New Tinius Olsen VectorExtensometer U70 testing
Published

How to Form a Hole with a Core Pin: Part 1

Prevent wear, flash, and mold damage with these design tips.

Share

There are many ways to form a hole in an injection molded part. Some are simple and inexpensive. Others are a little more involved but have some added benefits. This month I will review various methods I have used over the years and explain the pros and cons of each.

Figure 1 depicts six common methods for creating a through hole in a molded part. All of them use a core pin. While Fig. 1 shows core pins mounted in a B-Plate, most of these methods can be applied to core pins mounted in the A-Plate, an ejector sleeve, or even a cam. Additionally, while the methods discussed here depict cylindrical core pins, they can also be incorporated into a variety of shapes with the same intent and purpose—for example, forming a keyhole in a molded part.

Forming a hole on a molded part with a core pin

FIG 1 Various methods to form a through hole.

On occasion, you may see a mold where the through hole in the part was formed in a solid—meaning the core was machined with a protruding shut-off and not with a replaceable pin. That type of design should only be used for prototype molds for several reasons—the biggest one being the difficulty and cost to repair.
 

Core Pins

Core pins used to make a hole in a part should be through hardened—not nitrided. Most ejector pins and sprue-puller pins are nitrided and have an exterior case hardness of 65 to 74 Rockwell C (Rc), with a core hardness varying from 40 to 55 Rc, depending on the supplier and the type of pin. The case hardening is typically only 0.001 in. to 0.007 in. deep and is extremely brittle. This can cause the edge on the face of the pin to chip during production.

However, some ejector pins are available in a through-hardened condition, typically 58 to 62 Rc, but can also be found with a lower hardness of 50 to 55 Rc. You might consider using one of these through-hardened ejector pins for a core pin application because the tolerance on the diameter of an ejector pin is about -0.0003 to -0.0005 in. In the event of future wear or corrosion, the ejector pin can then be replaced with a core pin, which has a tolerance of +0.0003 to +0.0008 in.

All core pins are not created equal.

Be very careful when selecting the type of pin to use. I found one supplier that says its core pins are through-hard but specify a surface hardness of 62 to 65 Rc, and a core hardness of 50 to 52 Rc. Additionally, since we are only human, you will find discrepancies between what is specified in a supplier’s catalog, its website, and the actual product itself. It is always a good idea to check the diameter of any pin to four decimal places, and occasionally check the Rockwell hardness on the diameter and in the center of the face. If you buy molds offshore, you might be very concerned about the measurements you get. You might also be surprised with the measurements you get from a domestic supplier that purchases its pins offshore.

Core pins are typically made of H-13 or M-2 tool steel. The H-13 pins have a standard hardness of 30 to 35 Rc throughout. Higher hardness H-13 core pins are also commercially available with a 50 to 55 Rc hardness, and M-2 pins with a 60 to 63 Rc hardness throughout. In cases where the core pin butts off against the cavity or another pin, I prefer the harder hardness. In cases where the core pin enters a hole in the cavity or other component, I prefer the softer core pins, which will wear faster. It is cheaper to replace the core pin if it wears than it is to repair a worn hole in the cavity.

Copper-alloy pins (Fig. 2) can also be used in any of the depicted methods. They can be very beneficial for maintaining circularity of a hole, especially in thick-walled parts. They can also help reduce the cycle time of the part. But you need water running around or inside the copper pin to keep it cool.

how to form a hole in a molded part with a core pin

FIG 2 Copper-alloy core pins can provide faster cycle times.

However, since copper-alloy pins are much softer (90 to 98 Rockwell B) than steel pins, their face can deform if there is excessive force applied. At least one mold-component supplier offers core pins made of 420 stainless steel with a through hardness of 50 to 52 Rc. These are also a very good choice for core pins in many applications—particularly those for the medical industry. But compared with copper pins and carbon-steel pins, stainless steel has considerably lower thermal conductivity. While a stainless pin is resistant to corrosion, it can also require an increase in the mold’s cycle time.

Regardless of the type of material a core pin is made of, the smaller the diameter and the longer the exposed length, the more difficult it will be to keep cool. I once saw a medical mold that had extremely small-diameter core pins—so small that even tiny bubblers made of stainless-steel surgical tubing could not be inserted into them. The designer knew that the exposed length of the core pins would get extremely hot, even with a water-wash around their shaft. Therefore, he designed the mold with air jets in the cavity directly opposite each of the core pins. When the mold started to open, the air blasted against the hot pins. It was certainly an unconventional design, but it worked well in solving the cooling problem.
 

Method A

Method A in Fig.1 is the least expensive way to form a through hole in an injection molded part. The face of the core pin butts off against the cavity impression. The downside of this method is that it can either make a slight impression in the cavity as the plates start to “settle in” over time, or it can generate side flash if the injection pressure opens the parting line a little. It is also important to consider the hardness of the steel that the head of the pin sits on. Plates made of prehardened 4130 steel (28 to 34 Rc) are preferred over the softer 1030 steel (80 Rb). Alternatively, a hardened retainer about 1/8-in. thick can be inserted in the softer plate. The counterbore for the head of a core pin should be 0.002-in. deeper than the thickness of the head itself.
 

