Plastics Technology - Dedicated to improving Plastics Processing.
Issue link: http://pty.epubxp.com/i/827817
This mold was clamped at four times the required tonnage, resulting in separation of the cavity block in two locations. FIG 3 FIG 4 Several factors contributed to the frame rails breaking on this vertical press. The mold required about 10 tons to be held shut against the forces of injection, but the clamp tonnage was set to 100 tons. Aspect Ratio = flow length/wall thickness = 3.00/0.100 in. = 30:1 With a very low aspect ratio, the required pressure to pack this part will lower, thus further reducing the required clamp tonnage to hold the mold closed. In addition to lower tonnage, the probability of molding sinks, short shots, or out-of-tolerance parts is reduced. Next, we need to evaluate the gate size as well as the number of gates. A larger gate diameter has a much larger area for material to flow through, thus requiring less pressure from the injection unit to fill and pack the part. The number of gates will affect the injection pressure during both filling and packing. If there is a greater number of gates, a larger gate area, or if sequential filling requires less injection pressure, the result is a lower required tonnage. Our example of an LDPE lid has a single gate with a diameter of 0.030 in. Gate Area = diameter x diameter × 0.7584 = 0.030 × 0.030 x 0.7854 = 0.0007 in. 2 This is a very small surface area to inject 23 g of material per cavity. Given this amount of mate- rial and the gate size, expect the injection pressure during the packing phase to be slightly higher and therefore the tonnage requirements would be a little greater than 3 tons/in. 2 . Ultimately, the best method to determine required tonnage is to run a simulation with correct geometry, characterized material, and full runner system. Without simulation, we can draw conclusions from evaluating part geometry, material selection, gate size, gate location, and gate quantity. Based on these factors, an expected range for this mold would most likely be 339 to 508 tons (at 2 to 3 tons/in. 2 ). A good starting point would be 420 tons of clamp force. EFFECTS OF TOO MUCH TONNAGE Now that we have focused on how to determine appropriate clamp tonnage, let's take a look at the effects of applying excessive tonnage. Here are some typical molding defects that can be seen on parts almost immediately when tonnage is too high: • Burns; • Short shots; • Gloss level changes. CW LARGE ROBOT, LARGER VALUE AUTOMATION FOR LARGE PARTS On the battleground for large part molding, the MaxCW10 – MaxCW50 robots offer efficiency, raw strength and fast payback. Check out our entire line of servo robots for machines from 30 to 4000 tons. • Suitable for 470 – 4000-ton machines • Payloads as high as 264 lbs. • Full servo 3 axis with option for full servo 5-axis A+C • Vertical stroke up to 3000 mm • Traverse stroke up to 4500 mm • Aggressively competitive price ABSOLUTE ROBOT www.absoluterobot.com 38 JUNE 2017 Plastics Technology PTonline.com K now How