In this article we’ll examine ramp rate and how determining the correct rate is key to achieving optimal resin flow and effective fiber impregnation.
- Ramp rate controls how fast resin flows around and saturates the fibers, and the total cure time
- The Premier ESR supports ramp rates from 0.2 °C to 50 °C per minute, with forced-air cooling up to 30 °C per minute
- Alpha tested 1, 2.5, 5, 10 and 20 °C per minute; time to full cure temperature ranged from roughly 30 to 120 minutes
- 5 °C per minute gave the best storage modulus and shorter initiation and termination intervals
Alpha Technologies evaluated the influence of processing parameters on the rheological behavior and mechanical properties of prepreg composites using an Encapsulated Sample Rheometer (ESR). The parameters included cure temperature, ramp rate and out-time.
This is the second in a three-article series exploring the results of this study and the role of Alpha’s Premier ESR® in advancing a company’s process optimization goals for their prepreg materials. To learn more about the other key processing parameters we studied, download the full study.
What Is Ramp Rate?
Ramp rate is the rate at which the temperature is increased during the prepreg material curing process. The ramp rate determines how long it takes for the curing chamber to reach full curing temperature, which in turn determines the total duration of curing time.
Ramp rate is especially important in prepreg production because of how prepregs are constructed. Ramp rate affects the rate at which resin flows around and saturates the reinforcing fibers. Achieving the correct ramp rate is key to achieving optimal resin flow and effective fiber impregnation. These two parameters in turn determine both the mechanical integrity and void content of the cured prepreg material.
Premier ESR thermal control
Heating ramp rates
Forced-air cooling
The Premier ESR from Alpha Technologies is equipped with a digitally controlled thermal system, giving users tremendous control over processing temperatures. This precise control allows for more accurate simulation of different production processes and more valuable data for making confident decisions.
How Ramp Rate Impacts Prepreg Final Properties
For purposes of the study, Alpha Technologies tested ramp rates of 1 °C, 2.5 °C, 5 °C, 10 °C, and 20 °C per minute for prepreg materials. The time required to reach full curing temperature ranged from roughly 30 to 120 minutes.
Fast ramp rate
Faster ramp rates do not give the resin adequate time for optimal flow and impregnation. This can lead to uneven resin distribution, increased dry spots and voids, and incomplete curing.
Slow ramp rate
A slower ramp rate gives the resin more time to flow and thoroughly wet the fibers, reducing voids, dry spots, and other problem areas. This enhances mechanical properties and overall quality.
However, the slowest possible ramp rate is not always the best option. During this particular experiment, Alpha found that a ramp rate of 5 °C per minute delivered the smooth resin flow needed for optimal mechanical performance, as well as the best storage modulus at the end of the cure. It also resulted in shorter initiation and termination intervals.
Pinpointing the ideal ramp rate for a given prepreg material is key to optimizing efficiency in production and achieving superior quality in the final product.
Troubleshooting Prepreg Failures with the ESR
For a detailed study of common prepreg cure failure modes and how to troubleshoot issues with rheological measurements, download our white paper.