As plastic components become increasingly common across industrial sectors, professional welders are encountering more requests to weld thermoplastics – including those produced using 3D printing technologies. For those trained in traditional metal welding, transitioning to plastics, especially 3D printed ones, presents both new opportunities and unique challenges.
This article outlines the critical considerations for welders when dealing with 3D printed plastic parts, covering the impact of different 3D printing technologies, the behaviour of common 3D printable polymers, and the viability of welding techniques like hot air welding, extrusion welding, and ultrasonic welding.
Understanding 3D Printing Technologies and Their Impact on Weldability
At Midlands 3D, we specialise in a range of plastic 3D printing methods, including:
- Fused Deposition Modelling (FDM)
- Multi Jet Fusion (MJF)
- Selective Laser Sintering (SLS)
- Stereolithography (SLA)
Each of these technologies produces parts with differing microstructures, porosities, and mechanical properties, all of which affect the weldability of the final product.
FDM (Fused Deposition Modelling)
FDM prints thermoplastic filaments layer by layer. Parts produced by FDM often have visible layer lines and may contain gaps or internal voids, especially if printed with low infill or sub-optimal parameters. These voids can interfere with consistent heat transfer during welding, leading to weak joints.
Weldability: FDM parts can be welded using hot air or extrusion welding, particularly when the same base material is used. However, ensure parts are printed with high infill and minimal gaps for best results.
MJF (Multi Jet Fusion)
MJF builds parts from powdered nylon (typically PA12 or PA11) using a fusing agent and infrared energy. The result is a dense, isotropic part with excellent surface finish and mechanical integrity.
Weldability: MJF-printed nylon parts are excellent candidates for hot plate and ultrasonic welding, thanks to their uniform density and low porosity. Proper fixturing and control of heat application are essential to avoid degradation.
SLS (Selective Laser Sintering)
SLS also uses powdered thermoplastics (commonly PA2200) but sinters them with a laser. Parts tend to be strong and slightly porous.
Weldability: Like MJF, SLS parts perform well under hot plate and ultrasonic welding, although surface preparation may be needed to improve fusion at the weld interface.
SLA (Stereolithography)
SLA creates parts by curing photopolymer resin with UV light. These resins are typically thermosets, not thermoplastics.
Weldability: SLA parts are not weldable using conventional plastic welding techniques. Thermoset resins do not melt; they degrade under heat. Bonding should be achieved using adhesives designed for cured resin.
Common 3D Printable Plastics and Their Welding Characteristics
When welding plastic, material compatibility is critical. Below is a summary of key thermoplastics used in 3D printing and their respective weldability profiles.
PLA (Polylactic Acid)
- Used in: FDM
- Pros: Easy to print, low warping
- Cons: Brittle, low heat resistance
- Weldability: Fair. PLA has a low melting point (~180-220°C), but it tends to degrade quickly when overheated. Hot air welding is possible but requires careful temperature control.
ABS (Acrylonitrile Butadiene Styrene)
- Used in: FDM
- Pros: Tough, better heat resistance than PLA
- Cons: Warps easily when printing
- Weldability: Good. ABS welds well with hot air and extrusion welding. Use of compatible welding rods is essential.
PETG (Polyethylene Terephthalate Glycol)
- Used in: FDM
- Pros: Strong, chemical-resistant
- Cons: Stringy, can be hygroscopic
- Weldability: Good. PETG’s melting point (~230°C) makes it suitable for hot air welding.
PA12 (Nylon 12)
- Used in: MJF, SLS
- Pros: Strong, durable, flexible
- Cons: Absorbs moisture
- Weldability: Excellent. Nylon 12 welds very well using hot plate and ultrasonic welding due to its semicrystalline nature.
TPU (Thermoplastic Polyurethane)
- Used in: FDM, SLS
- Pros: Flexible, wear-resistant
- Cons: Difficult to print
- Weldability: Fair to good. Welding flexible parts requires special care with fixturing and lower temperatures.
Welding Techniques for 3D Printed Plastics
Hot Air Welding
This technique uses a stream of hot air to soften the surfaces and apply a filler rod of the same material.
- Best for: FDM parts made from ABS, PLA, PETG
- Considerations: Surface preparation is key. Sanding to remove oxidation and contaminants helps achieve better bond.
Extrusion Welding
Material is plasticised in a welding gun and deposited as a bead.
- Best for: Larger, thicker FDM parts
- Considerations: Similar to hot air welding but suitable for higher throughput and thicker sections.
Ultrasonic Welding
High-frequency vibrations create localised frictional heat.
- Best for: Dense, precise parts from MJF and SLS
- Considerations: Requires compatible part geometry and precise fixturing. Not suitable for FDM due to internal voids and lower density.
Hot Plate Welding
Both surfaces are pressed against a heated platen, then joined.
- Best for: Semi-crystalline thermoplastics like PA12
- Considerations: Ideal for production environments with flat, planar joint areas.
Design and Preparation Considerations
Welding success isn’t just about the material or technique—part design and prep are equally critical.
- Wall Thickness: Aim for uniform wall thickness in weld zones.
- Moisture Control: Many 3D printed parts (especially nylons) are hygroscopic. Dry thoroughly before welding.
- Surface Finish: SLS and MJF parts may require sanding or machining to create smooth welding surfaces.
- Fixturing: Proper alignment during welding is vital to avoid stress and ensure strength.
When Not to Weld
Certain combinations simply won’t work:
- SLA parts cannot be welded – these resins are thermosets and degrade with heat.
- Mixed materials – welding PLA to ABS or PETG to Nylon will result in weak joints or no bonding.
- Highly porous FDM parts – avoid welding low-infill prints.
In such cases, adhesive bonding or mechanical fastening is preferable.
Resources and Further Reading
- TWI (The Welding Institute): https://www.twi-global.com
- British Plastics Federation: https://www.bpf.co.uk
For bespoke advice, Midlands 3D offers consultation services to help select the right 3D printing material and process to ensure post-processing and welding compatibility.