Plastics : Part Ejection Damage in Plastic Injection Molding

Part ejection damage refers to physical defects that occur when a molded plastic part is forcibly removed from the mold cavity during the ejection phase of the injection molding process. These defects compromise both the aesthetic appearance and potentially the structural integrity of the finished product.

 
Common types of ejection damage:

 
1. Scratches and drag marks:

• Appearance: linear or streak-like marks on the part surface
• Real-life example: consumer electronics cases like phone covers showing visible scratch lines that follow the direction of ejection
• Industry impact: rejected parts in high-end consumer products where appearance is critical 

2. Ejector pin marks:

• Appearance: circular depressions, protrusions, or discolorations where ejector pins contact the part
• Real-life example: visible circular marks on the inside of plastic containers, bottle caps, or on the non-visible sides of automotive dashboard components
• Industry impact: may require secondary operations to remove or hide in visible areas

3. Deformation/Warping:

• Appearance: bent, twisted or misshapen parts
• Real-life example: plastic trays or containers that don't sit flat, warped housing components in appliances
• Industry impact: assembly issues, functional failures, and high rejection rates

4. Stress whitening:

• Appearance: cloudy white areas in otherwise clear or colored plastic
• Real-life example: white stress marks on colored plastic handles or buttons where ejector pins apply excessive force
• Industry impact: aesthetic defects requiring part rejection in consumer products

5. Fractures and cracks:

• Appearance: visible breaks or hairline cracks in the part
• Real-life example: broken mounting tabs on automotive interior trim pieces, cracked edges on plastic housings
• Industry impact: complete part failure, safety concerns, and product recalls

Common causes:

 
1. Insufficient draft angles:

• Explanation: lack of taper on vertical walls creates friction during ejection
• Real-life example: medical device components with straight walls that stick to the mold surface
• Solution: design with minimum 1-2° draft angle on all vertical surfaces

Lack of Taper: absence of a gradual slope or angle on vertical walls of a molded part that helps facilitate easier removal from the mold, resulting in increased friction during ejection and potential part damage.

2. Undercuts:

• Explanation: features that physically prevent part removal in the ejection direction
• Real-life example: snap-fit features on electronic enclosures that catch on the mold
• Solution: design with side-action tooling or eliminate undercuts when possible

Snap-fit Features: interlocking components designed to connect parts by temporarily flexing during assembly, then returning to their original shape to create a secure connection without requiring fasteners or adhesives.

3. Poor mold surface finish:

• Explanation: rough mold surfaces increase friction during ejection
• Real-life example: textured plastic parts showing inconsistent surface quality or tear marks
• Solution: proper mold polishing and maintenance

4. Improper ejection system design:

• Explanation: poorly placed or insufficient ejector pins
• Real-life example: thin-walled containers with ejector pin deformation at specific points
• Solution: optimize ejector system layout for even force distribution

5. Insufficient cooling time:

• Explanation: parts ejected while still too soft
• Real-life example: high-production packaging items with consistent deformation patterns
• Solution: increase cooling time or improve cooling system efficiency

Industry impact and solutions:

 
Manufacturing costs:

• Scrap rate increase: companies like automotive parts suppliers may see 5-15% rejection rates from ejection damage
• Production slowdowns: additional inspection steps for ejection damage can reduce throughput by 20-30%

Quality control measures:

• Automated vision systems: used by medical device manufacturers to detect subtle ejection marks
• Statistical process control: tracking ejection damage rates to identify trends before major issues occur

Design solutions:

• CAE simulation: companies like toy manufacturers use flow analysis to predict and prevent ejection issues
• Material selection: choosing resins with proper release characteristics for specific applications

CAE Simulation: computer-aided engineering simulation uses software to analyze and predict how products will behave under real-world conditions before physical prototyping, allowing engineers to test designs virtually.

Economic impact:

• Example: A major consumer electronics manufacturer reported saving over $2 million annually by redesigning a problematic component with optimized draft angles and ejection system

Preventative measures:

• Increase draft angles (slight taper or slope added to vertical surfaces of a mold to facilitate easier part ejection by reducing friction and preventing sticking) to minimum of 1-2° on all vertical surfaces
• Optimize gate location and size to control shrinkage and warpage
• Use proper mold release agents to reduce sticking
• Implement balanced ejection systems with properly sized and placed ejector pins
• Ensure adequate cooling time before ejection
• Polish mold surfaces to appropriate finish levels
• Add texture patterns strategically to reduce surface contact area
• Consider material selection factors like shrinkage rate and flexibility