Knowledge Hub · Plastics processing series

Six ways to shape plastics
how the same pellet becomes very different parts

A bottle cap, a jerry can, a pipe, a food tray, a melamine plate and a water tank all start as plastic pellets or powder, but go through very different machines. This page compares the six main processes in one table, then takes you through each machine on its own page.

Where processing sits in the value chain

A plastic part goes through four stages: resin → additives → processing → part. Petrochemical producers make the base resin, additives and masterbatch tune its properties for the job, and converters turn it into parts. ACS works in the second stage. We do not mould parts ourselves, but knowing how each machine works is essential to choosing a formulation: time, temperature and shear differ so much between processes that the same additive can perform very differently.

The six processes compared

Tooling cost, output speed and economic volume are general relative comparisons; they depend on part size and machine.
Process	Shape made	Typical parts	Main resins	Tooling cost	Output speed	Economic volume	Wall-thickness control
Injection molding	Solid, complex 3D parts	Caps, boxes, crates, automotive and appliance parts	PP, HDPE, ABS, PS, PC, PA	High	High	High	Very good, set by both mould halves
Blow molding	Hollow parts with a narrow neck	Bottles, jerry cans, chemical drums	HDPE, PP, PET	Medium	Medium to high	Medium to high	Moderate, via parison or preform control
Extrusion	Constant cross-section, continuous length	Pipe, profile, sheet, film, cable	PE, PP, PVC	Low to medium (die)	High, continuous	High	Good across the section
Thermoforming	Thin-walled, open shapes from sheet	Cups, trays, blisters, clamshells	PS, PP, PET, PVC	Low to medium	High	Low to high	Limited; walls thin out at corners and deep draws
Compression molding	Flat or thick parts from thermosets, rubber or composites	Melamine plates, rubber seals, electrical insulators, SMC panels	Thermosets (phenolic, melamine, UP/SMC), rubber	Medium	Low to medium (curing time)	Medium	Good, suits thick sections
Rotational molding	Large, seamless hollow parts	Water tanks, kayaks, wheeled bins	LLDPE / MDPE powder	Low	Low	Low to medium	Moderate; fairly even walls, corners tend to be thicker

How to choose a process

Start with the shape of the part, then look at volume and material. These questions narrow the options quickly.

Hollow with a narrow neck, such as bottles and jerry cans: blow molding. Very large and low volume, such as water tanks: rotational molding.
Same cross-section along the whole length, such as pipe, profile, sheet or film: extrusion.
Thin-walled and open on one side, such as cups and trays: thermoforming from extruded sheet.
Complex 3D shape, tight dimensions and high volume: injection molding.
Thermoset, rubber or composite material: compression molding.
Volume versus tooling cost: an expensive mould pays off when spread over many parts. At low volume, a process with cheaper tooling may give a lower cost per part.
Resin grade follows the process: injection uses higher-MFI grades; blow molding and extrusion use low or fractional MFI so the melt can support its own weight; rotational molding uses powder ground from dedicated grades.

The process also decides how additives go in. Injection, blow molding and extrusion can dose masterbatch at the hopper throat. Rotational molding uses powder, so additives usually have to be compounded into the pellet before it is ground. Details are on each process page.