Even if you have the perfect wavelength match (as discussed in Section 2), your UV adhesive can still fail to cure if the light lacks sufficient intensity—the raw power delivered to the adhesive surface. This power, or irradiance, is measured in units like milliwatts per square centimeter (mW/cm2). Insufficient intensity leads to a sluggish or incomplete chemical reaction, resulting in a soft, tacky, or weak bond. The Two Main Causes of Low Intensity A. Too Much Distance (Inverse Square Law) UV light energy follows the Inverse Square Law, meaning that intensity drops off rapidly as the light source moves farther away from the target. If you double the distance, the adhesive receives only one-quarter of the light intensity. The Fix: Position your UV lamp as close as possible to the adhesive bead or bond line. For fixed-position industrial lamps, follow the manufacturer’s recommended working distance precisely. For handheld hobbyist lights, aim for a distance of 1 to 5 centimeters for optimal results. B. Low Output or Degradation The lamp itself might not be providing the necessary power due to age, cleanliness, or simply being a low-grade unit. Aging Lamps: Traditional mercury vapor and fluorescent UV bulbs degrade over time. The bulb may still glow, but its UV output gradually diminishes, meaning your curing time needs to be progressively extended to compensate for the lost power. Dirty Lenses: Any dirt, dust, or overspray on the lamp’s lens or reflector will scatter or block the UV light, significantly reducing the energy that reaches the adhesive. Low-Power Units: Cheap, underpowered "UV flashlights" designed for checking bank notes often lack the necessary mW/cm2 output to reliably cure adhesives, especially those requiring fast curing times. Genuine Solutions for Insufficient Intensity 1. Measure and Monitor Irradiance (Industrial/Critical Users) For applications where bond strength is critical, the best solution is to measure the light’s output using a UV Radiometer(or UV light meter). This tool tells you the exact mW/cm2 being delivered. Action: Establish a minimum mW/cm2 value required to achieve full cure for your specific adhesive and then use the radiometer to ensure your light source meets that minimum intensity before every critical curing cycle. 2. Implement a Strict Cleaning Schedule Regularly clean the optical surfaces of your lamp using an approved solvent (like isopropyl alcohol) and a lint-free cloth. This simple step can restore significant lost intensity. 3. Adjust Curing Time (The Compensation Method) If you cannot measure or boost the intensity, you must compensate by increasing the exposure time. Action: If the manufacturer suggests 10 seconds at 100mW/cm2, and you suspect your lamp is only delivering 50mW/cm2 (half the required power), you might need to double the curing time to 20 seconds to achieve the same total dose of UV energy. Always test and verify the final hardness. 4. Invest in the Right Equipment For serious DIY or industrial use, avoid novelty UV lights. Purchase a dedicated, high-intensity UV LED curing lamp that specifies its irradiance (e.g., "3W at 365nm") and is explicitly designed for the purpose of curing adhesives. Checklist for Addressing Intensity Problems Problem AreaObservationAction to TakeDistanceLamp is held far away, or the fixed height is too high.Bring the light closer (1-5 cm is ideal for…

UV adhesives cure (or polymerize) when photoinitiators within the formula absorb specific wavelengths of UV light(usually UVA, or 365 nm to 405 nm) and convert that energy into free radicals. These radicals then initiate a rapid chain reaction that links the liquid monomers and oligomers together, transforming the sticky liquid into a hard, solid plastic. If this polymerization process is interrupted or incomplete, the result is tackiness—a weak, partially cured surface that often fails prematurely. Primary Causes of Incomplete Cure (Tackiness) The problem almost always boils down to one thing: Insufficient or incorrect UV energy reaching the adhesive. 1. Wavelength Mismatch UV curing lamps don't all emit the same type of light. Some adhesives are formulated for UVA (longwave) light (most common for consumer/hobbyist formulas), while others are optimized for UVV (shortwave) or specific LED wavelengths (common in industrial settings). The Fix: Always verify the recommended wavelength range for your specific adhesive. If the adhesive requires 365 nm light, a 395 nm or 405 nm light source might not contain enough energy at the critical wavelength to fully activate the photoinitiators. 