What are cold slugs? What are small foreign bodies lurking in injection molding and what can be done about them?

1. Introduction: What are ‘foreign bodies’ and what are they?

In plastic injection molding, ‘black spots’ and ‘foreign bodies’ can cause abnormal appearance and reduced strength. Among these, a surprisingly little-known molding defect is the **cold slug** introduced in this article.

This phenomenon may look small but has a significant impact on product quality. This article provides a detailed explanation of cold slugs, including their true nature, causes of occurrence and countermeasures. 2.

2. what are cold slugs?

Cold slug is a molding defect in which a lump of resin that has cooled and begun to solidify is introduced into the molded product. Normally, the resin that is first injected through the nozzle is cooler than the resin inside the mold and is more susceptible to cooling. This reaches the cavity through the runner or gate and causes defects.

3. main causes of occurrence

The main causes of cold slugs are:

● Cooling of the nozzle tip resin before injection starts.

Resin cooled at the nozzle tip during stoppage or after long periods of idleness, causing the resin to mix into the cavity.

● Heat loss at the sprue or runner

Improper nozzle temperature or sprue bushing design can lead to cooling at the very front of the flow.

● Cold slug wells not installed or poorly designed.

Increased risk of direct contamination on the product side if there is no ‘escape route’ for the cold resin.

Improper setting of injection speed and temperature.

If the resin temperature and injection speed are not matched, uneven cooling and flow of the resin will lead to defects.

4 - Cold slug countermeasures: measures that can be taken on site

The following measures can significantly reduce the occurrence of cold slugs

Thorough purging: always perform purging before and after stopping the molding process.

Review of molding conditions: optimise nozzle and sprue temperatures and injection speed

Appropriate cooling and preheat management: especially when restarting the molding machine.

Improve mold structure: installation of cold slug wells is effective.

5. use of cold slug wells

A structure that prevents the occurrence of cold slugs is the ** ‘cold slug well’. This is a ‘depression’** in the runner or just before the gate that traps cold resin, preventing it from flowing into the product side.

Example design dimensions.

Diameter: 1.5 to 2 times the runner diameter

Shape: cylindrical, tapered is common

An effective mechanism for separating only the cold slugs without adversely affecting the product.

6. be on your guard even with hot runners

It is easy to assume that using a hot runner will result in ‘no slugs’ at all, but it is not completely zero.

Care should be taken in the following cases:

The runner tip is cold immediately after start-up or at the end of a shutdown

Faulty heaters or temperature sensors

Dead space in the gate structure

Manage the risk of cold slugs even on hot runners with proper temperature control and start-up procedures.

7. summary and one-point advice

Cold slugs, although very small foreign particles, can cause many problems such as appearance defects, strength failures and carbonisation.

💡 The key point is how to deal with the ‘cold resin’ at the very front of the flow.

Purging, temperature control and slug well design - the combination of these factors can significantly reduce the risk of cold slugs.

We hope that this article will help you to reduce defects in your daily molding operations!