Ejector Pin Design Tips

If you’re currently designing a mold, have you taken into account the ejector pins that will help remove your part from the mold? Using the wrong type of pins or using them in the wrong location can prevent your part from being ejected in one piece that’s free of defects. Let’s talk about ejector pins and what you need to know to incorporate them into your mold design.

What Are Ejector Pins?

Ejector pins push a part out of the mold once it has cooled, occasionally leaving marks in the process. They are located in the B-side of the mold, which is where the part stays once the mold opens. After the mold opens, the pins extend into the cavity of the mold, push the part out, then retract so the mold can close and be refilled.

In order to minimize marks and maximize the effectiveness of the ejector pins, there are things you need to know and incorporate into your mold design.

Ejector Pin Types

There are three common types of ejector pins. While each type is excellent, they have their own characteristics and are designed for different environments. Using the wrong type of pin could result in extra expenses, either due to the pins having a shorter life and needing to be replaced more frequently or due to using a more expensive pin than necessary.

Through Hard Pins

Mostly suitable for plastic injection molds, through hard pins have the same hardness throughout the ejector pin’s diameter. Through hard pins last longest in temperatures below 200° C, so you’ll need a different pin type if you require higher temperatures.

Nitride H13 Pins

Best for die casting, nitride H13 pins have a surface hardness level of HRC 65 to 70, which allows them to survive longer in working temperatures up to 600° C.

Black Ejector Pins

Since nitride H13 pins don’t work well in temperatures above 600° C, black ejector pins were developed to withstand working temperatures up to 1000° C. These pins have a special black surface coating that helps them withstand the high temperatures and provide additional self-lubrication to the pin, which is perfect for automotive applications. While these ejector pins work well in most temperatures, they are more expensive than other pin types.

Other Types of Ejectors

In some cases, ejector pins simply can’t do what you need them to. Here are some other types of ejectors you may come across.

Ejector Sleeves

Ejector sleeves are hollow ejector pins that have a hard-surface sleeve pin with a hole and a core pin that fits in the hole. The holes in the sleeves protect and guide the pin.

Ejector Plates

Ejector plates work alongside ejector pins. They hold the heads of the pins to prevent them from coming out during the injection molding process.

Ejector Blocks

Designed for use on the surface of products with high warpage and to reduce ejector pin marks, ejector blocks have a lubricating hollow groove and are applied to the surface of thin products with a high surface finish.

Factors to Consider When Designing Your Mold

It’s important to consider how to remove the part from your mold when you’re designing it, and ejector pin placement is a crucial part of how the part comes out of the mold. Here are some factors to keep in mind when designing your mold.

Clearance and Dimensions

Here are some important dimensions and clearances to consider when you are designing your mold:

• Ejector pin holes should be at least 3/32” away from other holes
• Minimum clearance of 1/64” for pin holes on the mold plate
• Minimum clearance of 1/32” for pin holes on the ejector plate
• Clearance between the pin and the hole should be 0.02mm or less
• For plastic materials like PP, PE, and Nylon, hole diameter = pin diameter + 0.01mm; for plastic materials like HIPS, PC, and ABS, hole diameter = pin diameter + 0.02mm

Resin Type

Your choice of resin may affect pin size or placement. For example, some resins are “stickier” than others and require more force to release from the mold. Softer resins may also be at risk of marring or puncturing, requiring wider or more pins to spread the force out.

Part Shape

Pins need a flat area to push against, and the surface of that pad must be perpendicular to the direction of pin movement, so slants and curves can cause problems with your ejector pin placement. Additionally, if a part surface is textured, the smooth surface of the pad will be apparent. Try to incorporate smooth, flat areas into your mold for the ejector pins.

Surface Area

Ejector pins need a surface area to push against. If you’re making a grate, for example, where all that faces into the B-side mold half are the tops of ribs without enough surface area for the pins to push against, you may need to add some bosses to act as ejector pads.

Liquid Silicone Rubber (LSR)

Ejector pins are not used to remove parts made from liquid silicone rubber. Instead, they are manually pulled from the molds.

Other Factors to Consider

In addition to factors specific to ejector pins, keep in mind these 10 Tips for Designing Quality Molded Parts.

Common Problems and Solutions

Like anything, injector problems can have – or even create – problems. Two of the most common problems with injector pins is them breaking or leaving marks on your part.

Breaks

It takes a lot of force to push a part out of a mold, and if you have too few pins or the pins are too small in diameter, they can break. The best way to reduce the likelihood of broken pins is to use plenty of large-diameter pins to help distribute the force over a larger area.

Ejector Pin Marks

Ejector pin marks are “dents” left on the part by the pins while they push the part out of the mold. While pin marks may just be cosmetic, they may also lead to cracks, so you want to try to design your injection system to prevent ejector pin marks. Here are some tips:

• Locate the pins on hard areas of the part like ribs, pillars, and metal inserts.
• Arrange the pins so they provide consistent force across the part.
• Design the position of the ejector pins on flat surfaces rather than on slopes.

Let Us Help You Design Your Parts

At Universal Plastic Mold, we have more than 50 years of experience with plastic injection molding, and we know how tricky it can be to design the perfect mold, including the ejector pins. Let us help you with your injection molding needs. Contact us today.