How we eliminated the uneven curing issue with adhesive in in automotive interior production: A practical solution

Photo: Homa Appliances / Unsplash

Automotive interior production places high demands on the precision and quality of bonded joints. One of our customers faced a recurring issue—uneven curing of a two-component polyurethane adhesive, which led to assembly defects and complaints. After analyzing the process, we found that the key factor was not just the chemical formulation itself, but also the application and storage conditions of the raw materials. In this article, we share the specific steps that led to resolving the issue and improving production stability.

The Problem: Uneven Adhesive Curing and Its Impact on Production

In the automotive industry, the quality of adhesive bonds is crucial for the safety, durability, and aesthetics of interior components. During the production of one of our key components for a leading car manufacturer, we encountered a recurring issue: uneven curing of a two-component polyurethane adhesive. This defect manifested as local soft spots, reduced bond strength, and in some cases, visible surface deformations. The problem occurred irregularly, making it difficult to identify and resolve.

Analysis revealed several causes. First, fluctuating temperature and humidity in the production hall affected the curing rate. Second, insufficient mixing of the adhesive components led to local concentration differences in the catalyst. Third, variations in the thickness of the adhesive layer caused uneven heat distribution during curing. These factors resulted in production downtime, increased scrap rates, and the risk of complaints, threatening supplier relationships and the project's economics.

Diagnostics: How we identified the causes of uneven curing

To systematically address the problem, we first implemented detailed monitoring of the production process. Using thermal imaging cameras, we tracked the temperature distribution on bonded parts during curing and found that differences between individual zones reached up to 12 °C. We also analyzed adhesive samples using standard test methods to verify mixture homogeneity. The results confirmed that in some cases, the optimal ratio of components A and B was not achieved.

The next step was to verify the influence of the surrounding environment. Measurements of relative humidity and temperature in the hall showed that the values fluctuated within a range of 45–65% and 18–26 °C, which is too wide an interval for sensitive adhesive systems. It was also crucial to find that the time between adhesive application and the start of curing was not always consistent, leading to premature reaction initiation in some parts. These findings allowed us to focus on specific areas for improvement.

Photo: Lenny Kuhne / Unsplash

Solution: Process optimization and selection of suitable raw materials

Based on the diagnostics, we implemented several key measures. First, we adjusted the air conditioning in the production hall to maintain temperature and humidity within a narrow range of 22 ± 2 °C and 50 ± 5 %. We also introduced an automated adhesive dosing and mixing system, ensuring the precise ratio of components and their thorough mixing. To monitor the thickness of the bonded layer, we implemented a laser measuring device that alerts to deviations in real time.

A significant change was also the selection of a more suitable type of adhesive. We switched to a two-component polyurethane system with a slower but more uniform curing process, which is less sensitive to temperature and humidity fluctuations. This system also contains additives that stabilize reaction conditions, further reducing the risk of local defects. The adhesive change was carried out in compliance with REACH requirements and the customer’s internal specifications to avoid jeopardizing the certifications of the final product.

Results: Improved Quality and Production Efficiency

After implementing the changes, we observed an immediate improvement in the quality of bonded joints. The number of defects decreased by over 80 %, and the strength of the joints increased by an average of 15 %, as verified by standard tensile tests. Uniform curing also eliminated surface deformations, improving the aesthetics of the final parts and reducing the need for additional adjustments. The stabilization of the production process led to time and material savings, which had a positive impact on costs.

We have achieved long-term reliability in deliveries and strengthened customer confidence in our solutions. This project also served as a best-practice example for other production lines, where we applied similar principles to other adhesive systems. The key to success was systematic diagnostic approaches, collaboration with raw material suppliers, and a willingness to invest in technological improvements, which quickly paid off in the form of higher efficiency and customer satisfaction.

Photo: Simon Kadula / Unsplash

Implementation of Changes: Practical Steps and Process Validation

After identifying the key causes of uneven curing, we proceeded with the systematic implementation of the proposed solutions. The first step was adjusting the adhesive application technology. We introduced an automated dosing system with precise flow control, ensuring consistent adhesive application at a rate of 120–150 g/m², with a maximum deviation of ±5 %. Simultaneously, we optimized the temperature profile of the drying line: the temperature was gradually increased from 60 °C to 90 °C across five zones, with a total exposure time of 15 minutes. This profile was established based on thermogravimetric analysis (TGA) of the adhesive used, which showed the optimal temperature window for uniform polymerization.

A critical point was the validation of the new process. We conducted a series of tests on samples measuring 300 × 200 mm, monitoring the degree of curing using a Shore durometer and tensile tests according to standard methods. The results confirmed that 98 % of the samples achieved the required tensile strength (minimum 8 MPa) and uniform curing across the entire surface. For final verification, we produced a pilot batch of 500 parts, monitoring both mechanical properties and visual defects. Quality deviations were reduced to less than 1 %, which was below the acceptable threshold for serial production.

Team Training and Problem Recurrence Prevention

Technological changes alone are not enough—proper operation and equipment maintenance are also key. We conducted comprehensive training for the production team, focusing on three main areas: correct setup of the application technology, monitoring of drying line parameters, and visual inspection of parts. Operators were trained to identify common defects, such as local under-curing (manifested by surface stickiness) or, conversely, over-curing (indicated by a change in adhesive color). We also introduced control checklists that must be completed during each shift, including measurements of temperature, humidity in the production hall, and exposure time in the drying line.

For long-term prevention, we have implemented a system of regular process audits. Every two weeks, we perform control measurements of the strength of bonded joints on randomly selected parts, and once a month, we conduct a complete review of the application technology. Data is recorded in a central system, which allows us to monitor trends and promptly identify deviations. Thanks to this approach, we managed to reduce the number of complaints due to faulty bonding by 85% within the first six months after implementing the changes.

Economic and Environmental Benefits of Optimization

Process optimization has not only improved quality but also brought significant economic savings. By reducing the number of defective parts, we achieved material savings of approximately 12 tons of adhesive per year, which, at a price of 45–60 CZK/kg, represents savings in the order of hundreds of thousands of crowns. Additionally, energy consumption in the drying line was reduced by 18%, as the optimized temperature profile requires a shorter heating time. These savings were achieved without the need for investments in new equipment—it was sufficient to use the existing technology more efficiently.

From an environmental perspective, the solution has a positive impact on waste reduction. By reducing the number of defective parts, the volume of waste designated for recycling or disposal has decreased. Furthermore, we switched to an adhesive with a lower content of volatile organic compounds (VOC), which contributed to improving the working environment and reducing emissions. This step was in line with REACH regulations and helped meet the company’s internal sustainability goals. Overall, process optimization has delivered a synergistic effect—higher quality, lower costs, and a smaller ecological footprint.

Need a Custom Solution?

Every production process has its specific requirements. GCG Group supplies not only high-quality additives and adhesives for the automotive industry but also technical support, including process analysis and optimization recommendations. Detailed technical data sheets and safety data sheets are available for every raw material. Contact us – or browse our catalog of over 1,300 products.