ROSEMONT, ILL. — When a caps molder starts production with a new multi-cavity mold, one of the first questions frequently asked is “Why can't I mold the same parts?”
John Beaumont, president of Beaumont Technologies Inc., addressed that issue during a session at Plastics Caps & Closures 2014, held in Rosemont.
According to Beaumont, there are several conditions necessary across all cavities of a multi-cavity mold to ensure that all parts are produced the same. These include identical steel, thermal/cooling, process and material conditions.
“As it turns out, what the industry has labeled naturally balanced runners creates the greatest variables in injection molding today,” Beaumont said. “A simple short-shot analysis of a multi-cavity injection mold is the most effective method of evaluating molding variations and the formation of a plastic part.”
As a result, the perfect mold will have identical steel conditions, cooling, process and material properties.
“Rheological variations exist below the surface of your parts and this degrades every aspect of your part and part cost,” Beaumont pointed out.
As a result, unmanaged shear-induced melt variations can influence the formation of every injection molded plastic part. The issues can include warpage, residual stresses and mechanical properties.
A key to solving this issue is understanding the development of shear-induced rheological imbalance. Shear imbalances are a large part of imbalance problems.
“The flow of plastic in an injection mold is laminar,” Beaumont explained. “Significant shear in the outer laminates create dramatic rheological variations from the outer laminates through to the low sheared inner laminates.”
As a result, branches in a runner cause the shear-induced melt variations to be non-uniformly distributed to downstream branches and part-forming cavities.
These shear imbalances affect both the mold efficiencies and product quality. Patented melt rotation can strategically manage the non-homogeneous melt conditions.
“These systems, such as MeltFlipper, can control material properties across the runner system and within each molded part,” Beaumont said. “The result can be a true rheological balance in multi-cavity molds.”
Beaumont's company developed and sells the Melt Flipper.
Beaumont's company conducted a comprehensive study to evaluate the ability of CAE to predict shear-induced melt variations and their influence on mold filling.
The study was a collaboration between Beaumont and Autodesk Moldflow.
“We used multiple molds, identical material and process conditions,” he said.
The study included evaluation of nine different 3-D mesh variants and hybrid single dimension and 3-D meshes.
Solution variations included iteration limits, convergence error limits, modeling with and without a machine nozzle, removal of melt temperature constraints and turning on the advanced solver.
“We are picking up 90° temperature variations in the melt within a cavity,” Beaumont noted.
The research also detected melt temperature spikes of up to 400° F in some circumstances.
“You fill the mold in a fraction of a second and it is immediately cooling, so these temperature spikes are possible,” Beaumont added.
Ultimately, degradation is a time-temperature function.
“We could put a man on the moon in the 1960s, but we can't measure the temperature in a mold,” Beaumont concluded.