2. Insufficient UV Intensity (Power) The lamp might be the correct type, but the power output (irradiance) may be too low, especially if the light source is aging, dirty, or positioned too far from the adhesive. The Fix: Distance: Position the light source as close as possible to the adhesive bead without touching it. UV intensity drops significantly with distance (following the Inverse Square Law). Cleanliness: Regularly wipe the light source lens and the adhesive application surface to ensure maximum light transmission. Lamp Age: UV bulbs (especially fluorescent or arc lamps) degrade over time, losing intensity. If you rely on a fixed curing time, the actual energy delivered will slowly decrease. Consider measuring the irradiance with a UV radiometer if consistent, high-strength curing is critical. 3. Insufficient Exposure Time Sometimes, the solution is simple: the adhesive didn't sit under the light long enough to fully polymerize all the material. The Fix: Increase the curing time, often by 50% or more beyond the manufacturer's minimum recommendation, especially for thicker applications or when using a lower-power light. It's almost always better to over-cure than under-cure. 4. Excessive Bond Line Thickness UV light can only penetrate so deeply. If the adhesive layer (the bond line) is too thick, the UV light can fully cure the top surface, but the adhesive deeper inside remains liquid or tacky. The Fix: Control Application: For optimal strength and cure, UV adhesives are generally designed for thin bond lines (often <0.5 mm). Apply only the minimum amount of adhesive needed. Cure in Layers: For applications requiring a thicker layer, apply and cure the adhesive in multiple thin layers rather than one thick bead. Ensure each layer is fully cured before applying the next. Consider Volume Cure: Some industrial adhesives are formulated with secondary cure mechanisms (like moisture or heat) to cure areas that UV light cannot reach. 5. Oxygen Inhibition (The Surface-Tack Problem) 🌬️ This is arguably the most common cause of tackiness, which often presents as a fully hard, strong bond underneath a thin, sticky film on the surface. Atmospheric oxygen can interfere with the free-radical polymerization reaction right at the air-adhesive interface. This…

If the adhesive sets before the parts are properly placed and aligned, it leads to a poorly positioned bond or high internal stress. Consequences Misalignment: Parts cannot be adjusted after initial contact, leading to scrap or rework. Incomplete Wet-Out: The adhesive gels before it fully spreads across the entire bond area, resulting in a starved joint with low strength and gaps. High Cure Stress: Rapid curing generates heat and shrinkage stress quickly, increasing the risk of cracking or bond failure upon removal from the fixture. Solutions Increase Working Time: Switch to a UV adhesive formulated with a slower photoinitiator package, increasing its "open time" or working time before it fully sets under light. Reduce UV Exposure: If using a high-intensity lamp, reduce the UV irradiance during the initial "tack cure" phase. This slows the polymerization slightly, allowing extra time for minor adjustments. Separate Application/Cure: If the parts require complex alignment, use a UV adhesive that relies on a specific high-intensity UV wavelength for cure. Apply the adhesive under a different (non-curing) wavelength of light (e.g., standard room lighting shielded with UV filters) to maximize working time, then move to the high-intensity light for immediate cure. 2. Problem: Cure is Too Slow (Movement Before Set) If the adhesive remains liquid for too long after placement, the parts are susceptible to shifting, or the assembly process is significantly bottlenecked. Consequences Part Shifting/Slump: Gravity, vibration, or handling during transfer can cause parts to shift relative to each other before the adhesive develops sufficient strength ("green strength") to hold them in place. Slow Production Rate: Long cure times tie up fixtures and floor space, limiting the throughput of the assembly line. Flow-Out: As noted previously, on vertical or inverted joints, slow curing allows the liquid adhesive to flow out of the joint due to gravity. Solutions Increase Cure Intensity/Time: Increase UV Irradiance: Use a higher-power UV light source (e.g., switch from an LED to a high-intensity arc lamp) to speed up the reaction kinetics. Increase Exposure Time: Slow the conveyor or increase the dwell time under the lamp to ensure the part receives the full required dose quickly. Implement Tack Cure: Use a small, focused UV spot lamp for a flash cure (1-3 seconds) on a small, non-critical area of the joint. This rapidly achieves enough green strength to fix the parts in position, preventing movement, before the final, full cure is performed. Use Dual-Component Systems: If the assembly is complex and needs high green strength before UV exposure, consider a dual-cure system where a small amount of a fast-setting catalyst is mixed in, which initiates a quick, initial cure while the UV remains the primary final cure mechanism.

