Compared with metal parts, plastic electroplating products can not only achieve good metal texture, but also reduce the weight of products. While effectively improving the appearance and decoration of plastics, it also improves its electrical, thermal and corrosion resistance. However, the selection of plastic materials for electroplating should comprehensively consider factors such as material processing performance, mechanical properties, material cost, electroplating cost, difficulty of electroplating and dimensional accuracy.
Due to its structural advantages, it not only has excellent comprehensive properties, is easy to process and shape, but also has a high coating bonding force due to the easy erosion of the surface of the material, so it is currently widely used in electroplating.
Anti-corrosion mechanism
Step 2: When the corrosion reaches the bright nickel layer, since the potential of the microporous nickel is higher than that of the bright nickel (about 20mv), the bright nickel is preferentially corroded as an anodic coating, and the corrosion in the microporous nickel is terminated.
Step 3: When the corrosion extends further down in the bright nickel to the semi-bright nickel, since the potential of the semi-bright nickel is also higher than that of the bright nickel (about 100mv), the bright nickel is also regarded as an anodic coating, and it extends downward at this time. The corrosion is terminated in the semi-bright nickel, and the corrosion direction is carried out laterally in the bright nickel, which further delays the time of corrosion to the substrate and greatly reduces the speed of corrosion.
1. Selection of plastic parts
There are many types of plastics, but not all plastics can be electroplated. Some plastics and metal layers have poor bonding force and have no practical value; some physical properties such as expansion coefficients of some plastics and metal coatings are too different to ensure their performance in high temperature environments. At present, the most used for electroplating is ABS, followed by PP. In addition, PSF, PC, PTFE, etc. also have successful electroplating methods, but they are more difficult.
2. Plastic part modeling design
Under the premise of not affecting the appearance and use, the plastic part modeling design should meet the following requirements as far as possible:
(1) The metallic luster will make the original shrinkage more obvious, so it is necessary to avoid uneven wall thickness of the product to avoid shrinkage, and the wall thickness should be moderate to avoid the wall being too thin (less than 1.5 mm), otherwise, It will cause poor rigidity, easy deformation during electroplating, poor bonding force of the coating, and easy deformation during use, causing the coating to fall off.
(2) Avoid blind holes, otherwise, the treatment solution remaining in the blind holes will not be easy to clean, which will cause contamination in the next process, thereby affecting the quality of electroplating.
(3) The electroplating process has the phenomenon of sharp edge thickening. Sharp edges in electroplating can cause tip discharge, which can cause edge plating to bulge. Therefore, the transition of rounded corners should be adopted as far as possible, and the radius of the rounded corners should be at least 0.3 mm. Flat-shaped plastic parts are difficult to be electroplated. The center part of the plated parts is thin, and the coating is thicker toward the edge. The whole coating is in an uneven state. The flat shape should be changed to a slightly arc surface or a matte surface with orange peel . The larger the surface area of electroplating, the greater the difference in gloss between the center and the edge. A slight paraboloid can improve the uniformity of the gloss of the plating surface.
(4) Minimize grooves and protrusions on plastic parts. Because the deep concave parts are easy to be exposed during electroplating, and the protruding parts are easy to be scorched. The depth of the groove should not exceed 1/3 of the groove width, and the bottom should be arc. When there is a grid, the hole width should be equal to the beam width and less than 1/2 of the thickness.
(5) Sufficient installation and hanging positions should be designed on the plated parts, and the contact surface with the hanging tool should be 2 to 3 times larger than that of the metal parts.
(6) The design of the plastic parts should make the parts easy to demould when they sink. Otherwise, the surface of the plated parts will be stretched or sprained during forced demoulding, or the internal stress of the plastic parts will affect the bonding force of the coating.
(7) When knurling is required, the knurling direction should be consistent with the demolding direction and in a straight line. The distance between the knurled stripes and the stripes should be as large as possible.
(8) Try not to use metal inserts for plastic parts, otherwise the inserts will be easily corroded during pre-plating treatment.
(9) The surface of the plastic parts should be guaranteed to have a certain surface roughness.
3. Mold design and manufacture
In order to ensure that the surface of plastic plating parts has no defects, no obvious directional structure, and internal stress, the following requirements should be met when designing and manufacturing molds:
(1) The mold material should not be made of beryllium bronze alloy. It should be made of high-quality vacuum-cast steel. The surface of the cavity should be polished along the direction of mold release to a mirror surface with an unevenness of less than 0.2 m. The surface is preferably plated with hard chrome.
(2) The surface of the plastic part faithfully reflects the surface of the mold cavity, so the mold cavity of the electroplated plastic part should be very smooth, and the surface roughness of the mold cavity should be 1 to 2 grades higher than the surface roughness of the part surface.
(3) Parting surface, welding line, and core inlay line cannot be designed on the plated surface.
(4) The gate should be designed at the thickest part of the part. In order to prevent the molten material from cooling too fast when filling the mold cavity, the gate should be as large as possible (about 10% larger than the ordinary injection mold).
(5) Exhaust holes should be left to avoid defects such as air filaments and bubbles on the surface of the workpiece.
(6) When selecting the ejector mechanism, it should be ensured that the part can be demolded smoothly.
4.Selection of injection machine
Improper selection of the injection machine will sometimes cause large internal stress on the part due to excessive pressure, inappropriate nozzle structure, or mixing, which will affect the bonding force of the coating.
5. Plastic molding process
Injection molding parts inevitably have internal stress due to the characteristics of the molding process, but proper control of the process conditions will minimize the internal stress of the plastic parts and ensure the normal use of the parts.
On the contrary, if the process is not properly controlled, there will be great internal stress in the part, which not only reduces the strength and performance of the part but also warps and even cracks during storage and use, resulting in cracking or even falling off of the coating.
Therefore, the control of process parameters should make the internal stress of the part as small as possible. The process conditions to be controlled include raw material drying, mold temperature, processing temperature, injection speed, injection time, injection pressure, holding pressure, holding time, cooling time, etc.
6. The influence of post-processing of plastic parts on electroplating
Due to injection molding conditions, injection machine selection, part modeling design, and mold design, there will be internal stress in different parts of the plastic parts to varying degrees, which will cause insufficient local roughening, make activation and metallization difficult, and eventually cause metal The chemical layer is not resistant to collision and the bonding force is reduced.
Experiments show that both heat treatment and finishing agent treatment can effectively reduce and eliminate the internal stress of plastic parts, and increase the bonding force of the coating by 20-60%. In addition, the molded plastic parts should be specially packaged and isolated, and it is strictly forbidden to bump or scratch the surface, so as not to affect the appearance of electroplating. During the inspection, the inspector should wear degreased gloves to prevent contamination of the surface of the plated parts and affect the bonding force of the plating.