2. Prerequisites for the best lacquering results
The lacquering process – a short introduction
The lacquer is mixed in 1k systems with a solvent and for 2k systems also with a hardener component. The mixture is pressurized and then sprayed with a spray gun on the plastic part (substrate). The lacquer that does not hit the substrate is captured by water curtains and a water basin, respectively. Various chemicals are added there which allow the lacquer sludge to be filtered out.
Depending on the desired properties or functions that the lacquer should have, several lacquer layers are often needed. After each coating layer, the solvents must evaporate, which takes place in evaporation chambers in the manual painting or in evaporation zones in the painting plants. The evaporation phase is called flash-off.
If you lacquer in several layers and the underlying layer is still not completely dry after flash-off, this is called wet-on-wet painting, because the lacquer is not yet fully cured. This technology provides shorter process times.
Wet-on-wet painting is most common at AMB as it is also the most cost-effective way to lacquer. However, there are a few lacquering systems that require full curing of each layer before the next layer can be applied and therefore not lacquered wet-on-wet.
When all lacquer layers have been applied, the remaining solvents must evaporate, and the lacquer must harden. The hardening is accelerated by heat and therefore the lacquering fixtures with the lacquered details are placed in hardening furnaces in the manual section. In the automatic lacquering systems, the fixtures are instead run continuously through an oven. After the oven, most lacquers are sufficiently hardened, and the plastic parts can be handled manually or automatically in connecting processes. For some paints, such as piano lacquer, however, further curing must take place before the parts can be handled.
7 things to keep in mind when lacqering plastics
In the sections below, we tell you about the most important conditions that should be considered before an industrial lacquering project, whether it is manual or robotic.
-
The construction of the part
Some points need to be considered already at the design stage if the plastic part is to be surface treated. Therefore, feel free to contact us already during the design phase to discuss surface treatment so we can provide some valuable tips. Here are a few general tips to take consider:
- Avoid sharp edges by rounding them (also applies to holes).
- Create attachment points that can hold the part on the fixtures during painting.
- Lacquer highlights irregularities rather than hides them. Therefore, carefully consider the type of the injection moulding inlet and the method for its separation you will choose, and keep in mind that any sink marks will be more visible.
-
Purity of the substrate
Plastics are usually charged by static electricity and therefore attract dust particles. Some of these particles are so small that they are barely visible to the naked eye, but they will be clearly visible after painting when they are enclosed in the cured lacquer. Therefore, the surfaces must also be as dust-free as possible.
For the plastic surfaces to be lacquered to be as clean as possible, there are some measures that should be considered by the manufacturer of the plastic part:
- Release agents in the mould tool should be avoided.
- The ejectors in the mould should be treated so that they do not need to be lubricated.
- Suction cups or other automatic picking devices at the injection moulding machine must be made of materials that do not leave an imprint on the substrate. Consult with us.
- Manual handling may only be done with lint-free gloves.
- Ideally, the plastic parts should be packed dust-free as soon as possible or placed in protective form-fitting transport trays with lids.
For efficient lacquering in larger series, it is therefore often smart to have AMB to injection mould the substrate, which minimizes contamination and eliminates logistics costs – and thereby becomes more cost-effective.
In principle, the above also applies to metal substrates. Metals are often more contaminated with dirt and oil than plastics and at AMB we do not clean substrates. However, anodised or otherwise cleaned and dust-free metal parts are usually good to take directly into our production.
3. Choice of plastic material
In addition to the fact that the surface to be laquered must be clean from dust, grease, and other contaminants, one of the most important conditions is that the lacquers must be able to adhere to the substrate. Nobody wants a beautiful and functional lacquer to peel off. Therefore, the first step is to investigate whether the selected plastic material is generally suitable for lacquering or whether it must be treated in different ways to achieve good adhesion. Sometimes the surface treatment conditions are documented in the material’s technical data sheet (TDS).
In general, plastics have a relatively low surface tension that does not provide sufficient wetting of the surface and therefore various pre-treatments that activate the plastic surface may also be necessary. The surface tension of the substrate must exceed the surface tension of the coating, otherwise the lacquer materials cannot “wet” the surface. Plastics such as ABS or PC can usually be painted directly, but some plastics such as polypropylene (PP) must first be coated with a primer, which provides good adhesion. Additional lacquer layers can then also be applied to this layer.
In addition to using a primer, AMB can also improve adhesion in another way for some plastics with low surface tension. This is done in a vacuum chamber where a plasma is created and charged particles such as ions and electrons affect the plastic surface in different ways. The plasma cleans the surface of any remaining microscopic organic contaminants and activates the substrate so that the varnish can bond to the surface. Plasma treatment means that several difficult-to-wet plastics can be used as substrates.
4. Surface finish
All surfaces are more or less smooth or rough. Many factors contribute to the surface finish in the manufacture and for injection moulded plastics it is the surface of the mold tool that defines the surface finish of the part. The surface finish is a measure that describes the character of a surface and it is standardized by ISO 1302.
For moulds that are processed with erosive techniques, there are definitions for different surfaces at German VDI, and you can also buy tiles with different defined surface finishes that VDI has developed.
