Hard coating the screw offers the moulder significant advantages by dramatically reducing wear to the screw and screw tip assembly, resulting in less downtimes and increased profits.


Two of the most popular methods are PVD (Physical Vapour Deposition) and the HVOF method


The PVD-process (Physical Vapour Deposition) is based on vaporisation [ACFl) or sputtering of the metal components making up a solid metallic structure.


Importantly, the coating application temperature is less than the hardening and tempering  temperature of most original moulding and extrusion screws, thus ensuring that the existing molecular structure is maintained so hardness-losses are impossible.


The most commonly used hard (material) coatings are CRN, CRCN, TIN, and TICN.


PVD Coatings greatly increase durability resulting in a very hard surface e.g. chromium nitride 1800 - 2500 HV (2-3 mm the depth of nitriding) The same surface conditions exist after the application of the hard coating.


Hard coatings will increase the life of the screw substantially, but even after the coating has suffered excessive erosion (wear), the coating can be replaced and brought back to tolerance, reducing the need for new injection or extrusion screws.


The depth of these coating directly influences their durability and hence the life cycle of the component.




Colour: Gold

Application: Screws and Screw Tip Assemblies

Application temperature: up to 600 °C

Hardness; 1800-2500 HV

Friction: 0.65

Coating-thickness: from 1 to 6 µm




Colour: Grey

Application: Screws and Screw Tip Assemblies

Application temperature: up to 700°C

Hardness; 1800-2500 HV

Friction: 0.50

Coating-thickness: from 1 to 6 µm


Coatings for plastic processing

THE HVOF METHOD - (see also the PVD process)

The HVOF-Method is a process for the dressing, quenching and tempering of screws and utilizes cutting-edge thermal spray technology to apply extremely wear and corrosion resistant protective coatings to any size injection molding or extrusion feed screw.


The coatings are hard (62-72 HRC), dense (98% density) and crack-free. Proprietary compositions of hard carbides, ceramics, and alloys are combined to achieve abrasion and corrosion resistance unmatched by conventional hard facing alloys.


The process completely encapsulates any substrate with coating thicknesses of .005” or .010” (.125 -.250 mm). Post application diamond polishing provides a super 6-10 RMS (μinch) (.2-.4 Ra μmeter) mirror surface finish.


Carbide type and volume percentage are critical to the abrasion resistance of any material used for wear resistance. Tungsten carbide is one of the hardest and most economical compounds formed when combined with carbon.


Cemented carbides are carbides in particle form bonded with a metal usually Cobalt or Nickel. Hardness varies but all cemented carbides are harder than the hardest metals.


Tungsten carbide with Vickers hardness 2200 (85 HRC equivalent) is composed of up to 90% carbide particles by weight, nearly 80% by volume. Most standard wear resistant alloys contain a maximum 35% by volume of softer chromium carbide.

The Process

Image showing substrate of moulding screw coatings



Surface preparation is critical to the success of any thermal spray coating. Coating adhesion and quality are directly related to the cleanliness and the surface roughness of the substrate prior to coating.


Surface roughening promotes compressive surface stresses, interlocking surface laminations (layers), increases the bond area and decontaminates and De-oxidizes the prepared surface.



The screw made from tool steel first receives a roughened surface, then using a specialist high pressure gas mixture application gun the metallic powder particles are melted and are applied with a speed of 750m/s onto the screw.


The collision creates kinetic energy that generates extra heat, which supports the bonding and increases the compactness until the desired degree of hardness is reached.

image showing the process of applying hard wearing coatings
image showing the equipment in action applying hard wearing coatings

The TAFA JP8000 generates high temperatures (6000ºF, 3300ºC) which melts the metal portion of the carbide powder while high exhaust velocities (2500 f/sec, 750 m/sec) accelerates the ultra-fine particles toward the substrate.


On impact, the particles flatten, solidify and form an interlocking bond within the coating layers. This high kinetic energy released on impact contributes a strong bond (>15,000 PSI, 100 MPa) and high hardness values. Deposit densities typically exceed 98%.

The Process Step by Step

This technique ensure the screws is totally encapsulated the component


A lamination depth of up to 0.25 mm is possible.


On the surface the screw has the characteristics of a hard metal-screw combined with the advantage of the toughness in the centre.


Using this unique process we can guarantee at least twice the life for most application even if your existing screw is made from power metallurgy steels (HIPS)


The coatings are excellent for screws exposed to filled resins, rubbers or other abrasive compounds. Corrosion resistance, due to the high density of our finished coating is excellent.


Our coating is used to protect feed screws processing CPVC, fluoropolymers as well as halogen containing and halogen-free flame retardant additives.

Copyright Invotec Solutions Limited 2013 - Invotec Extra is a trading name of Invotec Solutions Limited