In our last blog, the team at Resin Designs walked you through how to troubleshoot voids in urethane adhesives and encapsulants. This week, let’s explore how to properly troubleshoot shrinkage.
How Shrinkage Occurs
Shrinkage takes place in elastomers when polymers and catalysts react. Under normal processing conditions, shrinkage rates remain at 1-2%. Warping will occur with excessive shrinking which can lead to cracking – these cracks can grow into large surface cracks.
Causes of Shrinkage
Mismatched Resin & Mold Temperatures
The urethane reaction is exothermic which means once the material mixture starts to react the blend will heat up. Differences between the mixture and mold temperature will cause the material to cure at difference speeds depending on which interface the material is touching. This dissimilar cure speed leads stresses on the material. As the mixture changes from liquid to solid, it’s crucial that the two temperatures remain in balance. Gelling reactant will shrink if the mold temperatures are too low, often leading to cracking. When the temperatures are too high, there can be warping.
Localized Temperature Disparities
Internal stresses can occur due to localized hot and cold spots in the mold. This often leads to shrinkage, cracking and warping.
Exotherm & Mold Temperatures Should Be Balanced
Peak exotherms are the highest reaction temperatures reached by mixtures and each polyurethane system has a different peak exotherm. Mixtures of a larger mass typically have a higher peak exotherm than smaller masses. The goal is to have the mold temperatures balanced within ±5°C of the peak exotherm.
Thoroughly Heat the Mold
Air in the oven must circulate so that the temperature is consistent throughout. Make sure that the mold remains in the oven until there is a uniform temperature.
Keep an Eye on Raw Material Temperatures
When degassed and mixed, raw materials tend to lose heat. They can also lose heat when sitting before mixing which means they need to be reheated so the correct temperature is maintained. Cracking can occur if the temperature is too high. It’s advised to use lower system temperatures and high mold temperatures.
If you need custom urethane adhesives and encapsulants, contact Resin Design at 781.935.3133. Our elite team of Polymer Scientists have years of experience designing and developing adhesives and encapsulants for a wide number of industries including aerospace, automotive and electronics. Entex 52101 and Entex 53060 are both made with aliphatic isocyanate backbones. Urethanes made from this chemistry are slower to cure, and therefore reach lower peak exothermic temperatures during cure which minimizes shrinkage. Additional benefits of using urethanes based on aliphatic structures are clear final products and UV stability.