Adhesive Wear (Galling) Microwelding and Material Tearing in Injection Molds

author: CHEN

2025-06-04

While abrasion and corrosion often steal the spotlight in mold wear discussions, adhesive wear—or galling—is an equally destructive yet frequently underestimated phenomenon. Unlike surface erosion that chips away material over time, adhesive wear is the result of intense localized friction, causing two surfaces to weld together microscopically and then tear apart during movement. This micro-welding effect can lead to severe damage, especially in high-pressure, high-friction molding environments.

Understanding adhesive wear in detail is vital for tool designers, process engineers, and material selectors aiming to extend tool life, preserve cavity precision, and maintain consistent part quality.

What Causes Adhesive Wear in Molds?

Adhesive wear—also known as galling—is a form of surface damage that occurs when two solid materials slide against each other under pressure, leading to microwelding and subsequent material transfer or tearing. In injection molding, this typically happens between the mold cavity surface and the plastic resin—or between moving steel components like cores, slides, and ejector pins.

Adhesive wear begins at the atomic level. When two metal surfaces are pressed together under load and move relative to one another, microscopic high points (asperities) come into contact. Under the high pressure and heat common in molding, these asperities can weld together temporarily—especially when:

- There's no lubrication,

- High clamping force is applied,

- Or material pairs have chemical affinity (e.g., similar steel grades).

As the components move apart, the welded junctions tear, pulling fragments from one surface and depositing them onto the other, forming scratches, gouges, or raised deposits.

Injection Molding Conditions That Promote Adhesive Wear

Contributing Factor	Impact on Galling Risk
High Contact Pressure	Amplifies asperity contact and heat generation
Poor Surface Finish (too smooth or too rough)	Can increase surface adhesion or create micro-traps
Lack of Lubrication or Mold Release Agent	Direct metal-to-plastic or metal-to-metal contact
Soft or Untreated Steel Surfaces	More prone to plastic deformation and welding
High Mold Temperatures	Promotes atomic bonding at contact points
Repeated Sliding Motion (ejector pins, cores)	Increases cumulative wear with each cycle
Resin Sticking (e.g., TPEs, nylons)	Increases friction and adhesion between part & mold

Material and Metallurgical Factors

Certain material combinations are naturally more prone to galling:

- Similar metals tend to weld more easily than dissimilar ones.

- Austenitic stainless steels like 304 and 316 have a higher tendency to gall due to their ductility and low hardness.

- Steels lacking surface treatments (e.g., nitriding, plating, or PVD coatings) are especially susceptible.

Polishing a mold cavity does not guarantee galling resistance. In fact, extremely polished surfaces can increase the real contact area between two surfaces, ironically making adhesive wear more likely.

Adhesive wear in molds is primarily caused by:

- High-pressure, high-heat environments,

- Material pairings that promote atomic adhesion,

- Lack of surface hardness or friction-reducing treatments,

- And repeated mechanical interaction (ejection, sliding).

Preventing galling starts with the right steel selection, surface treatment, and component design—especially in high-friction mold zones like ejectors, cores, or parting lines.

Which Steels Are Vulnerable (and Why)?

Not all mold steels perform equally under adhesive stress. The risk of galling is closely tied to surface hardness, chemical composition, and frictional behavior.

Steel Type	Galling Risk	Notes
Carbon Steel	High	Softer surface easily deformed; limited alloying to resist adhesion
Stainless Steel (e.g., 420, 440C)	Moderate	Higher hardness helps; surface polish is critical
Alloy Steel (e.g., H13, S7)	Low	Hardened and tough; responds well to surface treatments

Note: Even high-hardness steels can suffer galling without proper surface finish or lubrication.

Best Practices to Prevent Adhesive Wear

1. Heat Treatment & Hardening

- Through-hardening or nitriding improves resistance to micro-deformation.

- Ensure ejection pins and sliding components are at least 2 HRC harder than mating surfaces.

2. Surface Coatings

Apply low-friction, high-hardness coatings to prevent welding and dragging:

- DLC (Diamond-Like Carbon): Extremely low friction; excellent for medical molds.

- TiCN or TiAlN (PVD coatings): Strong, slick barriers that withstand heat and pressure.

- Chrome plating: Time-tested solution for food-grade or packaging molds.

3. Mold Design Considerations

- Add draft angles and venting to reduce adhesion between part and cavity.

- Minimize undercuts or deep core features that increase contact force during ejection.

- Use vented ejector pins or air-assist ejection where galling is frequent.

4. Material Matching

- Avoid using identical materials for sliding surfaces (e.g., steel-on-steel).

- Pair dissimilar materials with complementary hardness and friction profiles (e.g., S7 ejector pins in H13 molds).

How Adhesive Wear Progresses in Injection Molds

Adhesive wear—often called galling—is not a one-time event but a progressive failure mechanism that evolves over repeated molding cycles. What begins as microscopic material bonding can rapidly lead to mold damage, dimensional inaccuracies, and production downtime if left unchecked.

Understanding the stages of wear progression is crucial to designing more durable tools and planning effective maintenance.

Stage-by-Stage Progression of Adhesive Wear

1. Initial Asperity Contact

Even the most polished metal surfaces are rough at a microscopic level. During mold closure or part ejection:

- Tiny peaks (asperities) from opposing surfaces come into contact.

- If the contact pressure and temperature are high, localized microwelding begins.

- This is especially common in slides, lifters, and ejector pins where relative motion exists.

2. Microwelding Formation

Under repeated cycles:

- Local hotspots from friction and pressure cause atomic-level fusion at contact points.

- This fusion isn’t uniform—certain zones bond more due to surface energy, pressure, or material compatibility.

- Dissimilar metals or hardened vs. soft surfaces may delay this stage, while similar untreated steels accelerate it.

3. Material Tearing and Transfer

As the mold opens or moving parts slide:

- The microwelded points are sheared apart.

- One surface loses material, and the other gains a transferred fragment or “build-up”.

- This causes surface roughness, scratches, or bulges.

4. Wear Accumulation

With repeated molding cycles:

- Torn metal accumulates, raising friction further.

- Wear scars act as stress concentrators, which trap heat and debris.

- Surface deposits may scratch the molded part, affecting finish and dimensional accuracy.

5. Accelerated Deterioration

If left unaddressed:

- The damaged area expands rapidly.

- Material loss creates pits, gouges, or steps in critical sealing or cavity regions.

- In severe cases, components may seize—especially in tight-tolerance zones.

Signs of Progressive Adhesive Wear

- Shiny bands or smearing on mating steel parts

- Raised metal transfer patches or galling streaks

- Increased ejection force or sticking parts

- Part scratches matching mold component travel paths

- Seizing of slides or pins during cycling

Galling in Ejector Pins

In a production mold for a glass-filled nylon automotive connector:

420 stainless steel ejector pins without surface treatment began showing drag marks after ~20,000 cycles.

Wear progressed to galling and pin sticking by 60,000 cycles.

Retrofitting with TiN-coated H13 pins extended functional life to over 300,000 cycles.

Adhesive wear begins at a microscopic level but progresses rapidly once microwelding and tearing begin. Without preventive measures—such as proper material selection, coatings, and lubrication—this mechanism can significantly reduce mold life and part quality.