A brief discussion on the surface technology of car lamp plastic parts

Time:2023-11-18 08:45:31 / Popularity: / Source:

In order to obtain better appearance or performance, metal products often undergo special surface treatments, such as nitriding, chromium plating, galvanizing, etc. Similarly, in order to obtain these, plastic products also need some surface technology blessings. Based on my understanding of surface technology of plastic parts, let’s briefly talk about the two major surface technologies of automotive lamps-vacuum aluminum plating and surface spraying.

1. Vacuum aluminum plating

1.1 What is vacuum aluminum plating?

Break up aluminum products (aluminum wire, aluminum rings, aluminum blocks), distribute aluminum atoms evenly to surface of plastic parts in a vacuum environment, then form a protective film of silicon dioxide (actually, silicon dioxide is formed by spraying silicone oil first and then reacting with laughing gas). The entire process is completed in an aluminum plating machine. Evaporation and sputtering aluminum plating are similar, so I will not describe them in detail here.
surface technology of car lamp plastic parts 

1.2 What are functions and advantages of vacuum aluminum plating?

Referring to picture above, coating a layer of aluminum on the surface of a plastic part can not only give plastic part a certain metallic texture, but also reflect light emitted by light source like a mirror. Therefore, in automotive lighting manufacturing industry, vacuum aluminum plating is very commonly used.
Compared with traditional chrome plating and zinc plating, advantages of vacuum aluminum plating are still very obvious. The first is low pollution. Although aluminum vapor is also harmful to human body, it is still dwarfed by heavy metals such as chromium and zinc. There is no waste liquid to be processed in the whole process, which is very environmentally friendly. The second is high efficiency. Our company's aluminum plating cycle takes about 30 minutes, while previous unit took about 15 minutes. This depends on factors such as size of aluminum plating machine, parameter settings, and worker operation.

1.3 What should we pay attention to when designing aluminum-plated products?

①Draft angle should be as large as possible, and no structure should be too deep. Refer to picture below.
surface technology of car lamp plastic parts 
②When selecting materials, if product has function of reflecting light, it is best to choose black or other dark-colored materials. If it is just for decoration, it is best to use white or other light-colored materials.
For products with reflective functions, leakage plating is absolutely not allowed (a common defect is that aluminum is not plated where aluminum plating is required), because this will reduce reflection of light and affect light distribution function of lamp. If black materials are used, it will be easy for inspection personnel to detect when leakage occurs. On the contrary, if light-colored materials are used, even if leakage occurs, it will not be easily detected.

1.4 How to solve common aluminum plating problems?

There are still many common aluminum plating defects, such as yellowing, blueness on the surface of aluminum plating layer, aluminum explosion (aluminum products are not completely vaporized and adhere to the surface of product in the form of blocks), etc. There are also failures in performance tests, such as failure in "hundred grid test" (that is, insufficient adhesion of aluminum layer), failure in surface corrosion resistance testing, failure in high temperature testing, etc. For reasons of space, let’s give an example of low adhesion.
surface technology of car lamp plastic parts 
This is a product made of PC material. After aluminum plating, adhesion test failed. Aluminum layer was "uprooted" wherever tape passed. Later, we changed aluminum plating process. We first sprayed a layer of silicon dioxide on substrate for "priming" treatment, then performed regular aluminum plating. In this way, there will be no problem in adhesion testing.

2. Surface spraying

2.1 What kind of paint is used on plastic parts of car lights, and what is its function?

Generally speaking, main headlight cover needs to be spray-painted. It is sprayed with hardened paint on outside and anti-fog coating on inside (low-end cars generally do not spray it).
surface technology of car lamp plastic parts 
①Hardened paint
Most car headlight shades are made of PC material. Surface of PC shade is very soft after molding, and you can leave obvious marks with your fingernails. After spraying a layer of hardened paint on the outer surface of PC shade, surface is hard and can avoid minor scratches.
surface technology of car lamp plastic parts 
②Anti-fog coating
If you observe carefully in life, you can find that a layer of fog will condense in headlights of some cars parked on roadside. We learned in elementary school that this is small water droplets formed by pre-cooling of water vapor. Purpose of spraying anti-fog coating inside lampshade is to increase tension on inner surface of lampshade, turn small water droplets into a water film, reduce dispersion of light, and reduce impact of fog on light distribution of lamp.

2.2 How to solve common spraying problems?

There are also many spraying defects, such as orange peel, sags, particles, etc. Spraying process personnel are certainly familiar with this. Related parameters such as "flow rate, atomization, fan shape, UV, IR, time, air cleanliness" can be debugged slowly. I will not describe them in detail here.

2.3 What should we pay attention to when designing spray products?

surface technology of car lamp plastic parts 
Ordinary people can't imagine how tough a normal PC headlight lampshade is. I used to hit it with a hammer and it was difficult to break it. It often only left a mark on the surface. But there is a lampshade. After spraying anti-fog coating, if you hit wire drawing area inside lampshade with your fist, lampshade will crack. Why does this happen?
We cut out wire-drawing part inside lampshade and compared it with normal lampshade, as shown in picture below.
surface technology of car lamp plastic parts 
It turns out that edges and corners of inner drawing line of this product are too obvious, causing high demoulding resistance and causing cracks. After anti-fog coating is sprayed, crack defects are further expanded, just like triangular notch on plastic packaging designed to facilitate opening of packaging. Later, structure was optimized, wall thickness was increased, and it was tested by impact strength test (i.e., falling ball test) to meet international standards.

3 Summary

There are many plastic surface techniques. In order to make decorative parts inside lamp more "black" and increase visual effect, plastic parts are sprayed with black paint; in order to improve smoothness of surface of plastic parts and increase adhesion of aluminum layer, some spray a layer of adhesion paint; use laser or ink to engrave or draw on the surface of plastic part, etc.

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