1. Basic Mandatory Requirements
1. Thermoplastic Materials Are Required
Weldable materials:
PP, PE, ABS, PC, PA nylon, POM, PBT, PET, and other thermoplastics.
Non-weldable materials:
Epoxy resin, phenolic resin, melamine, epoxy fiberglass sheets, and other thermosetting plastics.
Thermosetting plastics will only carbonize when heated. They cannot melt and fuse, making them completely unsuitable for rotary friction welding.
2. Workpiece Structure: The Welding Surface Must Be Circular
The welding surface must be a circular rotating surface.
Non-circular parts cannot be processed by rotary friction welding. Hot plate welding or vibration friction welding should be considered instead.
2. Differences in Rotary Friction Welding Characteristics Between Crystalline and Amorphous Plastics
1. Semi-Crystalline Plastics: The Main Materials for Rotary Friction Welding
Typical materials include PP, PE, PA6/PA66, POM, and PBT.
Characteristics:
Narrow melting temperature range and high latent heat of fusion, requiring sufficient frictional energy to generate heat.
Significant crystallization shrinkage occurs after welding, which can result in greater internal stress.
Once the weld is properly formed, these materials generally provide excellent airtightness and solvent resistance.
They are preferred materials for applications such as filter elements, oil tanks, and pipe fittings.
Process requirements:
Crystalline materials require higher frictional energy, and the rotational speed and welding pressure must be properly matched.
Moisture-sensitive materials such as PA nylon must be dried before welding. Excessive moisture can cause bubbles, brittle welds, incomplete fusion, and water leakage.
2. Amorphous Plastics
Typical materials include ABS, PC, acrylic, and PC/ABS alloys.
Characteristics:
Have a broad softening temperature range.
Generate frictional heat relatively easily.
Generally have a lower welding threshold.
However, their high-temperature and chemical resistance may be lower than that of crystalline materials.
PC-based materials are also sensitive to moisture. Excessive moisture can cause silver streaks and bubbles in the weld.
Welding Difficulty Ranking
From easier to more difficult:
ABS > PC > PP > PE > PA > POM
POM has a relatively low coefficient of friction, which can cause slipping and make it difficult to generate sufficient frictional heat. Higher welding pressure and an optimized friction surface design may therefore be required.
3. Requirements for Same-Material and Dissimilar-Material Combinations
1. Same-Material Welding: The Preferred Solution
A plastic component should ideally be welded with the same grade of plastic.
The molecular structure and melting characteristics are closely matched, resulting in:
Higher weld strength, potentially reaching more than 90% of the base material strength
Better sealing stability
More consistent production performance
Therefore, same-material welding is the preferred option whenever possible.
2. Dissimilar Plastic Combinations: Limited Weldability and Validation Required
The following general industry criteria should be considered when evaluating dissimilar plastic combinations:
1. Melting temperature difference ≤ 30–40°C
The combination may have rotary friction welding feasibility when the melting temperature difference is within approximately 30–40°C.
If the temperature difference is too large, one material may already be overheated or degraded while the other has not yet melted, resulting in incomplete fusion.
2. Similar molecular polarity is preferred
The closer the polarity of the polymers, the better the compatibility at the molten interface.
Large differences in surface energy can reduce interfacial wetting and increase the risk of delamination and cracking.
3. Combinations with significantly different polarity should be avoided
For example, PP + PC is generally unsuitable for reliable rotary friction welding.
Examples of potentially weldable combinations:
PP ↔ PE
PC ↔ PC/ABS
ABS ↔ PC/ABS
Not recommended:
PP ↔ ABS
PE ↔ PC
These combinations generally have poor stability for mass production and require careful validation before use.
4. Requirements for Filled and Modified Plastics
1. Glass-Fiber-Reinforced Plastics
Examples include glass-fiber-reinforced PP and glass-fiber-reinforced PA.
Compared with ultrasonic welding, rotary friction welding can be more suitable for some glass-fiber-reinforced materials. However, as the glass-fiber content increases, the weld may become more brittle.
Reason:
Glass fibers do not melt and can remain at the welding interface, forming a barrier layer between the molten resins.
When the glass-fiber content exceeds 30%, the risk to sealing performance increases significantly.
