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Introduction: The Challenge of Shrinkage in High-Voltage Component Manufacturing
When working with resin and urethane systems for encapsulating or casting high-voltage components, shrinkage is a critical factor that can significantly impact the performance, safety, and reliability of the final product. In the context of electrical insulation and protection, shrinkage isn’t merely a cosmetic or dimensional concern — it can lead to voids, cracks, and compromised dielectric strength, jeopardizing the integrity of the entire system.
In this article, we’ll unpack what shrinkage is in resin and urethane systems, why it occurs, and how to mitigate its effects to ensure robust, high-performing encapsulation solutions.
What is Shrinkage?
In resin and urethane casting, shrinkage refers to the reduction in volume that occurs during and after the curing process. As the liquid resin transitions to a solid state, it undergoes chemical and physical changes that can lead to volume contraction.
This volumetric reduction can create internal stresses, voids, or cracks, ultimately affecting:
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Electrical Insulation: Gaps or voids can reduce dielectric performance and lead to partial discharge or breakdown.
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Mechanical Integrity: Cracks or stress points may propagate under thermal or mechanical cycling.
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Dimensional Stability: Critical dimensions may shift, affecting the component’s fit and function.
Why Does Shrinkage Occur?
Shrinkage in resin and urethane systems is primarily driven by two factors: chemical reactions and physical changes.
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Polymerization and Crosslinking
During curing, monomers and oligomers chemically react to form long, crosslinked polymer chains. This network formation often results in a denser molecular structure, causing the resin to contract as the system transitions from liquid to solid.
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Evaporation of Solvents or Volatiles
If the formulation includes solvents or other volatile components, these may evaporate during the curing process, contributing to additional volume loss.
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Exothermic Reactions and Thermal Effects
Exothermic curing reactions cause localized heating, which can accelerate polymerization and further enhance shrinkage. Additionally, differences in the coefficient of thermal expansion (CTE) between the resin and encapsulated components can create residual stresses as the system cools.
How Do We Eliminate or Mitigate Shrinkage?
Completely eliminating shrinkage in resin and urethane systems is often unrealistic due to the inherent nature of polymerisation. However, several techniques and best practices can minimize shrinkage and its negative effects:
1.
Material Formulation and Selection
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Low-Shrinkage Resins: Choose formulations designed for low volumetric shrinkage, often incorporating fillers or reactive diluents to reduce density changes.
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Reactive Fillers: Incorporating mineral fillers (e.g., silica, alumina) can reduce shrinkage by occupying space and mitigating polymer chain contraction.
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Optimised Processing Parameters
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Controlled Curing Profiles: Implement carefully ramped temperature cycles to manage exothermic reactions and prevent premature crosslinking. Gradual curing helps reduce internal stresses and uneven shrinkage.
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Vacuum Degassing: Always degas resin prior to casting to eliminate entrapped air and volatiles, which can expand or contract during curing and cause dimensional inconsistencies.
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Thermal Equilibrium: Uniform Cooling Before Cure
Pre-Cure Temperature Stabilisation: One of the most overlooked contributors to shrinkage is thermal imbalance during the curing process. When encapsulating electronic assemblies or large components such as transformers, it is essential that the entire assembly reaches thermal equilibrium before curing begins.
In practical terms, this means ensuring that the internal components — whether they are heat sinks, windings, or inserts — have cooled to match the ambient temperature of the resin system. If the internal mass remains significantly warmer, it can create localised hot spots during cure, accelerating crosslinking in those areas and initiating premature polymerisation. This differential curing leads to stress concentrations and uneven shrinkage throughout the structure.
Proper pre-cure cooling not only improves cure uniformity but also enhances the dimensional stability and dielectric integrity of the finished part.
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Design Considerations
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Mould and Tooling Design: Anticipate and compensate for shrinkage during design by building in dimensional tolerances or using flexible tooling where applicable.
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Anchoring Inserts and Fixtures: Use precision jigs and mechanical supports to maintain alignment during encapsulation, especially when dealing with heavy cores or embedded hardware.
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Post-Curing Treatments
Stress Relief via Post-Cure Cycles: Implement a post-cure stage to gradually bring the component to full crosslink density, relieving residual stresses and improving thermal and mechanical performance over time.
Conclusion: Stop Letting Shrinkage Sabotage Your Projects
If you’re reading this, chances are you’re already fighting the silent war that shrinkage wages on your encapsulation process. You’ve seen the voids, the stress cracks, the breakdowns — and you know how costly a single failed cast can be.
The good news? You’re not alone. And you don’t have to solve this alone.
By applying the practices outlined in this article — from resin selection and thermal control to careful cure management — you can get remarkably close to a shrinkage-free process. But if you’ve already spent too much time, money, and energy chasing elusive consistency, maybe it’s time to take a smarter route.
Let us take the pressure off. At PLEXNETIC, we live and breathe this stuff — solving complex encapsulation challenges is what we do. Whether you need material guidance, process engineering, or a complete turnkey solution, we’re ready to help you get it right the first time.
Don’t let shrinkage hold your projects back. Reach out today, and let’s solve this together.
For more information on Shrinkage in Resin and Urethane Encapsulation: A Technical Overview talk to PLEXNETIC LTD