Causes of Rust

1. High Environmental Humidity

• In high - humidity environments like the southern Meiyu season (where relative humidity can reach 80% or more), water vapor permeates and forms an invisible water film on the mold surface. Water reacts with the iron in the mold. Iron, as the negative electrode, loses electrons to become ferrous ions. These combine with dissolved oxygen in water to form ferrous hydroxide, which further oxidizes to rust (Fe?O?). Even in dry areas, poor ventilation can cause local humidity build - up and lead to mold rust.

2. Decomposition of Plastic Raw Materials

• Some high - temperature - processed plastics, such as polyvinyl chloride (PVC), decompose and release hydrogen chloride gas during injection molding when the processing temperature approaches or exceeds their thermal stability limit. Hydrogen chloride, being strongly acidic, corrodes the mold surface, destroying its passivation film and exposing the bare metal, thus accelerating rust. Plastics with brominated flame retardants may also release corrosive hydrogen bromide at high temperatures.

3. Release Agent - Related Issues

• Using low - quality release agents with impurities, water, or easily - oxidizing components can cause problems. Instead of isolating the product and the mold, they can contribute to rust. For example, some water - based release agents leave residues after rapid water evaporation at high temperatures, creating conditions for rust. Excessive spraying of release agents leads to accumulation in mold grooves and gaps, which is hard to clean, absorbs water vapor, and indirectly corrodes the mold.

4. Mold Processing Defects

• High surface roughness (Ra value of 10 microns or higher) in the mold cavity increases the contact area with external substances, making it easier for water vapor and corrosive gases to adhere. Tool marks left during processing have concentrated stress, and under corrosive media, they form corrosion pits that spread. Improper heat treatment processes like quenching and tempering make the mold's internal structure unstable, increasing surface metal activity and rusting rate.

5. Lack of Maintenance

• Failure to clean the mold after use allows plastic particles and flash debris to absorb moisture. Additives and plasticizers in these residues can erode the mold. Lack of regular inspection and anti - rust maintenance means that small amounts of corrosive substances accumulate over time. When the mold is idle for long without protection (such as oil coating and wrapping), it can rust within months.

Elimination Methods

1. Environmental Control

• Install humidity monitors in the workshop. When humidity exceeds 60%, use industrial dehumidifiers to reduce it to 40% - 50%. Improve ventilation by arranging vents properly. For high - precision molds, use small temperature - and - humidity - controlled storage cabinets.

2. Optimization of Raw Materials and Media

• Adjust injection - molding parameters for easily - decomposing plastics. For PVC, lower the processing temperature by 5 - 10 °C and shorten material residence time. Choose high - quality release agents, preferably fluorine - or silicon - based ones. Follow the product manual's dosage and spraying method.

3. Mold Repair and Strengthening

• For molds with processing - defect - related rust, use grinding and polishing to reduce the cavity surface roughness Ra value to 0.2 - 0.8 microns. Micro - grind stress - concentrated areas like tool marks and check for residual stress with an ultrasonic flaw detector. Eliminate residual stress with stress - relief annealing. Repair and strengthen worn parts with technologies like brush plating and laser cladding.

4. Standard Maintenance Process

• After each use, clean the mold with a high - pressure hot - steam spray gun, then dry it with cold air. Spray a special mold anti - rust agent. For long - term idle molds, apply anti - rust oil, wrap with anti - rust paper and plastic film, and store on a dry shelf. Regularly check the package.

5. Advanced Rust - Removal Treatment

• For slight rust, soak in white vinegar for 30 minutes to 1 hour, rinse, polish with 0 - grade fine sandpaper, and then prevent rust. For moderate rust, use chemical rust removers (e.g., those with phosphoric acid or citric acid), cover with plastic film for 2 - 3 hours, rinse, clean with an ultrasonic cleaner, and then dry and prevent rust. For severe rust, use mechanical rust - removal like glass - bead sandblasting (80 - 120 - mesh glass beads, 0.3 - 0.5 MPa pressure), then clean, test, and comprehensively repair and prevent rust.