While I can’t cover all the possible problems within this column, I’d like to mention a couple of items that can improve your moldings and help eliminate defects. First, you should mold SMC with vacuum assistance. Vacuum-assisted molding has become standard and helps reduce or eliminate any issue related to air entrapment. Blisters, porosity and even some non-fills are significantly improved. The part’s surface is smoother. If you aren’t doing vacuum-assisted molding, your competitors are and they have the advantage.
Second, some molding presses – especially if they are PLC-controlled – have a menu that lets you control the rate of tonnage build. The tonnage build is a continuation of the press closure. Getting the press closure right is a balancing act: It should be slow enough to avoid defects like blisters, fractures, knit lines and resin richness, but fast enough to avoid pre-gel, non-fills and laking (an irregular, dull surface appearance often associated with insufficient compression). However, the press doesn’t stop moving once closure is indicated and can continue to move slightly as the tonnage builds. The use of a slow tonnage build after press closure has proven helpful in minimizing and eliminating both wash and wrinkles. There are also indications that it helps eliminate blisters.
In some cases, a gradual reduction of tonnage may be required after full tonnage has been applied as a way to improve certain aesthetic defects, such as surface ripples or crazing. This would be characterized as a quick build to full tonnage for a preset amount of time (usually five to 15 seconds), followed by a reduction in tonnage by some percentage for the remainder of the cure cycle.
Most issues in molding SMC can be resolved by being attentive to changes in the operation and by ensuring that the materials and process are running to established parameters. There is no substitute for good practices. However, when a problem seems to exceed these, try reducing variation and seeking help through troubleshooting guides and other technical resources that can aid in focusing on details previously missed.
