Injection mold interlocks play a critical role in the alignment of injection mold components, yet their importance is often overlooked or minimized. They are typical of other industrial supplies that are taken for granted.
In the course of my career I have made and installed a wide variety of injection mold interlocks. Every now and then a new version appears with the hope that the mold halves will reliably line up and stay that way. Injection mold making demands high degrees of accuracy and these simple components are indispensable.
I recall one time, as an apprentice, saying to my boss when he questioned me about the alignment of my cavity blocks: “Oh, it’s bound to work out”, I said. His reply: “There is no ‘bound to!'”. He was right.
Fitting injection mold interlocks
As an apprentice I was given the task of grinding the male and female components so that there was only .0005 in. gap between the two pieces when closed, and ensuring that the each interlock was the correct height to match the “A” and “B” plates. These were basic D-M-E round pieces that are simple to install and time on a surface grinder.
After 20 minutes or so of removing steel off the top of the male side I went to the foreman and complained that it wasn’t working. As soon as I said it I saw that no matter how much I removed, the gap would never change, and the overall length would not change. All I had to do was grind the back to suit. Should have read the instructions, duh?
Include the interlocks in the design
In injection mold design, interlocking refers to the way the fixed half and moving half of a mold are locked together Interlocking makes use of angled surfaces on both halves of the mold which engage when the mold is closed.
The purpose of interlocking is to prevent any small sideways movement of the moving side relative to the fixed side so that part wall section and weight remains even for every cycle. If the alignment is off, everything else is out-of-whack as well.
At the first sign of wear on an interlock steps should be taken to remedy the cause, if not the problems only increase over time.
Interlocking is one of the most overlooked areas of plastic injection mold design. The two halves must be rigidly lock in place every cycle if quality plastic parts are to be produced for the life of the mold.
Another issue with proper interlock design is the choice of tools steels and their heat treatment. Using dissimilar tool steels of differing hardness is a common approach to ensure the longevity of the components.
Why you can’t just use leader pins and bushings
The reason you can’t just use leader pins and bushing is quite simply because they are not designed to act as mold interlocks.
Most leader pins and bushings have at least .001 in. clearance, which makes them great for what might be called “rough alignment”. They work great for aligning the stationary and moveable halves of the mold so it will open and close easily, especially for mold maintenance.
Then there is the fact that some molders remove the pins and bushings to help eliminate the possibility of grease contamination during the molding process. Some safety regulations may also apply in some cases.
Enter the mold interlock
There are many approaches to using precision mold interlocks. Details can be machined into the core and cavity, round or angular interlocks can be located in the core and cavity, they can also be located on four sides of the A and B plate faces.
Some interlocks are located on the outside perimeter of the A and B plates, using commercial components or custom made ones. Using the outside of the mold is quite common and can be machined at the same time as the mold base, which saves setup time.
Here is a list of commercially available injection mold interlock manufacturers.