Pretreatment process includes surface finishing, internal stress inspection, oil removal and roughening. The details are as follows.
① Surface treatment. ABS plastics should be surface treated before treatment, because there will be residual stress during plastic injection. In particular, internal stress will occur at the gate and the corresponding part of the gate. If it is not eliminated, these parts will bubble in electroplating. In the electroplating process, if it is found that the same part of the product is easy to blister, it is necessary to check whether it is the gate or the corresponding part of the gate, and also to check the internal stress, but in order to prevent problems before they happen, it is necessary to relieve the stress in advance.
Generally, surface finishing can be soaked in 20% acetone solution for 5 ~ 10s.
The way to relieve stress is to treat it in a constant temperature oven or water bath at 80℃ for at least 8 hours.
② Internal stress inspection method. At room temperature, the injection-molded ABS plastic product was soaked in glacial acetic acid for 2 ~ 3 minutes, and then the surface was carefully cleaned and dried. Observe the surface with a magnifying glass or stereomicroscope at 40 times. If the surface is white and there are many cracks, it means that the internal stress of the plastic is large and it cannot be electroplated immediately, so it should be stress-relieved. If the plastic primary color appears, it means that there is no internal stress or the internal stress is very small. When the internal stress is serious, the cracks on the plastic surface can be seen without a magnifying glass after the above treatment.
③ Oil removal. There are many kinds of degreasing agents available on the market, and the following formulas can also be adopted:
Sodium phosphate 20g/L.
Emulsifier lmL/L
Sodium hydroxide 5g/l
Temperature 60℃
Sodium carbonate 20g/l
Time is 30 minutes.
After degreasing, wash with hot water first, then clean water, then neutralize with 5% sulfuric acid, and then enter the coarsening process, which can protect the coarsening liquid and prolong its life.
④ coarsening. There are three kinds of roughening methods for ABS plastics, namely, high sulfuric acid type, high chromium acid type and phosphoric acid type. From the point of view of environmental protection, high sulfuric acid type should be adopted now.
A. High sulfuric acid type roughening liquid
Sulfuric acid (mass fraction) 80%
The temperature is 50-60 DEG C.
Chromic acid (mass fraction) 4%
Time 5 ~ 15 minutes
The effect of this coarsening solution is not as good as that of high chromium acid type, so it is better to take a long time.
B. High chromium acid type roughening liquid
Chromic anhydride (mass fraction) 26% ~ 28%
The temperature is 50-60 DEG C.
Sulfuric acid (mass fraction) 13% ~ 23%
Time is 5 ~ 10 minute.
This roughening liquid is universal and suitable for different brands of ABS. If there are few ingredients, it is necessary to extend the time or raise the temperature slightly.
C. phosphoric acid type roughening liquid
Phosphoric acid (mass fraction) 20%
Temperature 60℃
Sulfuric acid (mass fraction) 50%
Time 5 ~ 15 minutes
Chromic anhydride 30g/l
The coarsening effect of this coarsening liquid is better and takes longer, but the cost will increase with more components, so it is generally useless.
The life of all coarsening liquids is directly proportional to the number and time of processing plastic products. With the increase of coarsening amount and time, the amount of trivalent chromium will increase, and the effect of coarsening solution will decrease, which can be analyzed and supplemented. However, when there is too much trivalent chromium, the color of the treatment solution will be dark green, and chromic acid should be added after discarding part of the old solution.
Rough parts should be thoroughly cleaned. Because of the high concentration of chromic acid, it should be recovered in the recovery tank first, then washed several times, soaked in 5% hydrochloric acid, and then washed before entering the next process.
(2) Electroless plating process
Electroless plating process includes sensitization, activation, electroless copper plating or electroless nickel plating. Because electroless copper plating and electroless nickel plating require different processes, two different processes will be introduced respectively.
① Electroless copper plating process
A. sensitization
Stannous chloride 10g/L
Temperature l5 ~ 30℃
Hydrochloric acid 40ml/L.
