PU Release Agent Guide: How to Prevent Sticking, Cracks, and Surface Defects in Shoe Sole Molding
Why PU Shoe Sole Molding Often Fails at Demolding Polyurethane (PU) shoe soles are widely used in the global footwear industry for their flexibility, durability, and comfort. However, many manufacturers face recurring production problems—especially during the demolding process. Issues such as sticking, cracks, surface whitening, and bubbles can significantly reduce product quality and increase scrap rates. In most cases, these defects are not caused by the mold or the PU formulation itself, but by incorrect release agent selection or improper application. This guide explains the most common demolding problems in PU shoe sole molding and provides practical advice on how to choose and apply the right PU release agent for stable, defect-free production.
1. Common Defects in PU Shoe Sole Molding
According to the feedback provided by our customers, PU shoe sole manufacturers often face the following issues:
1.1. Sticking or incomplete demolding
A poorly matched release agent may fail under temperature or pressure, causing the PU to adhere to the mold.
1.2. Surface cracks
Uneven mold release can create micro-cracks, leading to visible defects and reduced durability.
1.3. Air bubbles or pinholes
Trapped air or an unstable release layer can cause bubbles that affect appearance and performance.
1.4. Uneven gloss or whitening
Over-spraying or using the wrong type of release agent often results in dull surfaces or whitening marks.
1.5. Mold contamination or residue
Low-quality release agents may leave waxy or greasy buildup, increasing maintenance time and affecting molding consistency.
This article will not cover defects resulting from process-related causes; most of the issues described above can be effectively addressed through proper selection and application of release agents.
2. Why Sticking Happens — The Root Causes
Sticking is the most common and most costly problem in PU shoe sole molding. The major causes include:
| The Causes | DETAILS |
|---|---|
| Insufficient temperature resistance | The release agent breaks down when mold temperature is too high. |
| Uneven or excessive spraying | Thick or inconsistent layers prevent uniform demolding. |
| Fast PU reaction | Rapid curing can trap the release layer, especially on textured molds. |
| Mold temperature imbalance | Cold spots cause partial sticking or uneven surfaces. |
| Wrong type of release agent | Oil-based vs water-based must match the PU system and mold temperature requirements. |
Selecting the right release agent resolves most of these root causes and stabilizes the entire production line.
3. Water-Based vs Solvent-Based PU Release Agents
Each type of release agent has its own advantages, and understanding the differences helps factories choose the most suitable product. In particular, our improved oil-based release agent addresses environmental concerns by achieving low VOC levels and reducing residues.
| Comparison Aspect | Water-based PU Release Agent | Solvent-based / Oil-based PU Release Agent |
|---|---|---|
| Main Solvent/Carrier | Water | Organic solvents |
| Environmental & Safety | Excellent. Low or zero VOC (Volatile Organic Compounds), Non-flammable, Low odor, and non-toxic. | Good quality. After improvement, Low-VOC formulation. |
| Mold Residue & Cleaning | Low residue. Decomposed products are often inorganic, mold is easier to clean, Leading to longer cleaning cycles. | Low residue, Reduces carbon buildup and deposits, Prevents sludge formation, and minimizes the need for frequent mold cleaning. |
| Mold Corrosion | Minimal to no corrosion. | Minimal to no corrosion. |
| Release Performance | Uniform and long-lasting. Easily forms a semi-permanent, Even film, suitable for complex and precise molds. | Strong release force, Forms a thicker film, lower release cycles per application. |
| Drying Speed | Slightly Slowe. Highly affected by humidity and temperature; requires hot air drying or extended dwell time. | Fast. Solvents evaporate quickly, Resulting in rapid drying. |
| Effect on Product Surface | Low. Does not affect or only slightly affects subsequent processes like gluing, painting, or bonding of the PU product. | Low. Does not affect or only slightly affects subsequent processes like gluing, Painting, or bonding of the PU product. |
| Cost | Raw material cost is often higher, But due to higher release cycles and less mold maintenance, The overall comprehensive cost is low. | Raw material cost is low. |
More factories are switching to water-based PU release agents due to environmental regulations and cleaner production requirements.
4. How to Choose the Right PU Release Agent for Shoe Sole Molding
To ensure stable demolding and consistent product quality, consider the following factors:
4.1. Mold temperature：
For low-temperature molds, oil-based release agents are preferred. For medium- to high-temperature molds, water-based release agents are recommended.
4.2 PU hardness and density
Soft foams require higher slip and better temperature stability.
4.3. Surface gloss requirements
If a glossy finish is required, choose a low-residue formula.
4.4. Mold material
Steel, aluminum, and textured molds require different lubrication levels.
4.5. Production speed
High-speed cycles require fast drying and high-stability agents.
5. Best Practices — How to Use PU Release Agent Correctly
Proper application is just as important as choosing the right product.
5.1 Spray a thin, even layer：
Avoid buildup, which causes whitening and dull surfaces.
5.2 Maintain the correct distance (20–30 cm)：
Ensures uniform coverage without over-wetting.
5.3 Preheat the mold to stable temperature：
Cold molds drastically increase sticking problems.
5.4 Apply multiple light coats instead of one heavy coat：
Thin and uniform layers perform better under pressure. If the release agent is applied too thickly, it can be manually leveled using a cloth or spatula coated with the excess release agent to achieve an even layer.
5.5 Clean molds regularly：
Remove accumulated residue to keep surface texture consistent.
Following these guidelines significantly reduces surface defects and improves overall demolding performance.
6. Why Many PU Shoe Sole Manufacturers Choose Our Release Agent
Yunzhu New Materials provides mature PU release agent solutions widely used by Chinese manufacturing groups. Our formulas are designed specifically for shoe sole factories requiring stable, high-quality demolding at fast production speeds.
Key advantages include:
- Smooth, consistent demolding
- Prevents sticking, cracking, and whitening
- High temperature resistance
- Suitable for textured and complex molds
- No transfer to PU surfaces
- Cleaner molds with fewer shutdowns
- Formula proven in large-scale Chinese factories
Reliable demolding leads to higher product quality, better surface appearance, and improved production efficiency.
7. Conclusion — Stable Demolding Starts with the Right Release Agent
Demolding is one of the most critical stages in PU shoe sole production. Choosing the right release agent can dramatically reduce defects such as sticking, cracks, and surface whitening—while improving mold cleanliness and extending mold life.
If you are facing demolding problems or want to improve production stability, we can provide:
- Free samples
- Technical support
- Product selection guidance
- Custom solutions for different PU systems
Contact us today to get professional advice and recommendations for your PU shoe sole manufacturing needs.