Why is surface finish so important? The same surface treatment can give completely different effects for the color experience and haptics (how the surface feels). It is easy to understand if you think that you, for example, spray paint on a glass pane or a textured wallpaper. Reflection and shadows will be completely different for the different surfaces and the layer thickness to get coverage with the varnish will also vary. Therefore, it is important to be clear about what surface a product should have in the end already during the project phase. We recommend that you produce samples on the surface you have in mind.
Feel free to try sample painting on different surfaces to see the differences, and it is important to then use the same reference surfaces internally and in communication with us.
5. Area of use and requirements specification
Plastics are fantastic materials that, when used properly, enable a huge variety of products, some of which cannot be replaced with other materials at all. However, no material has all the properties you want and therefore it must be chosen with care for the product to work as well as possible. This is also important to keep in mind when it comes to lacquering.
The lacquer and the substrate will form a unit and in addition to adding a color, a lacquer should primarily protect the part. Like the plastic substrate, lacquers also have different properties. In fact, several lacquer systems after drying/curing also form plastics.
In addition to the lacquer working with the substrate material, the following should be considered:
- Where should the product be used? Inside/outside? In what climate (for example, a marine environment that causes salt, moisture and UV radiation, or in care facilities or laboratories where the part often has to be washed with alcohol or other disinfectants)?
- What requirements must the product meet? For example, climate resistance, abrasion resistance, scratch resistance, chemical resistance, cleaning with chemicals (which ones?) and possible biocompatibility.
AMB’s knowledgeable lacquer suppliers can then recommend which paints may be suitable and how they are supposed to meet the given specifications. There are general descriptions of the performance of paints in their technical data sheets and sometimes concrete data for various tests performed according to certain specifications, not infrequently from the automotive industry as this is one of the largest users of paints in many different stressful climates and environments. In part, you can thus lean towards these tests, but in the end you must define for yourself what the product should be able to do and preferably also do your own tests for the specific area of use.
6. Masking
Sometimes, certain parts of the substrate should not be lacquered, and then these areas must be masked. You must also keep in mind that the lacquer is applied with pressure and as finely divided drops and this paint mist settles not only on the surfaces to be lacquered but also on, for example, the back of a detail or other places where you may not want the coating. This is called over-spray. Therefore, some form of masking of these surfaces may be needed, as the lacquer can then interfere with a following assembly or the like.
Depending on the shape of the part, more or less complex solutions are needed. We have produced a basic geometry guide for designers that you can read on request.
Especially for high-volume industrial series, the masking must be rational and easy to mount on and off, which can be done manually or fully automated. There are many different solutions and we try to find the best solution from a production perspective, taking into account series sizes and economy. Sometimes it becomes rational and in fact absolutely necessary to use an injection moulded part as a mask (see below in the section on fixtures) in order to meet customer requirements and at the same time be able to produce efficiently. AMB therefore collaborates with Tingverken, which for more than 25 years has specialized in developing masking solutions through injection molded parts.
A recurring question is whether it is possible to lacquer two colors directly next to each other or whether you can mask for, for example, a display on a painted surface. In general, it is almost impossible to have two paints next to each other without a visible edge between them. A tip is then to insert a small (but as wide as possible) groove between the surfaces so that a masking detail can create a smooth transition. However, there will still not be a completely sharp boundary between the surfaces.
When it comes to display surfaces, the same reasoning applies, but in some cases you can remove the lacquer with a laser (ablation). Depending on the choice of plastic material, this affects the display surface somewhat and it is relatively expensive.
7. Fixtures
AMB mainly lacquers in automated lacquering lines, but also manually for smaller series. Regardless of the technology used, the plastic substrates must be held during the process. We have developed various standardized fixture systems on which component fixtures for the individual part can be attached. The component fixtures are product unique and are often combined with masking solutions for each substrate. Sometimes a simple clamp is enough, but often more advanced component fixtures are needed in different materials or as injection moulded parts.
The smaller a plastic part, the more difficult it becomes to fixate it. AMB has, together with Tingverken, for many years developed mainly injection molded solutions for this. For each substrate, a plastic fixture is then designed which, by press fit, holds the part in place and also masks the surfaces that are not to be painted. The requirements for a component fixture in plastic are generally high and increase the smaller the parts are. During lacquer curing, heat is applied, and the press fit must retain the substrate without expanding it. In addition, there must be a small air gap around the substrate so that the film-forming lacquer does not glue it to the fixture.
The fixture also has other tasks:
- In the lacquering lines, AMB mainly uses cylindrical spindles that are rotated during the painting application and the substrate is exposed to a strong centrifugal force. Therefore, the plastic part must be securely fastened, especially if the part is held by a press fit.
- The plastic part must be exposed to the maximum to the lacquer cloud that is sprayed against it so that as much of its surface is covered as evenly as possible.
- We try to place as many parts as possible on each fixture for the best production economy, without compromising quality.
It is sometimes more difficult to develop an optimized fixture solution than to develop the best painting parameters. Our experienced technicians and designers of injection moulded component fixtures have many years of experience from many different projects and can therefore usually find solutions even for complex plastic parts with different masking requirements.
A big advantage of our component fixtures is that we can use them several times because we clean them regularly. Instead of throwing away the fixture after one or a few rounds of painting, we thus contribute to greater sustainability. Many times, a component fixture can be used throughout the life of the product.