often an "art of trade-offs." Increasing temperature improves flowability but may cause decomposition; increasing speed and shortening cycle time may lead to appearance defects. Optimal balance must be found based on product's primary requirements.
Standardization and Data-Driven Processes: Establishing scientific machine setup procedures and utilizing data-driven tools such as mold cavity pressure curves for monitoring and optimization shifts process from "experience-dependent" to "scientifically controllable."
Prevention is Key: Thorough pre-production preparation (drying, cleaning) and reasonable mold design can prevent numerous defects at the source, achieving twice result with half effort compared to post-production adjustments.
By closely integrating theoretical knowledge with production practice, through continuous accumulation and reflection, you can gradually overcome frustration of "unexplained defects" and truly understand "why it is so," gradually transforming into a practical expert.
Of course, the entire injection molding process is a complex process involving multiple intersecting factors. Current scientific injection molding concepts and tools also have more specialized theoretical tools, such as material viscosity curve testing, filling pressure drop analysis, gate freezing tests, DOE experiments, etc.