Injection molding is the most popular process in the plastics industry. However, few people know what exactly happens inside molds and their molded parts. Many companies rely on experienced and seasoned molders to set up process conditions for production by trial and error from time to time and batch to batch. And still, often, companies and molders puzzle about the inconsistent quality of their molded parts.
To have an efficient and effective injection molding operation, not just adjusting process parameters on the machine screen by trial-and-error guesswork, molders must have insights into the injection molding process by standing at the plastic material’s point of view. Herein, it is essential to understand the physical relationship among the plastic P-V-T (Pressure – Volume – Temperature) across the entire injection molding process cycle.
Generally, plastic material is classified into two categories: amorphous and semi-crystalline. Taking amorphous material as an example, Figure 1 illustrates the typically dynamic P-V-T (Pressure – Volume – Temperature) relationship across a single injection molding process cycle. Herein, volume refers to the specific volume of plastic material and is defined as the volume per unit mass, meaning the reciprocal of material density. Based on the plastic’s point of view, the physical phenomenon of a complete injection molding cycle can be typically presented in Figure 1, that a plastic’s specific volume increases (expands) with heating (increasing temperature) at a certain pressure, decreases (compressed) with force (increasing pressure) at certain temperature and decreases (shrinks) with cooling (decreasing temperature) at a certain pressure. Further interpretation of the P-V-T chart is as follows.
(1): Injection starts – to build the molten plastic pressure at the setting melt temperature.
(1) to (2): Filling stage – melt pressure increases at the setting melt temperature while the specific volume decreases as pressure increases.
(2): Filling stage ends; packing stage starts – the part outline is roughly fulfilled molten plastic material at setting melt temperature; cooling starts taking significant effect in reality.
(2) to (3): Packing stage – melt pressure keeps increasing while melt temperature starts decreasing; the specific volume continues decreasing as melt pressure keeps increasing.
(3): The melt pressure reaches peak value; process switches into holding stage; the packing stage ends.
(3) to (4): The melt pressure releases when switching from packing to holding stage; the material temperature keeps decreasing; the melt’s specific volume increases a bit at first with pressure release then decreases again as holding pressure takes effect.
(4): Holding stage starts.
(4) to (5): Holding stage – the melt temperature keeps decreasing because of the ongoing cooling effect in reality; the melt pressure decreases a bit as the melt temperature decreases; the melt specific volume keeps decreasing as the temperature keeps decreasing.
(5): Cavity gate is frozen; the holding stage finishes; cooling stage starts.
(5) to (6): Cooling stage – the melt temperature decreases; the melt pressure keeps decreasing; the melt’s specific volume keeps decreasing.
6): Cooling stage – the plastic’s pressure decreases to atmospheric pressure; dimensions of the molded part are the same as the dimensions of the mold cavity with the plastic’s specific volume Vm; the molded part starts shrinking in mold.
(6) to (7): Cooling stage – the molded part continues being cooled at atmospheric pressure; the plastic’s temperature keeps decreasing at atmospheric pressure; the plastic’s specific volume keeps decreasing at atmospheric pressure; the molded part shrinks in the mold cavity.
(7): The cooling stage finishes; the mold opens and ejects the molded part; the plastic’s pressure keeps at atmospheric pressure while the plastic’s temperature decreases to ejection temperature, at which the
plastic’s specific volume decreases to Ve.
(7) to (8): The molded part is ejected and cools in the air at atmospheric pressure; the plastic’s temperature keeps
decreasing; the plastic’s specific volume keeps decreasing; the molded part continues shrinking in the air.
(8): The molded part reaches balance status at atmospheric pressure and room temperature with the final specific volume Vf of the plastic material.
Based on the fundamental understanding of the plastic material P-V-T chart mentioned above, more insights it brings are worthy of notice.
1. The plastic’s specific volume difference between Ve and Vf represents the amount of volume shrinkage that the molded part shrinks freely without any constraint in the air after being ejected from the mold. For example, as Figure 2 shows, using a shorter cycle time to speed up production means the molded part is ejected at a higher temperature, leaving a more specific volume difference (Ve – Vf) and more amount of free volume shrinkage of the molded part than the longer cycle time. In such a situation, the size or dimension of the molded part becomes smaller and the molded part tends to warp or deform if the inherent volume shrinkage is not uniform.
2. Each path from point (1) to point (8) on the P-V-T chart represents the plastic material’s thermal and pressure histories at a certain location of the molded part and mold cavity under an injection molding process condition. The material at different locations of the molded part and mold cavity have different paths to represent their individual thermal and pressure histories at those locations under a single cycle, as Figure 3 shows. Theoretically, the plastic material of molded part near the gate has in-mold shrinkage (Vm – Ve) and post shrinkage in the air (Ve – Vf) the least, while that at the end-filling location the most for both.
3. Each path from point (1) to point (8) on the P-V-T chart represents not only the plastic material’s thermal and pressure histories at a certain location of the molded part and mold cavity under an injection molding process condition but also represents the material’s thermal and pressure courses of a single shot (cycle). Individual paths of material at the same locations of the molded part and mold cavity should be kept consistent as much as possible from shot to shot and time to time, which means parts are produced in consistent quality for the long term, as Figure 4 shows.
4. For a multi-cavity mold, plastic material at the same location of the molded part but from different mold cavities have individual paths as well to represent the material’s thermal and pressure histories at different cavities. If the process cannot provide balanced filling, packing, holding, and cooling effects on the molded parts from different cavities, the thermal and pressure histories of the material on the molded parts, even at the same locations, will be different from one cavity to another.
5. By employing proper part and mold designs, it can be achieved that more uniform volume shrinkage, i.e. Vm – Ve, from point (6) to (8) of the P-V-T chart, across the entire molded part from a location near the gate to the end-filling extremity.
6. Employ process optimization skills to develop a robust injection molding process condition, which aims to make the P-V-T path pattern at every location of the molded part repeat as much as possible from shot to shot and from time to time.
Fully understanding the plastic P-V-T relationship in the injection molding process does help companies and molders get aware of the inherent complexity of this most popular process, and give more respect to it. Next time, when any parameter of the injection molding process condition on the machine screen is going to be changed, have second thoughts on how it will influence the subsequent molded part quality by thinking about how the change will differ the plastic P-V-T path pattern from that of the previously produced parts. Next time, when setting up a new injection molding condition for new batch production, consider whether the setup will differ in the plastic P-V-T path pattern from the last well-done batch production.
About the Author:
Hank Tsai is the owner and consultant of Effinno Technologies Co., Ltd. ( https://en.effinno.com ) in Taiwan, an injection molding training and consulting service provider. He has been an SPE member since 1995 and has more than 25-year experience in the injection molding industry. He has expertise in injection molding technologies and practices, production efficiency management, part cost structure analysis, troubleshooting, simulation, mold/process/machine performance evaluation, and process optimization by Taguchi DOE. He is also the co-developer of a cloud-based software system and solution that aims to facilitate the digitalization of the industry’s machine utilization and production efficiency management. Contact: hank.tsai@effinno.com.
