VACUUM RESIN INFUSION – HOW NATURE AFFECTS THE RESULTS
Vacuum resin infusion is a process to manufacture high quality composites components. The driving force that pushes the resin through the fiber stack is the diferential between atmospheric pressure ouside the closed mold system and the pressure inside the system.
The atmospheric pressure that we experience on earth is caused by the weight of air above the measurement point and for this reason it varies with the altitude above sea level. The standard atmosphere (atm) however is an established constant. It is approximately equal to typical air pressure at earth mean sea level and is defined as follows:
1 atm = 760 Torr = 29,92 “Hg = 14.7 psi = 101.3 kN/m2 = 1013 mbar = 1013 hPa
In practice, atmospheric pressure at sea level will vary from about 980 mbar to about 1030 mbar. At the summit of Mount Everest atmospheric pressure averages about 300 mbar. The barometer in our home gives an absolute (atmospheric) pressure which means that the reading is the pressure above the absolute zero of pressure. The vacuum resin infusion process set-up is usually measured with an analog dial gauge, mounted on the vacuum pump or resin catch pot. The reading of such a gauge is relative to the external atmospheric pressure. This relative measurement is called gauge vacuum and is the pressure difference with the atmospheric pressure as reference point. The vacuum gauge is a manometer and works the opposite way to the barometer.
Thus the absolute pressure Pabs in the vacuum bag or resin trap is equal to the current atmospheric pressure Patm minus the gauge vacuum pressure Prel (Pabs = Patm – Prel). Relative pressure is the driving force for the resin to get into the part. It is the difference between the atmospheric pressure and the absolute pressure in the bag (Prel = Patm – Pabs ). The higher Prel the greater the distance one can push the resin through the laminate from a single injection point.
VACUUM RESIN INFUSION AT LOW ALTITUDE ENABLES GREATER DISTANCE
In a location such as Florida (sea level with high atmospheric pressure) one can achieve a higher relative pressure and can infuse over a longer distance than someone doing infusion in the mountains of Colorado. Weather affects the infusion process in the way that good weather provides higher Patm and bad weather lower Patm. For instance at sea level and standard weather (Patm = 30 inHg/1000mbar) a gauge vacuum of 27 inHg (Prel) is equivalent to an absolute in-bag pressure of 3 inHg. In other words, the vacuum gauge that reads, for example 27 inHg is actually reporting 3 inHg/100 mbar Pabs.
LESS AIR ENABLES HIGHER QUALITY
Whereas the value of Prel has an impact on the maximum flow distance of resin through the laminate Pabs is a parameter for the amount of air that is left in the system. The higher that value the more air is left and the more voids can arise during the vacuum resin infusion process. In the example above the gauge vacuum of 27 inHG (Prel) and in-bag pressure of 3 inHG (Pabs) indicates that 10% of air is left in the bagged system. For a high quality infusion process we strive to achieve the lowest possible Pabs, at least in the range below 20mbar (0.29psi or 0.59inHG), the lower the better.
If we want to confirm that a bag has been evacuated below 0.5inHG of vacuum, the gauge used to measure this value needs to be accurate in this region.
A conventional analog vacuum gauge will experience inaccuracies of at least ±25 mbar from weather variations alone, plus variations due to the accuracy of the mechanism within the gauge, typically between 1 and 3% of full scale, perhaps another ±30 mbar. It is apparent that an analog vacuum gauge with a total accuracy of ±55 mbar (1.62inHG) is not an appropriate instrument for measuring an in-bag vacuum of 10 to 20 mbar.
A DIGITAL ABSOLUTE PRESSURE GAUGE ENSURES HIGH QUALITY
For the measurement of such values we recommend to use a digital absolute pressure gauge. We have gained excellent experiences with the Made in Germany device GDH 200-14 in countless vacuum resin infusion projects. It responds to pressure changes with a resolution of 1mbar down to absolute zero in an instant which does not only save time with setting up the vacuum system and finding leaks. It is also a perfect performance indicator for the quality of the vacuum resin infusion process.