Ejector return springs

An ejector return spring is a compression spring that pushes the ejector back after it has pushed the part out. On the injection molding machine, return springs sit on the guide pins of the ejector's connecting plate, between the plate and the moving platen, as the drawing shows; they push the plate and the ejector rods back so the rods are clear before the mold closes. Many molds have return springs of their own on the mold's ejector plate. In both places the springs only return the ejector: the force that ejects the part comes from the ejector cylinder.

How return springs work

1. When the ejector moves forward, the connecting plate compresses the springs.
2. The springs store that force while the ejector is forward.
3. When the cylinder retracts, the springs help push the plate back and keep it pressed against its back position until the next cycle.

In the mold, springs around the return pins or the ejector guide pins push the mold's ejector plate back as soon as the machine's rods withdraw, before the mold closes. The return pins are the positive backup: when the mold closes, the fixed half pushes them, and the plate with them, fully back.

The return springs we supply are made of 65Mn spring steel, shot-peened or zinc-plated, for ejector systems and toggle mechanism returns. They are listed for Haitian MA and MA II and Yizumi A5 series machines.

Types of springs used in ejectors

• Round-wire compression springs, the general type, used on many machine ejectors.
• Die springs, wound from rectangular wire, which carry more load in the same space and are common in molds. They are often color-coded by load class, but the color codes differ between standards and suppliers, so match a replacement by its dimensions and load rating, not by color alone.

What matters when choosing a replacement is the outside and inside diameter (to fit the guide pin and the spring pocket), the free length, the load at the working length, and the maximum compression the spring is rated for.

Signs of a broken or tired return spring

• The ejector returns slowly or not fully, and the controller shows an ejector-not-back alarm.
• The ejector plate tilts as it returns, because the springs on one side are weaker.
• Pins stand proud in the mold as it closes, leaving marks on the part or getting damaged.
• A spring that is shorter than the others when the ejector is fully back: it has taken a permanent set.
• Broken coils, often found in pieces at the bottom of the spring pocket.
• A rattling or clicking noise from the ejector.

Why return springs fail

• Fatigue: every cycle compresses and releases the spring, and over a long period this cracks the wire.
• Over-compression: a stroke longer than the spring allows squeezes it to solid (coil bind) or beyond its rated travel, which shortens its life sharply.
• Corrosion from water leaks or a damp environment, which starts fatigue cracks.
• Heat close to hot molds, which weakens the spring over time.
• Wrong spring: a replacement with a different rate or length, so the springs on the plate no longer match.

Checking and replacing return springs

1. Retract the ejector, switch off and lock out the machine, and release the hydraulic pressure.
2. Remove the connecting plate or the spring retainers, as the machine design requires. The springs are under load: release them in a controlled way.
3. Clean the springs and the guide pins, and check each spring for cracks and broken ends.
4. Measure the free length of each spring and compare it with a new spring or the specified length.
5. Replace the springs in complete sets, all of the same type, so the plate is pushed evenly.
6. Check the guide pins for wear and burrs, refit everything, and run the ejector slowly to check that it returns fully.

Check that the ejector stroke set on the controller does not compress the springs beyond their rated travel. For springs in the mold, follow the mold maker's specification.

What to send for a replacement quote

Send the machine brand, model and serial number; where the spring is fitted; its outside diameter, inside diameter, wire size and shape, and free length; the number of springs; and a photo, ideally next to a ruler. Request a quote, and see the ejector guide and the ejector plate page for how the springs fit with the rest of the system.