To build painting preprocessing production line, firstly it needs to finish technological design and then can proceed with nonstandard equipment designing, manufacturing and installment. Thus technological design is the base for building production line; correct and reasonable route will result in good influence for production operation and products quality. Main contents of technological design includes: treatment method, treatment time and technological process etc.
1. Treatment method
Product treatment method means which method can make product and tank liquor contact to achieve chemical pretreatment purpose; include complete soaking, complete spraying, combined type of spraying& soaking, brush painting etc. It is depend on and influenced by product geometric dimension and shape, floor area, scale of investment, production capacity and other factors etc. For example, spraying method isn’t suitable for complicated geometric dimension product; some products such as fuel tank and oil barrel aren’t easy to be submerged so they aren’t suitable for soaking method.
1.1. Complete soaking method
Soak the product completely in tank liquor, take out it after some time treatment, it is a common method to achieve un-oiling or rust removal or Parkerizing purposes. Product has different geometric shape(some complex and some simple); as long as liquor can get to it, all can achieve treatment purpose; this is soaking method’s special advantages which spraying and brush painting can’t be analogized. However its defect is it hasn’t auxiliary use of mechanical washing thus treatment speed is relatively slow; time is rather long; especially for continuous hanging or conveying product; besides product operation time in tank, there is still uphill and downhill time so it makes equipment longer and this causes floor area and investment increasing. Only for Parkerizing, currently foreign countries tend to adopt complete soaking method; it is said complete soaking Parkerizing is easy to form high iron-content granulate crystal phosphate film which has good mating prosperities with negative pole electrophoresis.
1.2. Complete spraying method
Use pump to inflate liquor with 0.1～0.2Mpa pressure to make it become vaporific and spray on product to achieve the effect. As there is mechanical washing and liquor update use in spraying, thus treatment speed is faster and time is shorter. Production line is also shorter which also saves floor area and equipment; however, its defect is it can’t get to complex geometric shape easily such as inner chamber, corner so treatment effect isn’t good; so it is only suitable for simple geometric shape product. Spraying method is also not fit for acid-washing rust removal for it will bring equipment corrosion and in-process rust etc. a series of problems; thus it is very cautious to choose spraying acid-washing. As reported complete Parkerizing can easily cause crystallizing dendritic thick, low iron content phosphate film and foreign countries don’t advocate it as preprocessing for negative pole electrophoresis base before painting. Complete spraying method is mainly applied to domestic appliance, component& parts powder spraying, static electricity painting and positive pole electrophoresis etc.
1.3. Combined type of spraying& soaking
Combined type of spraying& soaking, usually means in some process, product is firstly sprayed and then soaking in tank and spray again after taking out from tank; all spray and soaking use same tank liquor. This combined type keeps spraying’s efficiency and high treatment speed as well as has soaking process which can make product’s all parts can be treated effectively. So this type can make preprocessing both short time for finishing treatment procedure, equipment floor area occupies little space and can get satisfactory treatment effect. Currently they all tend to use this type for automobile industry which has high requirement on preprocessing at home and abroad.
1.4. Brush painting method
This method is directly to brush treating fluid to product surface manually to achieve chemical treatment purpose; it usually can’t get very good effect and it is seldom used in factory; it is considered to use this method for some large-scale, simple shape products.
2. Treatment temperature
Considering energy-saving, improving working environment, reducing production cost, chemical reaction speed, treatment time and production speed, the general used temperature in production application is low temperature or middle temperature preprocessing technology.
Except liquid oil contamination, there is still a little solid oil; at low temperature, it is very hard to eliminate solid oil so it should choose mesotherm range for degreasing no matter it’s spraying or soaking.
It should choose mesotherm acid washing for common corrosion and oxide skin products to ensure eliminating completely corrosion and oxide skin. Except there is enough reason; otherwise it usually doesn’t choose low temperature or no temperature acid washing to eliminate rust. Low temperature acid washing is only limited to such as: product little corrosion and no oxide skin; eliminating time is unlimited; muriatic acid is permitted for acid washing in this case.
Usually no need temperature increase for surface adjusting process but adopts common temperature treatment.
