Determining Rotomolded Part Cure

At Entec Polymers we believe that it is critical for our customers to understand how to properly process plastic materials to achieve the best physical and mechanical properties. Because part cure can affect many different properties, including cost, scrap, and field failures, it is vitally important that parts are properly cured. A properly cured part will provide you with the best physical and mechanical properties and these properties will correlate most closely to the properties found on the materials technical data sheets.

Below are some examples of how to determine if you have processed the material properly and reached the proper cure:

Visual ways of determining part cure:

1. Look for a smooth inner surface on the inside of your rotomolded part. This indicates a proper cure.
2. A smooth inner surface should not be too shiny, otherwise this could indicate over-cure.
3. Lumpy, wavey, or rough inside surfaces are indicative of under-cure.
4. Cut a section of the part and using a utility knife slice a thin piece off and visually examine the cross section looking for bubbles inside of it. Too many bubbles indicate an under-cure condition while no bubbles can indicate over-cure. For a ten-inch section of a part you want to see no more than 4 or 5 bubbles located close to the inside part surface. If the bubbles are in the middle of the wall thickness or toward the outside surface of the part, then you are under-cured. You may need to use a handheld magnifying glass to better see the bubbles. It is our experience that seeing a few bubbles on the very inside surface indicates that you are not over cured.

Technical ways of determining part cure based on PIAT and Density:

1. Purchase temperature strips to place on a nylon zip tie down through the vent tube to record the maximum temperature inside the mold during the process. Wire the zip tie to the vent tube to hold it in place during the molding process. Ensure that the temperature strip is located about 2-3 inches outside of the vent tube on the inside of the mold cavity in order for it to work properly. Note that Wahl temperature strips will not give you the exact temperature as they typically read in 10-degree increments. You should also know the recommended PIAT of the material being molded.
2. Use a data logger temperature recording device to understand the exact peak internal air temperature during the process. You should also know the recommended PIAT of the material being molded.
3. Send molded samples to a lab and check the as-molded density of the part. If the density results are within the specification range recommended by the supplier, then the material has been processed correctly and is properly cured.

If you have any questions or would like to have a more detailed discussion on part cure, please reach out to your Entec Account Manager. Entec Technical Service is available to assist you with your roto-molding questions or needs.