Sofia Martinez
Plastic welding is a relatively easy process, but it is important to identify the type of plastic before beginning any welding work. There are many different types of plastics, but only a few are commonly used on auto parts, such as polypropylene blends, which are used for around 95% of late-model car bumpers. Other common plastics used in automotive applications include polyethylene, ABS, nylon, and polycarbonate. To identify the type of plastic, one can perform a short test weld or a burn test, which involves exposing a small piece of the plastic to an open flame and observing characteristics such as flame colour, odour, and smoke formation. Alternatively, a Rod Test Kit (RTK) can be used to verify the plastic material type by testing for adhesion between the parent material and the provided rods. Once the plastic type is identified, the appropriate welding rod and technique can be selected to achieve strong structural welds.
| Characteristics | Values |
|---|---|
| Identification method | Flame test, Rod Test Kit (RTK), or a short test weld |
| Flame test characteristics | Observe flame colour, smoke formation, dripping, etc. |
| RTK characteristics | Easy and safe, provides over 40 materials for ID testing, compatible with a wide range of plastics |
| Common plastics | Polypropylene, ABS, HDPE, LDPE, PP, PVC, Polyethylene, Nylon, Polycarbonate, Polyurethane |
| Polypropylene blends | Usually black, dark grey, or white with a PP symbol on the backside, identified as TEO or TPO |
| Polyurethane | Usually yellow on the backside with PUR or RIM symbols, use an airless plastic welder |
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What You'll Learn
Flame/Burn Test: Observe flame colour, smoke, drips, odour, etc
The flame test is a simple and low-cost method to determine the type of plastic for welding. It involves taking a small sliver of the parent material and, using a pair of tweezers, igniting it over a Bunsen flame.
The Bunsen burner, invented by Robert Bunsen in 1855, is particularly useful for flame tests due to its non-luminous flame, which does not disrupt the colours emitted by the test materials. When performing the flame test, observe the following:
- Flame colour: Different plastics will emit different colours when burned. For example, in a flame test, caesium ions will emit a sky-blue colour, while rubidium ions will emit a dark red colour.
- Smoke: Observe the smoke formations produced by the burning plastic.
- Drips: Note whether the plastic drips when ignited.
- Odour: Pay attention to any odours or smells produced during the test.
By examining these factors, you can accurately identify the type of plastic. However, it's important to note that this method only works for a limited number of plastics, and there are safety concerns associated with burning plastic. As such, it is recommended to use a Rod Test Kit (RTK) for plastic identification, which is safer and can test a wider range of materials.
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Rod Test Kit: Test for adhesion between the parent material and the rods
When welding plastic, it is essential to use the correct welding rod for each type of plastic. Using the wrong type of rod can lead to poor adhesion, weakened bonds, and poor longevity of the welds. This is where the Rod Test Kit (RTK) comes in. The RTK is a plastic ID test kit that provides the welder with over 40 different materials for ID testing. It is the most straightforward and safe way to verify your plastic material type. The RTK rods have been specially selected for weldability and compatibility with a wide range of commonly found plastics in the automotive repair and fabrication industries.
The RTK can be used to determine the type of material in unknown situations. The kit includes all the major typical types of materials used for most consumer and industrial products for testing adhesion to the parent material. The kit includes at least three pieces of each of the following:
- Polypropylene (PP)
- Polyethylene (PE)
- High-Density Polyethylene (HDPE)
- Low-Density Polyethylene (LDPE)
- Polyvinyl Chloride (PVC)
- Acrylonitrile Butadiene Styrene (ABS)
These are the most common base or parent materials used for plastic parts and thus the best to use for plastic welding. LDPE can sometimes be used on HDPE materials in certain applications.
To identify the correct plastic type, you can also look for the recycling code from 1 through 7 on the product to determine the parent material. These are typically surrounded by the popular triangle-arrow shape. You can also look for letters that are enclosed between the greater-than and less-than symbols, for example,
Once you have identified the plastic type, you can then select the correct welding rod for your specific welding needs.
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Welding Compatibility: Only like plastics can be welded
Welding plastics can be a complex process, and it is crucial to identify the type of plastic before attempting to weld. The success of the weld depends on using "like" plastics, and there are a few methods to identify the plastic type.
