Injection Molding and Plastics Processing

The injection molding cycle

Plastic pellets are melted in a heated barrel by a rotating screw, injected at high pressure into a closed steel mold, held under pressure while the part freezes, then the mold opens and the part is ejected. The cycle is the sum of the fill (1 to 3 s), packing and holding (3 to 10 s), cooling (usually the largest part) and mold opening, ejection and closing (2 to 6 s). Production molds are expensive (10 000 to 200 000 or more) but the cost per part is low, so the process wins from a few thousand parts up. For prototypes and short runs see 3D Printing and Prototyping Techniques.

Clamp force and press size

The injected plastic pushes the mold halves apart with the cavity pressure acting on the projected area of the part, so the press must hold them shut: F = P_cav × A_proj. Average cavity pressures are 20 to 40 MPa for thick, easy-flowing parts and up to 60 to 100 MPa for thin-walled or long-flow parts. Machines are rated in tonnes (or kN) of clamp force; choose one with 10 to 20 percent more than needed, and a shot capacity of 30 to 80 percent of the barrel’s.

Cooling time

t_cool = h² ÷ (π² α) × ln [ (4 ÷ π) (T_melt − T_wall) ÷ (T_eject − T_wall) ]

Worked example: the guard cover

Shrinkage and warpage

Plastic shrinks as it cools, by 0.5 to 3 percent depending on the polymer: crystalline plastics (PP, PE, PA, POM) shrink more than amorphous ones (ABS, PC, PS), and glass-filled grades much less and unevenly (more across than along the fibers). The mold cavity is made larger than the part by the shrinkage: for a 120 mm part the cavity is 120 ÷ (1 − s):

Plastic	Typical shrinkage	Cavity length for 120 mm
ABS	0.5 %	120.60 mm
PC	0.6 %	120.72 mm
PP	1.8 %	122.20 mm
PE-HD	2.5 %	123.08 mm
PA66 (unfilled)	1.6 %	121.95 mm
PA66 + 30 % glass	0.5 %	120.60 mm
POM	2.0 %	122.45 mm

Warpage results from uneven shrinkage: thick and thin sections cooling at different rates, flow orientation, uneven mold cooling. Keep walls uniform, add ribs instead of thickness, gate at the thick section, and cool the two mold halves equally. Sink marks appear opposite ribs and bosses: the rib thickness should be at most 0.5 to 0.6 of the wall (1.5 mm here).

Design rules

• Uniform wall thickness: typically 1.5 to 4 mm; change gradually (taper 3:1); core out thick sections.
• Draft of 1° to 2° (more on textured surfaces) so the part releases.
• Radii: inside corner radius at least 0.5 × the wall; sharp corners concentrate stress and block the flow.
• Ribs 0.5 to 0.6 of the wall thick, height up to 3 times the wall, with a draft; bosses with an outside diameter about twice the hole and joined to walls with ribs.
• Undercuts need slides or lifters, which raise the tool cost; design snap fits that can be molded straight (see Prototyping Techniques for snap-fit strain).
• Gate and weld lines: a weld line (where two flow fronts meet) is weaker and visible; place gates so weld lines fall in low-stress areas. Fill at 12 cm³/s here, through a 2.5 × 1.5 mm gate (330.7 m/s) is close to the limit for shear heating.
• Materials: choose from the stress, temperature, chemicals and appearance; add glass fiber for stiffness, UV stabilizers outdoors, flame retardants where required.

Other plastics processes

Process	How	Parts and volumes
Extrusion	melt pushed through a die, cooled and cut	pipes, profiles, sheet, cable insulation; continuous
Blow molding	a hot tube (parison) inflated inside a mold	bottles, tanks, ducts; millions
Thermoforming	heated sheet drawn over or into a mold by vacuum	trays, covers, vehicle panels; low tooling cost
Rotational molding	powder in a heated mold rotated on two axes	large hollow parts: tanks, kayaks; low tooling cost
Compression and transfer molding	thermoset charge pressed in a heated mold	electrical parts, composites
Reaction injection molding (RIM)	two liquid components react in the mold	large polyurethane parts
Vacuum casting	resin cast in silicone molds	prototypes and 10 to 50 pieces
3D printing	material added layer by layer	prototypes, one-offs, jigs (Material Selection, Testing and Failure)

Related

• Previous: Sheet Metal Forming. Next: Composites and Fiber-Reinforced Plastics.
• 3D Printing and Additive Manufacturing.
• Prototyping Techniques for tooling break-even.
• Materials and Processes: the whole guide.

FAQ

Why is the cooling time the biggest part of the cycle?

Plastics conduct heat poorly, and the time grows with the square of the wall thickness. Thin walls, good mold cooling and a higher ejection temperature all shorten it.

What does a hot runner do?

It keeps the plastic in the runner molten, so no runner has to be cooled, ejected and recycled. It saves material and cycle time for a higher tool cost.

Can I mold a part from any thermoplastic?

Most, but each has its own temperature, shrinkage and drying needs (nylon and PC must be dried). Take the data from the supplier and a mold-flow simulation for critical parts.

What does “short shot” mean?

The cavity did not fill: too little plastic, too cold a melt or mold, too slow an injection or a blocked gate or vent.

References

• Rosato, D. V., Rosato, M. G., Injection Molding Handbook, Springer; Beaumont, J. P., Runner and Gating Design Handbook, Hanser; Osswald, T. A., Polymer Processing Fundamentals, Hanser.
• Bryce, D. M., Plastic Injection Molding: Manufacturing Process Fundamentals, SME; Kazmer, D. O., Injection Mold Design Engineering, Hanser.
• ISO 20457 (plastic molded parts: tolerances) and DIN 16742; resin suppliers’ molding guides (Covestro, BASF, SABIC).

Last reviewed: October 2026. Cavity pressures, shrinkage, diffusivities and costs are typical illustrations.