Die springs unnecessary in modern-day molding

Are you using die springs in your mold’s ejector housings for safety reasons to return the ejector plate? If so, read on. According to Alex Mora, director of operations at Formula Plastics Inc., springs could be hazardous to your mold.

The industry started using springs early in the history of injection molding, when molding machines did not contain a hydraulic ejector plate. At the time, the use of die springs was the equivalent of a mechanical knockout. When the machine opened, the mold hit the stationary bars, which pushed the mold ejector plate forward, and the springs returned the pins at closing. Later, when IM machines became more modern and had a hydraulic ejector standard, molders continued using the springs for safety, especially on molds with slides.

Even with current technological advances, die springs are still a common standard for many new molds. However, our company has rejected this standard and we remove the springs from the molds when they come in—except when a stubborn tooling engineer forces us to leave them in the mold. Why did we challenge this paradigm that I call “plastic mythology”?

Moment of Clarity

Years ago we ordered all of our molding machines with adjustable pressure or force on the ejector system. It has been a standard on all of our new machines for years. This allows us to control the force and use only enough pressure to eject the part out of the cavity without losing speed. The purpose of using only the required pressure is to prevent damage to a mold when a core pin, ejector pin, ejector sleeve, or a lifter starts to gall or seize.

If this happens when using only the required pressure, the plate stops moving and no further damage is done to the mold. We record the ejection pressure just as we record the injection pressure on the cycle card. (It’s also important to note that the pressure should be adjusted only by someone who is authorized to do so.)

It wasn’t until years after we began ordering our machines with adjustable pressure that I realized that mold springs might be more harmful than helpful thanks to an incident with a severely damaged mold. The mold was damaged when an ejector pin seized up and broke. I asked the plant manager how much ejector pressure the machine had, and he said 800 psi. Most of the molds run at less than 500 psi and some as low as 200 psi. Why was he running the pressure so high? Because it took that much pressure to collapse or push the springs.

The pressure required to collapse the four or more springs on a mold depends on the length and the size of each spring and is incremental by the coils of the spring; therefore, the pressure has to be set high enough to collapse all of the coils for the stroke. When a pin starts to gall or seize, the pressure is too high for the piston to stop, so the pin seizes and damages the mold. What could be a minor repair becomes a major repair because of the die springs. This is why we remove the springs from our molds.

If a mold has slides or side actions, install an early return mechanism and a limit switch for protection. Springs can be hazardous to your molds, not to your safety.

What prompted me to spring into action and write this article? A month ago I saw a box full of new springs in our tool shop. Wondering where they came from, I realized that they were all of the springs we removed from new molds our customers sent us.