Why do multi-material parts fail, and how is it prevented?

Multi-material parts usually fail when the tolerances and how the different materials react together aren’t fully understood before the mold is cut. It’s a chemical-engineering-and-experience business: getting a soft material and a hard material, or a plastic and a metal, to hold together the way a drawing says they will takes real expertise in how those materials mesh and work with each other, checked before tooling starts, not after.

Bonding and adhesion failures

A multi-material part fails at the bond line more often than anywhere else on the part, when the two materials were never a good chemical match or the bond had no mechanical backup to lean on.

Not every plastic bonds to every other plastic. Some pairs, like certain TPEs to a compatible substrate, form a genuine chemical bond during molding; others simply don’t want to stick. When a pairing isn’t checked for compatibility first, the part can look fine off the press and then separate weeks or months later, once it’s been flexed, heated, or handled in the field.

The fix that holds up is usually a mechanical interlock built into the mold, a groove, undercut, or textured surface that gives the second material something to grip rather than just sit on top of.

The material pairs Fudakin runs most are soft-on-hard plastic overmolding, TPE, TPU, or TPR overmolded onto a rigid substrate like PC, ABS, PP, or nylon, for grips, seals, and straps, and insert molding, plastic molded directly over a metal part. Both run across our overmolding and two-shot (2K) presses.

Shrink-rate mismatch and warping

A part warps or the bond line stresses when the two materials in it shrink at different rates as they cool. This is a design and material-selection problem, and it has to be caught before the mold is cut, not after.

Every plastic shrinks a different amount going from molten to solid, and the rate depends on the resin family, the fill content, and the wall thickness. When two materials shrink at noticeably different rates, the part pulls itself out of shape as it cools: a flat surface bows, a bond line separates a hair’s width, or a feature that should sit flush sits proud or sunken. None of this shows up off the press; it shows up later as a part that doesn’t fit or doesn’t seal.

The standard guidance is to keep the substrate wall roughly twice the thickness of the overmold wall at the bond, so the thinner, later-cooling material isn’t fighting a thicker one that already set up and shrank. Getting that ratio wrong is one of the most common causes of a warped multi-material part, and it’s decided on paper, during design, long before steel is cut.

Fudakin’s DFM review checks shrink behavior and bonding for the chosen materials on every multi-material project, before any mold is cut, not only when a customer flags a concern.

Shop-floor control

Even a well-chosen material pair and a well-built mold can still fail to bond if three process conditions aren’t controlled on the floor. These aren’t design problems; they’re run conditions that have to be managed shot after shot.

• Moisture. Hygroscopic resins such as PC, nylon (PA), PBT, and some TPU grades pull moisture out of the air. If that moisture isn’t dried out of the resin before molding, it turns to steam at the interface between the two materials and blocks adhesion.
• Contamination. A mold release agent, oil, or even a fingerprint left on the first-shot part or insert acts as a barrier. The second material ends up bonding to the contamination instead of the part underneath it.
• Temperature. A substrate that has cooled too far before the second shot goes in, or a melt-temperature gap of roughly 40°C or more between the two materials, is a commonly cited warning sign. Either one can leave the interface only partly fused.

A good material pairing and a well-designed mold set the part up to succeed. Drying, cleanliness, and temperature control during the run are what actually deliver the bond.

Paint adhesion on mixed-material parts

Paint that flakes, bubbles, or lifts off an overmolded or mixed-material part almost always comes down to surface energy, not the paint itself. Different plastics accept paint differently, and a coating that bonds perfectly to one material can fail on the material right next to it.

Rubbers, TPEs, and some engineering plastics have low surface energy, so paint has nothing to grab onto unless the surface is treated first. A primer formulated for that specific material, or a surface treatment like flame or plasma treatment before painting, raises that surface energy enough for the coating to bond properly. Skip that step, or use one primer across two materials that need different ones, and the paint holds on one material and lifts on the other, usually right at the seam between them.

A multi-stage paint process matters here too: surface treatment where needed, then a primer matched to each material, then the base coat and finish, with cure time between stages rather than one pass sprayed over everything. Before a part ships, bonds and paint are checked by visual inspection and Fudakin’s full-time QC staff.

Tooling and gate design

A mold built for one material with a second step bolted on tends to produce parts with the first three problems built in from day one. Multi-material tooling has to plan for a second material before the first cavity is cut, not after.

The gates for the second shot need to be positioned and timed so the second material reaches the bond surface while it can still fuse into the first, not after that surface has cooled and set. Cooling channels have to manage two materials with different thermal behavior in the same tool, and for two-shot work the press and mold need to hold the part in exact register between shots. None of this is visible in the finished part, but it’s the difference between a tool designed for a multi-material part and a single-material tool with an extra station added on after the fact.

What a factory that runs molding, painting, and assembly itself does differently

A factory that runs molding, painting, and assembly on its own floor can catch a bonding, shrink, or paint problem at the handoff between stages, before the part moves further down the line. A factory that ships the part out to a separate paint shop or assembly vendor doesn’t get that chance; by the time a bond or paint failure turns up, it has already left the building that could have caught it.

More than one in four new projects at Fudakin arrive with one of these problems already showing: bonding, warping, or paint adhesion.

One example: a customer came to Fudakin still in the design phase of a dial-and-ratchet strap system, an attachment assembly used to tighten and secure a sport product to the body, combining several materials in one small assembly. Every factory the customer had approached before Fudakin had turned the project down or been unable to make it work, not over cost, but because of the number of materials that had to come together and the process needed to join them without one working against another. Fudakin worked through the material and process combination with the customer while the product was still in design, before tooling was cut, and the product has since launched.

Comparison: failure modes and what prevents them

Failure mode	What you see	Root cause	How it’s prevented
Delamination	Materials separate at the seam	Incompatible pairing, or a chemical bond with no mechanical backup	Verify compatibility before tooling; add a mechanical interlock at the bond line
Warping	The part bows, twists, or a flat surface isn’t	Shrink-rate mismatch, often worsened by wall-thickness ratio	Substrate wall thicker than the overmold wall; check shrink and bonding in DFM before cutting steel
Paint flaking or lifting	Coating lifts or bubbles at the seam	Low surface energy, or one primer used for two materials	Match primer/treatment to each material; multi-stage paint; check bonds and paint by visual inspection and full-time QC before shipping
Off-register or short shot	Second material doesn’t fully cover the intended surface	Gate placement or timing wrong, or a mold not built to hold register	Design gate position and shot sequencing into the tool from the start
Weak bond despite compatible materials	Parts separate even though the pairing should bond	Moisture in the resin, contamination on the first part, or temperature	Dry hygroscopic resins, keep first-shot parts and inserts clean, control substrate and melt temperatures

A multi-material part that holds up is decided mostly on paper: material pairing, wall thickness at the bond, gate position, and the finishing plan for each material. Fixing any of these after tooling is expensive. Getting them right before tooling is what a DFM review is for.

To check a material pairing before tooling, use the overmolding material compatibility chart.

Further reading: PlasticsToday: adhesion failures in overmolding, Fictiv: overmolding material compatibility