The screw and barrel are the most expensive wear parts of an injection molding machine. Replacing them means not only the cost of the parts but also days of downtime. Even so, in most plants wear is only noticed when production goes wrong.
Yet wear progresses slowly and gives early signs. Reading these signs turns an unplanned overhaul into planned maintenance.
Wear is not a single mechanism
The screw and barrel wear in three different ways. For the right protection you need to know which one dominates.
| Mechanism | How it happens | Typical source |
|---|---|---|
| Abrasive | Hard particles scrape the metal surface | Glass fiber, mineral filler, metal particles |
| Corrosive | Chemical attack damages the surface | PVC degradation, flame retardants, moisture, some additives |
| Adhesive | Metal-to-metal contact, the surfaces stick together and tear | Loss of the lubricating film, the screw touching the barrel |
In practice these mechanisms usually work together: the clearance opened up by wear disturbs the centering of the screw, which increases metal contact; a surface weakened by corrosion becomes more exposed to abrasive wear.
Early warning signs
The first signs of wear appear not in the quality report but in the process data:
- Plasticizing time is getting longer — if the screw turns for longer for the same shot, the barrel-screw clearance has increased and material is slipping back
- The cushion does not hold — if the remaining cushion fluctuates from cycle to cycle, there is backflow
- Shot weight is fluctuating — if the weight varies while the dosing and mold side are constant, the screw side should be looked at
- Back pressure is ineffective — if increasing back pressure no longer provides the homogeneity it used to
- Black specks and color residue are increasing — material catches on the worn surface and in deepening scratches, degrades and then breaks off into the part
- Color changes are taking longer — if more purge shots are needed than before, the surface has roughened
Some of these signs overlap with the quality problems in the defect troubleshooting guide; what sets them apart is that they all get worse over time.
How is clearance measured?
A definitive assessment is made by measuring the screw and barrel:
1. The screw diameter is measured with a micrometer at different zones (feed, compression, metering zone) 2. The barrel inside diameter is measured with a bore gauge 3. The difference gives the radial clearance
In a new system the clearance is very small. As the clearance increases, material starts to slip back over the screw flight — plasticizing efficiency falls and pressure transfer weakens. The manufacturer's tolerances must be taken as the basis; as the limit is approached, replacement must be planned, not waited for.
During measurement the following are also checked: the sharpness and coating condition of the screw flight, wear of the non-return valve (check valve), scratches and corrosion marks on the barrel surface.
Protection: by cause
1. Metal particles
The fastest and most severe damage comes from metal particles. A piece of blade from regrind, wire from packaging or a dropped bolt cuts a deep scratch in the screw and barrel in a single pass.
The way to protect against this is to place magnetic separators at suitable points in the line. Details, and where metal enters the line: Metal separation guide for plastics production
Products: Mensink metal separation systems · Michel magnetic separators
2. Abrasive fillers
Glass-fiber reinforced and mineral-filled materials are abrasive by nature; this cannot be eliminated, but it can be slowed down:
- Choosing a wear resistant material for the screw and barrel (see below)
- Avoiding unnecessarily high screw speed
- Setting the temperature profile to suit the material — in a zone running cold, the filler behaves more harshly
The same problem occurs in the conveying line: glass-fiber raw material wears through standard pipes and elbows. That is why wear resistant pipes and elbows exist — and the particles that break off a worn-through pipe eventually reach the screw.
3. Excessive screw speed
What determines wear is not the number of revolutions per minute but the speed covered by the screw surface. A thick screw, even turning at the same speed as a thin one, has an outer surface that travels much faster; it strains both the raw material and its own surface more. That is why machine catalogs give the limit not in revolutions but in meters/minute — and when it is exceeded, two kinds of damage are seen at once: the material degrades and the metal surface wears rapidly.
To calculate the permitted speed for your screw diameter: injection molding calculators · Peripheral speed limits by material: material processing values
4. Corrosive materials and moisture
When PVC degrades, corrosive compounds are released; flame retardant additives and some pigments have a similar effect. In addition, moisture in undried raw material leads to hydrolysis at high temperature and feeds the corrosive environment.
Protection: proper drying, not holding the material in the barrel longer than necessary, and choosing a corrosion-resistant surface.
- Drying equipment: raw material drying systems
- Residence time depends on the ratio of shot volume to screw volume — it should not fall below 20%: screw capacity calculation
5. Running dry and empty
Turning the screw without material removes the melt film between the surfaces and creates direct metal contact. Not running the screw empty during stoppages and cleaning prevents this kind of sudden damage.
6. Using regrind
Regrind carries risk in two ways: it may contain metal particles, and because of its dusty structure it behaves differently in the feed zone. Controlled use and metal separation before granulating reduce this risk: Regrind and scrap recovery
Screw and barrel material options
| Application | Suitable approach |
|---|---|
| Standard thermoplastics (PP, PE, PS) | Nitrided steel is sufficient — economical, widespread |
| Glass-fiber and filled materials | Bimetallic barrel and hard-coated screw — abrasion resistant |
| High filler content, continuous production | Tungsten carbide based coatings — the highest resistance, the highest cost |
| PVC and corrosive materials | Corrosion-resistant alloy; nickel-based coatings |
| Mixed production (both filled and standard) | Bimetallic barrel + coated screw is a balanced choice |
A wear resistant material raises the initial investment. When deciding, what should be compared is not the part price but the expected life and downtime cost: a set that costs twice as much but lasts three times as long usually works out cheaper, because it also reduces the number of stoppages.
Inspection plan
Include the screw and barrel in the maintenance program:
1. At every color/material change — look at the purge shots that come out: are there black specks or metallic glints? 2. Monthly — record plasticizing time, cushion stability and shot weight deviation; the trend shows the wear 3. Annually or at a set shot count — remove and measure the screw and barrel, check the non-return valve 4. Record every measurement — what is meaningful is not a single measurement but the rate of change
The same approach applies to the mold side: Mold maintenance and periodic inspection plan
In summary
It is not possible to eliminate screw and barrel wear; the aim is to make it predictable. Stopping metal particles before they enter the line, respecting the peripheral speed limit, drying the raw material correctly and choosing a surface suited to the material — these four markedly extend service life.
For protective equipment and material selection for your line, you can consult our technical support team.
Screw and barrel life starts with choosing the machine to suit the raw material it will process. Series with special equipment for corrosive materials such as PVC and abrasive ones such as glass-fiber compounds are listed separately in the plastic injection molding machine range.