Rotational Moulding Troubleshooting Guide: How to Fix Common Rotomolding Problems

Rotational molding, or rotomolding, is one of the most flexible plastic manufacturing methods used today. It’s perfect for creating durable, seamless, and hollow parts—everything from tanks and containers to playground equipment and industrial components. But even the most advanced process can face challenges. 

In this guide, we’ll walk you through rotational molding troubleshooting—explaining how to spot, diagnose, and fix common rotomolding defects before they affect product quality or production efficiency. Whether you’re new to the process or a seasoned manufacturer, this comprehensive guide will help you optimize every stage of your roto moulding process. 

What Is Rotational Molding?

Before diving into troubleshooting, it’s important to understand what rotational molding is and how it works. 

Rotational molding is a simple yet powerful plastic forming process. It works by heating a mold filled with polymer resin powder. As the mold slowly spins on two axes, the resin melts and spreads evenly inside. After cooling, the part comes out smooth, seamless, and with walls that have the same thickness all around. 

Unlike injection molding or blow molding, rotomolding doesn’t use any external pressure. That’s why it works so well for making big, hollow, or complex parts — and it’s cheaper when it comes to tooling, too. But here’s the catch: the process really depends on getting the temperature, rotation speed, and mold quality just right. If those aren’t controlled properly, defects can easily show up. 

Why Rotational Molding Troubleshooting Matters

Even with advanced machinery, small mistakes in cycle timing, material handling, or mold maintenance can easily cause rotomolding defects like bubbles, thin walls, or rough surfaces. These issues not only waste material but also reduce product quality and affect customer satisfaction. 

Effective rotational molding troubleshooting plays a big role in keeping production smooth. It helps spot problems early, so manufacturers waste less material and save more energy. Plus, good troubleshooting improves part consistency and keeps dimensions accurate. It also helps molds and machines last longer. When that happens, downtime drops, production speeds up, and profits naturally increase. So yeah, that’s why troubleshooting really matters. Now, let’s take a closer look at the most common rotomolding problems — and how to fix each one step by step. 

Common Rotomolding Defects and How to Fix Them

Bubbles or Voids in the Part

Sometimes, you may find small air pockets or empty spaces inside the molded part. These are some of the most common rotomolding defects and are typically caused by trapped air or incomplete melting of the resin.

This problem often occurs due to insufficient heating time, a mold that’s too cool, moisture in the resin, or blocked mold vents. To fix it, try increasing the oven temperature or extending the heating cycle slightly. Make sure to pre-dry the resin before loading it, and regularly check mold vents to keep them clean and open. It’s also important to ensure that the mold rotates evenly during the heating phase.

Pro Tip: Use a moisture analyzer or preheat your resin storage area to prevent humidity issues that cause air pockets.

Thin or Uneven Wall Thickness

Uneven or thin walls are another major issue in the rotational molding process. When some parts of a product are too thin, it can weaken the structure and affect the overall look of the piece.

This defect usually comes from incorrect rotation speed, uneven heating or cooling, or poor mold design with sharp edges or deep cavities. To solve it, balance the rotation ratio between the primary and secondary axes. You can also use rotomolding simulation software to identify where resin distribution may be uneven. Redesigning sharp corners into smooth transitions can also help. Lastly, make sure your oven’s airflow is stable and uniform.

Remember: One of the main benefits of rotational molding is even wall thickness—achieving that requires proper rotation and precise temperature control.

Warping or Distortion After Cooling

If your finished part looks twisted or bent after cooling, warping may be the cause. This issue happens when certain sections cool faster than others, causing uneven shrinkage.

The common reasons include uneven cooling rates across the mold, removing the part too early, or inconsistent wall thickness. To fix this, allow enough time for even cooling before demolding. Using a controlled air or water cooling system helps achieve uniform results. Try to avoid rapid cooling unless your specific material requires it.

Tip: If warping occurs often, consider adjusting the mold material or adding insulation to help maintain a consistent cooling rate, especially for complex parts.

Surface Roughness or Pinholes

A rough surface or tiny pinholes on the product’s exterior often point to contamination or overheating. These rotomolding defects can make your part look unattractive and reduce its durability.

Usually, the problem comes from a dirty mold surface, overheated resin, or trapped moisture. To prevent this, clean the mold carefully after each cycle and keep resin storage containers tightly sealed. Adjust the oven temperature so the resin melts evenly without burning.

Insight: A stable oven temperature and a clean mold are essential for producing smooth, flawless rotomolded parts.

Incomplete Fusion or Under-Cured Resin

In some cases, you may notice that the inner layers of a product aren’t fully fused, or that resin particles remain visible. This happens when the resin doesn’t get enough heat to melt and blend properly.

The main causes include too little heating time, a low oven temperature, or using low-quality resin with uneven particle size. To fix this, slightly extend your heating duration or raise the temperature to ensure complete melting. Always use certified, high-quality polyethylene powder to get better results. It’s also helpful to check the oven’s temperature distribution using a thermal probe.

Flashing or Excess Material Around Edges

Flashing occurs when thin strips of excess material appear along the parting line of a mold. While a small amount is normal, too much flash means there’s a problem in your setup.

This defect usually happens due to worn or damaged mold seals, too much internal pressure, or misaligned mold halves. To correct it, replace old gaskets or seals and make sure the resin loading amount is accurate. Also, inspect and realign the mold to ensure both halves fit perfectly.

Quick Tip: A little flash is fine—it should be easy to trim. But if it happens often, it’s a sign your mold maintenance schedule needs attention.

How to Approach Rotational Molding Troubleshooting Step-by-Step

Step 1: Inspect the Mold

Check for signs of wear, corrosion, or residue buildup. Molds with poor surface finish or vent blockages often cause multiple rotomolding problems.

Step 2: Review the Process Parameters

Record your temperature curve, rotation speed, and cycle time. Compare them with recommended settings for your specific resin type.

Step 3: Examine the Material

Moisture, contamination, or poor-quality powder can all lead to rotomolding defects. Always use pre-dried, high-grade materials from reliable suppliers.

Step 4: Check Machine Calibration

Make sure your rotational molding machine is running with proper alignment, even heating zones, and accurate timing controls.

Step 5: Monitor Cooling and Demolding

Uneven cooling or premature demolding often causes part distortion. Ensure cooling systems are consistent and operators follow standard timing.

Best Practices to Prevent Rotomolding Defects

Preventing defects is always better than fixing them. Here are a few proactive rotational molding troubleshooting strategies:

• Regular Maintenance: Keep molds, vents, and clamps clean and aligned.
• Process Monitoring: Use sensors to record temperature and rotation data.
• Operator Training: Skilled operators identify early signs of issues.
• Material Handling: Store resins in dry, climate-controlled areas.
• Testing: Perform visual and dimensional checks after every batch.

Pro Insight: Investing in automated rotational molding equipment with smart temperature and motion control can reduce human error significantly.

Key Takeaways

• Understanding what rotational molding is helps identify root causes faster.
• The most common rotomolding defects include bubbles, warping, thin walls, and surface imperfections.
• Effective rotational molding troubleshooting combines process control, machine maintenance, and operator training.
• Partnering with a trusted equipment manufacturer like Benfan ensures long-term performance and efficiency.

Conclusion

Rotational molding troubleshooting doesn’t have to be complicated. With the right knowledge, consistent process control, and high-quality rotomolding machines, you can significantly reduce defects, increase yield, and deliver products that meet global standards.

If you’re looking to enhance your roto moulding process, contact Benfan today. Our experts are ready to help you identify the right equipment and techniques to make every part perfect.