Injection Molding Process Values and Calculations (PDF Reference)
Setting up or quoting an injection molding job needs a handful of numbers over and over: clamp tonnage, shot size, pressures, cooling time and drying conditions. This reference collects them in one searchable table with the formula, a typical range and the unit, and exports to PDF or Excel for the floor.
| Parameter | Formula or rule | Typical value or range | Unit | Note |
|---|---|---|---|---|
Clamp force required | Projected area x cavity pressure x number of cavities | 2 to 5 for general parts | US tons per sq in of projected area | Thin wall and long flow paths need more. Add a margin and check the machine rating. |
Clamp utilization | Required clamp force / machine clamp force | Up to about 80 percent | percent | Running near 100 percent risks flash and mold wear. |
Projected area | Part area seen in the direction of mold opening, plus runner area | Part dependent | sq in or sq cm | Include runner in cold runner molds. |
Shot size (volume) | Part volume x cavities + runner volume | Part dependent | cu in or cm3 | Add cushion, usually a few millimetres of screw travel. |
Shot weight | Shot volume x melt density | Resin dependent | g or oz | Use melt density, which is lower than solid density. |
Barrel utilization | Shot size / machine shot capacity | About 20 to 80 percent | percent | Very small shots risk long residence time and degradation. |
Melt pressure | Hydraulic pressure x intensification ratio | 5,000 to 20,000 | psi | Ratio is machine specific. Electric machines read pressure differently. |
Hold pressure | Set as a fraction of injection pressure | About 50 to 80 percent | percent of injection pressure | Set using a gate seal study. |
Back pressure | Set at the screw during recovery | 50 to 300 | psi (hydraulic) | Higher improves mixing and color but raises shear and recovery time. |
Injection time | Fill time to transfer point | 0.5 to 5 | s | Depends on wall thickness and flow length. |
Cooling time | t = s squared / (pi squared x alpha) x ln[ 4/pi x (Tmelt - Tmold) / (Teject - Tmold) ] | Often the largest part of the cycle | s | s is wall thickness, alpha thermal diffusivity. Cooling grows roughly with the square of wall thickness. |
Cycle time | Injection + hold + cooling + mold open and close + ejection + dwell | 10 to 60 for typical parts | s | Cooling usually drives it. Measure with a stopwatch before estimating. |
Parts per hour | 3600 / cycle time x cavities | Part dependent | parts per hour | Multiply by expected yield to get good parts. |
Screw recovery rate | Shot weight / recovery time | Machine rated | g/s or lb/h | Check plasticizing capacity against shot weight and cooling time. |
Screw L/D ratio | Flight length / screw diameter | About 20:1 | ratio | General purpose screws for common resins. |
Mold shrinkage allowance | Cavity size = part size x (1 + shrinkage) | Resin and fill dependent | percent | Use the resin supplier's shrinkage data and verify with a first article. |
Wall thickness uniformity | Keep walls even, ribs about 50 to 60 percent of nominal wall | 1 to 4 mm for most parts | mm | Thick sections cause sink and long cooling. |
Drying: nylon (PA6, PA66) | Hot air or desiccant dryer | About 80 C for 4 hours | C and h | Dew point near minus 40 C. Follow the supplier datasheet. |
Drying: polycarbonate (PC) | Desiccant dryer | About 120 C for 3 to 4 hours | C and h | Moisture causes splay and loss of strength. |
Drying: ABS | Hot air or desiccant dryer | About 80 C for 2 to 4 hours | C and h | Follow the grade datasheet. |
Drying: PET | Desiccant dryer | About 150 C for 4 to 6 hours | C and h | Very sensitive to moisture. |
Drying: polypropylene (PP) | Usually not required | No drying for most grades | - | Check filled and specialty grades. |
Typical melt temperature: PP | Supplier datasheet range | About 200 to 250 | C | Always use the datasheet range for the exact grade. |
Typical melt temperature: ABS | Supplier datasheet range | About 210 to 250 | C | Higher heat can discolor and degrade. |
Typical melt temperature: PC | Supplier datasheet range | About 280 to 320 | C | Hot, dry resin needed. |
Typical melt temperature: PA66 | Supplier datasheet range | About 270 to 295 | C | Narrow processing window. |
Typical mold temperature: PP | Supplier datasheet range | About 20 to 60 | C | Hotter molds improve surface and reduce warpage but lengthen cooling. |