Equipment degradation directly impacts the UV dose (Dose=Intensity×Time). A. Lamp Aging (Reduced Intensity) Mechanism: All UV lamps, whether traditional Mercury Arc bulbs or modern LED arrays, degrade over their operational lifetime. Arc Lamps: The electrodes slowly erode, and the quartz envelope becomes clouded by deposits (solarization), which reduces light transmission. The peak UV intensity (mW/cm2) steadily declines. LED Arrays: The LED diodes themselves degrade due to heat and current over thousands of hours, resulting in a reduction of the optical output power. Result: Since the production speed (cure time) is usually fixed, the reduced intensity means the parts receive a significantly lower UV dose (J/cm2), leading to an under-cured adhesive. B. Dirty Optics (Reduced Transmission) Mechanism: Dust, adhesive fumes, overspray, or splatters can accumulate on the surface of the reflector, filter, or lens (including the light guide or fiber optic tip). Even a thin film acts as an effective UV filter and light blocker. Result: The dirty optics physically block the UV light, reducing the intensity reaching the adhesive, similar to lamp aging, causing an inconsistent and inadequate cure. 2. Solutions: Implementing a UV Maintenance Plan A rigorous schedule for monitoring and cleaning is essential for reliable bonding. A. Regular UV Radiometry (The Essential Check) Measure Output: The single most important maintenance step is the regular use of a UV radiometer (light meter)to measure the UV intensity (mW/cm2) and/or total energy dose (J/cm2) delivered to the actual bond line location. Establish a Baseline: Measure the output when the lamp is new and establish a minimum intensity threshold (the point where the bond strength begins to fail). Scheduled Monitoring: Check the UV output daily or shift-by-shift for critical applications. Once the output falls below the established minimum threshold, the lamp or LED system is due for replacement or the process time must be adjusted. B. Cleaning and Replacement Schedule Clean Optics Daily: Establish a procedure to gently clean the lens or UV light guide tip at the start of every shift using a lint-free UV-safe wipe (e.g., micro-fiber) and the manufacturer-recommended solvent (often high-purity IPA). Replace Consumables: Mercury Arc Lamps: These typically need replacement after 1,000 to 2,000 hours of operation, regardless of visible condition, to prevent a steep drop-off in UV output. LED Systems: While having longer lives, they require regular calibration or replacement of modules based on the manufacturer's recommended output degradation curve. Clean Reflectors/Cooling: Regularly inspect and clean the reflectors (for arc lamps) and ensure the cooling fans/vents are free of dust to prevent thermal overload, which accelerates LED degradation and shortens the life of arc lamps.

Sunlight causes degradation through a process called photo-oxidation, a chain reaction accelerated by heat and oxygen. UV Photons: The high-energy UV radiation (especially UV-A and UV-B wavelengths) in sunlight is absorbed by the polymer chains. This energy breaks the chemical bonds in the adhesive's backbone, a process called photolysisor chain scission. Free Radical Formation: The broken bonds create new, highly reactive free radicals. Oxidation: These radicals react with oxygen in the atmosphere, leading to the formation of unstable peroxide and hydroperoxide groups. These groups subsequently decompose to form stable, but often colored, compounds like carbonyl groups (C=O). Resulting Defects: This process manifests as: Yellowing/Discoloration: The formation of C=O groups creates chromophores, which absorb blue light and make the adhesive appear yellow or brown. Loss of Strength: The chain scission weakens the overall polymer network, leading to reduced tensile strength, lower flexibility, and increased brittleness. Surface Chalking/Cracking: Continued degradation can cause a powdery layer on the surface or visible micro-cracks. 2. Prevention and Mitigation Strategies Preventing environmental degradation requires selecting the correct adhesive chemistry and using protective measures. A. Material Selection Aliphatic Formulations: Use adhesives based on aliphatic (non-aromatic) monomers and oligomers. Aliphaticstructures are significantly more stable and resistant to UV and oxidation than aromatic structures, offering superior non-yellowing performance. UV Stabilizers and Absorbers: Select adhesives that contain integrated UV absorbers (UVA) and Hindered Amine Light Stabilizers (HALS): UVA absorbs the incoming UV energy and dissipates it harmlessly as heat. HALS scavenge the free radicals created by initial UV damage, stopping the oxidative degradation chain reaction before it can cause widespread damage. B. Joint Design and Protection Physical Shielding: If possible, design the assembly so that the adhesive joint is physically shaded or positioned away from direct sunlight. Coatings: Apply a UV-blocking overcoat or clear lacquer to the cured adhesive bond. This top layer absorbs the sunlight's UV rays, protecting the structural adhesive underneath. Substrate Protection: When bonding through a transparent substrate (like glass), ensure the substrate itself has an inherent UV filter (e.g., laminated safety glass).

