Ultrasonic Welding for Injection Molded Parts: Design Details You Must Pay Attention To

In the injection molding industry, we frequently encounter this scenario: The product design looks great, the functionality is perfect, but when it's time to assemble two plastic parts by welding, problems appear—weak welds, air leaks, visible marks on the surface...

More often than not, these issues don't originate from the welding process itself. They start at the design stage of the injection molded parts.

Today, let's discuss: If your product requires ultrasonic welding, what design details must you consider before the mold is made?

Part 1: First, How Does Ultrasonic Welding Work?

Before diving into design details, it's helpful to understand the basic principle of ultrasonic welding.

The ultrasonic welding process can be summarized as: High-frequency vibration → Friction heat → Plastic melting → Molecular bonding.

In concrete terms:

• The welding equipment converts standard 50/60Hz electrical current into high-frequency electrical energy (20-40kHz)

• A transducer converts this electrical energy into mechanical vibration at the same frequency

• The vibration is transmitted through a horn (sonotrode) to the plastic part

• At the joint interface, the vibration energy is concentrated by an energy director, generating friction heat

• The plastic melts and flows, creating a molecular-level bond between the two parts

This entire process typically takes only 0.2 to 1.0 seconds, making it extremely efficient.

Key point: Ultrasonic energy needs to be "concentrated" to effectively melt the plastic. This is why energy director design is critical—it's the starting point of the entire welding process.

Part 2: The Heart of Weld Joint Design: The Energy Director

An energy director is a pre-designed raised feature on the injection molded part, typically triangular in cross-section. Its function is to concentrate ultrasonic energy at an extremely small contact point, generating heat quickly to initiate melting.

2.1 Key Design Parameters for Energy Directors

2.2 Energy Director Selection by Material Type

Amorphous Plastics (ABS, PC, PS, PMMA, etc.)

• Energy director apex angle: 90° (right triangle, 90° at the apex)

• Welding characteristics: Efficient energy transmission, easy to weld

• Recommended joint types: Basic energy director, step joint, tongue-and-groove

Semi-Crystalline Plastics (PA, POM, PP, PBT, etc.)

• Energy director apex angle: 60° (equilateral triangle)

• Welding characteristics: Requires more energy, solidifies quickly after melting

• Recommended joint type: Shear joint

Why the difference?
Semi-crystalline plastics transition from solid to molten state very rapidly, over a narrow temperature range. If you use a standard energy director, the melted plastic may solidify before properly fusing with the mating part. This is why semi-crystalline plastics typically require shear joints to ensure weld strength.

2.3 Material Compatibility: Not All Plastics Weld Together

This is one of the most overlooked factors. Ultrasonic welding requires the two materials to be chemically compatible.

Special attention: If plastics contain additives like flame retardants, mold release agents, or lubricants, welding performance may be affected. Welding tests are recommended in advance.

Part 3: Five Common Ultrasonic Weld Joint Designs

Different welding requirements call for different joint designs. Here are the five most common approaches:

3.1 Basic Energy Director Joint

The most common and simplest design, suitable for most applications that don't require sealing.

Design:

• Triangular energy director on one part

• Flat surface on the mating part

• Energy director height: 0.25-0.75mm

Best for:

• Amorphous plastics

• Applications not requiring hermetic seals

• Primary focus on weld strength

Advantages: Simple design, easy mold manufacturing
Disadvantages: Possible flash, affects appearance; cannot guarantee sealing

3.2 Step Joint

This design can hide welding flash, providing better appearance.

Design:

• Step features for part alignment

• Energy director can be added to the step

• Minimum wall thickness: 2mm

• 0.13-0.51mm gap on non-welding side for flash containment

Best for:

• Products with appearance requirements

• Applications needing self-alignment

• Structural strength needed without sealing

3.3 Tongue-and-Groove Joint

This is the preferred choice for hermetic seals and the most robust design.

Design:

• Tongue on one part, groove on the other

• Energy director on the tongue tip

• Requires thicker walls to accommodate tongue-and-groove

Best for:

• Products requiring air/water tightness

• Amorphous plastics

• Applications with space for tongue-and-groove features

Advantages: Self-aligning, flash contained in groove, excellent sealing
Disadvantages: Requires thicker walls, slightly higher mold cost

3.4 Shear Joint

This is the preferred choice for semi-crystalline plastics and offers the highest weld strength.

Design:

• Interference fit design: inner part slightly larger than outer part's inner diameter

• Minimal initial contact, parts "shear" together during welding

• Weld depth typically 1.25× wall thickness

• Vertical weld land height: 1.0-1.5mm (determines weld strength)

Best for:

• Semi-crystalline plastics (PA, POM, PP, PBT, etc.)

• Applications requiring high strength and sealing

• Small to medium parts

Advantages: Highest strength, best sealing, molten plastic protected from air
Disadvantages: Tight dimensional tolerances required, needs stable molding process

3.5 Peak-and-Valley Joint

This design provides self-alignment and is suitable for applications requiring complete sealing.

