Sofia Martinez
Identifying different plastics before welding is crucial as different types of plastics are generally not cross-compatible. There are two common methods for identifying plastics: the Flame Test and the Rod Test Kit (RTK). The Flame Test involves igniting a small sliver of the plastic over a Bunsen flame and observing its reaction. Alternatively, the RTK can be used to test for adhesion between the plastic and the rods. Other methods include observing toolmarks, performing a test burn to determine the melting point, and comparing the characteristics of the plastic with a plastic of known content.
| Characteristics | Values |
|---|---|
| Identification method | Flame Test, Rod Test Kit (RTK), or comparing with a plastic of known content |
| Recycling code | Numbers 1 through 7 on the product |
| Material | Polypropylene, Polyethylene, Nylon, ABS, Polycarbonate, Acetal, PTFE, PVC, HDPE, LDPE, PP |
| Melting point | Different plastics melt at different temperature ranges |
| Scratches | Strings or galling |
| Burning | Different plastics smell differently when burnt |
| Solubility | Acetone will attack PVC and ABS but not polypropylene, polyethylene, nylon, or Delrin |
| Density | Difficult to distinguish various types of polyethylene by density alone |
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What You'll Learn
- Identify the plastic type: look for recycling codes or a 'test burn' to determine melting point
- Flame Test: use tweezers to ignite a small piece over a Bunsen flame
- Rod Test Kit: test for adhesion between the parent material and the rods
- Observe: cut and clean a sample, note scratches and abrasion
- Compare: purchase welding sticks of known content to compare with unknown plastics
Identify the plastic type: look for recycling codes or a 'test burn' to determine melting point
Identifying the type of plastic is crucial before welding, as only "'like'" materials can be welded together. For instance, polypropylene, commonly used in car bumpers, can only be welded with a polypropylene welding rod.
One way to identify the plastic type is to look for recycling codes, which are typically surrounded by the triangle-arrow symbol. These codes range from 1 through 7 and indicate the parent material. For example, Polypropylene/ethylene can be identified easily as they float in water.
Another method is to perform a test burn or a "Flame Test" to determine the melting point and identify the plastic. Different plastics have distinct melting points and smell differently when burned. For instance, polyethylene and polypropylene have a candle wax odour, while nylon has a very distinct smell. A soldering iron can be used to heat the plastic and observe its reaction.
Additionally, the "Rod Test Kit" (RTK) is a recommended option to test for proper adhesion between the parent material and the rods. This kit provides a safe and controlled way to identify the plastic type without resorting to open flames.
Other characteristics, such as abrasion, toolmarks, and reaction to solvents like acetone, can also provide clues about the plastic type. For instance, acetone will attack PVC and ABS but not polypropylene, polyethylene, nylon, or Delrin.
Comparing with a plastic of known content is also helpful, especially for beginners. This can provide insight into the characteristics of an unknown plastic by observing similarities or differences.
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Flame Test: use tweezers to ignite a small piece over a Bunsen flame
When welding plastic, it is important to identify the type of plastic you are working with. This is because different plastics are generally not cross-compatible—they will not stick or weld with one another.
One way to identify plastic type is through a flame test. To perform a flame test, you will need a small sliver of the plastic you are working with, as well as a pair of tweezers and a Bunsen burner. Hold the sliver of plastic with the tweezers and ignite it over the Bunsen flame. Observe the colour of the flame as the plastic burns. Different plastics will burn with differently coloured flames.
For example, if the flame is yellow, the plastic is likely HDPE. A blue flame indicates LDPE, while a yellow-green flame indicates polypropylene. PVC produces an orange flame, and polystyrene produces a lilac flame.
A flame test is a simple and effective way to identify the type of plastic you are working with. However, it is important to exercise caution when working with open flames. Always take the appropriate safety precautions when performing a flame test.