Possible improvements:
Increase the width of the welding friction surface.
Reduce rotational speed.
Increase the holding pressure time.
Conduct airtightness testing to verify the welding performance.
2. Mineral-Filled Plastics
Common fillers include calcium carbonate and talc.
As filler content increases, the proportion of meltable resin decreases, resulting in a continuous reduction in weld strength and sealing performance.
For sealing applications, highly filled formulations should be avoided whenever possible.
3. Color Masterbatch and Recycled Materials
Dark-colored materials containing carbon black generally have limited impact on the welding process.
However, recycled materials may contain more impurities and experience molecular degradation, which can result in localized incomplete welding and brittle welds.
For pressure-bearing or airtight rotary friction welded products, recycled material content above 30% is not recommended.
4. Elastomers and Toughening Agents
When large amounts of soft TPE or POE toughening agents are added, the material can absorb frictional energy, slowing down heat generation.
In such cases, it may be necessary to increase rotational speed and provide additional frictional allowance.
5. Requirements for the Incoming Condition of Plastic Parts
Many welding defects are caused not by the welding equipment, but by the condition of the plastic parts themselves.
1. The Welding Surface Must Be Clean and Free of Contaminants
Oil, mold release agents, silicone oil, and dust can directly cause incomplete fusion and intermittent leakage around the weld.
During injection molding, the use of mold release agents should be minimized. If necessary, the welding surface should be cleaned before welding.
2. Moisture Content Must Be Controlled
Moisture-sensitive plastics require mandatory drying before welding.
PA nylon, PC, and PET are highly moisture-sensitive.
Recommended drying conditions include:
PA: 80–85°C for 4–6 hours
PC: 110–120°C for 3–4 hours
Welding these materials without proper drying can result in bubbles, cracking, and airtightness failure at the weld.
3. Injection-Molding Stress and Warpage
If the plastic part is warped, the upper and lower welding surfaces may not fully contact each other, resulting in incomplete fusion on one side.
Rotary friction welding cannot correct deformation caused by injection molding. Defective or excessively warped parts should therefore be screened out before welding.
4. Welding Rib and Friction Allowance Must Be Dimensionally Stable
Rotary friction welding relies on a defined welding allowance or displacement to achieve melting and fusion.
If the height of the welding rib varies significantly between injection-molding batches, mass-production defects may occur.
The equipment should preferably operate in displacement-controlled mode rather than relying solely on time-controlled welding for production.
6. Rotary Friction Welding Risk Summary by Plastic Type
| Plastic Type | Welding Difficulty | Main Risk | Typical Applications |
|---|---|---|---|
| PP Polypropylene | Moderate | Significant crystallization shrinkage and tendency to generate flash | Filter elements, housings, water-path sealing components |
| PE Polyethylene | Moderate | Relatively low coefficient of friction and slow heat generation | Containers, pipe fittings |
| ABS | Easy | Yellowing caused by overheating | General housings, non-pressure-bearing parts |
| PC | Easy | Moisture-induced bubbles and high-temperature stress cracking | Transparent parts, structural components |
| PA Nylon | Relatively Difficult | Moisture-induced bubbles and significant cooling shrinkage | Automotive valves, high-temperature-resistant components; drying is mandatory |
| POM Polyoxymethylene | Relatively Difficult | Slipping and difficulty generating frictional heat | Precision small rotating components |
7. Industry Practices to Avoid
❌ For airtight and pressure-bearing products, avoid highly glass-fiber-filled and highly mineral-filled formulations whenever possible.
❌ Do not put dissimilar plastic combinations directly into mass production. Sample validation must be performed first, including tensile strength and airtightness testing.
❌ Do not put moisture-sensitive plastics directly into production without proper drying.
❌ Do not attempt rotary friction welding with thermosetting materials. They will only carbonize and become scrap when heated.
8. Additional Guidance: Matching Plastics with Rotary Friction Welding Equipment
Servo rotary friction welding machines, with fully controllable rotational speed, welding pressure, and displacement throughout the process, can compensate for some material-related limitations.
However, equipment cannot change the fundamental weldability of the plastic itself.
If the material is inherently unsuitable for rotary friction welding, even the most advanced equipment cannot achieve stable mass production.
