What causes ejector marks on injection-molded parts?

Product design:

• Insufficient wall thickness: Thin sections make it harder to fill the cavity and increase the likelihood of visible marks during ejection.
• Complex geometry: Deep ribs or sharp corners may lead to uneven ejection forces.

2. Mold Design Issues

• Poor gating system: Sharp corners, narrow runners, and long flow paths increase stress at the pin location.
• Gate design: Small gates cause high flow resistance and orientation stress, leading to marked areas with high stress.
• Incorrect ejector pin layout: Too few, too small, or poorly placed ejector pins lead to uneven force distribution.
• Uneven cooling: Irregular temperature gradients within the mold cause differential shrinkage.
• Inadequate venting: Trapped air causes vacuum forces that pull the part toward the core during ejection.

Types of Ejector Pins in Injection Molding

The type of ejector pin used can affect both part quality and the likelihood of visible marks. Common types include:

• Straight Ejector Pins: The most widely used type, suitable for general applications. Can leave round marks on flat surfaces.
• Shoulder Ejector Pins: Have a larger diameter at the base to prevent movement and wear; often used for more precise alignment.
• Sleeve Ejector Pins (or Tube Ejectors): Surround a core pin or boss and are used for ejecting cylindrical parts like tubes or deep holes.
• Blade Ejector Pins: Thin, flat pins ideal for narrow ribs and slots; reduce stress in delicate areas.
• Custom Ejectors (e.g., stepped or contour): Tailored to match the shape of the part and reduce marking risk.

Tip: Choosing the right ejector pin type, along with correct placement and surface finish, can significantly reduce the chance of visible pin marks.

Machine Parameters:

• Injection settings: Incorrect control of pressure, speed, and switch-over point.
• Holding pressure: Errors in pressure, speed, switch-over point, or backpressure adjustment.
• Mould temperature: Unsuitable temperature settings for the core, cavity, or cooling channels.
• Material temperature: Improper barrel temperature settings across different zones.
• Ejection process: Incorrect ejection speed, pressure, or method applied.
• Clamping force: Wrong machine tonnage selection or clamping force setting.

Product design:

• Ensure appropriate wall thickness based on material and product structure, generally not less than 2.5mm for simple structures and 2.8mm for complex structures with ribs.
• Design ribs and bosses to distribute stress and allow balanced ejection.

Mold design:

• • Adjust the sizes of the sprue and runner to alleviate injection pressure overshooting caused by filling difficulty.
  • Properly locate the gate to facilitate cavity filling and minimize stress concentration.
  • Design ejector pins on ribs to distribute ejection force evenly.
  • Consider ejecting at stress points on deep ribs to avoid excessive force causing ejector marks.
  • Add sufficient venting to prevent vacuum formation.
  • Improve cooling channel layout for uniform cooling.

Injection Machine parameters:

• • Select an appropriate injection molding machine.
  • Control process conditions, including mold temperature, processing temperature, injection speed, pressure, holding pressure, time, and cooling time.
  • Optimize processing temperature, injection pressure, speed, and time.
  • Adjust holding pressure and time when the product is more than 90% injected.
  • Ensure sufficient cooling time before ejection to prevent defects.

Ejector pin marks represent a prevalent yet preventable issue in injection moulding. By optimising product design, mould structure, and processing parameters, manufacturers can substantially minimise these surface defects and enhance part appearance. DSW specialises in precision mould making and provides expert solutions to reduce ejector marks in high-volume plastic production.