By Dallas Cada. DCC Consulting
The following tech brief examins the phenomenon of mold filling orientation. The information can be used as a good basic understatnding of what happens once the material enters the mold. Thematerial used can be made up of most resins and filled matrixes.
As we know, most parts have some degree of “frozen in” molecular orientation. The molecular orientation will be influenced by molecular weight and relaxation characteristics, by process conditions during production. The orientation can also be minimized through design and process variables that minimize mold filling pressure requirements.
Residual (frozen-in) orientation is equal to the orientation level due to flow, which is equal to the relaxation of the molecules. A warmer tool and material temperature will promote molecular relaxation. This will usually result in a longer cycle time however, will decrease molded in stress. This is because of the material mold filling orientation the polymer at the cavity sees. The higher temperatures, will allow the flow-induced stresses and molecular orientation to relax after mold filling. The injection speed used will also influence the degree of orientation at the time of fill. Faster fill leads to an increase in viscous heating and a decrease in the amount of conductive cooling. As a result, there is more molecular mobility at the instant of fill so relaxation can occur.
All polymers experience a pseudoplastic laminar profile (see figure 1). Basically, molecular orientation develops during the mold-filling phase. Polymere chains become stretched out due to velocity gradient associated with laminar flow behavior. While most orientation occurs at the surface of the part, the molecules remain in a coil configuration at the core, Because the cavity and wall freeze first this leads to high interfacial shear stresses. Orientation will continue during process especially near the gate region. Packing and holding can be modified to increase or decrease orientation. Orientation problems are more significant for higher molecular weight and fiber reinforced polymers.
In summary, orientated molecules tend to radiate from the gate towards the end of flow. This results in relatively high stresses in the gate as the polymer molecules attempt to recover. This can also lead to dimensional distortion at elevated temperatures. Annealing can relieve internal stresses however the part should be fixtured in order not to distort it. Gating the part correctly will help with molded in stress and molecular orientation. Positioning the gate properly will promote orientation in the direction of the maximum stress associated with the end use application. The use of a hot runner system will also reduce molded in stress by increasing the flow of the polymer. Last but not least, is process optimization. Reducing molded in stress by increasing heat and velocity can help with molecular orientation.
About the Author
Dallas Cada is a highly trained plastics engineer with over 20 years of sales support experience. Owner of a plastic consulting business (DDC Consulting), his experience includes technical service, application development, market engineering, injection molding, design, tooling, material suggestions and problem solving for plastic manufacturing companies. For more information with troubleshooting plastic problems or helping with new plastic applications, contact Dallas Cada by e-mail at dallascada@charter.net. Contact Dallas by phone (507) 458-5785 or (507) 452-1584.
