Selecting a Process
Converting to closed molding calls for a change in culture where workers need to be more attuned to technical
skills than craftsmanship. Initial factors to consider when selecting a process include the size, geometry,
mechanical and physical requirements, and volume. Other selection-influencing factors are investment capital,
tooling, plant size and layout, workforce capabilities and management preferences.
The natural progression is to an intermediate closed mold process that technologically falls between
traditional open molding and matched metal-die compression molding of SMC. Intermediate closed mold
processes are also called liquid composite technologies because the resin is introduced to the manufacturing
process in a liquid state. While there are numerous variations, intermediate processes generally fall
within three major categories: resin transfer molding (RTM), RTM Light and Infusion.
There are no rules or mathematical equations that can simplify process selection. The comparison charts and
descriptions that accompany this discussion are intended to serve solely as guides in making a preliminary
decision. The AOC Closed Mold Technical Center has materials and process expertise that can help manufacturers
make a more informed choice.
Open vs. Closed Molding: Basic Process Comparison
| Process |
Part Volume |
Part size |
| OPEN MOLDING |
| Hand lay-up |
Low |
Small to Large |
| Spray-up |
Low |
Medium to Large |
| CLOSED MOLDING |
| RTM |
Medium to high |
Small to Medium |
| RTM Light |
Medium |
Medium |
| Infusion |
Low |
Large |
Intermediate Closed Mold Processes: Basic Process Comparison
| |
RTM |
RTM Light |
Infusion |
| Injection Pressure |
40 to 50 PSI |
12 to 14 PSI |
N.A. |
| Vacuum |
N.A. |
-10 to -15 inch Hg |
-25 to -26 inch Hg |
| Tooling A |
Rigid |
Rigid |
Rigid |
| Tooling B |
Rigid |
Flexible / Semi-Rigid |
Polyfilm |
| Max. Fiber Content |
60% |
40% |
40% |