Injection Molding Of Polymeric Materials by Taylor A. Groll Padnos College of Engineering and Computing Grand Valley State University Laboratory Module 6 EGR 367 – Manufacturing Processes Instructor: Dr. P. N. Anyalebechi October 18, 2004T. Groll 1. Abstract Low-density polyethylene (LDPE), high-density polyethylene (HDPE), and santoprene plastics were used to injection mold seven specimens each to determine the affects of polymer material composition, barrel temperature, cycle time and air supply pressure on the quality of injection molded parts and the number of rejects. As the air supply pressure decreased, the cycle time had to be increased to maintain complete filling of the mold cavity, regardless of the type of plastic used. Increasing the barrel temperature allowed the cycle time to be decreased to completely fill the mold cavity. The parts produced with a barrel temperature of 320 °C had the best surface finish of all of the parts made. The HPDE parts were the most consistent with respect to maintaining the same dimensions across all seven processing conditions. Plastics whose thicknesses do not greatly change over different processing conditions are excellent choices for processes that have some fluctuation inherent in the system settings. 2. Introduction Injection molding is the most widely used of all plastic processing methods. The process involves the feeding and subsequent heating and mixing of thermoplastic material into a barrel or hopper. The plastic mixture is then forced into a mold cavity where it cools and hardens into the shape of the mold cavity. The forcing of the plastic material into the mold cavity is accomplished though the usage of either a ram or screw-type plunger. This particular experiment was performed using the bench-top version of the ram plunger type injection molding machine. With the ram plunger type injection molding, the plastic pellets are melted and a ram plunger is extended into the cylindrical barrel which forces the molten plastic through a nozzle and into the mold cavity. The ram plunger is forced into the barely by air pressure. Insufficient air pressure can result in a short shot condition in which the mold cavity is incompletely filled resulting in a rejected part. With the screw plunger type injection molding, plastic pellets are forced down the barrel by way of a large screw. The screw plunger type injection molding process is faster than 2T. Groll the ram plunger type because the rotating screw distributes the heat that melts the plastic pellets more evenly. Once the plastic has been sufficiently mixed and melted, the screw is forced down the barrel in the same manner as the ram plunger and the molten plastic is injected through the nozzle and into the mold cavity. 3. Experimental Procedure To begin the experiment, the heaters were turned on and the barrel temperature was set to 205 °C and allowed to warm up. The temperature was set at a higher temperature than required for melting because faster cycle rates were desired. The nozzle temperature was set to 220°C and allowed to warm up. The nozzle temperature was set slightly higher than required to keep the molten plastic from solidifying prior to its injection into the mold cavity. Due to the fact that the warm up of the barrel and nozzle take approximately one hour, the pre-heating was performed by the instructor. Before cycling any plastic through the machine, a mold release agent was sprayed on the mold cavity, the upper platen and the ram plunger to keep the plastic from sticking to the injection molding components. Raw plastic pellets were then poured into the hopper and barrel. Three different types of plastic were used: low-density polyethylene (LDPE), high-density polyethylene (HDPE), and santoprene. Pellets of HPDE were used first. The material was allowed to melt in the barrel for approximately 5 minutes. An additional amount of plastic was added to the barrel so that the hopper was filled to within 0.5 cm of the fill line. The additional amount of un-melted plastic acted as a buffer between the ram and the molten plastic and made it significantly less likely for the plastic to stick to the ram. Before the first shot, the nozzle and barrel assembly were purged for approximately 5 seconds to ensure proper operation. The system pressure was set to 4500 psi. Three separate parts were made at cycle times of 2, 5, and 10 seconds. All of the parts were marked with a paint-pen to differentiate between system settings. Next, two more parts were made at a cycle time of 5 seconds with the barrel temperature of 160 °C and 225 °C. In both cases, the nozzle temperature remained at 220 °C and the system pressure was 4500 psi. Finally, two more specimens were made with a barrel temperature of 205 °C, nozzle temperature of 220 °C, cycle time of 10 seconds and the system pressures of 3000 and 6000 psi. After all of the specimens were labeled, 3T. Groll thickness measurements were taken at three different points on each part. The results were recorded in a table. A flash weight test was also performed on each part. Each part was weighed before and after trimming off the excess material from the mold. The results were recorded in a table. The barrel and nozzle were purged after running each plastic to ensure that no mixing of the plastics occurred in the molded parts. 4. Results The data in Table 1 show the settings of each condition in the experiment. Table 2 shows the mass and recovery data taken for each of the three plastics tested (HDPE, LDPE, Santoprene) at each of the seven processing condition. Condition 1 produced incomplete mold fills for each of the three plastics tested (Table 2). Condition 1 produced the greatest recovery rate of 95.83% for HDPE. Conditions 1, 4, and 7 produced recovery rates greater than 90% for LDPE. Conditions 1 and 4 produced recovery rates greater than 90% for Santoprene. Figure 1 shows a graphical representation of the data in Table 2. Condition 1 produced the greatest percent recovery for the three plastics as a group (Figure 1). Condition 3 produced the lowest percent recovery for the three plastics as a group (Figure 1). Table 3 shows the thickness measurements taken of each part for each of the seven different conditions. Table 3 shows that the santoprene maintained the greatest average thickness for all seven processing conditions. The HDPE thickness