Abstract
We investigated the ejection/demolding of continuous production of injection molded Poly(Lactic Acid) tensile testing specimens and analyzed the effect of 1 wt% Calcium-Stearate additive as demolding agent (sliding or mold release agent) on this process. We demonstrated that the Poly(Lactic Acid) specimens could get stuck in the mold or even break during demolding during continuous injection molding in a certain type of mold, which has a low draft angle and varying cavity width along the flow path. The standard dumbbell-shaped tensile testing specimen is produced in such a mold. Demolding was rated with the use of a high-speed camera into three categories (problem-free demolding / stuck, but demolded undamaged product / stuck and damaged product). Moreover, the ejector force required to push the product out of the cavity was monitored over 30 continuous injection molding cycles. We also investigated the effect of processing parameters, such as injection rate (screw speed), holding pressure, holding time, mold temperature, melt temperature, backpressure, screw rotational speed, and pre-process drying (drying or not drying the pellets before injection molding). We managed to avoid product breakage during demolding with the proper settings of certain process parameters and the use of Calcium-Stearate, an effective demolding agent. This ensured problem-free demolding and thus continuous injection molding.
Highlights
Biopolymers are receiving more and more attention. They are considered the eco-friendly versions of "ordinary" plastics (crude oil-based and non-biodegradable), since biopolymers are renewable resource based (bio-based) and inherently biodegradable
Nowadays, biopolymers are receiving more and more attention
The injection molding of Ca-ST filled Poly(Lactic Acid) (PLA) resulted in undamaged products with much lower tendency to get stuck into the mold, which was shown by the ejector force, which levelled off at lower value (2.1 kN)
Summary
Biopolymers are receiving more and more attention. They are considered the eco-friendly versions of "ordinary" plastics (crude oil-based and non-biodegradable), since biopolymers are renewable resource based (bio-based) and inherently biodegradable. We demonstrated this type of ejection defect during the series production of injection molded PLA tensile specimens, and investigated how ejection is affected by the processing parameters.
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