There is no obvious speed difference for low and middle temperature Parkerizing; both they can form phosphate film in short time. If the product after Parkerizing requires long time storage, it should choose middle temperature Parkerizing and it will have good antirust effect.
All preprocess can adopt common temperature or no temperature washing method; if the last washing is by hot water drying, temperature should be above 80℃.
3. Treatment time
After treatment method and temperature is decided, treatment time is depended upon products oil contamination and corrosion extent. Usually it is referred to time requirement on preprocessing medicament user manual.
4. Technological process
It is divided different technological process as per product requirement on oil contamination, corrosion extent and base paint.
4.1. Completely rust-less product
Pre-degreasing—degreasing—washing—surface conditioning-parkerizing— washing—drying (it can directly goes into electrophoresis tank even the base paint isn’t dry). This is standard 4 process and widely used; it is applicable in various kinds of cold-rolled plate and machining rust-less products preprocessing; it can put surface conditioning agent into degreasing tank which can reduce one procedure.
4.2. Mixed products of common oil contamination, corrosion and oxide skin
Degreasing& rust removal “two-in-one” –washing –neutralizing –surface conditioning –Parkerizing – washing – drying (or directly electrophoresis). This set technology is mostly wide-used in China and it is applicable for every kind product (except heavy oil contamination) preprocessing; if it adopts middle temperature Parkerizing, it can omit surface conditioning procedure; for simple plate type product, it can also omit neutralizing procedure so it can become standard on rust 4 procedure technology.
4.3. Heavy oil contamination, corrosion, oxide skin products
Pre-degreasing—washing-- Degreasing& rust removal “two-in-one”-- washing –neutralizing –surface conditioning –Parkerizing-- washing – drying (or directly electrophoresis). For heavy oil contamination product, firstly it need pre-degreasing to eliminate most oil to ensure its metal surface is completely cleaning after next step Degreasing& rust removal “two-in-one” treatment.
5. Several notice
Pay attention to some little point in technology design even some is relevant to equipment design; if not considerate, it will cause many disadvantage effects to production line and worker operation such as procedure interval time, overflow washing, phosphate scumming, product technological hole, tank body and heating pipe material.
5.1. Procedure interval time
If each procedure interval time is too long it will cause product is rusted twice in operation process; especially when it has acid-washing process. Product after acid-washing is very easy to be oxidized in air to get rusty and turn green; it would be better to set inter process water film protection which can reduce rust. Product will turn yellow or green after it’s rusted which seriously affect Parkerizing effect and cause product become grey and yellow so it can’t form complete Parkerizing film; thus inter process interval time should be shortened as much as possible. If time is too short, water on product storage can’t be drained effectively which will cause channeling phenomenon; especially in spraying method, it will cause mutual spraying splashing channeling which make it is hard to control tank liquor composition even the liquor will be destroyed. Therefore in consideration of procedure interval, it should choose a proper interval time as per products geometric dimension and shape.
5.2. Overflow washing
It is advocated to use overflow washing to guarantee product is completely cleaned and to reduce channeling phenomenon. It should inflow from bottom and overflow from the opening pole on diagonal top.
5.3. Product technological hole
For some tubular or easily formed dead angle water storage product, it must choose proper point to drill technological hole to guarantee water can be completely drained away in very short time. Otherwise it will cause channeling or it needs long time to be drained in air but this will cause twice rust which affects Parkerizing effect.
5.4. Phosphate scumming
For any kind phosphate fluids it will generate dregs more or less (light railway series have little dregs in color Parkerizing); it should mark there is Parkerizing device in design, especially spraying Parkerizing, it is indispensable. Typical scumming devices available are: tilted plate separator, high-order precipitation tower, centrifugal pulp cleaner and paper bag type strainer.
5.5. Tank body and heating pipe material
Though it’s not technological design’s content to choose tank body and heating pipe material; if it’s not reminded during design, it would cause design personnel’s ignorance thus it would affect whole production line operation. For sulfuric and hydrochloric acid washing, tank body material can only be glass fiber reinforced plastics, granite and plastics; heating pipe material can only be lead-antimony alloy pipe or ceramic pipe but can’t use stainless steel material. If it uses phosphoric acid washing, both tank body and heating pipe material can be stainless steel; of course it also can be glass fiber reinforced plastics, granite or plastics.
