Plastic injection molding is the most widely used manufacturing process for producing plastic parts, accounting for over 75% of all plastic products globally. From automotive components to hotel equipment, this process enables the mass production of complex plastic parts with high precision and low per-unit cost. This guide walks through each step of the injection molding process.
Overview of Injection Molding
Injection molding involves melting plastic pellets and injecting the molten material into a mold cavity, where it cools and solidifies into the final part. The process is cyclical and can produce parts ranging from a few grams to several kilograms.
Key advantages include:
- High production speed (10-60 second cycle times)
- Low scrap rate (typically under 5%)
- Ability to produce complex geometries
- Consistent part quality across thousands of cycles
- Compatible with most thermoplastics
Step 1: Material Preparation
The process begins with selecting and preparing the raw plastic material:
- Material selection: Common materials include PP (polypropylene) for general use, HDPE for durability, ABS for impact resistance, and PC for transparency. For ESD-safe products, carbon-filled compounds are used.
- Drying: Many materials, especially hygroscopic ones like PC and nylon, must be dried to moisture levels below 0.02%. Drying is typically done at 80-120°C for 2-4 hours in a dehumidifying dryer.
- Coloring: Masterbatch colorants are mixed with natural pellets at 1-4% ratio. Pre-colored pellets are also available for consistent color matching.
- Blending: Virgin material is often blended with regrind (recycled runners and rejected parts) at 15-30% ratio to reduce material costs without compromising quality.
Step 2: Mold Installation and Setup
The mold is installed in the injection molding machine and prepared for production:
- Mold mounting: The mold is bolted to the machine platens. Alignment is critical—even 0.1mm misalignment can cause flash or parting line damage.
- Clamping force calculation: Required clamping force = projected area × cavity pressure. For a 200cm² part at 300 bar cavity pressure, minimum clamping force is 60 tons.
- Sprue alignment: The mold sprue bushing must align precisely with the machine nozzle to prevent material leakage.
- Cooling line connection: Water cooling channels are connected to maintain mold temperature (typically 20-80°C depending on material).
Step 3: Plasticization (Melting)
Plastic pellets are fed from the hopper into the barrel, where they are melted by a combination of heater bands and screw rotation:
- Barrel temperature: Set in zones from feed to nozzle, typically 180-280°C for PP, 200-300°C for PC. Temperature increases progressively from rear to front.
- Screw rotation: The reciprocating screw rotates at 50-200 RPM, generating shear heat that supplements the heater bands. This ensures uniform melting.
- Back pressure: 5-20 bar back pressure is applied during plasticization to remove air bubbles and ensure homogeneous melt.
- Shot size: The screw retracts to a preset position, accumulating the correct volume of molten plastic for the next shot.
Step 4: Injection
The molten plastic is injected into the mold cavity:
- Injection speed: Typically 50-150 mm/s. Slow at first to prevent air entrapment, then faster to fill the cavity before the melt cools.
- Injection pressure: 800-1800 bar. Sufficient to fill all cavities completely and pack the material.
- Cavity filling: The melt flows through the sprue, runner system, and gates into the cavity. Flow length is determined by material viscosity, wall thickness, and mold temperature.
- Venting: Air in the cavity must escape through mold vents (typically 0.01-0.03mm deep). Inadequate venting causes burn marks and short shots.
Step 5: Packing and Holding
After the cavity is filled, holding pressure is applied to compensate for material shrinkage:
- Holding pressure: 50-80% of injection pressure, applied for 3-15 seconds
- Cushion: 3-5mm of material remains in front of the screw to ensure pressure transmission
- Gate freeze: Holding continues until the gate solidifies, preventing backflow of material
- Shrinkage compensation: Typical shrinkage rates: PP 1.5-3%, HDPE 1.5-3%, ABS 0.4-0.7%, PC 0.5-0.8%
Step 6: Cooling
The part cools in the mold until it is rigid enough for ejection:
- Cooling time: Typically 10-40 seconds, depending on wall thickness. Thicker walls require exponentially longer cooling.
- Cooling channels: Water at 20-80°C circulates through channels machined into the mold. Baffles and bubblers improve cooling in hard-to-reach areas.
- Mold temperature: Higher mold temperatures improve surface finish but increase cycle time. Lower temperatures speed up cooling but may cause stress.
- Rule of thumb: Cooling time ≈ (wall thickness in mm)² × 2 seconds for PP
Step 7: Ejection
The mold opens and the part is ejected:
- Mold opening: The moving platen retracts, separating the mold halves at the parting line
- Ejector pins: Hydraulic or mechanical pins push the part off the core. Pin placement is critical to avoid warping or marking.
- Air blast: Compressed air may be used to assist ejection and remove any debris
- Robot extraction: For precise or delicate parts, a robot arm removes the part and places it on a conveyor
Step 8: Post-Processing
After ejection, the part may require additional processing:
- Runner separation: Runners and sprues are removed manually or by a robotic cutter
- Deflashing: Flash (excess material at parting lines) is trimmed by hand or with a trimming fixture
- Inspection: Visual inspection for short shots, flash, sink marks, and weld lines. Critical dimensions are checked with gauges or CMM.
- Secondary operations: May include ultrasonic welding, pad printing, assembly, or packaging
Common Defects and Solutions
- Short shots: Incomplete filling. Increase injection pressure, speed, or melt temperature.
- Flash: Excess material at parting line. Reduce injection pressure, increase clamping force, or inspect mold for wear.
- Sink marks: Depressions on thick sections. Increase holding pressure and time, or reduce wall thickness.
- Weld lines: Visible lines where melt fronts meet. Increase melt temperature or adjust gate locations.
- Warping: Uneven shrinkage causing distortion. Ensure uniform wall thickness and optimize cooling.
Machine Specifications at LSY Plastic
LSY Plastic operates a comprehensive range of injection molding machines to serve diverse client needs:
- JSW (Japan Steel Works): 180T-500T machines for medium to large parts
- Mitsubishi Heavy Industries: 200T-1000T machines for large industrial components
- Shin-Nittetsu (NSC): 500T-2500T heavy-duty machines for extra-large products like ESD floor panels
This machine range covers everything from small precision parts to large logistics pallets and ESD floor boards used in forklock and freight scenarios.
For more in-depth guides on this topic, check our Injection Molding Hub.
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