Know How | 10 MINUTE READ

Part 2 The Basics of Tapered Interlocks

When, where, and how to use them.


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In Part 1 of this series in September, we reviewed the various types of tapered interlocks and the pros and cons of each. If you are going to use tapered interlocks in a mold, there’s a few additional things you should know. If they are not located, installed and maintained properly, they can be either completely ineffective or can be the cause of various molding issues. Tapered interlocks can do more than just prevent parting-line mismatch on a molded part. When used properly, they can help protect the mold from catastrophic damage.

Fitting Them to the Mold
It is not uncommon to buy interlocks whose flat faces touch, but the tapered surfaces have a slight gap.  If you come across this condition, grind the face of the female interlock until the tapered surfaces mate—like they should.

The height of most conical interlocks intentionally comes with extra stock on both the male and female halves. The backs of these interlocks are ground to the correct height when fitting the mold, to adjust for variations in the pocket depths and insert heights.

There is no need to fit the ends of rectangular interlocks. In fact, the length tolerance on rectangular interlocks is typically ± 0.010 in. Any clearance on the ends will not affect the function of the locks and it makes them easier to remove. The width tolerance is typically +0.000/-0.001 in., but the height tolerance can be as much as ±0.005 in. for each half. Therefore, to avoid having to use shim stock in a brand-new mold, measure the height of the interlocks prior to machining the pockets. One of the nice features about rectangular interlocks is that they are easy to adjust. They can be ground and shimmed in any direction if they should ever wear, gall or lose contact.

Measure the height of the interlocks prior to machining their pockets.

How Much Preload?

There is disagreement among mold-component suppliers and moldmakers as to whether or not the front faces of interlocks should seat at the same time the tapered surfaces do. Some say they should and offer interlocks with all of the surfaces mating. Others say they shouldn’t and offer interlocks with as much as a 0.040-in. gap.

I believe, as do other industry experts, that in order to function properly in the real world, tapered interlocks should have a slight preload. A preload of 0.0010 in. is common for interlocks with a 10° taper. You can use slightly more preload for tapers of less than 10° and slightly less for tapers greater than 10°. Do not confuse the amount of preload with the amount of gap between the faces. One has absolutely nothing to do with the other.

In order to function properly, tapered interlocks should have a slight preload.

Too much preload can cause the interlocks to wear out faster. Wear is caused by surface friction and is proportional to the amount of contact pressure. It is independent of the amount of surface area. Therefore, you can use the same amount of desired preload for both conical and rectangular interlocks, regardless of their size. When there is too much preload, the female half can potentially crack. Too much preload can also cause the interlock to “seat” into the mold base sooner. This is why I like to make the gap between the preloaded faces no more than 0.0005 to 0.0010 in.


Steel Types & Hardness

Conical and rectangular interlocks are available in various types of steels and in various hardnesses. However, every mold-component supplier makes both the male and female halves of the same steel type and hardness. In theory, the faces of these interlocks do not rub or slide against each other. Therefore, there is no need to make them out of different types of steel with different Rockwell hardnesses. But in reality, if there is any misalignment—which there always is—the faces of these interlocks do, in fact, rub and slide against each other. A light coating of grease can help extend the life of an interlock. It would be beneficial to molders if mold-component suppliers offered tapered interlocks that are designed to withstand these frictional forces.

If you are going to make your own interlocks, you might consider making the male half, which is subjected to compression, out of a graphitic tool steel, such as A-10 or O-6, with a 58 to 62 Rockwell C hardness. The female half, which is subjected to tension, could be made of a tougher, more ductile tool steel, such as H-13 or 420 Stainless with a 48 to 52 Rockwell C Hardness.


How Many Should You Use?

There is a rule of thumb that recommends using two conical interlocks for small molds, four for medium-sized molds, and six for large molds. You should know how I feel about rules of thumb. This is another one of those “rules” I disagree with.

Conical interlocks typically come in diameters of ½, ¾, 1, 1½, and 2 in. I recommend two relatively small conical interlocks for small molds, two medium locks for medium-sized molds, and two large locks for large molds. Two larger interlocks can provide better alignment and more longevity then four smaller ones because they can have more total bearing surface area. For example, two 1-in. interlocks have 30% more surface area than four ½-in. interlocks.

Also, if you have more than two conical locks, they all have to be perfectly positioned and there can’t be any differential thermal expansion. Otherwise, they will just fight each other, which will cause them to either wear, break, or try to prevent the mold from closing all the way.

Tapered Interlock BasicsFIG 1 A cavity insert cracked at an inside corner.


Interlocks Should Support the Cavity

As plastic pressure builds within a mold during the packing phase, the side walls of the cavity will try to move outward. If the steel around the cavity isn’t strong enough and the packing pressure is high enough, these can often result in one of four problems. The worst-case scenario is that the cavity cracks, typically starting at an inside corner, as shown in Fig. 1. The more common scenario is scuffing on the outside surface of the part, because the cavity wall tries to move back inward to its original relaxed position when the mold starts to open.

