Non-Painted Metallic Resins: Pigment Orientation Decides Whether the Joint Shows

Non-painted resins remove the painting operation and move the cosmetic risk into moulding and welding instead. The classic problem with them is a colour difference next to the joint: strength is fine, appearance fails.

Where the colour difference comes from

These metallic-effect resins rely on flake aluminium or pearlescent pigment aligned within the matrix. When the flakes lie parallel to the surface they reflect in one direction and read as metallic lustre; disturb the alignment and reflection scatters, so brightness and tone in that area differ from the surroundings.

Flow during moulding sets the orientation once. Welding makes the melt flow again, so orientation near the joint is rearranged and the difference appears. This is not a mis-set parameter; it follows from how the material produces its colour.

The same effect in moulding is called a weld line, with the same cause: disordered orientation where two melt fronts meet. Welding creates one more after moulding.

What can be done

First priority: move the joint out of the visible area. The only complete solution. Put the joint inside a side wall, on the base or behind a cover at the design stage.

Second: contain the melt. Use a shear joint so melt flows only between the mating faces and never spreads onto a visible surface. Melt from an energy director spreads sideways and readily leaves a ring on the visible face.

Third: hold energy to the minimum. Less melt means a smaller disturbed region. These parts should run in energy or depth mode with energy set precisely at fusion, not with a comfortable margin.

Fourth: limit the heat-affected region. High amplitude for a short time beats low amplitude for a long time: melting starts quickly and heat has less time to reach the visible surface, so the surface orientation survives more intact.

Fifth: keep the horn off the visible area. A witness mark disturbs the surface orientation and shows up more on these materials than on ordinary ones. Mirror-finish the horn face, add an interlayer, or move the contact point out of view altogether.

Filler and flow matter too

Like glass fibre, flake pigment concentrates at the fusion interface or lies unfavourably, affecting joint strength as well as appearance. Grades with high pigment loading weld weaker than natural resin of the same base, and the design should take the lower figure from the start.

Where both a metallic finish and high strength are required, the usual approach is to separate the cosmetic surface from the structural one: the visible part in the non-painted grade, the load-bearing structure in another, joined mechanically or welded out of sight.

Sample first, then set the standard

Results here are highly grade-sensitive, and suppliers and pigment loadings behave very differently. A change of grade requires revalidation; the old parameters do not carry over.

The sensible sequence is to weld samples in the actual grade and actual geometry, judge them under the agreed lighting and viewing angle, and set the production standard once feasibility is confirmed.

The effect of fillers and pigments on welding is in the material handbooks on the downloads page. Energy mode, depth mode and amplitude settings are on the ultrasonic welding machine pages.

Send the grade, the geometry and the cosmetic standard to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.