How to prevent the rotomoulded products from shrinkage and deformation?

Shrinkage and deformation are common seen problems to motomoulded products. Solutions to problems encountered will vary case by case. Most problem can be easily solved with a slight adjustment in operating procedures, or by choosing the right resin. Following are some general guidelines.

Mold cooling methods

Mold is usually cooled by fan while still rotating.This gradually lowers the temperature. The best way is to cool it down by air under lower density. The cooling rate must be kept within a certain range. Very rapid cooling (for example, water spray) would result in deforming and shrinking at an uncontrolled rate, producing a warped part.

Demoulding temperature

The longer the mold is cooled, the less shrinkage rate. On the premise of ensuring easy removal of the part., The lower the demolding temperature the better, so that the risk of shrinkage deformation can be minimized. Once the rotomoulded products are removed from the molds, they can not be transferred to other place with temperature difference, and it is best to immediately add the padding to keep the product in shape.

Proper mold rotation

Adjust ratio and speed of the rotating mold to obtain even coverage and adequate number of powder tracking, making the rotomoulded product with uniform wall thickness.

Redesign the mold

Review and change mold design if necessary, eliminating sharp corners and narrow passages, that may cause shrinkage and deformation

Resin shrinkage ratio

Use proper resin having adequate melt index and molecular weight distribution for application. For example, some rotomoulded products require precise size for the assembly. It is better to choose the material with lower shrinkage ratio

Pigments, additives and mold release agents

Select the proper and right amount of pigments, additives and mold release agents that don’t affect the shrinkage.

Modify the design

Modify the design an avoid large, flat panels in part design as possible as it could be.

Heating process

The rotomoulded products should be rotated through two or more axes, rotating at balanced and even speeds in order to avoid the accumulation of resin powder. The length of time the mold spends in the oven is critical: in case too long and the resin will degrade, reducing impact strength. If the mold spends too little time in the oven, the resin may not be melt completely. The resin power will not have time to fully melt and adhere to the mold wall, which results in large bubbles and deformation. This affects the mechanical properties of the finished product.

Adequate venting

During the first stages of cooling, there will be a rush of air into the rotomoulded products to fill the resultant partial vacuum. If there is inadequate venting, air will enter molten resin at the parting line and become stuck as the part wall solidifies. It this case, it need to vent the mold to atmosphere pressure, use the proper vent material and increase the vent size.

For more solutions about the problems to rotomoulded products in production, please leave a message or submit the inquiry form, the professional team will get back to you in the due course.