Clamp tonnage
Projected area times a tonnage factor for the resin. Projected area is the footprint of all cavities and the runner, seen from the direction the mold opens.
Rule-of-thumb factors. Thin walls under about 1 mm, long flow lengths and stiff or glass-filled grades need the high end or more. 1 U.S. ton = 0.907 metric tonnes = 8.90 kN.
Mold shrinkage and cavity size
The cavity is cut larger than the part by the shrinkage. Use the shrinkage from your grade’s data sheet when you have it.
Glass-filled resins shrink much less along the flow than across it, often 2 to 3 times less, so one number cannot describe both directions. Values are family-level typical figures from our processing parameters chart.
Cooling time estimate
Estimated from the thickest wall and the melt, mold and ejection temperatures. Cooling is usually the longest part of the cycle, and it grows with the square of the wall thickness.
A textbook estimate for a flat wall cooled from both sides. Real cycles depend on cooling-line layout, steel, part shape and how warm the part can be when it is ejected, so treat this as a starting point. Temperatures are midpoints from our processing chart.
Dryer and hopper size
The hopper has to hold enough resin to give every pellet its full drying time at the rate the press uses it.
Drying temperature and time come from our resin drying chart. Pellet bulk density is usually 33 to 40 lb/ft³; regrind and powder run lower. Drying times assume a dehumidifying dryer with a dew point of −40°F.
Regrind
At a steady regrind rate, part of every shot has been through the barrel more than once. This shows how much, and how to mix a batch.
Many customer specs cap regrind at 15 to 25%, and some medical, food-contact and safety parts allow none. Hydrolysis-sensitive resins such as nylon, PC, PET and PBT lose more with each pass, especially if regrind is not dried. See prime vs. off-grade resin.
Colorant let-down ratio
A let-down ratio of 50:1 means one part colorant to 50 parts natural resin.
Your colorant supplier’s recommended ratio comes first. Some suppliers state percent of the natural resin rather than of the blend; at low loadings the difference is small.
Material cost per part
Enter your own resin price. We do not publish prices; this uses only the figures you type.
Material only: no machine time, labor or packaging. Pounds per 1,000 parts are shown even without a price, which is useful for estimating orders; the order-size calculator turns them into gaylords and truckloads.
Common questions
How do I calculate clamp tonnage for injection molding?
Multiply the total projected area of the parts and runner, seen from the direction the mold opens, by a tonnage factor for the resin, usually 2 to 5 tons per square inch. Add about 10% as a safety margin. Thin walls and long flow lengths need the high end of the range.
How do I size a mold for shrinkage?
Divide the part dimension by one minus the shrinkage. For a 100 mm part in a resin that shrinks 1.5%, the cavity should be 100 / 0.985 = 101.52 mm. Multiplying by one plus the shrinkage gives a slightly smaller number and is a common source of undersized parts.
How big should a dryer hopper be?
Hopper capacity in pounds equals the press throughput in pounds per hour times the drying time in hours. A press using 40 lb/hr of a resin that needs 4 hours needs at least 160 lb of resin in the hopper at all times; most molders add about 20% margin.
What does a 50:1 let-down ratio mean?
One part colorant to 50 parts natural resin. That is 1.96% colorant by weight of the blend, which many people round to 2%.
Running the numbers on a new resin?
Send the grade you run now, or the part and conditions, and a polymer chemist matches a grade and quotes it.
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