Requirements for Plastic Workpieces for Aochuang Machinery Rotary Friction Welding Machines

Rotational Welding Principle

Rotational welding works by rotating two plastic components against each other at high speed. Friction between the contact surfaces generates heat, causing the plastic to melt. After rotation stops, pressure is maintained while the joint cools and solidifies, forming a strong welded connection.

Rotational welding is only suitable for thermoplastics. Thermosetting plastics cannot be rotationally welded.

1. Material Requirements - The Key Factor

✅ Plastics Suitable for Rotational Welding

1. Crystalline Plastics - Best Results

• PP (Polypropylene)
• PE (Polyethylene) - the most commonly used materials with mature rotational welding processes
• POM (Polyoxymethylene)
• PA (Nylon) - including PA6 and PA66; rotational speed and pressure need to be adjusted accordingly

Crystalline plastics have a relatively narrow melting temperature range, which allows efficient frictional heating and provides high welding strength.

2. Amorphous Plastics

• ABS
• PC (Polycarbonate)
• PMMA (Acrylic)
• PS (Polystyrene)
• SAN (Styrene-Acrylonitrile)

These materials can also be rotationally welded, but they have a wider melting temperature range and are more prone to excessive flash and yellowing. Therefore, welding parameters require more precise adjustment.

❌ Materials Not Suitable for Rotational Welding

1. Thermosetting Plastics

Examples include phenolic resin, epoxy resin, and BMC.

These materials do not melt when heated. Instead, they tend to carbonize, making them completely unsuitable for rotational welding.

2. Highly Filled Flame-Retardant Materials

Materials containing:

• More than 30% glass fiber
• Large amounts of talc
• Large amounts of calcium carbonate

These fillers reduce frictional heat generation and can result in extremely low welding strength and cracking.

3. Rubber and Elastomers

TPE/TPU materials tend to slip during friction and are difficult to weld. Certain soft TPE materials may require specially designed tooling and process parameters.

4. Incompatible Plastics

Different plastics that are not chemically compatible cannot be reliably welded.

For example, PP and PC are incompatible materials. Their molecular structures do not properly fuse, so even if they appear to melt together, the joint can easily separate afterward.

Same-material combinations are preferred. Compatible material combinations, such as ABS+PC alloys, can also be rotationally welded. Incompatible materials should not be used for rotational welding.

Tip: PP containing up to approximately 20% glass fiber can generally be rotationally welded. Materials with more than 30% glass fiber are not recommended.

2. Product Structure Design Requirements

1. The Welding Surface Must Be Circular

Rotational welding relies on circumferential rotational friction, so the welding area must have a circular or rotationally symmetrical structure.

Suitable designs include:

• Circular welding surfaces
• Annular welding surfaces
• Circular bosses

Square, irregular, or non-rotational welding surfaces are not suitable for rotational welding.

Large flat surfaces should not be used as the primary welding interface.

2. Stop/Positioning Structure

A positioning stop, shoulder, or other limiting structure should be designed to:

• Prevent component displacement during rotation
• Control welding depth
• Prevent excessive melting
• Prevent product collapse
• The recommended stop shoulder height is generally 0.8–2 mm.

This structure also helps prevent molten material from being squeezed into the interior of the product.

3. Welding Rib

It is recommended to design a circular welding rib around the welding area.

Recommended dimensions:

• Height: 0.6–1.2 mm
• Width: 1–1.5 mm

The welding rib helps:

• Concentrate frictional heat
• Control flash
• Maintain a stable welding area

Avoid flat-to-flat welding surfaces. Large flat friction areas can generate uneven heat and increase the risk of poor welding.

4. Wall Thickness

The recommended wall thickness in the welding area is 2–4 mm.

• Too thin: The part may deform or crack under pressure.
• Too thick: Frictional heating becomes slower and less efficient.

Avoid thin walls and large unsupported sections. Rotational welding generates torque during the welding process, which can twist or crack weak structures.

5. Reserve Space for Flash

Molten material and excess flash are normally generated during rotational welding.

The product structure should therefore include a flash groove or flash relief area.

Flash should not be allowed to directly affect critical sealing surfaces or visible cosmetic surfaces.

6. Torsional Strength of the Workpiece

Because rotational welding generates torsional force, the workpiece should not contain thin, brittle, or easily twisted structures.

For products with poor toughness, the rotational speed should be reduced appropriately.

3. Raw Material and Surface Conditions

1. Raw Material - Virgin Material Is Preferred

Virgin resin should be used whenever possible.

Recycled or reprocessed materials may contain impurities and have unstable melting characteristics, resulting in fluctuations in welding strength and increasing the risk of incomplete or weak welds.

2. Surface Condition - No Mold Release Agent, Silicone Oil, or Grease

Silicone oil, mold release agents, grease, and other contaminants can form a barrier between the welding surfaces and directly reduce welding strength.

The welding surfaces should be cleaned and degreased before welding if necessary.

3. Color

Color generally does not affect weldability.

However, dark-colored materials may absorb heat faster. The addition of masterbatch should not significantly alter the original melting characteristics of the material.

4. Process Matching - Corresponding Machine Parameters

The higher the rotational speed, the greater the frictional heat generated.

However:

Speed too high: Excessive flash, overheating, burning, or yellowing may occur.

Speed too low: Insufficient heat generation can result in incomplete or weak welding.

Therefore, rotational speed, welding pressure, welding depth, and hold time should be adjusted according to the material and product structure.

5. Common Welding Defects and Their Causes

1. Weak Welds, Water Leakage, or Air Leakage

Possible causes include:

• Incompatible materials
• Welding rib too small
• Insufficient rotational speed or pressure
• Mold release agent or other contaminants on the welding surface

2. Excessive Flash

Possible causes include:

• Rotational speed too high
• Hold time too long
• Stop/positioning structure not functioning properly

3. Product Cracking or Torsional Damage

Possible causes include:

• Wall thickness too thin
• Excessive torsional force
• Welding fixture does not provide sufficient anti-rotation support

4. Burning or Yellowing

Possible causes include:

• Rotational speed too high
• Excessive frictional heat
• Welding rib too large

6. Quick Rotational Welding Suitability Checklist

✅ Suitable for Rotational Welding

• Thermoplastic materials
• Circular or rotationally symmetrical welding surfaces
• Same or compatible materials
• Glass fiber content ≤20%
• No silicone oil or mold release agent on the welding surfaces
• Proper stop/positioning structure
• Proper circular welding rib

❌ Not Suitable for Rotational Welding

• Thermosetting plastics
• Non-circular or non-rotational welding surfaces
• Glass fiber content >30%
• Incompatible plastic combinations
• Surfaces containing excessive silicone oil or mold release agents

In short: Before selecting a rotational welding machine, first confirm the material, welding geometry, material compatibility, filler content, and product structure. Correct product design and process matching are essential for achieving a strong, leak-tight rotational weld.