Method B

Method B is the same as Method A, except that the pin is spring loaded to overcome any variation in plate thickness or injection pressure. Springs also help to protect small, fragile core pins. Bellville springs work very well for this application. One spring washer or a stack of washers may be required, depending on the size of the pin and the amount of preload desired.

I have used this method quite frequently on return pins and other mold components. It greatly reduces the amount of shock inflicted on the pin and on the mating plate or mold component. Since the pin may need to expand or contract slightly, the bore hole it rides in should be relieved. The amount of bearing surface or land length in the core insert should be at least twice the amount that the pin protrudes from the core. It is also a good idea to grind (“dust”) a small amount of steel off the back of the core pin to ensure it is perpendicular to its centerline. Now the spring will apply even pressure—preventing any forces that would try to push the pin slightly to one side or the other.

Consider the hardness of the steel that the head of a core pin sits on.


Method C

Method C uses a core pin that pilots into a hole in the cavity, which is the same diameter as the pin. The pin is bullnosed in case of any misalignment. This design eliminates the need for any spring loading, but it is prone to deforming the through hole and possibly the core pin itself if there is significant misalignment. As the through hole or the pin begins to wear, it can generate down-flash.

If the hole in the part that the core pin forms is tall, the tip of the pin needs to be retained by some method, such as entering a hole in the cavity. If you think about a pen barrel or a syringe, you can easily see that core shift is big concern. When a core pin is supported on both ends, core shift is less of an issue.
 

Method D

Method D is what I call a double-piloted core pin. As the mold closes, the smaller-diameter tip of the pin enters a mating through hole in the cavity. The clearance between this portion of the pin and the through hole is typically 0.0005 in. per side. Once engaged, the full pin diameter then enters a counter-bored hole in the cavity, which typically has 0.0010 in. of clearance per side—or less depending on the recommended vent depth for the molding material. This double-pilot or prealignment method helps protect the larger hole from damage and wear.
 

Method E

Method E is similar to the single-piloted core pin in Method C, except that the lead-in diameter is reduced. This is also an inexpensive method of forming a through hole. The advantage to this method is that if the through hole in the cavity begins to wear, the chance of down-flash is minimal due to the step on the core pin, which shuts off on the face of the cavity. The downside of this method is basically the same as the downside of Method A. The core pin can make a slight impression in the cavity as the plates start to flatten over time, or it can generate side flash if the injection pressure opens the parting line a little.
 

Method F

Method F is a combination of the spring-loaded Method B, and the stepped diameter of Method E. This is probably one of the best designs for extremely long-running molds with tight-tolerance through holes.
 

Venting and Cleanout

The hole in the cavity that the core pin engages should not be blind. It should be machined all the way through the cavity insert or A-Plate. In fact, it is best if the through hole tapers outward or otherwise increases in diameter after a sufficient amount of land length. Everything from dirt, sub-gate flakes and flash to grease and other debris will eventually fill up the hole. That can cause a pin to stick, compress, bend, buckle, or possibly break.

Designing an injection mold to work well and last a long time with minimal repair costs and downtime is not difficult if you pay attention to the details.
 

ABOUT THE AUTHOR:  Jim Fattori is a third-generation molder with more than 40 years of experience in engineering and project management for custom and captive molders. He is the founder of Injection Mold Consulting LLC  in Pennsylvania. Contact:  jim@injectionmoldconsulting.com;
injectionmoldconsulting.com

Dover Clear
mold, mould track, digital tracking, molding
Konica Minolta
Dri-Air
Maguire Ultra
Gardner Business Media, Inc.
Guill - World Leader in Extrusion Tooling
Cranes, Conveyors, Racks, Loaders, Accessories
Shuttle Mold System
Konica Minolta CM-36dG
AM Workshop
Uway LLC

Related Content

best practices

Understanding Strain-Rate Sensitivity In Polymers

Material behavior is fundamentally determined by the equivalence of time and temperature. But that principle tends to be lost on processors and designers. Here’s some guidance.

Read More

Tunnel Gates for Mold Designers, Part 1

Of all the gate types, tunnel gates are the most misunderstood. Here’s what you need to know to choose the best design for your application.

Read More

The Effects of Temperature

The polymers we work with follow the same principles as the body: the hotter the environment becomes, the less performance we can expect.

Read More

Density & Molecular Weight in Polyethylene

This so-called 'commodity' material is actually quite complex, making selecting the right type a challenge.

Read More

Read Next

sustainability

Lead the Conversation, Change the Conversation

Coverage of single-use plastics can be both misleading and demoralizing. Here are 10 tips for changing the perception of the plastics industry at your company and in your community.

Read More
Extrusion

How Polymer Melts in Single-Screw Extruders

Understanding how polymer melts in a single-screw extruder could help you optimize your screw design to eliminate defect-causing solid polymer fragments.  

Read More
Insert and MedTech molding automation solutions