Getting the proper cure for a composite application is important to meet performance requirements, such as longevity, corrosion and fatigue resistance, low odor and more. Simply put, curing refers to hardening polymer materials by cross-linking the polymer chains. So how do you achieve the proper degree of cure, which can range from less than 90 percent to almost full cure at 99.9 percent? I like to think of it as getting the proper amount of energy into the part while it is being made.
Energy comes from the resin itself – the temperature of the materials used, the mold and the exotherm of the part – as well as ambient conditions during production and after making the part, like the post cure. The final source of energy derives from the peroxides used and the levels of those peroxides.
Let’s focus on two things that composite fabricators can control – temperature and peroxides.
Pay Attention to Temperature
Make sure the resin is stored and pumped within a reasonable temperature range. The ideal is 70 to 80 F, but anywhere between 60 to 90 F is acceptable. You can achieve this through simple strategies, such as holding resins in a temperature-controlled storage tank or in drums on a pallet so they are not sitting directly on a cold concrete shop floor.
If you plan to mix fillers and additives into the resin before use, they also should be at the proper temperature. Otherwise, the mixture needs to be adjusted to a reasonable temperature before it is used. Adjusting the resin or mix is a good way to control the temperature at the beginning of the process. One note of caution: If you use a high-speed mixer to adjust the temperature, be careful not to whip air bubbles into the resin.
The mold temperature also is very important. Once the resin or mixture makes contact with the mold surface, it starts warming or cooling to that temperature. A mold should be controlled to a narrow temperature window, otherwise the final cure of the resin can vary dramatically.
Some commonly used processes are spray-up open molding, pultrusion, resin transfer molding, sheet molding compound/bulk molding compound, infusion and centrifugal casting. The mold surface in these processes is a major factor in the energy put into the process to drive the cure. A colder mold surface, whether it is a spray-up mold or a die for a pultrusion, can make the resin gel cure slowly and lower the exotherm of the resin, making it unable to drive the cure to a sufficiently high level. If the surface temperature is also not uniform in temperature, the finished composite may have parts that are well cured and others that are poorly cured.