Time l ~ 3min
The production of tetravalent tin can be suppressed by putting pure tin blocks into sensitized solution. The sensitized part must be cleaned with distilled water before it can be activated to prevent chloride ions from being brought in and consuming silver ions.
B. silver salt activation
Silver nitrate 3 ~ 5g/L.
room temperature
Add ammonia water to make it transparent.
Time is 5 ~ 10 minute.
The advantage of this activating solution is that the cost is low, and it is easier to judge the activation effect according to the color change of the activated surface. Because the activation layer of silver nitrate reduced to metallic silver is brown, if the color is very light, it means that the activation is not enough, or the time is prolonged, or the activation solution needs to be supplemented. Palladium activation method can also be used. At this time, the colloidal palladium method already introduced can be used, or the following step-by-step activation method can be used. If it is colloidal palladium method, the previous sensitization can be omitted, and a depolymerization can be added after activation.
C. palladium salt activation
0.2-0.4g/l palladium chloride
The temperature is 25-40 DEG C.
1~ 3ml/l hydrochloric acid
The time is 3 ~ 5min.
After activation and thorough cleaning, plastic products can enter the chemical plating process. If the product not cleaned by activating solution enters the electroless plating solution, it will cause autocatalytic decomposition of electroless plating, which must be paid attention to.
D. Electroless copper plating
Copper sulfate 7g/L
Formaldehyde 25ml/L
Nickel chloride 1g/L
The temperature is 20 ~ 25℃
Sodium hydroxide 5g/l
The PH value is 1 1 ~ 12.5.
Potassium sodium tartrate 20g/L
Time l0 ~ 30min
The biggest problem of electroless copper plating is insufficient stability, so it should be carefully maintained, and it is best to filter while stirring with air. When the consumed raw materials are supplemented, it is calculated as 4mL reducing agent of lg metal.
② Electroless nickel plating process
A. sensitization
Stannous chloride 5 ~ 20g/L
The temperature is 25-35 DEG C.
2 ~10ml/l hydrochloric acid
The time is 3 ~ 5min.
B. activation
0.4-0.6g/L palladium chloride
The temperature is 25-40 DEG C.
Hydrochloric acid 376 ml/l
The time is 3 ~ 5min.
Electroless nickel plating can only use palladium as activator, but it is difficult to use silver as catalyst, and the palladium ion concentration is also high. At present, electroless nickel plating is mostly carried out by one-step activation method. Namely one-step activation by colloidal palladium method. Due to the progress of surfactant technology, the content of palladium in commercial activators has been greatly reduced, and 0. 1g/L palladium salt can play an activating role.
C. Electroless nickel plating
Nickel sulfate 10 ~ 20g/L
PH value (adjusted with ammonia water) 8 ~ 9
Sodium citrate 30 ~ 60g/L.
The temperature is 40-50 DEG C.
Sodium chloride 30 ~ 60g/L.
Time 5 ~ 15 minutes
Sodium hypophosphite 5 ~ 20g/L
Electroless nickel plating has better conductivity and luster than electroless copper plating, and the stability of the solution itself is also relatively high. The usual supplement can be carried out by nickel salt concentration colorimetry. During replenishment, nickel sulfate and sodium hypophosphite are added according to 50% ~ 60% of the new amount respectively. After each shift is completed, the pH value can be reduced to 3 ~ 4 with sulfuric acid, which can be stored for a long time without failure. When the processing capacity is large, it should be filtered every day, usually at least once a week.
(3) Electroplating process
Electroplating process can be divided into three categories: thickening electroplating, decorative electroplating and functional electroplating.
① Thickening electroplating. Because the chemical coating is very thin, in order to achieve the metallization effect of plastic, the coating must have a certain thickness, so it must be thickened and electroplated after electroless plating. At the same time, thickening electroplating also increases the reliability of further decorative or functional electroplating. If thickening electroplating is not carried out, there will be quality problems in various conventional electroplating solutions in many occasions, mainly due to incomplete electroplating or partial dissolution of chemical coatings, resulting in waste products.
