Automotive Bumper Injection Molding Process

Automotive bumpers are divided into front and rear bumpers, typically made of modified polypropylene (PP+EPDM+talc) material. They are produced using large horizontal injection molding machines. The entire manufacturing process consists of the following steps: raw material pre-treatment → injection molding → part removal and trimming → pre-spray treatment → painting → assembly and inspection → storage. Below is a detailed explanation of each stage.

 

I. Raw Material Pre-treatment (Ingredient Drying)

 

1. Material Blending

Main material: Modified PP pellets; Auxiliary materials: Color masterbatch, anti-aging agent, toughening agent, and recycled material (within compliant proportions).

Add ingredients according to the formula into a high-speed mixer and blend thoroughly to ensure consistent color and performance.

 

2. Drying and Dehumidification

PP has low moisture absorption, but excess moisture may cause silver streaks and bubbles in the product. Use a hot air dryer to dry at 80–90°C for 2–4 hours, maintaining moisture content below 0.05%.

 

3. Feeding and Conveying

After drying, the pellets are automatically conveyed into the injection molding machine hopper via an automatic feeding system, ensuring a fully enclosed process to prevent secondary moisture absorption and dust contamination.

 

 

II. Injection Molding (Core Process)

 

1. Mold Preheating and Equipment Setup

Large bumper molds are equipped with hot runner systems. First, heat the hot runner to 200–230°C, and control the mold temperature via a mold temperature controller at 40–60°C. Adjust injection pressure, speed, holding pressure, and cooling time parameters.

 

2. Clamping

The injection molding machine's clamping unit applies high pressure to close the mold. The clamping force typically ranges from 1800 to 4000 tons (bumper parts are large thin-walled components), ensuring no flash occurs at the parting line.

 

3. Injection and Filling

Barrel heating is zonally controlled: rear zone at 170°C → middle zone at 190–210°C → nozzle at 210–230°C. After screw feeding, melt PP material is injected at high pressure and velocity through the hot runner system and gate into the bumper cavity, ensuring complete filling of thin-wall areas without short shots or weld lines.

 

4. Holding Pressure and Packing

After filling, switch to holding pressure to compensate for shrinkage during solidification, preventing sink marks, warping, and dimensional deviations in the bumper.

 

5. Cooling and Solidification (Critical Step)

Internal circulating cooling water forces rapid mold cooling, accounting for over 60% of the total cycle time. With a wall thickness of 2.5–3.5 mm, cooling time ranges from 40 to 90 seconds, ensuring sufficient cooling to prevent deformation upon ejection.

 

6. Pre-Release, Mold Opening, and Ejection

The screw pre-relaxes to release pressure, and the machine slowly opens the mold. The mold ejector pins and ejector blocks simultaneously push out the bumper, while a robotic arm enters the mold to remove the part.

 

7. Robotic Part Removal and Placement on Rack

A dedicated six-axis or three-axis robot gently removes the bumper and places it flat onto a custom-shaped rack. Stacking or squeezing is strictly prohibited to avoid deformation or scratches.

Injection cycle per bumper: 70–120 seconds, depending on part size and cooling efficiency.

 

III. Offline Post-Processing (Injection Molding Semi-Finished Products)

 

1. Manual/Robotic Trimming: Remove gates, runners, flash from parting lines, and excess material at snap-fit areas; for complex contours, use CNC trimming robots to ensure smooth edges.

2. Initial Defect Inspection: Visual inspection for short shots, bubbles, silver streaks, shrinkage, weld lines, scratches, deformation, and color differences; defective parts are either reworked separately or crushed for recycling.

3. Shaping and Aging: Place semi-finished parts on temperature-controlled racks for 1–4 hours to relieve internal stresses and minimize subsequent spray coating deformation; minor deformations are corrected using cold fixtures.

4. Sanding and Dust Removal: Lightly sand areas requiring coating to improve adhesion; use high-pressure air guns combined with electrostatic dust collectors to remove surface dust.

 

 

IV. Spray Painting Production Line (Core Process for Bumper Appearance)

 

1. Pre-treatment: Degreasing and Water Washing

Alkaline degreasing → water washing → surface conditioning → phosphating → deionized water rinse → drying. Removes mold release agents, oil stains, and dust. Phosphating film enhances paint adhesion; drying temperature: 80–120°C.

 

2. Electrostatic Dust Removal

Ionized air neutralizes surface static electricity and blows away fine dust particles to prevent spray defects.

 

3. Primer Coating (Epoxy / Plastic-Specific Primer)

Automated electrostatic spray applies primer with a film thickness of 15–25 μm, followed by low-temperature curing. Purpose: covers the substrate, improves adhesion for mid-coat, and conceals minor imperfections.

 

4. Mid-Coat Application

Fills fine scratches and pinholes, enhances topcoat fullness, then cured through baking.

 

5. Color Topcoat (Body-Matching Color)

Automated electrostatic spraying applies body-matched color paint, ensuring no color difference across the vehicle. Metallic finishes require two coats: base color + clear coat.

 

6. Clear Coat Application (Gloss Protection)

Transparent clear coat provides gloss, weather resistance, and scratch resistance with a film thickness of 30–50 μm.

 

7. High-Temperature Curing

Baked in an oven at 80–140°C for 20–40 minutes to achieve complete film curing.

 

8. Final Inspection

Check for color variation, sagging, orange peel, pinholes, miss-spraying, and gloss uniformity. Defective parts are reworked and resprayed.

 

V. Assembly and Finished Product Inspection

 

1. Component Assembly: Install accessories including fog light brackets, grilles, radar mounting bases, clips, buffer foam, trim strips, and chrome trim pieces.

2. Dimensional Full Inspection: Use coordinate measuring machines or inspection fixtures to verify installation hole positions, contour dimensions, and fit clearances to ensure proper vehicle fitment.

3. Functional and Visual Re-inspection: Check for paint scratches, loose assembly, broken clips, and accuracy of radar hole positioning.

4. Labeling and Film Application: Apply vehicle model labels and protective films to prevent scratches during transportation.

6. Packaging and Warehousing

Finished bumper covers are wrapped in PE protective film and placed layer by layer on dedicated transport racks, then shipped to the vehicle manufacturer's warehouse for assembly.

 

Supplement: Scrap Material Recycling Process

Sprue gates and defective parts generated during injection molding and trimming are crushed by a shredder, dried, and then blended back into the raw material at a specified ratio for re-injection molding (recycled material content ≤15% to prevent reduction in impact strength).

 

Simplified Flowchart (Shortcut)

Raw material drying and mixing → mold clamping in injection molding machine → injection → holding pressure → cooling → ejection and part removal → trimming and flash removal → aging and shaping → pre-treatment for spraying (degreasing and phosphating) → primer → base coat → color coat → clear coat → baking → assembly of accessories → dimensional and appearance inspection → lamination, packaging, and storage