If you look around, from your phone case and computer keyboard to car interiors, appliance casings, even medical equipment and toy models—vast majority of these precisely shaped, mass-produced plastic products come from same manufacturing process: injection molding.

 

For newcomers to injection molding industry, "injection molding" may sound both familiar and unfamiliar. Familiar because its products are ubiquitous; unfamiliar because underlying process is complex and precise. Today, we'll use a long article, accompanied by a comprehensive flowchart, to thoroughly break down the entire injection molding process for you, showing you "journey" from a single plastic granule to a finished product.
Essence of Injection Molding: Reshaping from Plastic Granules to Finished Products
Simply put, injection molding is a processing method that involves heating and melting plastic material, then injecting it under high pressure into a sealed mold cavity. After cooling and solidification, a plastic product perfectly conforming to mold shape is obtained.
Core of this process lies in characteristics of "thermoplastic" plastics: melting when heated, solidifying when cooled, and this process can be repeated. Below, we'll unfold process step by step, following logical sequence of production.
A Diagram to Understand the Entire Injection Molding Process

 

If any one of these elements fails to meet requirements, defects may occur in finished product. It is necessary to identify source of problem before making improvements.
Step-by-Step Explanation: Every Key Link from Granules to Finished Product

1. Raw Material Drying
Why is drying necessary? Many plastics (such as PA nylon, PC polycarbonate, ABS, etc.) are hygroscopic. Even trace amounts of moisture will vaporize in high-temperature barrel, leading to defects such as silver streaks, bubbles, and reduced strength on product surface.
How to dry? Use a dedicated dryer (hot air, dehumidification, vacuum, etc.). Depending on material, set an appropriate temperature (e.g., PC 120℃) and time (usually 2-4 hours or more) to reduce moisture content to process requirements (e.g., below 0.02%).

 

Three-phase integrated dehumidification dryer
Key point: Dried material should be used immediately or stored in an insulated hopper to prevent re-absorbing moisture.
2. Color matching and mixing
If a specific color product is required, color masterbatch or color powder must be uniformly mixed with raw material granules in a mixer according to specified ratio.
Key point: Uniform mixing is crucial; otherwise, it will lead to color differences and color streaks in product.

Raw material enters injection molding machine barrel from drying hopper, beginning plasticizing stage.

 

1. Plasticizing process
Heating: Heating coils outside barrel heat plastic granules in stages, gradually raising temperature from a solid state to a viscous melt.
Shearing and Mixing: Core component—rotating screw—while retracting and conveying material, uses powerful shearing action to further homogenize material and expel residual gases.
Three key elements of plasticization: temperature, back pressure, and screw speed.
Temperature: Segmented control, gradually increasing from feed port to nozzle to prevent high-temperature decomposition of material or screw backflow.
Back Pressure: Resistance encountered by screw during retraction. Appropriately increasing back pressure enhances plasticization uniformity and removes air bubbles, but excessive pressure can lead to material overheating or decreased production efficiency.
Screw Speed: Affects plasticization time and shear heat. For heat-sensitive plastics (such as PVC), speed should not be too high.
2. Injection and Mold Filling: Once plasticized melt reaches predetermined metering, screw stops rotating.
Mold clamping system tightly locks mold, withstanding upcoming enormous injection pressure.
Injection Stage: Driven by a hydraulic cylinder, screw acts like a precision syringe, injecting molten material from barrel tip through nozzle, mold runner, gate at high speed into mold cavity at a set speed and pressure.
Key Points for Mold Filling:
Injection Speed: Too high a speed may cause jetting marks and poor venting; too low a speed may cause insufficient filling and obvious weld lines. Generally, a multi-stage control of "slow-fast-slow" is used.
V/P Switching: This is one of core key points of machine adjustment. When melt fills approximately 95%-98% of cavity, switch from speed control to pressure control (holding pressure). Timing of switching point is crucial.

1. Holding Pressure Stage: After mold filling is complete, although mold cavity is filled, melt will shrink during cooling.
Holding Pressure: Screw continues to apply a low but sustained pressure, forcing additional melt into mold cavity to compensate for shrinkage, prevent product shrinkage, ensure dimensional stability and structural density.
Holding Pressure Parameters: Holding pressure (typically 40%-80% of injection pressure), holding time.
2. Cooling and Shaping: During holding pressure, mold cooling system (internal water pipes) begins to function. Cooling water (usually room temperature water, ice water, or mold temperature water) continuously circulates, carrying away heat from melt and causing it to gradually solidify from surface to core.
Cooling Time: This accounts for majority of the entire molding cycle (approximately 70%). Time depends on product wall thickness, plastic type, and cooling efficiency. Sufficient cooling is essential; otherwise, ejecting product before it is fully hardened will result in whitening and deformation.
Mold Temperature: Precisely controlled by a mold temperature controller; higher mold temperatures are required for higher surface finish requirements.

1. Mold Opening: After cooling time, injection molding machine's clamping mechanism moves moving mold section backward, separating it from fixed mold section; this is mold opening.
Mold opening speed is usually controlled in stages: slow start (protecting product) → fast mold opening (improving efficiency) → slow termination (buffering).
2. Ejection: After mold opening, product is usually attached to moving mold side.
Ejection system (ejector pins, ejector plates, ejector sleeves, etc.) moves forward under drive of ejection cylinder, smoothly pushing product away from mold core.
Key Points for Ejection:
Ejection Speed/Pressure: Must be stable and moderate; excessive speed or force will damage product (whitening, puncture).
Ejection Timing: Must be performed after product has completely cooled and solidified.
Draft Angle: Product design must have sufficient draft angle; otherwise, ejection will be difficult or appearance will be damaged.

1. Robotic Arm Part Retrieval
In modern automated workshops, manual part retrieval has been gradually replaced by injection molding-specific robotic arms.
Robotic Arm Process: After mold opening and ejection, robotic arm quickly extends into mold, precisely grabbing product or sprue using a pneumatic suction cup or clamp, then removing it and placing product in a designated location (conveyor belt, packing area), while simultaneously feeding sprue into a crusher.
Advantages of Robotic Arms:
High Efficiency: Synchronized with injection molding machine cycle, significantly shortening production cycle time.
High Stability: Avoids human error, ensuring continuous production.
High Quality: Prevents products from being contaminated or scratched by human contact.
Safety: Prevents workers from coming into contact with high-temperature molds and moving parts.
Linked Control: Robotic arm and injection molding machine are interconnected via signals to achieve fully automated cycles.
2. Post-Processing
Sprue Separation: Robotic arm or a person separates product from gating system (main runner, branch runner).
Sprue recycling: Separated sprues (usually same type of material) are crushed and can be mixed into new material at a certain proportion (usually ≤30%) for reuse to reduce costs. However, attention should be paid to degradation of recycled material performance and potential pollution.
Post-processing: Depending on requirements, secondary processing may be performed such as deburring, drilling, hot stamping, spraying, printing, and assembly.
Quality inspection: Inspections are conducted on dimensions, appearance, and function.

 

At this point, the entire production cycle of an injection-molded product is completed. Once injection molding machine locks mold, next cycle begins immediately, thus achieving efficient and continuous mass production.
For further reading, please refer to PART 02: Basic Structure of an Injection Molding Machine: A Comprehensive Understanding of Three Maj.