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Cavity Area = length × width = 4.6 in × 4.6 in = 21.16 in. 2 Parts Area = cavity area × cavities = 21.16 in. 2 × 8 = 169.28 in. 2 Tonnage =parts area × tonnage factor = 169.28 in. 2 × 3 tons/in. 2 = 507.84 tons Now we have a starting point to determine what clamp tonnage is correct for this mold. But other factors must also be taken into consideration to determine if the tonnage required will be above or below 3 tons/in. 2 . • Melt flow index or rate (MFI, MFR); • Flow length; • Wall thickness; • Gate location; • Gate size; • Number of gates; • Volume of resin. The first item that needs to be considered after calculating the tonnage based on projected area is how the material flows. MFI or MFR can be found on the material data sheet from the supplier. A material with a high MFI or MFR will require less pressure from the injection unit to complete the packing phase; therefore, the tonnage required will be lower. When evaluating the MFI or MFR, it is imperative that a comparison be done only between grades of the same type of resin. The reason for this is the ASTM test has a different temperature, orifice size, and weight for each type of resin. These values do not corre- late directly with molding, because of how the ASTM test is performed and how molding machines actually operate. The MFR or MFI relate more closely to the packing phase since it is typically at a much lower flow rate than the filling phase (see Fig. 2). When reviewing our example of molding a lid, the material selected was LDPE. This material will likely have a very low viscosity, indicating that the pressure required from the injection unit to pack the part will be fairly low. Therefore, based on material selection, the tonnage required will likely be less than the typical 3 tons/in. 2 . Next, we need to evaluate both the flow length and the wall thickness. This is referred to as the aspect ratio, comparing the flow length (from gate to end of fill) to wall thickness. Generally speaking, an aspect ratio less than 150:1 has a low risk of sinks, short shots, or dimensional issues. In our example of molding a lid, the flow length is 3.00 in. and wall thickness is 0.100 in.: When evaluating the MFI or MFR, it is imperative that a comparison be done only between the same types of resin. Melt viscosity is related to clamp-tonnage requirements, but MFI does not reliably correspond to actual melt viscosity in molding and cannot be compared accurately between resin types. For example, PC will always show high viscosity in molding, regardless of whether the MFI is 2 or 25 g/10 min. Also note than an MFI of 10 g/10 min for PP does not mean an equal molding viscosity for the same MFI with PC. Comparing MFI Range and Melt Viscosity in Molding High Viscosity Medium Viscosity Low Viscosity ABS HDPE LDPE PC PMMA POM PP PS Low MFI numbers indicate high viscosity. 2 g/10 min 25 g/10 min FIG 2 1 g/ 10 min 68 g/ 10 min 2 g/ 10 min 23 g/ 10 min 1 g/10 min 24 g/10 min 1 g/ 10 min 23 g/ 10 min 2 g/ 10 min 53 g/ 10 min 2 g/ 10 min 40 g/ 10 min 1 g/ 10 min 14 g/ 10 min Make Perfect Parts and Save Money! • Preload cores for perfect parts. • Save $$$$ with a smaller cylinder. • Simplify mold operation and design. • Smaller mold base = low cost parts. • Fully Preloads at only 1,500 psi. • Stays locked without hydraulics. • 400˚ F systems available. • Rush 0-2 day shipping available. pfa-inc.com N118 W18251 Bunsen Drive Germantown, WI 53022 (262) 250-4410 Fax (262) 250-4409 KOR-LOK ® , Hydra-Latch ™ and Hydra-Jaws ™ are trademarks of PFA, Inc. ©2016 PFA, Inc. All Rights Reserved. Mold OPEN Core Out Mold CLOSED Core OUT Mold CLOSED Core IN Core PRE-LOADED Against FULL Injection Pressure CAN'T DO IT! CAN'T DO IT! KOR-LOK ® TRADITIONAL SIDE-ACTION VS. SIDE-ACTION SYSTEMS SYSTEMS YES! YES! YES! YES! YES! YES! SIDE-ACTION SYSTEMS Designed & Made in USA HYDRA-JAWS ™ Every Mold, Every Machine, Every Time! Adjusts to Fit Every Mold Quick and Secure Consistent Clamping Force 36 JUNE 2017 Plastics Technology PTonline.com K now How