Problems And Solutions Related To Planar Deformation In Rotational Forming.

I. Root Causes of Large-Surface Deformation
Rotomolding is a cooling process, with deformation primarily occurring during the cooling phase. Its core physical principle is uneven shrinkage.

Uneven Cooling Rate:

This is the primary cause. When natural cooling or improper airflow settings occur, the cooling rate of the large-surface area of ​​the mold differs from that of corners, edges, and ribs.

Principle: Thicker sections (such as corners) or sections that dissipate heat quickly (such as near metal inserts) cool slowly, while the central area of ​​the large-surface area cools quickly. The sections that cool first solidify and shrink first, creating tensile stress on the sections that cool later. When the later-cooled sections (such as corners) finally shrink, they pull inwards on the already solidified large-surface area, causing it to cave in.

Internal Stress Accumulation:

During the heating and cooling cycle, the plastic molecular chains are stretched and rearranged. If cooling is too rapid, the molecular chains are "frozen" in a non-equilibrium state, and internal stress cannot be fully released. After demolding, these residual stresses gradually release, leading to warping and deformation.

Structural Design Defects:

Excessively Large and Insufficiently Supported Planar Surfaces: A large, flat surface is like a thin sheet of paper, lacking sufficient rigidity to resist shrinkage stress.

Uneven Wall Thickness: If mold design or manufacturing results in excessive differences in product wall thickness, the interface between thick and thin sections becomes a stress concentration point and the starting point of deformation.

Inappropriate Demolding Angle: An excessively small demolding angle causes the product to tightly wrap around the mold core during cooling and shrinkage, resulting in additional stretching and deformation during demolding.

Inappropriate Process Parameters:

Mismatched Heating/Cooling Times: Insufficient or excessive heating, or improperly set cooling procedures (such as the time and sequence of air cooling and water cooling).

Inappropriate Rotation Speed: An inappropriate speed ratio between the main and auxiliary shafts can lead to uneven powder distribution and inconsistent wall thickness in large planar areas.

Mold Design Issues:

Uneven Thermal Conductivity of Mold Material: For example, the mold itself may have hot or cold spots.

Insufficient Mold Structural Rigidity: Micro-deformations of the mold itself during high temperatures and cooling will be directly replicated on the product.

II. Systematic Solutions Solving large planar deformation requires a systematic approach, with each step interconnected from product design and mold design to process adjustments.

1. Product Structure Design (The Most Effective and Fundamental Solution)

This is the preferred and best approach, increasing rigidity from the source to counteract shrinkage stress.

Adding Reinforcing Ribs:

This is the ultimate method for dealing with large planar deformation. Ribs not only significantly increase rigidity, but they also shrink upon cooling, thus "holding" the plane and counteracting its shrinkage tendency.

Design Considerations:

The height of the rib should be 2-3 times the wall thickness.

The root width of the rib should be approximately 0.5-1 times the wall thickness.

The top of the rib should be rounded to avoid stress concentration.

All rib roots must have rounded corners to ensure smooth powder flow and strength.

The rib layout should be symmetrical and uniform to avoid creating new unbalanced stresses.

Designing an Arched or Wavy Shape:

Changing the flat surface to a slightly arched curve (like the arch of a bridge) utilizes structural mechanics principles to distribute stress evenly, significantly improving resistance to deformation. This is a more advanced solution than stiffeners.

Designing a Flanged Edge:

Design a vertical or angled edge (flanged edge) at the edge of the large flat surface, forming an "L" or "U" shaped cross-section. This is like adding an edge to a piece of paper, resulting in an exponential increase in bending stiffness.

Ensuring Uniform Wall Thickness:

Avoid sudden changes in wall thickness in the design, ensuring a smooth transition.

2. Mold Design Optimization

Optimizing the Cooling System:

Airflow Guidance: Use ducts, hoods, etc., to ensure cooling air is evenly distributed across the entire large flat surface, rather than just blowing onto a single point. Avoid direct, concentrated airflow impacting the center of the surface.

Segmented Cooling: First, slowly cool the back of the mold and non-cavity surfaces with water mist or a gentle breeze, allowing the product time to relax stress within the mold, before forcibly cooling the large flat surface.

Adjusting Mold Material: For particularly difficult-to-machine flat surfaces, consider adding heat-conducting fins to the back of the mold to balance heat dissipation throughout the mold.

Ensuring Mold Rigidity: The mold must have sufficient thickness and support to prevent deformation during processing.

Appropriate Demolding Angle: Ensure a sufficient demolding angle (generally 1°-3° is recommended for both inner and outer walls) to reduce demolding resistance.

3. Molding Process Adjustment: Once the design and mold are fixed, improvements can only be made through fine-tuning the process.

Adjusting the Cooling Program (Most Critical):

Delayed Cooling: After heating, do not immediately activate strong cooling. Allow the mold to rotate slowly in the air for a period of time ("smothering") to ensure more uniform temperature distribution inside and outside the product and release some stress.

Segmented Cooling: First, use low-volume cold air for slow cooling for a period of time, then turn on full air volume and water spray for rapid cooling. The key is to cool thick-walled areas (such as corners) and thin-walled areas (such as flat surfaces) as synchronously as possible.

Asymmetrical Cooling: If one side of the product has reinforcing ribs while the other side is flat, try cooling the ribbed side first.

Optimize Heating Cycle:

Ensure sufficient heating time and appropriate temperature to ensure complete powder melting and good plasticization, forming a dense structure and reducing stress caused by internal defects.

Adjust Rotation Speed:

Fine-tune the speed ratio of the main and auxiliary shafts to ensure uniform powder deposition over large planar areas, achieving the most consistent wall thickness possible.

4. Post-Processing Methods
Shaping Fixture:

This is the last resort. Create a shaping frame (jig) that matches the ideal shape of the product. After demolding, place or fix the product on the shaping frame while still hot until it has completely cooled to room temperature. This method effectively corrects deformation but increases labor and cost.

Summary and Recommendations
To solve the problem of large planar deformation in rotational molding, the following approach should be followed:

First, design solutions: Introduce reinforcing ribs, arched structures, or flanges during the product design phase. This is the most economical and effective method.

Second, mold optimization: Design reasonable cooling channels to ensure mold rigidity and uniform heat conduction.

Then, fine-tune the process: focus on adjusting the cooling program, employing strategies such as delayed, segmented, and asymmetrical cooling to balance shrinkage.

Finally, post-processing remediation: when unavoidable, use shaping fixtures to maintain the product's shape.

In actual production, these methods are usually combined, and the optimal solution for a specific product is found through repeated trials. Remember, "prevention is better than cure"; considering and mitigating the risk of deformation from the initial product design stage is the most effective approach.