Two identical-looking plastic parts can need completely different adhesives. The deciding factors are the polymer’s surface chemistry and the way the joint is built on your line. This guide is a working method for getting both right.

Start by Identifying the Polymer

Every plastic bonding decision begins with knowing exactly what the substrate is. Check the molding identification code, then place the material in one of two groups by surface energy, measured in mN/m.

High surface energy plastics, above about 38 mN/m, include ABS, polycarbonate, acrylic, rigid PVC, and PET. Adhesives wet these surfaces and form strong bonds with normal cleaning.

Low surface energy (LSE) plastics, below about 34 mN/m, include polypropylene at roughly 29, polyethylene at roughly 31, and fluoropolymers near 18. These resist wetting and need either a specialty adhesive, a primer, or surface activation such as plasma or flame treatment.

Confirm the group with a dyne pen before committing. A part that reads 30 mN/m will not hold a bond designed for 40 mN/m stock.

The Four Working Chemistries

Acrylic (MMA)

Two-part structural adhesive, lap shear of 2,000 to 3,500 psi, elongation of 30 to 100 percent, fixture in 5 to 20 minutes. Many grades bond untreated polyolefins. Best for dissimilar-material joints, panels, and structures that flex or take impact.

Epoxy

Two-part thermoset, tensile of 3,000 to 5,000 psi, low shrinkage, gap fill to several millimeters, cure in hours. Best on rigid plastics like polycarbonate and ABS for static structural loads and chemical exposure. Poor on untreated LSE plastics.

Cyanoacrylate

Single-part, cures in seconds with surface moisture, shear of 1,500 to 3,000 psi, bond line under 0.15 mm. Best for small parts and high-speed lines. Brittle, limited gap fill, softens above about 80°C. An LSE primer extends it to polyolefins.

UV-curing

Single-part, fixtures in 2 to 10 seconds under 365 to 405 nm light, cures water-clear with low shrinkage. Best for transparent acrylic and polycarbonate assemblies, optical parts, sensor housings, and electronics enclosures. At least one substrate must transmit UV. See UV glue versus epoxy for transparent bonding. Incure’s Uni-Weld plastic bonder line covers PC, ABS, and PMMA across viscosity and strength grades.

Design the Joint for the Adhesive

Adhesives are strong in shear and weak in cleavage and peel. Design accordingly.

• Favor lap, scarf, and sleeve joints that load the bond in shear.
• Size the overlap so the calculated shear stress stays below 25 percent of the adhesive’s rated strength, leaving margin for aging and stress concentration at the edges.
• Hold a consistent bond gap. Most structural adhesives perform best between 0.1 and 0.5 mm; too thin starves the joint, too thick wastes strength and increases shrinkage stress.
• When bonding plastic to metal or glass, expect the expansion mismatch to load the joint every thermal cycle. Choose a more elastic adhesive or add a compliant bond-line thickness. See how CTE mismatch causes adhesive bond failure.

Fit the Adhesive to the Line

The best chemistry on paper fails if it does not fit the process.

1. Cure speed versus line rate

If parts must move in seconds, use cyanoacrylate or UV. If a few minutes of fixture time is acceptable, acrylic or fast epoxy opens up.

2. Dispensing

Match viscosity to the equipment: thin liquids for jet valves and capillary joints, thixotropic pastes for beads on vertical surfaces.

3. Environment and safety

Solvent-free UV and 100 percent solids epoxy reduce VOC handling. Cyanoacrylate needs vapor control for operator comfort.

4. Rework

If field repair is expected, avoid a permanent rigid epoxy where a serviceable acrylic or a mechanical assist would do.

If you want a chemistry shortlist for a specific part, Email Us with the polymer and the joint drawing.

Validate Before Production

Bond production substrates with production surface prep. Condition samples to the service temperature and humidity extremes, then run shear and peel tests. Section a few bonds to check for voids and full wet-out. Only a passing aged-strength result justifies committing a line.

Frequently Asked Questions

Q: What bond gap should I design for?

A: Most structural plastic adhesives perform best between 0.1 and 0.5 mm. Below that the joint is starved and brittle; above it, shrinkage stress rises and shear strength drops. Design mating features, ribs, bosses, or a recessed pocket, that hold a consistent gap rather than relying on clamp pressure to set it.

Q: How do I bond polypropylene or polyethylene reliably?

A: Three routes. Use a two-part methacrylate rated for untreated polyolefins, the simplest option. Apply a cyanoacrylate LSE primer that chemically activates the surface, then bond within the primer’s working window. Or treat the surface with flame, corona, or plasma to raise surface energy above 40 mN/m and bond promptly. Plain epoxy and unprimed cyanoacrylate will not hold.

Q: Why does my clear plastic craze near the bond line?

A: Crazing is fine cracking from solvent attack or released molding stress. Aggressive solvent-based adhesives and some cyanoacrylate vapors attack polycarbonate and acrylic. Switch to a UV or low-stress adhesive, and anneal highly stressed molded parts before bonding.

Working With Incure

Incure formulates acrylic, epoxy, cyanoacrylate, and UV-curing adhesives for the full range of engineering plastics, plus the curing equipment to run them at rate. Our specialists help you identify the substrate, design the joint, and match the adhesive to your process. Contact Our Team to discuss your plastic bonding application.

Visit www.incurelab.com for more information.