How to Minimize the Effect of Ejector Pin Marks on Your Injection Moulded Parts

For all rapid tooling and injection moulding projects at HLH, designing and positioning pins to minimize their effect on your parts is an essential part of our Design For Manufacturing (DFM) process. While our engineer typically determine pin placement, customers get to sign off on pin locations before an order is finalised. This can be an important decision, as ejector pin marks can not only affect the aesthetics but also the part’s form, fit, and functionality.

In this article, gain a better understanding of what ejector pin marks are, what causes them, and how to minimize them on your product’s surface.

What Are Ejector Pin Marks in Injection Moulding

Ejector pin marks (or ejector marks) are visible imprints or blemishes—typically circle, oval or elongated—left on the surface of moulded parts during the injection moulding process. These marks are typically created by ejector pins that extend into the mould cavity after it opens and play a critical role in safely and efficiently pushing the part out of the mould.

Ejector marks typically only affect the appearance of a part and are mostly harmless. However, if located in critical areas like sealing surfaces and areas exposed to high stress, it can interfere with the part’s intended function and hinder assemblies or operation of moving components. It can also weaken the structural integrity of a part.

What Causes These Marks?

1. Flawed product design, such as overly thin walls, inadequate draft angles, or multiple rib positions.
2. High internal stress in the moulded products due to excessive injection pressure, rapid dwell time, elevated mould temperature, or unbalanced cooling times.
3. Improper ejector placement or inadequate use of mould-release agents.
4. Inadequate design of the ejector unit, leading to fast ejection speeds.

How to Minimize Ejector Pin Marks in Injection Moulding

In many cases, it is not possible to completely eliminate ejector pin marks since ejector pins are essential for pushing the part out of the mold. However, they can be minimized. Here are some specific solutions:

• Thoroughly spray the mould-release agent.
• Avoid using small gates and excessively long runners to reduce flow pressure loss.
• Be mindful of the distance between the ejector pins and the gates to prevent excessive pressure.
• Place ejector pins in areas that can withstand greater force, particularly in deeper sections of the mould or where there is significant ejection resistant.
• Ensure the mould cavity surface is as smooth as possible.
• Increase the draft angle to facilitate easier part ejection from the mould. (Read our draft design guide).
• Reduce both injection pressure and dwell pressure.
• Reduce the injection rate to ensure quality mould filling.
• Limit the amount of melt injected into the mould.
• Control the mould temperature and extend the cooling time to allow uniform cooling of the entire part.

Source High Quality Injection Moulded Parts

Tooling & plastic injection moulding are very complex processes that require experience to get right. With HLH’s engineering team, you can expect quality and consistency throughout the process. To get started on your injection moulding process, simply submit your 3D CAD drawings to our site contact form along with your project details and we will get back with a rapid quote and thorough DFM prior to order.

Discover more guides to optimise your design for injection moulding:

How to Design Ribs for Injection Moulding
How to Design Bosses For Injection Moulding
Identifying Injection Moulding Defects and Solutions
Choosing a Gate Type for Injection Moulding