A plastic mold works if the part can leave it and the melt can reach every corner of it: walls taper with a draft angle of 1-3 degrees so the part releases, wall thickness stays uniform so it cools evenly, internal corners are rounded, and undercuts are avoided. The mold is two halves clamped together with a cavity, a runner and gate to carry the melt in, vents for trapped air, and ejector pins to push the part out.
Part of Plastics and Polymers
Mold design is the layout of the cavity, runner, gate, vents and ejection of a tool that gives a molded plastic part its shape.
The Parts Of A Mold
Plastics Manufacturing names the parts:
- Two halves that clamp together and form a cavity in the shape of the part.
- A gate, the opening where molten plastic enters the cavity.
- A runner, the channel from the injection point to the gate.
- Ejector pins, which push the finished part out when the mold opens.
Add to those vents: small shallow slits or holes at the points that fill last, where air would otherwise be trapped and compressed. The corpus’s Plastic Blow Molding makes the same point for blow moulds, where air trapped at the corners is vented through small holes. Venting is from general knowledge here for injection moulds.
Four Design Rules From The Carrier
- Draft angle of 1-3 degrees. Walls must taper so that the part releases. Straight walls grip the mold as the plastic shrinks onto the core.
- Uniform wall thickness. Thick sections cool more slowly than thin ones, which causes warping and internal stress. Keep walls even.
- Rounded corners. Sharp internal corners concentrate stress and the part cracks there. Put a radius on every corner.
- No undercuts. A shape that locks into the mold prevents ejection. If you cannot avoid one, you need a split mold with a side-action mechanism.
Where To Put The Gate
Put the gate at the thickest section so that the melt flows from thick to thin. If it enters a thin section first, it freezes there and the thick section shrinks with nothing feeding it, leaving a sink mark or void. Place the gate where a small scar will not matter, since the gate must be cut off. Keep the runner short and no thinner than the gate, because it also freezes.
Choose The Mold Material By How Many Parts You Need
| Mold material | Good for | Limit |
|---|---|---|
| Steel | Long runs, high pressure, hot plastics | Hard to machine |
| Aluminium | Low-volume production, easy to work | Wears faster, lower pressure |
| Hardwood | Very small runs of low-temperature plastics (starch, casein) | Chars on hot plastic |
The table follows the carrier. Mould surfaces need a smooth finish, because every scratch is copied into the part.
Worked Example: Sizing The Cavity For Shrinkage
Thermoplastics shrink as they cool. The corpus gives no shrinkage figure for plastics, so measure it on a test shot. Suppose a test part meant to be 100.0 mm comes out at 98.5 mm. The shrinkage is (100.0 - 98.5) / 100.0 = 1.5%. To make a 100.0 mm part, cut the cavity to 100.0 / (1 - 0.015) = 101.5 mm. The 1.5% is an invented illustration of the method, not a material property.
Common Mistakes
- Making the cavity exactly part size — it comes out small.
- Thick boss on a thin wall — sink mark opposite the boss.
- No vents — short shots and burn marks at the last-filled corner.
- Forgetting the parting line — flash appears where halves do not seat.
Hot mold, flying plastic
A mold at working temperature burns skin, and a mold that is not fully clamped sprays molten plastic out of the parting line. Wear gloves and eye protection and keep your face clear of the parting line during injection.
What’s Next
- Learn next: Molding Process — heat, inject, cool, eject
- Combine with: Compression Mold — the thermoset mold, which needs no runner