A. a first thickening solution
Nickel sulfate150 ~ 250g/l
The pH value is 3-5
Nickel chloride 30 ~ 50g/L.
The cathode current density is 0.5 ~ 1.5a/dm2.
Boric acid 30-50g/L.
Time depends on the requirements.
The temperature is 30-40 DEG C.
B. Second thickening solution
Copper sulfate150 ~ 200g/l
Sulfuric acid 47 ~ 65g/l
Temperature l5 ~ 25℃
0.5 ~ 2ml/L of additive
The cathode current density is 0.5 ~ 1.5a/dm2.
Phosphor-copper anode special for acid copper plating on anode
Cathode movement or electroplating solution stirring
Time: Any commercial brightener on the market can be used as electroplating additive as required.
C. the third thickening solution
Copper pyrophosphate 80 ~100g/l
The pH value is 8-9
260-320g/L of potassium pyrophosphate
The temperature is 40-45 DEG C.
Ammonia 3 ~ 6 ml/l
The cathode current density is 0.3 ~ 1A/dm2.
The above three thickening baths are all suitable for electroless nickel plating, but the thickening bath is suitable, and sulfate copper plating can be used for electroless copper plating.
② Decorative electroplating
A. Acid bright copper plating
Copper sulfate185 ~ 220g/l
The cathode current density is 2 ~ 5a/dm2.
Sulfuric acid 55 ~ 65g/l
Commercial brightener1~ 5ml/l
Phosphor-copper anode for acid copper plating on anode
Temperature l5 ~ 25℃
The cathode is moved for 30 minutes.
Anode movement
B. Bright nickel plating
Nickel sulfate 280 ~ 320g/L.
The temperature is 40-50 DEG C.
Nickel chloride 40 ~ 45f/l
The pH value is 4-5
Boric acid 30 ~ 40f/l
The cathode current density is 3 ~ 3.5a/dm2.
Commercial brightener 2 ~ 5ml/L
Time l0 ~ 15 minutes.
Both of the above processes require cathode movement or agitation of electroplating solution. It is best to use circulating filtration, which can not only stir the plating solution, but also keep the plating solution clean.
Decorative electroplating can be a copper-nickel-chromium process, or it can be polished copper before other fine decoration, such as bronzing after brushing, or it can be polished copper before plating bright nickel and then plating imitation gold.
C. Decorative chromium plating
Chromic acid 280 ~ 360g/L.
The battery voltage is 3.5 ~ 8V
Sodium fluorosilicate 5 ~ 10g/L
The cathode current density is 3 ~ 10a/dm2.
Silicic acid 0.2 ~ 1 μ g/L.
The time is 2 ~ 5min.
The temperature is 35-40 DEG C.
This is a low-temperature decorative chromium plating suitable for plastic electroplating, but because the use of chromium is increasingly restricted, there will be many other chromium plating layers to choose from.
The recommended electroplating solution for replacing chromium is as follows:
26-30g/L stannous chloride
Zinc chloride 2 ~ 5g/L.
Cobalt chloride 8 ~12g/l
Potassium pyrophosphate 220-300g/L.
Chromium -90 additive 20 ~ 30ml/L..
The cathode current density is 0. 1 ~ 1A/dm2.
2 ~ 8 ml/L of chromium stabilizer
Anode pure tin plate (No.0 tin)
The pH value is 8.5 ~ 9.5
Time l ~ 5min
The temperature is 20-45 DEG C.
Cathode moving or circulating filtration
Chromium -90 additive is a common commercial additive in China at present.
③ Functional electroplating. As mentioned earlier, in addition to decorative parts, plastic electroplating is also widely used in structural and functional devices. In fact, the function of functional use mainly depends on the properties of the surface coating. In fact, in a broad sense, decoration is also a function, but the function mentioned here still refers to physical or chemical or electrical properties. Such as conductive coatings, reflective coatings, electromagnetic shielding coatings, solderable coatings, etc.