What tonnage does a case injection molding machine need for a 480 mm cavity?

Answer: 617 tonnes at 35 MPa, 710 with the 1.15 factor, so specify 800. A 650-tonne press flashes the seal lip.

Does a bigger case injection molding machine shorten the cycle?

Answer: Only if the screw grows too, else plasticising lengthens and 72 seconds becomes 78.

Can screw diameter simply follow tonnage?

Answer: No. Tonnage clamps, the screw meters. A 2.5 kg shot suits 65 mm; 80 mm leaves melt standing 3 minutes longer.

Must a deep core carry its own water?

Answer: Above 180 mm, yes. Without it the core reaches 90 degrees and four corners lift 0.8 mm.

How is clamp force proven at trial?

Answer: Over 30 shots there should be no 0.1 mm flash on the lip and a 0.05 mm feeler should stay out.

A 480 mm cavity drawing: sizing a case injection molding machine

Quick answer: Fix clamp force from the pressure-bearing area, choose the screw from shot weight, then set the cycle from cooling area. Run those steps in order and a case injection molding machine lands on the right frame size.

Reading the drawing for three loads
Projected area times cavity pressure equals clamp force
Pick the screw by shot weight
Mould and cycle follow water lines and ejection
Turning four numbers into a trial checklist
FAQ

1. Reading the drawing for three loads

An instrument-case builder brought a drawing last month and asked: a 480 mm by 360 mm by 180 mm cavity, 3.0 mm walls, 2.2 kg per part, would the old 650-tonne press in the bay run it? A case injection molding machine is specified by four linked numbers, and the first one decides the rest.

Three groups of figures matter. The pressure-bearing outline is not the cavity outline: 480 mm by 360 mm, plus outer walls and a 12 mm seal lip, reaches 508 mm by 388 mm, and once the lip is deducted the projected area is 1730 square centimetres. Weight comes next, 2.2 kg of part plus 0.3 kg of runner, a 2.5 kg shot. Then cooling area, 0.65 square metres at a 3.0 mm section, which fixes cooling time, cycle and output.

From the drawing	Converted to	Machine parameter it drives
Cavity 480 by 360 mm, 12 mm lip	1730 sq cm bearing area	Clamp force
Part 2.2 kg, runner 0.3 kg	2.5 kg shot	Screw capacity
0.65 sq m, 3.0 mm section	32 s cooling	Mould circuit

2. Projected area times cavity pressure equals clamp force

Clamp force is the first figure a case injection molding machine must meet, and cavity pressure tracks depth. At 180 mm the melt runs 300 mm from the gate, so 35 MPa is needed to pack the corners; a 60 mm lid needs 25 MPa. Take 1730 square centimetres at 35 MPa and the clamp force is 6055 kN, or 617 tonnes.

Case Injection Molding Machine output: matte black case face-on, screw bosses inside the handle, three square feet below

Platens absorb the surge of packing, so a 1.15 factor lifts 617 tonnes to 710. One frame size up, 800 tonnes covers it. A 650-tonne press falls 60 tonnes short and leaves 0.1 mm of flash along the seal lip, which is exactly where an O-ring has to seat.

Oversizing has its own bill: tie bars 1050 mm apart leave 100 mm of slack around a 900 mm by 850 mm mould and add 40 minutes per changeover, while a 620 mm stack on a press rated from 500 mm needs a spacer that eats 30 mm of ejector stroke.

3. Pick the screw by shot weight

Buyers usually match the screw to tonnage, the second place a specification slips. Capacity drives the choice: 2.5 kg divided by 0.8 calls for 3.1 kg of polystyrene equivalent, and a 65 mm screw delivers 3.6 kg.

Fit an 80 mm screw to an 800-tonne case injection molding machine and nominal shot jumps to 6.0 kg; polypropylene held above 230 degrees for 6 minutes then degrades into yellow streaks while weight scatter widens from 1.5 percent to 3 percent.

Keep length-to-diameter at 22 to 24, compression ratio 2.5, back pressure 6 MPa and 90 rpm. Harder back pressure breaks glass fibres and costs 30 percent of strength. Drying follows the resin: 80 degrees for polypropylene, 4 hours at 100 for glass-filled PA66 to reach 0.15 percent moisture.

4. Mould and cycle follow water lines and ejection

Cycle time is where a case injection molding machine earns its keep, and 72 seconds is built rather than guessed: 8 seconds of clamp movement, 12 of hold, 32 of cooling and 5 for removal, with plasticising overlapped. Cooling is the only stage with slack, and cooling belongs to the water circuit.

Case Injection Molding Machine output: two-tone case upright, black front, pale grey back shell, black hinge pin, vertical parting line

A core 180 mm deep needs its own circuit. Fixed-half water alone lets the core climb to 90 degrees, the skins shrink 0.8 mm apart, and the bottom corners lift until half of them miss a bench.

Hold 28 degrees with a 3 degree rise and 60 litres per minute per cavity; at an 8 degree rise, two parts from one shot differ by 0.06 kg and operators chase hold pressure all shift.

Allow 1.6 percent shrink, so a 480 mm cavity is cut at 487.7 mm. Draft stays at 1.5 degrees; below 1 degree in the deep section, ejection climbs from 11 tonnes to 18 and prints 0.3 mm bumps where a liner has to sit.

5. Turning four numbers into a trial checklist

On one sheet they become a checklist ticked before a case injection molding machine is accepted.

Check	Target	Tolerance
Consecutive shots	30	No stop
First-part weight	2.2 kg	2 percent
Cavity length	480 mm	0.3 mm
Cycle time	72 s	4 s

The skipped item is usually the consecutive count: watch weight from shot 25 to 30, where a short screw shows first. Time the cycle by stopwatch rather than the screen, since a 4 second gap to reality is normal. Log measured tonnage too, because platens deliver about 90 percent of nominal and 760 tonnes holding 710 leaves 7 percent.

Conclusion

Sequence matters: area sets tonnage, shot weight sets the screw, cooling sets the cycle, ejection sets the tool. Swap two and the early saving returns doubled. Every millimetre on the drawing ends up as a figure on a plate, which is the whole art of specifying a case injection molding machine.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.