The most common plastics used in modern vehicles and auto parts are often stamped with plastics identification codes. However, when these codes are absent, a welding test is required to identify the plastic type. There are two popular methods for testing: the Flame Test and the Rod Test Kit (RTK).
The Flame Test involves taking a small piece of the plastic and igniting it over a Bunsen flame. By observing the burning behaviour, such as the flame colour, smoke formation, and dripping, one can identify the plastic type. For example, PVDF (Polyvinylidene Fluoride) produces an acrid odour, burns to a black ash, and self-extinguishes. On the other hand, TPUR (Polyurethane) produces a yellow flame, slight black smoke, and a faint apple-like odour. This test is simple but can provide a fairly accurate indication of the plastic type.
The recommended method for identifying plastics is the Rod Test Kit (RTK), which offers a safer and easier way to verify the plastic type. The RTK provides over 40 different materials for testing, ensuring weldability and compatibility with a wide range of commonly used plastics. This kit is designed for use in the automotive repair and fabrication industries, and its rods are UV-stabilized.
Additionally, plastics can be identified by their melting behaviour, as different plastics melt at various temperature ranges. Polypropylene, ABS, HDPE, LDPE, PP, and PVC are the most common base materials used for plastic welding due to their weldability and compatibility with various welding techniques.
It is important to note that some plastics, like polyurethane, are thermoset plastics, meaning they are not meltable and require different repair methods, such as using an airless plastic welder instead of a nitrogen welder.
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Material Identification: Look for symbols like PP, PUR, etc
Identifying the type of plastic is the first step in any welding job. Different plastic types are generally not cross-compatible, meaning one plastic type does not stick or weld with a different type. Therefore, it is crucial to identify the plastic type to ensure a successful weld.
One way to identify the plastic type is to look for symbols or letters enclosed between the greater-than and less-than signs, such as
Additionally, the recycling code on the product, typically surrounded by the triangle-arrow shape, can help identify the parent material. Codes range from 1 through 7, each representing a different type of plastic.
It is important to note that not all plastics have easily identifiable symbols or codes. In such cases, a test weld or a burn test can be performed to determine the plastic type. These tests involve exposing a small piece of the plastic to an open flame and observing its behaviour during and after burning.
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Float Test: Used to rule out certain plastics
The float test is one of the many ways to identify plastic types before welding. It is a good test for ruling out certain plastics, but it is difficult to specifically identify a plastic. The test has been around for years and is often coupled with the burn test.
The float test is performed by taking a small sliver of the plastic and placing it in water. Different plastics have different densities, so the time taken to sink and whether it floats or not can help identify the plastic type. For example, LDPE (Low-Density Polyethylene) is buoyant and will float in water for a long time.
The burn test, on the other hand, involves igniting a small sliver of the plastic over a Bunsen flame and observing the colour of the flame, smoke formation, and whether it drips. This test can give a good indication of the plastic type.
Another method is the Rod Test Kit (RTK), which is recommended by some as the most accurate, easy, and safe way to identify plastics. The RTK provides over 40 different materials for testing, and the rods are specially selected for weldability and compatibility with commonly found plastics. This method tests for adhesion between the parent material and the rods provided. Polypropylene, ABS, HDPE, LDPE, PP, and PVC are the most common base or parent materials used for plastic parts and are best suited for plastic welding.
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Frequently asked questions
There are several ways to determine the type of plastic you're working with. The first is to look for a plastics identification code, which is stamped on most plastics used in modern vehicles. If there is no identification code, a welding test is required. This can be done by performing a short test weld to see if the plastic welding rod stays affixed to the base material after it has cooled. You can also perform a burn test, where you expose a small piece of the plastic to an open flame and observe characteristics such as flame colour, smoke formation, and odour.
Some common types of plastics used in auto parts include polypropylene blends, polyurethane, polyethylene, ABS, nylon, and polycarbonate. Polypropylene blends are the most common, with about 95% of late-model car bumpers being moulded from this material.
The recommended method for identifying plastics before welding is to use a Rod Test Kit (RTK). This kit provides over 40 different materials for ID testing and is the easiest and safest way to verify your plastic material type.
It is important to note that only "like" materials can be welded together successfully. Additionally, different plastics melt at different temperature ranges, so this can be a decent way to identify the type of plastic you're working with.
