The Designer's Edge: Lifters

This detail shows a lifter at 10 degress with 2.9375 of ejector stroke

providing 0.518 of lifter travel.

As I mentioned last month, a lifter is a component that moves on an angle with the ejection to release an undercut on the part. To calculate the angle, you need to know if the depth of the undercut on the part, material shrinkage and the ejector stroke being used are available on the tool or machine. For example, if your detail or undercut on your part is .300 your lifter would need to travel this distance with the addition of part shrinkage and clearance in order for the part to be removed freely from the mold without hangups. 

The amount of required clearance varies with part size, the plastic being used and the shrinkage factor. I have seen clearance be designed in, but part shrinkage still causes the lifters to remain engaged, which makes part removal very difficult. For example, a 20-inch PP part could have part shrinkage of up to 0.400 inch over the total length, or 0.200 from the outside edges to the center of the part. This is essential to understand, as it can be overlooked in the design. If you were going to design in 0.100 inch clearance and your lifter was on the outside edge of the part (10 inches from center based on the shrinkage above) the lifter travel would need to be 0.600 inch, 0.300 for the undercut, 0.200 for shrinkage and 0.100 for clearance. Then let’s assume you have a four-inch ejector stroke for the lifter travel. We need to find out what angle on the lifter will give us 0.600 of travel to release the part. I always use trigonometry, but will use some simple math for this example. One degree will travel approximately 0.017 over one inch, so with 4 inches of ejector stroke available, one degree will travel 0.068 inch (4 x .017). Simply multiply 0.068 by any number to calculate at what angle the lifter must be. In this case, the lifter would be placed at nine degrees (9 x .068 = .612 travel).

Once you understand how angles are determined for lifters, it's important to understand how these angles impact mold maintenance. From a mechanical viewpoint, the greater the angle, the more concern with mechanical forces contributing to wear and failures. Unsupported length is another concern with angles. For example, you can have a steep angle lifter that is more robust than a shallow angle lifter with too much unsupported length. This boils down to an unsupported length/diameter ratio and the angle of the lifter.

Later this month I will discuss how to support lifters with steep angles or excessive unsupported length.