What Is Leader Pins?

Been running two-shot jobs for about fifteen years now and leader pins still cause more headaches than they should. Guys will spend weeks dialing in process parameters and completely ignore the fact that their alignment hardware is shot.

Leader pins do one job. They line up your A-side to your B-side. Simple. But in two-shot work, you've got two parting lines to worry about, sometimes three if you're running a stack configuration. Mess up alignment at either interface and you're chasing flash for the rest of the run.

The Basics

Your leader pin is just a hardened steel dowel that slides into a bushing. DME and Hasco sell them by the truckload. Progressive Components has their own line. Most shops I've worked with standardize on one supplier and stick with it because mixing brands creates fit problems down the road.

Standard setup is four pins at the corners. The pin presses into one plate, the bushing presses into the other. When the mold closes, pins engage bushings and everything lines up. Works fine for single-shot tools.

Two-shot is different.

Why Two-Shot Complicates Things

Say you're running a rotating platen setup. Arburg and Engel machines do this a lot. Your center section spins 180 degrees between shots. That center section needs its own set of leader pins on both faces.

So now you've got pins going from your A-plate into the front of the rotating section. And you've got another set going from the back of the rotating section into your B-plate. Eight leader pins minimum instead of four. Some bigger tools run twelve.

Here's where shops get into trouble. The rotating section has to index perfectly every cycle. If your pins on the rotating section aren't placed symmetrically around the rotation axis, the tool won't close the same way after it spins. I saw a shop in Grand Rapids fight this for two weeks before they figured out their pin pattern was 2mm off-center.

Material and Specs

Most pins come in 1.2767 or equivalent tool steel. Hasco uses their own proprietary grade but it's similar. Surface hardness runs 58-62 HRC after heat treatment.

For the bushing side, you've got two choices. Bronze or steel.

Bronze bushings are more forgiving. AMPCO 18 or similar alloys. They self-lubricate to some degree and they'll tolerate a little misalignment without galling. Downside is they wear faster. On a two-shot job running three shifts, I've seen bronze bushings go oval in under 200K cycles.

Steel bushings last longer but they need grease and they need perfect alignment. You can't cheat with steel. Most Meusburger and MISUMI bushings are steel with oil grooves cut in. Works great if your maintenance guys actually grease the tool.

Tolerances That Actually Matter

Pin diameter tolerance is usually h6. Bushing bore is H7. That gives you somewhere between ten and thirty microns clearance on a 25mm pin. Sounds tight but it's not tight enough for some two-shot work.

The problem is thermal growth. Your mold runs at maybe 80C on the first shot side and 60C on the second shot side. Or the reverse. Either way, your A-side and B-side are growing at different rates. That h6/H7 fit that worked great at room temperature might be binding at process temp.

I've gone to h5/H6 fits on a couple of medical jobs where the customer spec'd ±0.05mm at the material interface. Expensive but necessary.

Runout on the pin matters more than most people think. Spec says 5 microns max on the bearing surface. Real world, if you're over 8 microns you'll see it in the parts. The pin wobbles as it enters the bushing and that wobble transfers right to your cavity alignment.

The Fifth Pin Argument

Old school moldmakers say four pins is enough. They're not wrong for single-shot work.

Two-shot is different. The rotating section flexes. Especially on tools over 500mm. You clamp down with 300 tons and the middle of that section deflects toward the nozzle side. Four corner pins can't stop it.

Adding a fifth pin dead center of the parting line helps. Not everyone does it. Progressive Components sells a setup specifically for this. It's not cheap but it works.

One shop I consulted for was running a large automotive bezel. Two-shot soft-touch over rigid substrate. They had flash at the interface on every part. Added a center pin and the flash disappeared. Saved them from scrapping the tool.

Installation Notes

Pins go in cold. Literally. Dunk them in liquid nitrogen or pack them in dry ice for twenty minutes. The pin shrinks, you drop it in the bore, it warms up and locks in place. Standard interference is 20-40 microns.

Don't skip the nitrogen step. I've watched guys try to press pins in at room temperature. They score the bore and then you've got a pin that's loose six months later.

Bushings install the same way. Some guys heat the plate instead of cooling the bushing. Works either way. Just don't do both or you'll have too much clearance when everything normalizes.

Make sure your pins extend past the bushing face when the mold is closed. Three to five millimeters. This lets you eyeball engagement when you're setting the tool. If you can't see pin sticking out past the bushing, something's wrong.

Wear Patterns to Watch

Leader pins wear in predictable ways. The entry chamfer rounds off first. Then you get a wear band about 10mm into the bearing surface where the bushing lip contacts during opening.

Measure your pins every 100K cycles on a two-shot tool. Use a micrometer, not calipers. You're looking for diameter loss. On a 25mm pin, 20 microns of wear means replacement time. Don't push it. Worn pins let the tool shift and that shift gets worse every cycle.

Galling is the other failure mode. You'll see silver streaks on the pin surface. Means metal is transferring from the bushing. Usually happens when lubrication fails or when there's contamination in the bushing bore. Replace both components if you see galling. The bushing bore is damaged even if it looks okay.

Real World Problem

Had a job last year. Two-shot handle for a power tool manufacturer. Rigid nylon substrate, TPE overmold. Tool was about 400K cycles in and parts started showing wall thickness variation at the grip section.

First thought was process. We tweaked hold pressure, pack time, cooling. Nothing helped.

Pulled the tool and measured the leader pins. Three of them were fine. One was down 35 microns. The bushing on that corner had gone oval.

What happened was the grease port on that bushing was positioned wrong. It sat in a pocket where purge material collected. Maintenance couldn't get grease in there properly. That corner ran dry for probably 200K cycles.

Replaced the pin and bushing, cleaned out the pocket, repositioned the grease fitting. Problem solved.

Point is, leader pin problems don't always show up as flash or short shots. Sometimes they show up as dimensional variation that looks like a process issue.

Supplier Notes

DME is the standard in North America. Good quality, reasonable lead time, everybody stocks their sizes. Their LP series covers most two-shot applications.

Hasco dominates in Europe. Similar quality. Different catalog numbers. If you're building a tool in Germany and running it in Ohio, make sure you've got the right spare bushings on hand. They're not interchangeable.

Progressive Components makes some specialty items for two-shot work. Their center-lock system for rotating platens is worth looking at if you're building a new tool.

MISUMI and Meusburger both have comprehensive catalogs with good online configuration tools. Lead times can be long for non-standard sizes.

Avoid the cheap offshore pins. I've seen pins that were supposedly 58 HRC measure closer to 50. They wear fast and they're not round to start with.

Closing Thought

Leader pins aren't glamorous. Nobody tours your shop and asks about your alignment hardware. But they're fundamental to making two-shot work. Get the specs right, install them properly, maintain them on schedule, and they'll run for a million cycles without drama.

Skip any of that and you'll spend your days chasing problems that look like everything except worn leader pins.