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Rod Test Kit: test for adhesion between the parent material and the rods
When welding plastic, it is crucial to identify the type of plastic you are working with. Different plastics have different properties, and they generally do not cross-compatible; that is, one plastic type does not stick or weld with a different type.
One way to identify plastics is through the use of a Rod Test Kit (RTK). This kit provides the user with over 40 different materials for identification testing. The RTK rods are selected for weldability and compatibility with a wide range of commonly found plastics.
To use the RTK, you must first melt the plastic part. Simultaneously, melt one of the RTK rods, which you suspect might be the right welding rod. Hold the melts together and allow them to cool. If the rod sticks to the plastic, this is the correct welding rod for that plastic type.
Another way to identify plastics is through a flame test. This involves taking a small piece of the plastic and igniting it over a Bunsen flame. Each plastic has a unique burning characteristic. For example, PE tends to burn with a candle-like smell, while PVC produces a strong odour and black smoke.
Other methods of identifying plastics include checking the density of the plastic, colour-coding, and flexibility checks.
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Observe: cut and clean a sample, note scratches and abrasion
When identifying different plastics for welding, it is important to first cut and clean a sample. This initial step can reveal a great deal about the plastic type, though not the specific formulation. Observe the sample's reaction to cutting and cleaning: note any scratches and abrasion. For instance, strings or galling may appear when the plastic is scratched.
The machinability of the plastic can also be determined by examining the toolmarks. Some plastics, like PVC, machine well but do not have bearing qualities. Various nylons and Delrins are good when mechanically applied and are suited to honed HSS bits.
Additionally, the density of the plastic can provide some clues as to its identity. For example, it is challenging to distinguish various types of polyethylene by density alone. However, a burn test can help identify polyethylene as it emits a candle wax odour when heated. Nylon also has a very distinctive odour when burned, as does Delrin (which is acrylic).
It is also helpful to compare the characteristics of an unknown plastic to a plastic of known content. For example, polypropylene/ethylene is easily identifiable as it floats in water.
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Compare: purchase welding sticks of known content to compare with unknown plastics
Welding plastic requires the use of a welding rod made of the same type of plastic as the plastic being welded. Different plastics melt at different temperatures and have different properties, so it is important to identify the type of plastic before welding.
One way to identify the type of plastic is to look for a recycling code from 1 through 7 on the product, typically surrounded by a triangle of arrows. Another way is to purchase welding sticks of known content and compare them to the unknown plastic through a "test burn" to determine the melting point. Polypropylene, ABS, HDPE, LDPE, PP, and PVC are the most common base materials used for plastic parts and are therefore good candidates for comparison.
A third method of identification is the "Flame Test," which involves taking a small sliver of the plastic and igniting it over a Bunsen flame with a pair of tweezers. This test can be dangerous and is not recommended. A safer alternative is to use a "Rod Test Kit" (RTK), which tests for proper adhesion between the parent material and the rods provided.
Once the plastic has been identified, the correct welding rod can be selected. It is important to note that the welding temperature will depend on the type of plastic and the specific welder being used. For example, when welding polypropylene (PP), the welder needs to be set at approximately 572°F/300°C. Polyethylene (PE) is another easy plastic to weld, but it is important to remember that you can weld low-density polyethylene (LDPE) to high-density polyethylene (HDPE) but not the other way around.
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Frequently asked questions
Different plastics have different melting points, so a 'test burn' can be used to identify the type of plastic. You can also look for recycling codes on the product, which are typically surrounded by a triangle of arrows.
You can use a "Flame Test" by taking a sliver of the material and igniting it over a Bunsen flame. You can also use a "Rod Test Kit" (RTK) to test for adhesion between the parent material and the rods.
Polypropylene, ABS, HDPE, LDPE, PP, and PVC are the most common plastics used for welding.
Nylon has a distinctive odour, as does Delrin (acrylic). Polyethylene and polypropylene smell like candle wax when burned.
Examine the toolmarks present—most appear sawn. You can also look for characteristics such as abrasion, scratches, and colour.
