What Is a Runner Lock Pin (Z-Pin)? Its Function, Selection Criteria, and SG-WIND’s Solutions for Defects

Introduction

Injection molding dies contain numerous components that, while seemingly inconspicuous at first glance, play a crucial role in ensuring stable molding. One such component is the runner lock pin (Z-pin).

The runner lock pin is an essential component that securely holds the resin flowing from the sprue to the runner and ensures that the resin is pulled away from the fixed side when the mold opens.

Although this component often goes unnoticed, if it is not properly designed or selected, it can cause various problems, such as disruptions in the molding cycle or poor runner demolding.

Furthermore, in recent years, there has been growing interest in utilizing runner lock pins not merely as “runner-retaining components,” but also as a means of managing gas within the mold.

In this article, we will provide a detailed explanation, ranging from the basics of runner lock pins (Z-pins) to the defect-reduction benefits achieved by replacing them with SG-WIND pins.

What Is a Runner Lock Pin (Z-Pin)?

A runner lock pin is a component typically installed on the movable side (CORE) of the mold, and its role is to hold the runner in place after the resin has solidified.

Generally, the tip of the pin features an undercut or tapered shape, and the runner is secured as the resin penetrates this area.

During mold opening, it plays the following important roles:

・Pulling the runner out of the sprue bushing

・Holding the runner toward the core side

・Ensuring stable demolding

.

If the runner lock is not functioning properly,

• The runner remains on the CAV side

• The runner breaks off midway

• Automatic removal becomes impossible

• The molding cycle stops

such problems may occur.

Therefore, it can be considered a critically important component in automated molding lines.

Why It Is Called a “Z-Pin”

Runner lock pins come in various shapes, but those with a zigzag or Z-shaped tip are called “Z-pins.”

This unique shape enables:

・Mechanical interlocking with the resin

・High holding force

・Stable runner locking

In particular, it demonstrates high holding force with materials that have a high shrinkage rate, such as PP and PE.

Performance Requirements for Runner Lock Pins

A high-quality runner lock pin must meet the following performance requirements:

① Reliable runner retention

The runner must be securely held in place until the mold is opened, without slipping out midway.

② Demolding performance

If the holding force is too strong, the runner will break during demolding.

A balance between holding force and demolding performance is crucial.

③ Wear resistance

Since the resin comes into contact with the pin on every shot, it must be resistant to wear even after prolonged use.

④ Ease of Maintenance

It is also important that these parts are easy to replace and offer high reproducibility.

The Concept of Replacing Runner Lock Pins with SG-WIND Pins

In injection molding, various gases are generated as the resin flows at high temperatures and speeds.

For example,

these include volatile components in the resin, additives, moisture, decomposition gases, and air.

Normally, these gases are vented through the parting line or vent holes; however, if venting is insufficient, the gases can become adiabatically compressed between the resin and the mold, causing ignition and resulting in scorch defects—burn marks left on parts of the molded product.

While using vent inserts is one way to improve venting performance when venting is insufficient,

a simpler method to enhance venting performance is

to replace the runner lock pins with SG-WIND pins.

The SG-WIND pin is a vent pin originally developed for gas venting.

The interior of the pin features a fine venting structure designed to efficiently vent gas while minimizing resin leakage.

By installing this SG-WIND pin in the runner lock section,

gas that previously flowed through the runner lock section into the cavity can now be vented along the way.

In other words, the concept is to vent the gas before it enters the cavity.

Effects on Defect Reduction

In practice, the following improvements can be expected:

・Reduction in silver defects: Proven improvement of 40% or more

・Reduction in gas burn

・Improvement in short shots: Proven improvement of 80% or more

・Improved weld line quality

・Improved surface quality

・Stabilization of molding conditions

Furthermore, by reducing the amount of gas inside the cavity,

it helps stabilize the internal mold pressure,

which in turn helps suppress variations in molding conditions.

Simultaneously Achieving Runner Locking and Gas Control

Traditionally, runner lock pins were viewed simply as “components that hold the runner in place.”

However, by using SG-WIND, you can add a new function—gas venting—in addition to runner retention. In other words, a single component can fulfill two roles.

This is a significant advantage because it allows for quality improvements while minimizing the scope of modifications required to existing molds.

Summary

Runner lock pins (Z-pins) are critical components in injection molds that ensure the runner is securely held in place. However, since the area surrounding the runner lock pin serves as a gas passageway, depending on the molding conditions, gas may flow into the cavity, potentially contributing to defects.

Replacing it with an SG-WIND pin allows for efficient gas venting while maintaining the runner lock function, which can help reduce cosmetic defects such as silver streaks, gas burn, and short shots. This effect is particularly noticeable in high-speed filling, thin-wall molding, and molding with materials prone to gas generation.

The SG-WIND pin, which adds the new value of “gas control” to its traditional role of “holding the runner,” is an effective solution for improving quality without requiring major modifications to existing molds.

If you are currently struggling with issues such as silver streaks, gas burn, or weld line defects, why not take a closer look at the runner lock section and consider using the SG-WIND pin? A potential improvement point that you may have overlooked could be a major step toward reducing defects.