The molding cycle is heat the plastic, clamp the mold, inject, hold pressure while the part cools, open the mold and eject, and the hold stage lasts from 30 seconds to several minutes depending on the size of the part. Most defects come from three causes: melt too cold or too hot, pressure too low or too short, and uneven cooling, and each defect points to one of them.
Part of Plastics and Polymers
The molding process is the repeated cycle of melting, injecting, cooling and ejecting thermoplastic to make parts in a closed mold.
Run The Cycle In Order, Every Time
Step 1: Heat. Bring the plastic in the cylinder to its working range and no higher. Plastics Manufacturing warns that degradation begins 30-50 °C above the working temperature, and that is also where the toxic fume is worst. Hold the melt there long enough to be even, and no longer, since plastic that sits hot keeps degrading.
Step 2: Clamp. Close the mold and clamp it harder than the injection force will try to open it. See the arithmetic in Injection Molding.
Step 3: Inject. Drive the plunger steadily. The cavity should fill in one smooth push.
Step 4: Hold and cool. Keep pressure on while the gate is still molten so that more plastic feeds in to replace what shrinks. The carrier gives 30 seconds to several minutes. When the gate has frozen, pressure no longer matters, and the part cools to rigidity.
Step 5: Eject. Open the mold and push the part out with the ejector pins. A part ejected too early deforms. Trim the gate and runner, and put the trimmings back into the feed for sorting and remelting.
Defects Point Back To A Cause
| What you see | Likely cause | First thing to change |
|---|---|---|
| Part not full (short shot) | Melt too cold, pressure too low, vent blocked | Hotter melt, more force, clear vents |
| Thin fin along parting line (flash) | Clamp too weak, pressure too high | Clamp harder, lower pressure |
| Dimple over a thick section (sink mark) | Hold too short, thick wall | Hold longer, thin the wall |
| Warping | Uneven cooling, early ejection | Cool evenly, wait before ejecting |
| Brown streaks, burnt smell | Melt too hot or trapped air | Lower the temperature, add vents |
| Visible seam line (weld line) | Two flow fronts met too cold | Hotter melt, move the gate |
| Cracks at corners | Sharp internal corner, stress | Rounded corners; see Mold Design |
The causes are standard moulding practice, from general knowledge and not a corpus table.
Treat One Variable At A Time
Change only temperature, or only pressure, or only hold time, between shots. Two changes at once make it impossible to know which one worked. Write down each shot’s temperature, hold time and result, since a settlement has no tech support to ask.
Worked Example: Timing A Batch
A part needs 90 seconds of hold and cooling, and 30 seconds to open, trim and reload the mold. One cycle is 90 + 30 = 120 s, so one mold yields 3,600 / 120 = 30 parts per hour. Cooling is 90 of the 120 seconds, so cooling time is the limit. A second mold filled while the first cools could roughly double the output.
Common Mistakes
- Opening before the gate freezes — the melt flows back out of the cavity.
- Holding the heat during a long pause — plastic left molten in the cylinder degrades.
- Regrinding too much — each remelt weakens the plastic; see Remelting.
Fume, burns and trapped pressure
Burnt-smelling streaks mean the plastic is degrading and releasing fume: stop, ventilate, and find out what the plastic is before you resume. Molten plastic burns skin and sticks. Never clear a blocked nozzle while the cylinder is hot and loaded. See Fume Safety.
What’s Next
- Learn next: Safety and Quality — testing what you have made
- Combine with: Buttons, Handles and Hardware — parts worth molding first