Injection unit barrel and screw for thermoplastics: bimetallic wear protection and pressure rating

01

Choose the barrel wear protection: nitrided bore or bimetallic inlay

A Xaloy patent explains why bore wear matters: if the bore is enlarged by wear or corrosion, plastic leaks back through the increased clearance between screw and bore, and the proper pressure and injection action cannot be maintained. It compares two constructions. A nitrided bore gives a hardened layer only 0.005 to 0.020 inches deep, with nitride concentration falling rapidly with depth; a centrifugally cast inlay is about 0.060 to 0.125 inches thick, with uniform composition, corrosion resistance, hardness and wear resistance through its thickness, and makes the cylinder last 5 to 10 times as long as a nitrided barrel. [1]

Reifenhäuser Reiloy lists four bimetallic inlay alloys for injection moulding and extrusion barrels. Smart R121 is a martensitic iron-based alloy with chromium carbides, 1.5 mm layer, min. 65 HRC, for plastics with up to 30% glass fibre. Advanced R131 is iron-based with vanadium carbides, 1.5 mm layer, min. 60–64 HRC, for up to 40% glass fibre. Premium R241 is a nickel matrix with tungsten carbides, layer min. 1.0 mm, min. 59 HRC, for maximum wear and corrosion resistance. Select R115 is a nickel-cobalt alloy, min. 55 HRC (min. 47 HRC above 60 mm inner diameter), for maximum corrosion resistance. [3]

For PA, PBT, PEEK and PPS, Reiloy's barrel matrix recommends Smart or Advanced up to 30% filler, Advanced or Premium below 40% and Premium above 40%; for PTFE it lists Select. [3]

Editorial calculation: 0.060 to 0.125 in equals about 1.5 to 3.2 mm, so Reiloy's 1.5 mm layer sits at the thin end of the cast inlay range in the Xaloy patent, and both are roughly 3 to 25 times the 0.005 to 0.020 in (0.13 to 0.51 mm) depth of a nitrided layer. [1] [3]

Give the supplier the machine model and injection unit size, screw diameter and L/D, the resins and fillers (type and percentage), processing temperatures, any corrosive additives or flame retardants, maximum injection pressure, the current barrel and screw materials and the wear measured, and whether you need a new unit or a replacement barrel and screw.

02

Check the barrel's pressure rating and backing steel

Another Xaloy patent describes injection pressures of 20,000 psi and above at the high-pressure end of the cylinder. Its process heat-treats the backing steel to a bainitic structure of 30–50 Rockwell C while the hard ferrous liner transforms to martensite of 58–65 Rc. In its example the backing steel reached 50 Rc, 247,000 psi yield strength and 271,000 psi tensile strength, and the 64 Rc liner did not crack until 55,000 psi internal pressure; without the heat treatment the backing steel was only 18–20 Rc and 60,000 psi yield, so the liner would crack below 30,000 psi. [2]

The later Xaloy patent sets discharge-end pressure as high as 30,000 psi during the forward stroke of screw and check ring, and notes that casting heat causes grain growth in the cylinder steel while nickel or cobalt inlays need slow cooling to avoid cracking. Its example cylinder, 1.650 in bore × 5.078 in outside diameter × 24 in long with a tungsten-nickel inlay in ASTM 193B-16 steel, failed at 60,000 psi internal pressure; the inlay fractured but the crack did not extend into the steel. [1]

Reiloy lists the strength of its barrel base materials as Rm 980 MPa and Rp0.2 at 300 °C of 580 MPa for Reiloy Standard, 800 and 360 MPa for C60, and 630 and 300 MPa for NiCr22Mo9Nb; it specifies Reiloy Standard backing for its injection moulding barrels, with NiCr22Mo9Nb as an option for Select R115. [3]

Editorial calculation: 20,000 psi ≈ 138 MPa (about 1,380 bar) and 30,000 psi ≈ 207 MPa (about 2,070 bar); the 55,000 psi crack pressure in the first example is about 379 MPa, roughly 1.8 times 30,000 psi. [2] [1]

03

Compare barrel inlay alloys

Reiloy's datasheet lists these inlays; ranges apply to the stated alloy only.

04

Where wear and pressure act in the barrel, screw and check ring

In the Xaloy patents the bore inlay and the check ring decide whether the unit still builds pressure, while the backing steel decides how much pressure it can hold.

05

State pressure, bore size and alloy for a barrel and screw quotation

Pressure, bore diameter, barrel temperature and the inlay and backing materials are the values the reviewed sources give numbers for; the remaining rows depend on the machine maker.

06

Send the resin, pressure and wear data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Machine: [model; injection unit size]
Screw: [diameter; L/D; check ring type]
Resin: [type; filler and percentage; additives]
Conditions: [melt temperature; maximum injection pressure]
Barrel: [nitrided or bimetallic; inlay alloy; backing steel]
Wear: [measured bore and screw wear; current life]
Scope: [complete unit; barrel and screw only; nozzle and heaters]

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