Typical mold temperature: ABS | Supplier datasheet range | About 40 to 80 | C | Affects gloss and stress. |
Typical mold temperature: PC | Supplier datasheet range | About 80 to 120 | C | Affects stress and surface. |
Typical mold temperature: PA66 | Supplier datasheet range | About 70 to 100 | C | Controls crystallinity and shrinkage. |
Typical shrinkage: ABS | Datasheet range | About 0.4 to 0.8 | percent | Varies with grade and direction. |
Typical shrinkage: PC | Datasheet range | About 0.5 to 0.7 | percent | Varies with grade. |
Typical shrinkage: PP | Datasheet range | About 1.0 to 2.5 | percent | Semi-crystalline, direction dependent. |
Typical shrinkage: PA66 unfilled | Datasheet range | About 0.8 to 2.0 | percent | Glass fill lowers it, with directional differences. |
Values are typical starting points and rules of thumb for general grades, not specifications. Always use the resin supplier's datasheet, the machine manual and a documented process window for the actual job.
Compare quoted and actual process values
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How to use the reference
Type a parameter or resin name in the search box to filter the table. Use the formula column to calculate your own job values, then compare with the typical range as a sanity check. Export to PDF to pin it at the press or to Excel to add your own materials. Treat every range as a starting point to be replaced by datasheet values and your validated process window.
- Search by resin (PC, ABS, PA66, PP) or topic (drying, shrinkage)
- Use ranges as a sanity check, not as a setpoint
- Record the validated value on the process sheet
The key calculations
Clamp force is projected area multiplied by cavity pressure, then checked against machine capacity with margin. Shot size is part volume times cavities plus runner, then compared with barrel capacity. Cycle time is the sum of injection, hold, cooling, mold motion and ejection, and cooling dominates for thick walls because it grows with the square of thickness. Parts per hour is 3600 divided by cycle time, times cavities, before yield loss.
Reading the ranges
A range tells you whether a number is plausible, not whether it is right. A clamp requirement of 0.5 tons per square inch or 12 should prompt a recheck of area units. Melt and mold temperatures vary by grade, fillers and wall thickness, so use the supplier's window. Drying matters most for hygroscopic resins such as nylon, polycarbonate and PET, where moisture causes splay, brittleness and viscosity loss.
- Check units first when a result looks off by ten times
- Hygroscopic resins need drying before every run
- Verify shrinkage with a first article before cutting steel
Common mistakes
Frequent errors include forgetting runner area in the projected area, using solid density for shot weight, running a press at full tonnage, and copying a melt temperature from another grade. Another is quoting from the cycle time of a similar part rather than from a calculated cooling time. Record actual cycle times and scrap in the ERP so future quotes use real data.
Frequently Asked Questions
What is the formula for clamp tonnage in injection molding?
Clamp force equals the projected area of the part and runner multiplied by the cavity pressure, with a safety margin. A common starting point is 2 to 5 US tons per square inch of projected area, higher for thin walls and long flow paths.
How do I estimate injection molding cooling time?
Cooling time depends on wall thickness squared, the resin's thermal diffusivity and the melt, mold and ejection temperatures. The result is a starting estimate; confirm it with a trial and adjust for part geometry and mold cooling.
Are these values valid for every resin grade?
No. They are typical ranges for general grades. Filled, flame retardant and specialty grades differ, so always use the supplier datasheet and the process window validated on your machine and mold.
Can I get this as a PDF?
Yes. Use the export button to save the table as a PDF for the press, or as Excel to add your own materials and notes.
If your actual cycle times and scrap sit in the ERP and MES, we can help you build a private assistant on your own servers that compares quoted and actual process values by mold.
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