The occurrence of polyurethane foam defects during the foaming process can be attributed to various factors, including both chemical and mechanical aspects. The foam defects, general causes, and the solutions are summarized as follows:
Foam Cracking
Cause: During the late stage of foaming, the rate of foam formation exceeds the gelation rate.
Solution: Increase the use of tin catalysts to expedite the gelation rate.
Transverse Cracking in the Middle of Foam
Cause: Uneven distribution of materials leading to overlapping in some areas.
Solution: Ensure the even distribution of liquid materials and maintain proper alignment of the conveying paper mold.
Transverse Cracking on the Surface of Foam
Cause: Inappropriate conveyor belt speed causing a narrow white region and severe material overlap.
Solution: Adjust the conveyor belt speed as needed to address the white region and reduce material overlap.
Longitudinal Cracking Inside the Foam
Cause:
1.Variations in the foaming rate before and after, causing the foam to roll paper.
2. Leakage of amine catalyst in the pipeline, leading to excessive local catalyst within the foam.
Solution:
1. Adjust the foam progression rate to ensure consistency.
2. Regularly inspect all material pipelines to prevent leakage.
Foam Shrinkage and Closed-Cell Formation
Cause:
1. Low stirring speed.
2. Excessive use of tin catalyst.
3 Insufficient air intake (especially when material temperature is low).
Solution:
1. Increase stirring speed.
2. Reduce the amount of tin catalyst.
3. Increase the airflow to enhance cell formation.
Grooving in the Foam Body
Cause:
1. Excessive use of amine catalyst, high material temperatures, and excessive heat generation during the reaction.
2. Premature stacking of the foam body after foaming, preventing efficient heat dissipation.
Solution:
1. Adjust the formulation, reduce amine catalyst, and modify temperature settings.
2. Allow the foam to mature without stacking for at least 2 hours after preheating.
Stripe Formation
Cause:
1. Material whitening occurs too quickly.
2. The white region is too narrow, with no overlap before and after whitening.
3. Discharge speed is too fast, causing excessive air bubble formation.
Solution:
1. Identify and address the causes of premature whitening (such as excessive catalyst or high temperature), and adjust the white region to ensure partial material overlap.
2. Adjust the conveyor belt speed, increase stirring speed, and inspect the mixing head installation.
Macroscopic Pores
Cause:
1. Inadequate mixing, leading to uneven dispersion of the introduced air.
2. Precipitation of pigments.
3. Excessive discharge speed with excessive air entrainment.
Solution:
1. Improve the mixing efficiency of the mixing head and increase stirring speed.
2. Increase the ball milling time for pigment liquids.
3. Adjust the mixing head speed and blade angle.
Foam Body Fails to Form
Cause:
1. Material temperature is too low.
2. Catalyst is ineffective.
3. Polyether quality is subpar, such as excessive acidity.
Solution:
1. Maintain the temperature above 20°C.
2. Check the catalyst’s activity.
3. If issues persist, stop the process and conduct a thorough inspection.
