Injection Molding Laboratory Report
Department of Mechanical Engineering
University of Wisconsin- Madison
Wisconsin 53706, USA
The purpose of this lab was to explore the effects associated with changing switch over, shot size, pack/hold time, cooling time, pack/hold pressure, and mold temperature on the final product of an injection molded sample.
Keywords: Injection Molding, Process Parameters, Product Quality
Introduction
Injection molding is one of the leading plastic processes, both in money and quantity of parts produced. The process involves heating plastic pellets through friction and an outside heat source. The hot material is delivered to the mold through the use of a large screw. When the mold is closed, the screw extends forward laterally and drives the hot plastic into the mold, where it is cooled until fully solidified. More material is driven into the mold as the part cools to maintain correct dimensions due to parts shrinking during the cooling process. Material is driven into the mold up until the time the gate freezes. See figure A.
Figure A. Injection Molder
In the lab that was performed, we used high density polyethylene (HDPE). HDPE is the high density version of PE plastic. It is harder, stronger and a little heavier than LDPE, but less ductile.
Injection molding has been one of the most important fabrication tools for the plastics industry. The process is used to make automotive interior parts, electronic housings, house wares, medical equipment, toys, crates, and pails, thin-wall food containers, drink cups, lids, and milk bottle caps.
Results and Discussion
When our switch over was reduced to 4 mm, the weight of the sample increased approximately .5 grams from the initial, and the thickness stayed constant. It was a more complete fill than the original conditions. See Figure 1.
Figure 1. Thickness & Weight vs. Switch Over
When our switch over was increased to 11mm, the sample parts were incompletely filled, called short shot. Since it was an incomplete fill, the weight decreased from the original. The thickness was a little lower than the original, and the molten material did not make it to the second pressure tap. See Figure 1.
When our shot size increased to 42 mm, our weight increased, which is expected since we are forcing more material into the mold. Since we are forcing more material into a constant volume, the pressure was much larger than the original. There was an average increase in pressure of approximately 40 MPa. See Figure 2.
Figure 2. Thickness & Weight vs. Shot Size
When we decreased the Pack/Hold time, the pressures decreased as the time decreased. We assume that our first data point in an outlier, our data follows this trend. The weight and the thicknesses are approximately the same as the original. See Figure 3.
Figure 3. Thickness &Weight vs. Pack/Hold Time
From our data, there are no significant changes or patterns that one can see from changing cooling times. This would be intuitive because we are not changing the amount of material but rather the time that the mold is held together.
See Figure 4.
Figure 4. Thickness &Weight vs. Cooling time
As the Pack/Hold pressure increased the weight increased, and the pressure increased. As pressure increased, the force pushing material into the mold as it cooled increased, this increases the weight. As the amount of material filling a constant volume increases, the pressure increases as well. See Figure 5.
Figure 5. Thickness & Weight vs. Pack/Hold Pressure
When we decreased the mold temperature, the weights increased, the pressure decreased, and the thicknesses stayed the same. This is due to a cooler temperature in the mold causes the shrinkage to occur faster and therefore more material can be packed into the mold at a lower pressure. The thicknesses are the same because they still fill the same volume of the container, but the sample cooled faster has a slightly higher density. See Figure 6.
Figure 6. Thickness & Weight vs. Mold Temperature
See Appendix for Raw Data and P-t Trace Graphs
Conclusion
For actual manufacturing profit and product quality are main factors. If the temperature is too hot or cold, we will either see a short shot or thermal degradation. If the pressure is too high, flashing will occur, and if it is too low, we will have too much shrinkage and the parts will not be within specified tolerances. The product must be within certain process parameters to ensure product quality. The most profitable process is performed at the lowest acceptable temperature and from our data, the highest quality part is made using a higher pressure.
References:
Osswald, Tim A. Polymer Processing Fundamentals, Hanser Publisher, Munich, 1998.