Plastics Technology - Dedicated to improving Plastics Processing.
Issue link: http://pty.epubxp.com/i/839458
Let's say a fill rate of 1 in. 3 /sec is needed to fill each cavity in an eight-cavity mold. The amount of material flowing through the sprue orifice is therefore eight times that amount, or 8 in. 3 /sec. Depending on the material type, if the sprue orifice is under- sized, you will generate not only a large pressure drop, but also a significant amount of shear heat, which can cause the material to degrade and burn. This increased degrada- tion also generates more material outgases, which can accelerate the clogging and erosion of the vents in the mold. Assume the part in the eight-cavity mold above has a uniform wall thickness of 0.090 in. Depending on the material type, you might start out with the depth of the gate to each part being two-thirds of the wall thickness, and the width being twice the depth, or in this case, 0.060 in. deep × 0.120 in. wide. The total area of each gate is 0.060 × 0.120 = 0.0072 in. 2 . The total area of the eight gates combined is 0.0072 x 8 = 0.058 in. 2 . When you divide this area by the constant 0.7854, and then take the square root, you get an equivalent diameter of 0.271 these shrinkage differences apply to both. Since sprues are typically the thickest molded compo- nent, the amount of shrinkage is going to be greater, as is the likelihood of negating the taper lock. Therefore, smaller internal tapers, such as 1°, 0.75°, or even 0.5° per side can usually be accommodated without any difficulties. And the cost savings can be considerable. For example, reducing the internal taper from the standard 2.39° included angle down to 1.5° for a 6-in. sprue bushing with a 5/32-in. orifice diameter reduces the diameter of the sprue at the parting line by 23% and the area by 41%. It also reduces the weight of the sprue by 32%. Orifice Size: Oversized sprue-bushing orifice diameters waste material, time, and money. So how do you determine what size the orifice should be? Ideally, the area of the sprue-bushing orifice should be approximately equal to the total area of all the gates combined. Ignoring this is a common mistake and often the reason why it takes a lot of injection pressure to go through a sprue bushing—robbing you of additional pressure to pack out the cavities. Oversized sprues waste material, time, and money. Undersized sprues can cause sink and warp. 525 East Stop 18 Road Greenwood, Indiana 46142 Phone: 317-887-6352 Email: sales@TempTek.com For information online www.Temptek.com Factory Phone 317-887-6352 Outdoor Central Chillers • Complete Outdoor Chilling System with Refrigerant Zones and Pumping System • For Use In Most Outdoor Climates • Central Unit for Plant Wide Cooling for Many Processes • Integral Air-Cooled Condenser Indoor Portable & Central Chillers • Use Inside Plant Facility for Single Process or Plant Wide Cooling • Microprocessor Controller and Display • Small Footprint to save floor space • Environment Friendly Refrigerants • Internal Coolant Reservoir And Cirulation Pump Water Chillers with New Product Warranties, industry recognized components for Maximum Reliability, Economically Affordable capital equipment providing perfect bottom-line returns, and a network of factory trained technicians for Unbeatable Service. • Environment Friendly Refrigerants • Microprocessor Instrument Controls • Non-Ferrous Construction • Small Footprint • Other Products: Temperature Control Units, Pump Tanks, Cooling Towers, Material Dryers, Loaders, Granulators 30 JULY 2017 Plastics Technology PTonline.com K now How