Design:

• Sawtooth-like interlocking features

• 0.13-0.51mm gap on non-welding side

• Minimum wall thickness: 3mm

Best for:

• Applications requiring complete hermetic seals

• Products needing self-alignment

Part 4: Other Critical Design Considerations

Beyond the joint itself, several design details directly impact welding success:

4.1 Near-Field vs Far-Field Welding

Based on the distance from the horn contact point to the weld interface:

Near-Field Welding (<6mm)

• Distance from horn to weld interface less than 6mm

• High energy transmission efficiency

• Suitable for all materials, especially semi-crystalline plastics

• Shorter weld times, lower pressure requirements

• Preferred approach

Far-Field Welding (>6mm)

• Distance from horn to weld interface greater than 6mm

• Energy loses strength traveling through the part

• Only works with rigid amorphous plastics (PS, ABS, PMMA, etc.)

• Requires longer weld times and higher pressure

• Use cautiously, only when necessary

Design recommendation: Keep weld interfaces within 6mm of the horn contact surface whenever possible.

4.2 Wall Thickness Uniformity

Ultrasonic welding relies on vibration energy traveling through the part. Abrupt wall thickness changes affect energy transmission.

Design principles:

• Maintain uniform wall thickness

• Avoid localized thick sections that may cause sink marks (sink marks can collapse during welding)

• Ensure sufficient rigidity to transmit vibration energy

4.3 Avoid Sharp Corners

Sharp internal corners can create stress concentration points under ultrasonic vibration, potentially causing part cracking.

Design principles:

• Use radii on all corners

• Minimum radius: 0.2-0.5mm

• Round sharp edges to prevent energy concentration and cracking

4.4 Part Fit-Up Clearance

The fit between mating parts before welding matters significantly.

Design principles:

• Ideal clearance: 0.05-0.1mm (depending on part size)

• Too tight: Difficult to assemble, may crush energy director

• Too loose: Misalignment, uneven welding

4.5 Weld Surface Height Consistency

Ideally, the entire weld surface should lie in the same plane and parallel to the horn face.

If weld surfaces are not at the same height:

• High points contact first, melt first

• Low points may never make contact, resulting in poor welds

Design recommendation: Keep all weld surfaces at the same height. If impossible, ensure height differences are within acceptable limits.

4.6 Horn Contact Surface Design

The horn needs a contact surface to transmit vibration. Poor contact surface design leads to energy loss or surface marks.

Design principles:

• Provide adequate flat surface area for horn contact

• Use PE film buffer if surface protection is needed

• Polished or uneven surfaces are more prone to marking

Part 5: Often-Overlooked "Hidden" Issues

5.1 Moisture Absorption in Nylon

Nylon (PA) is highly hygroscopic. If nylon parts sit in air after molding, they absorb moisture.

Consequence: During welding, moisture turns to steam, creating bubbles and voids at the weld interface, severely weakening the joint.

Countermeasure: Weld nylon parts as soon as possible after molding ("dry" welding). If parts have been sitting, dry them before welding.

5.2 Mold Release Agent Effects

Some injection molded parts use mold release agents during production. Residue on weld surfaces prevents molecular bonding.

Countermeasure: If release agents are necessary, choose weldable grades or clean weld areas before welding.

5.3 Filler Material Effects

Plastics may contain fillers like glass fiber, carbon fiber, or talc. These fillers affect weldability.

General rules:

• Higher filler content = greater welding difficulty

• Fillers at the weld interface can become stress concentration points

• Conduct welding tests before final mold commit

Part 6: Summary: Ultrasonic Welding Design Checklist

Before finalizing product design and committing to molds, run through this quick checklist:

Material Selection

• Are the two part materials chemically compatible?

• For semi-crystalline plastics, is a shear joint selected?

• Is filler content within weldable limits?

• Is moisture absorption a concern that needs addressing?

Joint Design

• Is energy director height within 0.25-1.0mm?

• Does energy director angle match material requirements (90° amorphous, 60° semi-crystalline)?

• Is the correct joint type selected (strength/sealing/appearance)?

• For sealing requirements, is tongue-and-groove or shear joint used?

• For appearance requirements, is there flash containment?

Structural Design

• Is weld interface within 6mm of horn contact (near-field)?

• Is wall thickness uniform without abrupt changes?

• Are all corners radiused (R≥0.2mm)?

• Is there sufficient rigidity to transmit vibration?

• Are all weld surfaces at the same height, parallel to horn face?

Dimensional Control

• Is fit-up clearance between 0.05-0.1mm?

• For shear joints, is interference precisely controlled?

• Are there self-aligning features (steps, tongue-and-groove)?

Process Preparation

• Is adequate horn contact area provided?

• Is the contact surface flat and mark-resistant?

• Is mold release agent use considered?

• Are welding tests planned to validate design?

Part 6.5: Design Recommendations Summary Table

Final Thoughts

Ultrasonic welding is a process where design determines success. Welding equipment can only execute what the design allows—it cannot compensate for design flaws.

As injection mold suppliers, our advice is: Incorporate welding considerations at the product design stage, not after molds are made, trying to figure out how to "make it weld." Think through welding requirements early, choose appropriate joint designs, control critical dimensions, and production will run smoothly.

If you're developing a product that requires welding and you're unsure about your design, contact us. We can help with DFM analysis to identify potential issues before mold commitment, avoiding costly rework later.