TPE Injection Molding
Material Flow Behavior
Most Abery overmolding compounds have relatively low viscosity characteristics. They are shear responsive and their viscosity is reduced when they are processed at high shear rates. This helps them flow into and fill thin walled sections commonly encountered in overmolding. • The shear thinning behavior of GLS TPEs should be considered when designing injection molds and optimization of molding conditions. Figure 15 illustrates the range of viscosity of GLS overmolding TPEs. The lower end of the range is typical for most GLS TPEs. For the viscosity data of a specific GLS TPE, refer to the individual product technical data. Due to the shear sensitivity of TPEs, their injection molding process is slightly different than that of engineering thermoplastics. Unlike most thermoplastics, melt temperature alone is not the most effective method of controlling the TPE molding process. Ideally, the goal is to obtain the lowest melt viscosity of the molten TPE through a combination of shear rate and temperature adjustments. Shear rate in modern molding machines can be controlled through the use of: • TPE melt velocity. • First stage injection pressure. • Smaller gate size. General TPE Injection Molding Guidelines The following are basic principles for optimizing the molding process for GLS TPEs: • During start-up, the molding parameters should be set at the mid-point of the TPE supplier’s recommendations. • Shear rate adjustments should be the primary method used to control TPE melt viscosity. TPE melt temperature adjustments should only be used to fine-tune the process. • To minimize shrinkage issues, adjust second stage pressure as necessary to insure the TPE melt is fully packed into the mold cavity. • Utilize “cushion” properly during the mold filling process.
Machine Selection Molding machines with conventional reciprocating screws are recommended. Newer computer controlled machines have the ability to better control critical molding parameters and are preferred for multi-cavity tools and high production quantities. Machines with programmable injection rates to “profile” the shot, and independent first and second stage pressures can produce better quality parts. This results in finished components with improved part weight and dimensional control. Older molding machines that control the shot size by position are preferable to machines that only control the process by pressure and/or time. The clamp capacity necessary for general purpose GLS compounds is 2-3 tons per square inch multiplied by the total projected area of the cavities and the runner system. This is lower than most other TPEs. If possible, use a machine that utilizes 25% to 75% of the barrel shot size capacity. This will result in better temperature control of the material and minimal material residence time at high temperature. The typical material residence time for GLS overmolding compounds should be no more than 4-5 minutes maximum. In most two-shot molding applications, the TPE shot size is considerably smaller than the plastic substrate shot size. If available, use an injection unit sized to minimize the TPE residence time. During startup, choose a smaller nozzle diameter, which will help to produce shear heating during injection and generate less cold slug material. Suggested starting nozzle diameters are 1/16" to 3/16" (1.6 mm-4.8 mm). General-purpose (GP) screws are suitable for most GLS TPEs. Compression ratios between 2.0 and 3.0:1 will effectively control GLS overmolding TPEs during the injection molding process. Barrel/screw combinations should offer a minimum 24/1 L/D ratio. Purging If the press is idle for more than 10 minutes, purge thoroughly with a low flow (0.5-2.5 melt flow) polyethylene (PE) or polypropylene (PP) before restarting component production. To prevent flashing, restart the machine using a reduced shot size and gradually increase it to the previous setting. Injection Pressure and Injection Speed Typically, the required first stage injection pressure to fill a TPE cavity is 300 to 800 psi, depending upon the number of cavities and the mold runner layout. To achieve the benefits of shear-thinning, the injection speed should be adjusted to fill the mold in 1-3 seconds (varies with material). A higher shear rate can be readily achieved through greater TPE melt velocity (shot speed) and higher first stage injection pressure. Mold Temperature To prevent mold condensation and the resultant water contamination in the cavity, the mold temperatures should be set above the dew point temperature in the molding area. Mold temperatures may have to be increased if there are long or thin sections of the part that cannot be filled by adjusting other molding parameters. TPE mold temperatures of 70-120˚F (21-49˚C) are typical. Accurate mold temperature is critical when running molds with core lifters. When using these ejection systems, differential thermal expansion of the mold may cause the lifter to jam