When a production line suddenly starts showing weak bonding, irregular glue patterns, or unpredictable adhesion strength, technicians often check temperature settings, pump pressure, or machine calibration. Yet one subtle culprit is frequently overlooked: bubbles forming after the hot melt adhesive melts.
These bubbles are tiny, but their damage is real—poor bonding, wasted adhesive, and unstable coating results.

For workshops using Hot Melt Glue Sticks, Hot Melt Glue Guns, Bulk Hot Melt systems, or PUR Hot Melt technology, understanding why bubbles appear is essential for stable, long-term production.

Dongguan Saipu Electromechanical Device Co., Ltd., a leader in hot melt glue machinery solutions, has seen these issues across packaging, automotive sealing, electronics, masks and garment bonding, hygiene products, bookbinding, and filter manufacturing. Across industries, one truth remains the same: bubbles are the enemy of consistent adhesion.

■ The Three Sources of Bubble Formation After the Hot Melt Adhesive Melts

1. Moisture in the Adhesive — Steam Trapped in Viscous Glue

Moisture is the most common cause of bubbles after the hot melt adhesive melts.
When heated, moisture instantly vaporizes into gas, but because hot melt adhesive is extremely viscous, the vapor cannot escape easily. It stays trapped, forming bubbles that later appear in glue lines or coating layers.

This affects:

• Hot Melt Glue Sticks stored in humid environments

• Bulk Hot Melt pellets left in open bins

• PUR Hot Melt materials exposed to ambient moisture

If moisture enters, bubbles are inevitable.

2. Chemical Cross-Reactions Between Different Adhesives

Another frequent cause is mixing adhesives made with different base resins or additives.
Even small residual amounts left in the tank can react with newly added material, causing gas release during melting.

Examples include:

• Switching between brands of Bulk Hot Melt

• Using Hot Melt Glue Guns with mixed leftover sticks

• Changing from EVA-based to polyolefin-based formulas

• Residue left behind in a tank used for PUR Hot Melt

The chemical reaction produces gas bubbles that emerge as soon as the hot melt adhesive melts, leading to foam-like glue flow.

3. Air Being Pulled Into the System Due to Low Glue Levels

Sometimes the adhesive itself is not the problem—the tank level is.

In large-capacity systems, using only a few blocks of Bulk Hot Melt may leave too much empty space in the tank. When glue levels fall too low, both adhesive and air enter the pump inlet simultaneously.
This produces bubbles in the delivery line that ultimately reach the nozzle.

This is especially common when:

• Machines are oversized compared to daily glue usage

• Hot Melt Glue Guns with internal reservoirs run nearly empty

• The operator intentionally loads minimal adhesive to avoid waste

Even though the glue seems to flow normally, the real output volume decreases because the adhesive is diluted with air pockets.

■ How to Solve Bubble Problems Based on the Cause

A. If Moisture Is the Issue

• Inspect storage conditions for Hot Melt Glue Sticks and Bulk Hot Melt

• Dry the glue tank before loading new adhesive

• If the adhesive surface appears foggy or moist, pre-dry before use

• Keep PUR Hot Melt sealed tightly—PUR reacts aggressively with humidity

Good moisture control ensures stable melting behavior when the hot melt adhesive melts, reducing foaming and improving bonding strength.

B. If Chemical Reactions Are the Cause

• Thoroughly clean the glue tank when changing adhesive brands or formulations

• Avoid mixing old and new adhesives, even within the same category

• When switching Bulk Hot Melt types, flush the tank completely

• For Hot Melt Glue Guns, eject the old stick fully before inserting the new one

Different formulas = different chemical behaviors. Reducing cross-talk avoids unexpected gas generation.

C. If Air Entry Is the Cause

• Maintain a minimum tank fill level

• Choose equipment capacity appropriate for real glue consumption

• Avoid running Hot Melt Glue Guns or tanks until fully empty

• Add adhesive in smaller but more frequent amounts

Stable glue levels ensure that when the hot melt adhesive melts, it remains bubble-free and consistent in every application.

Industry Analysis: Effects of Bubble Formation on Production

This analysis clearly shows:
Bubble control equals cost control.

FAQ

Q1: Why do bubbles appear only sometimes?

Because moisture, chemical residue, or air levels fluctuate day by day. Even Hot Melt Glue Sticks can absorb moisture differently depending on weather.

Q2: Will bubbles affect adhesion strength?

Yes—bubbles break glue continuity, causing peeling, weak bonding, and voids, especially in PUR Hot Melt or high-speed Bulk Hot Melt lines.

Q3: Can I ignore small bubbles?

No. Small bubbles become large defects during pressurization or coating, especially when the hot melt adhesive melts unevenly.

Q4: How do I prevent bubbles permanently?

Consistent tank hygiene, controlled storage, avoiding adhesive mixing, and maintaining proper glue levels.

Call to Action

Dongguan Saipu offers:
✔ Technical support for bubble-related troubleshooting
✔ Free "Melt Behavior Inspection Checklist"
✔ Application testing service for Hot Melt Glue Guns, Bulk Hot Melt, and PUR Hot Melt users

You can request your free checklist or consultation anytime.

Conclusion: Bubbles Are Small, but Their Impact Isn’t

Bubbles are more than a cosmetic defect—they represent moisture issues, chemical reactions, or air intrusion affecting every adhesive system. Whether you're using Hot Melt Glue Sticks, high-capacity Bulk Hot Melt, precision Hot Melt Glue Guns, or moisture-sensitive PUR Hot Melt, bubble control is a direct path to stronger adhesion and better product quality.

Understanding what happens when the hot melt adhesive melts gives manufacturers the power to prevent defects before they happen. Reliable bonding begins with bubble-free glue.