Then there is the scenario where the cavity walls apply so much pressure on the part that the bolts retaining the core in the B-plate break, as shown in Fig. 2. This is fairly common with tall parts and an overpacked condition. Lastly, there is the bewildering scenario where the bolts holding onto the core are sufficiently strong, but the press doesn’t have enough force to open the mold. If you have ever had to heat up a mold, or use a hydraulic bottle jack between the platens, to get a mold to open, then you know what I’m talking about.

FIG 2 A core insert stuck inside a cavity.


FIG 2 A core insert stuck inside a cavity.


Interlocks can help prevent all these problems from occurring by providing a significant amount of lateral support. Rectangular interlocks have more surface area than conical interlocks and can resist larger lateral loads. However, in the scenario above, where the sidewall of a cavity tries to flex outward, a rectangular interlock mounted perpendicular to the sides of the mold will not help prevent this from happening. Rectangular interlocks would have to be mounted parallel to the sides of the mold. Then they would act as a heel and help prevent the plates or inserts from shifting. This is the only time you might consider mounting a rectangular interlock in a direction other than perpendicular to the side of a mold. Keep in mind that doing this could cause a problem if there is a large temperature differential between the mating surfaces.

If you think about it, proper interlocking allows you to minimize the size of a mold base when the cavity has a lot of outward projected area, such as in the case of a tall garbage can.


Interlocks help prevent problems by providing lateral support.



Mold Setup

One very important trick I learned over the years is to clamp the mold under pressure before fully tightening the mold clamps. This ensures that the interlocks are seated and the two halves of the mold are aligned. See the June 2017 article, “How To Mount An Injection Mold.”


Maintain Your Interlocks

When you see signs of wear or galling on the mating surfaces of an interlock, it should be corrected as soon as possible. If not, the problem will only get worse. If the faces of the interlocks are rusty, they’re not touching and therefore not doing what they were designed to do. This usually happens when the interlocks “seat” into the mold base over time. They need to be shimmed to their original height. If one side is rusty and the other side is shiny, you probably have an alignment issue, often caused by differential thermal expansion, as shown in Fig. 3. You should also periodically inspect the inside of the female half of the interlocks. You might be surprised at what you will find inside besides a little dirt or grease.

FIG 3 Wear on a conical interlock due to misalignment.


FIG 3 Wear on a conical interlock due to misalignment.




Molds with Cams

Interlocks are not always necessary to align the two halves of a mold. In fact, sometimes they can be a hindrance. For example, when a mold has two opposing cams, the heels and the angled back faces of the cams already act as interlocks and align the mold in one direction. Additional interlocks, working in the same direction as the cams, would fight one another and therefore are not required for alignment. This is why conical interlocks are not recommended in molds with more than one cam. They align a mold over a full 360°. Two rectangular interlocks are a much better choice in molds with two opposing cams. They can keep the cams centered, thereby protecting their sides from damage, as shown in Fig. 4.

Molds with four-way cams, which form a “plus” sign, typically do not require any interlocks at all. The four cams naturally align the plates in both directions. Adding interlocks would only make it difficult for the moldmaker to “time out” the cams and no additional benefits would be gained. Molds with just a single cam definitely require interlocks to align the mold in both directions. A single cam can apply a great amount of side load, causing the mold halves to misalign. This side load can be caused by preloading the heel that activates the cam, and by the injection pressure pushing against the face of the cam.


FIG 4 Wear on the side of a cam due to plate misalignment.


FIG 4 Wear on the side of a cam due to plate misalignment.



Cylindrical Interlocks Have Their Place

There is another type of interlock worth mentioning. It is probably one of the earliest types ever invented and is occasionally still used today. It is basically nothing more than a round shaft lying on, and parallel to, the parting line. The mold halves are assembled and a hole is drilled on the four sides of the parting line. Short sections of a hardened round shaft are then screwed into one side of the mold.

One application where this type of interlock is very beneficial is when a mold has a round core or core pin on the parting line, typically activated by a cam. Shutting off on a cylindrical surface is notorious for developing parting-line flash along the pin’s centerline. As long as the diameter of the cylindrical interlock is larger than the diameter of the core pin, it will help protect the core pin from damage due to any misalignment of the mold halves. The female half of the interlock will hit the male half before hitting the core pin just before the mold is fully closed, as seen in Fig. 5.

Tapered Interlock Basics FIG 5 A cylindrical interlock protecting a cylindrical core pin.​​​​​​​​​​​​​​


This is another “old-school” trick that seems to have been forgotten over the years. Unfortunately, many people consider this type of interlock to be “cheap” and associate it with a poor-quality mold built offshore. But for some applications, this type of interlock can significantly reduce your repair costs.

About the Author: Jim Fattori is a third-generation injection molder with more than 40 years of molding experience. He is the founder of Injection Mold Consulting LLC, and is also a project engineer for a large, multi-plant molder in New Jersey. Contact:


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