Choosing a Resin Pair and Colourants for Laser Welding

Choosing a Resin Pair and Colourants for Laser Welding

Key Takeaways

• Two requirements must both hold: the polymers must be melt-compatible, and their optical behaviour at the wavelength must differ appropriately.
• Colour is an optical specification. A colour change is a process change, and a matched pair is a matched pair at one wavelength only.
• Glass fill scatters, reducing effective transmission and making the transmitting part the limiting component more often than expected.
• Qualify the material pair before the tooling, because a weldability problem discovered after mould cutting has very few cheap answers.

The question “can these two parts be laser welded?” has two independent answers that must both be yes. The polymers must be capable of forming a melt bond with each other, which is a materials-compatibility question shared with every other welding method. And their optical properties at the working wavelength must differ in the right direction and by enough, which is specific to laser welding and is where most surprises occur.

Melt Compatibility Comes First

A weld forms when two molten polymer surfaces interdiffuse and solidify together. That requires overlapping processing windows — both materials molten at the same temperature without either degrading — and sufficient chemical similarity for the chains to entangle across the interface.

Same-polymer joints are the straightforward case and cover most production. Different grades of the same polymer generally weld, though differences in melt flow, filler content and additive package shift the parameters. Different polymers within a family sometimes weld acceptably. Chemically dissimilar polymers usually do not form a real bond regardless of how well the optics behave, and any adhesion obtained is mechanical rather than molecular and tends to be disappointing under load or over time.

This ordering matters because optical properties are easier to change than chemistry. If the materials are melt-compatible, absorption and transmission can be engineered with colourants and additives. If they are not, no amount of optical work produces a sound joint.

Colour Is an Optical Specification

The most common production surprise in laser welding is a colour change that nobody flagged as a process change. Colour is specified by designers, marketing and customers on visual grounds, and the resulting pigment system determines behaviour at the laser wavelength — a relationship with no reliable connection to appearance.

Pigments absorb selectively. Two blues that match visually can transmit very differently at 1064 nm because they use different pigment chemistries. A white loaded with titanium dioxide scatters strongly. Some organic pigments are effectively transparent in the near-infrared; some inorganic ones are not. A part that transmits adequately in natural colour may not in a pigmented version, and the failure is total rather than gradual.

The working discipline is to treat colour as a controlled process input: qualify each colour combination, require notification of colourant reformulation from the compounder, and measure transmission and absorption at the working wavelength as part of material approval rather than relying on visual matching.

Absorption in the Lower Part

Carbon black remains the reference absorber and works extremely well — strong, broadband absorption at modest loading, well understood, inexpensive. Its characteristics do vary with grade: particle size and structure affect both how strongly and how superficially it absorbs, which in turn affects where the heat is generated relative to the interface. Selecting a carbon black for welding is a real decision rather than a default.

Where black is unacceptable, near-infrared absorbing dyes and pigment systems allow coloured, natural and visually clear absorbing parts. These require the same qualification as any additive — dispersion uniformity, thermal stability at processing temperature, regulatory acceptability for the application — and their absorption is typically narrower in wavelength than carbon black, which makes the match between additive and source more critical.

Transmission Through the Upper Part

The transmitting component is limited by more than colour. Wall thickness sets path length and transmission falls with it, sometimes sharply. Crystallinity in semi-crystalline polymers scatters, and because crystallinity depends on cooling rate, mould temperature changes optical behaviour directly. Glass and mineral fillers scatter strongly, which both reduces the energy reaching the joint and defocuses it, lowering peak intensity where it is needed. Additives with their own absorption — ultraviolet stabilisers in particular — can overlap the laser wavelength. Surface texture scatters at the entry face before any energy enters the part.

Filled transmitting parts are the case most often underestimated. A 30 per cent glass-filled upper component may transmit a small fraction of what an unfilled one does, and the usual response — increase power — risks surface damage at the entry face before the joint reaches melt. Where a filled material is unavoidable on the transmitting side, thinner sections in the beam path, a strategy that tolerates lower intensity, or a different wavelength are the real options.

Dual-Purpose Additive Systems

Assemblies that must be both laser marked and laser welded present a genuine formulation problem, since both processes depend on the same optical properties and want different things from them. Additive systems formulated to serve both purposes allow one compounded material to do both, which simplifies material management considerably.

The requirement is that the two processes are developed together rather than sequentially. An additive loading optimised for mark contrast may make a transmitting part too absorbing to weld; one optimised for weld absorption may produce poor marking contrast. Establishing both process windows against the same formulation, early, is what makes the approach work.

Qualify Before Tooling

Weldability is a property of the combination — polymer pair, colourants, fillers, additives, wavelength — and it is inexpensive to establish before commitment and disruptive to discover afterwards.

A practical pre-tooling qualification measures transmission of the intended transmitting material at the intended thickness and colour, measures absorption of the intended absorbing material, and welds moulded plaques or prototype parts in the actual material combination to confirm that a workable process window exists with margin. That exercise takes days and answers the question that otherwise gets answered during production trials, when the moulds exist and the material choice is effectively fixed.

Related Reading

• Carbon Black Selection for Through-Transmission Welding
• Dual-Purpose Laser Additives
• Clear-to-Clear Laser Welding
• Laser Transmission Welding Explained
• One Masterbatch for Marking and Welding

Need help with this?

The Sabreen Group provides independent engineering support for material selection, colour matching and additive qualification for laser welding. Our engineering services team works with manufacturers on process development, material qualification and production troubleshooting. Contact us to discuss your application.

Frequently Asked Questions

Can two different polymers be laser welded together?

Only where they are melt-compatible: their processing temperature ranges must overlap so both are molten without either degrading, and they must be chemically compatible enough to form a bond rather than a mechanical interlock. Similar polymers from the same family generally work; dissimilar families usually do not, whatever the optical properties.

Why does a colour change require requalification?

Because colourants determine how much of the laser wavelength is transmitted and absorbed. Two parts that look similar to the eye can behave completely differently at 1064 nm, since pigments absorb selectively and visible appearance carries little information about near-infrared behaviour. A colour change alters the energy reaching the joint and therefore the process.

Does glass fill affect laser welding?

Significantly. Glass fibres scatter the beam within the transmitting part, reducing the energy that reaches the interface and spreading it out, which lowers peak intensity at the joint. Higher fill levels compound this, and glass at the joint surface also reduces the polymer available to form the weld.

What is a dual-purpose additive?

An additive system formulated to serve both laser marking and laser welding, allowing one compounded material to be marked and welded with the same wavelength. It simplifies material management for assemblies that need both operations, and it requires the two processes to be developed together since they compete for the same optical properties.

When should material weldability be established?

Before tooling is cut. Weldability is a property of the material pair, the colourants and the wavelength, and if the chosen combination does not weld, the remedies are a material change, a colourant change or an additive — all of which are straightforward at design stage and disruptive afterwards.

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