Twelve questions worth asking any factory before you send a deposit — including us. Written for buyers replacing or upgrading a screw barrel on an injection moulding or extrusion line.
Most buyers choose a screw barrel supplier on price and delivery date, then discover that quoted metallurgy and delivered metallurgy are not always the same thing. This checklist is the one we hand to customers who are comparing several quotes. Use it on us, use it on our competitors, and buy from whoever answers best.
Nothing here is theory. Every point below is something we get asked, or something we wish a customer had asked their previous supplier.
Each row gives you the question, why it matters, and our own answer in plain terms.
Ask this because: Every batch of base steel should arrive with a mill test certificate showing chemical composition and mechanical properties. Ask for sample certificates from previous orders before you place your own.
Our answer: We supply the mill certificate for the base steel and the certificate for the alloy powder, and we send samples to new customers who ask.
Ask this because: Nitriding depth and surface hardness should be specified, not promised. A number nobody writes down is a number nobody will defend later.
Our answer: HV 900–1020 at 0.5–0.8 mm for gas nitriding, HRC 55–72 through a 2–5 mm cast alloy layer for bimetallic. Both figures appear on your quotation.
Ask this because: Bore straightness, surface roughness on the flight lands, and concentricity decide whether a set performs or merely fits.
Our answer: Straightness 0.015 mm/m or tighter, flight-land roughness Ra 0.4 µm, concentricity held and measured against your drawing.
Ask this because: A mature factory can list its inspection documents without thinking. Silence, or 'don't worry, we check everything', is an answer in itself.
Our answer: Dimensional inspection report, hardness readings, material certificates, NDT of the bimetallic bond, and photographs of the finished parts before packing.
Ask this because: You are buying one set, not a shipment of a thousand. Sampling tells you nothing about the part in your crate.
Our answer: Every single set is hardness-tested and dimension-inspected. There is no exemption for single-piece orders.
Ask this because: Any genuine plant can walk you through its machining floor on a live video call. Reluctance usually means there is no floor to walk.
Our answer: Live walkthrough of the machining shop, the nitriding and centrifugal casting furnaces, and the inspection room — during a call in your time zone.
Ask this because: SGS, TÜV or Intertek pre-shipment inspection costs a few hundred dollars and removes an entire category of risk.
Our answer: We host third-party inspectors regularly and hand over dimensional data for verification without argument.
Ask this because: If you are sending an OEM reference drawing or an in-house developed design, intellectual property terms belong in writing before anything else.
Our answer: We sign NDAs as standard for custom drawing work, or we sign yours.
Ask this because: A deposit with the balance before shipment is normal. Be careful with anyone demanding 100% up front from a new customer, and check that the receiving company matches the export licence.
Our answer: T/T (30% deposit, balance before shipment); L/C at sight available. We quote to the arrangement that suits your company.
Ask this because: Warranty terms negotiated after a problem are not terms, they are favours. Get the period, the coverage and the replacement route in writing.
Our answer: Written warranty against material, heat-treatment and dimensional defects, with a clear route to replacement or correction.
Ask this because: The first order tells you everything: delivery accuracy, documentation completeness, and quality consistency against the drawing.
Our answer: No minimum order quantity. One set is a perfectly reasonable first order, and it is how most of our long relationships began.
Ask this because: A supplier who claims to build every type at every size is promising something no factory delivers. Real factories know their range and say what lies outside it.
Our answer: We machine to Ø300 mm. Beyond that we tell you in the first reply, because a job we cannot do well is worse than a job we decline.
Copy this table into your enquiry so every supplier quotes the same thing.
| Item | Nitrided set | Bimetallic set |
|---|---|---|
| Base steel | 38CrMoAlA, 42CrMo, 34CrAlNi7, 31CrMoV9 | Same grades, cast-lined |
| Surface hardness | HV 900–1020 | HRC 55–72 |
| Layer depth | 0.5–0.8 mm nitrided case | 2–5 mm metallurgically bonded alloy |
| Bonding method | Diffusion (gas nitriding) | Centrifugal casting |
| Best suited to | Unfilled PE, PP, PS, ABS | Glass fibre, mineral filler, FR grades, PVC, recycled |
| Cost per running hour | Lowest for light duty | Lowest for abrasive duty |
| Parameter | Standard to specify | Why it matters |
|---|---|---|
| Bore straightness | ≤ 0.015 mm per metre | Prevents uneven screw contact and localised wear |
| Flight-land roughness | Ra 0.4 µm | Affects melt film stability and output consistency |
| Bore roughness | Ra 0.4–1.6 µm after honing | Governs conveying efficiency and cleaning |
| Screw–bore clearance | Per machine builder's schedule | A 10% drop in flight OD can cost ~25% output |
| Concentricity | Held and recorded against the drawing | Decides whether the set fits without modification |
Four nitriding furnaces and four bimetallic casting furnaces, in-house, which is why our schedule is not another factory's queue.
