Rubber shrinkage is a critical phenomenon encountered in the manufacturing of rubber products. Understanding rubber shrinkage is essential for manufacturers across various industries. Precise control and management of shrinkage are crucial to ensure that rubber components meet design specifications and quality standards. As a professional rubber molding manufacturer, we can provide tailored solutions for custom molded parts. In this post, we will delve into the intricacies of rubber shrinkage, exploring its causes, effects, and solutions.
What is Rubber Shrinkage?
Rubber shrinkage refers to the phenomenon where rubber materials undergo a reduction in size or volume during the manufacturing process. This reduction occurs primarily during the cooling and curing stages of rubber molding, leading to changes in the dimensions of the final product. During the manufacturing process, rubber materials undergo various physical and chemical transformations. When rubber is heated and then cooled, it undergoes a phase change from a more fluid-like state to a solid form. As the rubber cools, molecular rearrangements occur, resulting in a reduction in volume. This reduction is known as rubber shrinkage.
Factors Contributing to Rubber Shrinkage
Rubber shrinkage is influenced by several factors related to the rubber compound formulation and the molding process parameters.
Rubber Compound Factors
Polymer Type: Different rubber polymers exhibit varying shrinkage rates due to their molecular structures. Generally, silicones and fluoroelastomers have higher shrinkage rates compared to nitrile rubbers (NBR) or EPDM.
Filler Type and Loading: The type and amount of filler in the rubber compound impact shrinkage. Compounds with higher filler loadings tend to have lower shrinkage rates. Carbon black and silica-filled compounds show lower shrinkage than calcium carbonate or clay-filled compounds.
Other additives：Other additives such as plasticizers increase elongation but also raise shrinkage. Stabilizers (like antioxidants and antiozonants) can reduce shrinkage by increasing crosslinking density. As for colorants, they generally have minimal effect on shrinkage.
Cure System: The cure system used (sulfur, peroxide, etc.) affects shrinkage, with peroxide-cured compounds often exhibiting higher shrink rates than sulfur-cured ones.
Durometer/Hardness: Softer rubber compounds with lower durometer values generally have higher shrinkage rates compared to harder compounds.
Molding Process Factors
Mold design: The design of the mold used in the manufacturing process can influence how the rubber material contracts during cooling.
Molding Process: In the molding process, injection molding is faster, resulting in more shrinkage, while extrusion molding is slower and leads to less shrinkage. Additionally, rapid cooling increases shrinkage.
Cure Time and Temperature: Higher cure temperatures and longer cure times lead to increased crosslink density, reducing the shrinkage rate.
Injection/Mold Pressure: Higher injection pressures during molding can result in increased shrinkage rates.
Mold Temperature: Significant differences between the rubber compound temperature and mold temperature can cause higher shrinkage.
Internal stresses: Residual stresses within the rubber material can contribute to shrinkage over time, especially in complex or thick-walled components.
Environmental Factors
Temperature: High temperatures accelerate the expansion and shrinkage of rubber, especially during vulcanization. Different types of rubber have varying sensitivity to temperature changes.
Humidity: Humidity has minimal impact on the shrinkage of most rubbers, but rubber containing moisture-absorbing fillers (such as silica) may absorb moisture and expand. Excessive moisture can soften the rubber, affecting dimensional stability.
Other Factors
Geometry: Thin parts tend to shrink more than thicker, more solid parts.
Inserts: Inserts (metal, fabric, etc.) can affect shrinkage. The bond between rubber and insert limits shrinkage in some areas while other parts may shrink more.
Post-Processing: Post-curing, demolding, and cooling can affect shrinkage. Even trimming or finishing operations can cause slight dimensional changes.
Consequences of Rubber Shrinkage
Rubber shrinkage can have significant effects on the quality and dimensional accuracy of molded rubber products. The key impacts of rubber shrinkage on product quality are:
Dimensional Inaccuracies
Rubber shrinkage causes the final part dimensions to be smaller than the mold cavity dimensions, leading to parts being out of specified tolerances. This can affect critical dimensions like sealing surfaces, interference fits, or precise geometries required for proper function.
Warpage and Distortion
Uneven shrinkage rates across different sections of a part can lead to warpage, twisting, or distortion of the final shape. This is especially problematic in complex geometries or parts with inserts/reinforcements that restrict shrinkage in localized areas.
Surface Defects
High shrinkage can cause surface sink marks, voids, or other cosmetic defects on the part surface due to uneven material flow during molding. This impacts the visual appearance and can affect functional properties like sealing ability.
Inconsistent Quality
Variations in shrinkage rate due to changes in compound formulation, cure conditions, or other process parameters can lead to inconsistent part dimensions across different production batches. This makes it difficult to maintain tight tolerances and consistent quality.
Impact on Mechanical Properties:
Rubber shrinkage can influence the mechanical properties of the final product, including its strength, flexibility, and resilience. Changes in dimensions and internal stresses resulting from shrinkage may alter the performance characteristics of rubber components, affecting their ability to withstand mechanical loads, environmental conditions, or service life requirements. Inadequate management of shrinkage can lead to inconsistencies in product performance and durability, potentially compromising product quality and reliability.
Solutions to Minimize Rubber Shrinkage
There are several common methods used to reduce rubber shrinkage during the molding process:
Select lower Shrinkage Rates Rubber Materials
- Choose rubber polymers with inherently lower shrinkage rates like nitrile (NBR) or EPDM instead of silicones or fluoroelastomers (FKM). The EPDM rubber shrinkage percentage is usually low, especially during the vulcanization process.
- Use harder durometer/higher stiffness rubber compounds which exhibit lower shrinkage compared to softer compounds.
- Opt for sulfur cure systems over peroxide cures as peroxide-cured compounds tend to have higher shrink rates.
- Increase filler loading, especially with carbon black or silica fillers, to restrict shrinkage.
Optimize Manufacturing Process
- Control cure time and temperature precisely to achieve optimal crosslink density without overcuring, which increases shrinkage.
- Maintain mold temperatures close to the rubber compound temperature to minimize thermal gradients causing shrinkage.
- Use lower injection/mold pressures during molding to reduce orientation effects that can increase shrinkage.
- Monitor and adjust process parameters in real-time.
- For silicone rubber, cure at lower temperatures (20-30°C) if possible to limit shrinkage.
Consider Shrinkage When Designing
- Account for expected shrinkage by intentionally oversizing mold cavities based on measured shrink rates for the specific rubber compound.
- Avoid complex geometries with thick sections or inserts that can restrict uniform shrinkage and cause warpage.
- For large silicone molds, incorporate fiberglass reinforcement layers to restrict shrinkage.
Post-Processing to Minimize Rubber Products Shrinkage
- Perform post-cure heat treatments, when applicable, to stabilize dimensions by driving off volatiles and promoting additional crosslinking.
- For silicone molds, promptly demold products that release heat during curing to prevent mold damage from shrinkage.
- Silicone rubber is a widely used high-performance material, but silicone rubber shrinkage cannot be overlooked. Proper storage of silicone molds with the master model inserted can help maintain dimensional stability over time.
Conclusion: Ensure Quality by Managing Rubber Shrinkage
Controlling rubber shrinkage is essential for producing durable, high-quality rubber components. By addressing this challenge early, manufacturers can save costs and deliver better results.
That’s where we are. With a focus on quality, consistency, and engineering support, we’re the trusted partner for companies seeking reliable rubber solutions.
Explore our quality control or contact us today to discuss how we can support your project.
