The choice between extrusion and moulding is usually settled by geometry before anyone reaches the cost discussion. A component with a constant cross section and no defined endpoint is an extrusion. A component with features that vary along its length, or with a bonded metal insert, is a moulding. Most enquiries fall clearly on one side of that line.
The cases that do not are where the decision gets expensive, because tooling for one process cannot be adapted to the other. Understanding what each process can hold dimensionally, and what it costs to change your mind, is worth more at the enquiry stage than at the quotation stage.
What the Rubber Extrusion Process Actually Does
Uncured compound is fed through a screw and forced through a die of the required profile, emerging as a continuous length that is then vulcanised in line, typically by hot air, microwave or salt bath.
Two consequences follow from the continuous nature of the process. The first is that length is effectively unlimited, and unit cost falls sharply with run length, because the die is cheap relative to a mould and the process does not stop between parts. The second is that the material behaves in ways the die alone does not control. Compound swells as it leaves the die orifice, an effect known as die swell, and hollow or complex sections can collapse slightly during vulcanisation unless supported on mandrels or formers.
Extruded rubber profiles are therefore the right answer for gaskets, seals, glazing profiles, edge trims, tubing and conveyor skirting, where the section is constant, and the length is a cut-to-order variable.
What the Rubber Moulding Process Does Differently
Rubber moulding forms discrete parts in a closed cavity under heat and pressure, and it exists in three variants that are often treated as interchangeable when they are not.
Compression moulding places a preform in an open cavity and closes the press. Tooling is the cheapest of the three; it suits large parts and lower volumes, and it accepts a wide range of compounds. It also produces the most flash and the longest cycle times.
Transfer moulding forces the preform from a pot through sprues into a closed cavity, which improves consistency and handles bonded inserts far better than compression.
Injection moulding feeds compound through a screw directly into a heated closed mould. Tooling cost is highest, cycle time is shortest, and repeatability across a long run is the best of the three.
Moulded rubber products cover anything with three-dimensional geometry: bearings, pads, bushes, diaphragms, grommets, sole plates and rubber-to-metal bonded assemblies.
The Tolerance Question, With Numbers
This is where the comparison stops being a matter of preference. ISO 3302-1 sets dimensional tolerance classes for both processes, and the gap between them is substantial.
For moulded products, the standard defines four classes, M1 for precision mouldings down to M4 where dimensional control is non-critical. At a nominal dimension between 10 and 16 mm, a Class M1 fixed dimension carries a tolerance of ±0.15 mm. For unsupported extrusions, the standard defines three classes, E1 to E3, and at the same 10 to 16 mm nominal dimension, a Class E1 cross section carries ±0.50 mm. Class E3 at that size is ±1.30 mm.
Extrusion is between three and eight times looser than precision moulding at the same nominal size, and the standard notes that some synthetic rubbers cannot achieve Class E1 at all. If a drawing calls for tolerances that only moulding can hold, the process decision has already been made regardless of what the geometry suggests.
Two further clauses in the standard are worth knowing before a drawing is issued. Moulding introduces a distinction between fixed dimensions and closure dimensions, the latter being those affected by flash thickness and mould part displacement, and closure tolerances are the wider of the two. For parts moulded by transfer or injection, however, all dimensions can be regarded as fixed. That is a concrete argument for transfer or injection over compression on a dimensionally critical part, independent of volume.
Where the Compound Constrains the Process
Material selection interacts with the tolerance decision more than most specifications acknowledge. ISO 3302-1 states that silicone rubbers, fluorocarbon elastomers, and other special-purpose compounds shrink more on cooling, which makes Classes M1 and M2 very difficult to achieve with them. Bonded non-rubber parts alter shrinkage behaviour again and widen the practicable tolerance further.
The standard also states plainly that a product can be no more accurate than the mould, and that greater accuracy makes moulds and their maintenance progressively more expensive. Precision is purchased, not requested.
Volume, Tooling and the Real Cost Comparison
The economics run in opposite directions. Extrusion has low tooling and unit costs, with the die typically produced quickly. Its weakness is that changing the profile means a new die, and that scrap during start-up and profile changeover is unavoidable.
Compression moulding carries moderate tooling cost with higher unit cost and slower cycles, which suits low to medium volumes. Injection moulding inverts that, with high tooling investment recovered across a long run through short cycles and reduced material waste.
The break-even point depends on annual volume, part complexity, and design stability. A design still under revision is poorly served by an injection tool.
Choosing Between the Two
The decision usually resolves against four questions.
- Does the section stay constant along the full length, or does the geometry vary? Constant favours extrusion; varying requires moulding.
- Does the part need a bonded metal insert? Only moulding accommodates this, and transfer or injection handles it better than compression.
- What tolerance class does the drawing demand? Anything at M1 or M2 rules out extrusion entirely.
- What is the annual volume and how settled is the design? High and stable justifies injection tooling. Low or provisional favours compression or extrusion.
Where Custom Rubber Manufacturing Fits
Most industrial requirements are not served by a catalogue part. Custom rubber manufacturing covers compound formulation against the operating environment, tooling design, and process selection against the tolerance and volume requirement, and the three decisions are connected rather than sequential.
Ameenji Rubber Limited operates both routes, producing extruded rubber profiles alongside moulded rubber products including bridge bearings, composite grooved sole plates, anti-vibration pads, rubberised level crossings, industrial sheets and UIC vestibules, in compounds including EPDM, neoprene, nitrile, silicone and natural rubber. Material and dimensional verification is carried out under NABL-accredited testing, with MORTH empanelment and RDSO registration covering infrastructure and rail supply.
The process is rarely the first decision to make. Define the geometry, the tolerance class, and the operating environment, and the process selects itself.