Q05 — TECHNICAL ANSWER
Machined or Moulded Plastic Parts — Which?
Machine a plastic part when the quantity is small, the geometry is simple to cut, or the material cannot be moulded. Mould it when a formed shape is needed repeatedly and identically. We run both processes, which means the answer here is a manufacturing decision rather than a sales one.
What each process is actually good at
Machining removes material to reach a shape, so it suits parts whose defining features are cut features — diameters, bores, faces, profiles along a length. A UHMWPE wear strip with a specific track profile and hole pattern is straightforward to cut and would be awkward to form.
Moulding reproduces a form, so it suits parts whose defining feature is the form itself — a deep-formed cup, a ribbed cover, a toothed gear, a profiled roller. The form comes out the same every cycle, which is what makes moulded parts interchangeable across a machine and replaceable years later to the same fit.
The dividing line is usually the shape rather than the quantity. A part that would take a long time to cut from solid, and produces a lot of swarf doing it, is telling you it wants to be moulded.
When the material decides for you
Some materials remove the choice. UHMWPE does not flow like a conventional thermoplastic, so it is machined to drawing from sheet or rod regardless of quantity — its molecular weight is exactly what gives it the wear resistance it is chosen for.
Others go either way. Nylon and acetal gears, sprockets and star wheels are machined or moulded depending on the part and the quantity — a single replacement sprocket with a keyed bore is usually faster machined; a repeat batch of the same form is better moulded.
HDPE, PP and ABS parts in this catalogue are moulded, because the components made in them are formed shells and covers rather than cut profiles.
How the decision is made on a job
It is made at the review stage, against your drawing, before anything is produced. We review the requirement, confirm manufacturability, material and process, and manufacture the finished component to it — the process is part of what gets confirmed, not an assumption.
That is worth knowing when you are comparing suppliers who only run one of the two. A moulder will find a way to mould your part and a machine shop will find a way to cut it; the answer you get depends on what the supplier owns.
Some components use both. A part can be moulded and then machined where a bore or a face has to be finished to the drawing, which is often the shortest route to a formed part with one precise feature.
Machining and moulding — related questions
- Is moulding always cheaper in quantity?
- Not automatically. The shape matters as much as the number — a simple profile is cheap to cut at any quantity, while a formed shell is slow and wasteful to machine even in small numbers.
- Which plastics can you not mould?
- UHMWPE. It does not flow like a conventional thermoplastic, so those parts are machined to drawing from sheet or rod. Nylon, HDPE, acetal, PP and ABS are moulded.
- Can a part be both machined and moulded?
- Yes, and it often is. A moulded part can be machined afterwards where a bore or a face has to be finished to the drawing.
- Who decides the process, us or you?
- You can specify it, and we manufacture to your drawing. Where it is left open we confirm the process with you at review rather than assume one.
Send your drawing with the material, quantity and application and we will confirm the process with you before production. If you are replacing an existing part and have no drawing, a sample with dimensions and photographs is a workable starting point.