How to remove powder coatings comparing the different methods
There are a number of different stripping methods to remove powder coatings.
Powder coatings have a great durability which makes them difficult to remove.
Yet in some cases stripping powder coatings is necessary to repaint a part. There are a number of appropriate stripping methods for the removal of powder coatings, but wet abrasive blasting is possibly the most efficient one.
Removing powder coatings can be done using different methods. The selection of the most cost-effective and efficient process depends on several aspects such as:
- the type, dimensions and material of the parts
- the desired stripping speed
- the desired surface finish
- the environmental or health restrictions
- the purchase and operations budget
There are four general stripping methods:
- mechanical stripping
- thermal stripping
- thermochemical stripping
- chemical stripping
Chemical stripping methods use hot or cold stripping chemicals in a batch-type process to remove the coating from parts. The corrosive hot strippers cause the coating to swell and dissolve, loosening it from the part. The degraded coating is then sprayed off or just falls off into the chemical bath. Cold chemical strippers are usually solvent-based products that are potentially dangerous.
Thermal cleaning incinerates the coating by exposing it to very high temperatures. The ash residue is usually washed off by water or removed by blasting. Since the temperatures can be as high 650°C, the parts must be resistant to these extreme temperatures.
There are three types of thermal cleaning methods:
- Bake-off stripping systems are batch-type processes where parts are loaded into the oven at temperatures of around 340-400°C. Most parts of these ovens are equipped with water-misting systems to optimize the burning rate and prevent damage to the parts. The parts must be cleaned after the burning process to remove the residual ash.
- Burn-off stripping systems use much higher temperatures – between 540°C and 650°C – to strip coatings from parts. These high temperatures ignite the coating quickly and burn them away from the part. Such systems are usually installed as inline processes. After the burning process, the fire is extinguished and the residual ash is removed using water quenching. This method is not suitable for all types of parts as the extreme temperatures can warp or distort the part.
- Fluidized bed stripping systems use inert media such as sand and oxides as a heat transfer medium. Under the influence of heat and gas or air pressure, the medium is fluidized. The parts are immersed into the tank where the hot medium (approximately 425°C) breaks down the coating, and the abrasive character of the medium removes all ashes. After this treatment the parts require no additional cleaning. These systems are mainly used in batch type operations but can be automated for inline processing of hangers.
Thermochemical stripping methods combine temperatures of about 425-485°C and chemical reactions to strip the coating from the part. This inline stripping methodology is appropriate for both products and hangers in a short cycle time. After treatment, chemicals and residue are sprayed off with water.
Mechanical stripping = blasting
Mechanical paint stripper uses blasting media such as sand, water, carbon dioxide pellets, glass bead, steel shot, plastic media, slag, oxides, garnet, etc. These media are ‘shot’ under high pressure at the coated surface, removing the coating by abrasion. Mechanical cleaning systems can be automated or batch operations.
Choosing the right abrasive media affects removal speed of the coating and the surface profile. Hard abrasive particles remove the coating fast but also leave the roughest profile on the metal’s surface. More gentle blasting media, like plastic or soda, leave the metal surface with a smooth profile but inquire longer cleaning times.
Wet abrasive blasting
Wet abrasive blasting is similar to traditional dry abrasive blasting such as sandblasting. The difference is that the blast media is moistened prior to impacting the surface. This reduces dust, allowing for operators to work in a wide range of environments with minimal set-up and clean-up costs. Wet abrasive blasting results in a cleaner, more consistent finished surface, ready to coat, with no embedded particles or clinging dust.
Selecting the optimal stripping method is often a consideration of stripping time and capital costs. However, sometimes the cheapest process to buy is the costliest to operate. The finishing requirements also play an important role in determining the best stripping method for the removal of powder coatings.
Graco’s EcoQuip vapour blasting equipment is the most efficient, cost-effective, clean, and environmentally friendly stripping method for powder coating removal. It generates 92% less dust than dry blasting and uses less water than traditional wet blasting. You can adjust the air, water, and media ratio to your needs.