Meaning
Latent heat absorption characterizes the transition of semi-crystalline resins during the melting phase of the production cycle. The enthalpy of fusion is measured as the area under the melting peak in a differential scanning calorimetry scan and is expressed in Joules per gram. This value represents the energy consumed to break the intermolecular forces holding the crystal lattices together.
Amorphous polymers do not exhibit this specific energy spike because they lack a defined crystalline structure to dismantle.
Thermodynamic Energy
Measuring the heat flow into the sample allows for the quantification of the energy state of the resin. A high enthalpy of fusion indicates a material that is heavily organized into crystals which requires more power from the heater bands to melt. This energy is not recovered during the process and must be managed through efficient heat transfer.
Understanding the energy profile helps in selecting the right size of injection unit for a specific material.
Crystallinity Calculation
Comparing the measured heat of a sample to the known value for a one hundred percent crystalline version of that polymer yields the degree of crystallinity. This ratio is vital for predicting the mechanical strength and chemical resistance of the finished part. If the enthalpy of fusion is lower than expected, the part may have been cooled too quickly, preventing the crystals from fully forming.
Such a deficiency can lead to post-molding shrinkage or warping as the material continues to organize at room temperature.
Processing Demand
Thermal management in the barrel is dictated by the energy needed to overcome the crystalline bonds. Materials with a high enthalpy of fusion can cause a drop in melt temperature if the screw speed is too fast and the residence time is too short. This leads to unmelted granules entering the mold which creates surface defects and structural weak points.
Balancing the energy input with the throughput rate ensures a homogenous melt and consistent part quality.