When the bond line is too thick, two primary failure mechanisms occur: A. Incomplete Cure (Light Attenuation) Light Blocking: UV light intensity attenuates (decreases) exponentially as it passes through the adhesive layer. In a thick joint, the light may not reach the deepest parts of the bond line, leaving the material at the core liquid and uncured. This core uncured material dramatically reduces the bond's mechanical strength and causes outgassing or leaching. Shadowing: The light may reach the sides but not the center of the deep gap, leading to a peripheral "shell" of cured material over a weak core. B. Stress Concentration and Shrinkage Increased Shrinkage Stress: UV adhesives cure by polymerization, which involves volumetric shrinkage. In a thick bond line, the total volume of shrinkage is greater, leading to higher internal stresses, which can cause cracking, crazing, or stress failure at the interface, especially with rigid substrates. Effective Bond Area: The stress applied to the joint is distributed over the cross-sectional area of the bond. If the gap is too large, it may be filled with weak, partially cured material, effectively reducing the strong, fully cured area, leading to premature cohesive or adhesive failure under load. 2. Solutions for Bonding Large Gaps Addressing overly large bond line gaps requires changing either the application method or the adhesive technology. A. Material Selection Use Dual-Cure Adhesives: For gaps over a few millimeters, switch to a UV/Thermal or UV/Moisture dual-cure adhesive. The UV light performs a quick surface tack cure, and a secondary mechanism (heat or moisture) penetrates the thick, shadowed core to achieve a 100% complete cure. Use High-Penetration Formulations: Select adhesives specifically designed for "deep-cure" applications. These typically utilize longer UV wavelengths (UV/Visible light, 385 nm or 405 nm) which penetrate thicker layers more effectively than standard 365 nm UV light. Use Filled Adhesives: Filled adhesives contain inert materials (e.g., glass beads or silica) which help control the bond line thickness and reduce the amount of reactive liquid monomer, thus minimizing shrinkage and its associated stress. B. Process Control Layering/Staging: For extremely deep gaps, apply and cure the adhesive in multiple thin layers. Cure each layer completely before applying the next. This ensures full cure in each section but is slow and labor-intensive. Pre-Set Spacing: Utilize shims, spacers, or filler beads to control the gap size precisely to the adhesive manufacturer's specified maximum. This ensures the adhesive is only used in the optimal thin-film geometry it was designed for. For optimal performance, UV adhesives should generally be used in bond lines less than 0.25 mm (0.010 in.) unless the material is specifically formulated and verified for deeper cure.

Substrate contaminants, such as mold release agents, lubricants, or processing oils, cause failure in two primary ways: A. Cure Inhibition (Chemical Interference) This occurs when a chemical on the substrate actively prevents the adhesive from polymerizing. Free-Radical Systems (UV Acrylates): Substances like amines, sulfur compounds, or certain waxes can scavenge the free radicals generated by the photoinitiator. This stops the polymerization chain reaction, resulting in a completely uncured or permanently tacky bond line. Cationic Systems (UV Epoxies): These systems are highly sensitive to basic (alkaline) contamination (e.g., amine-based cleaners or residue). Bases neutralize the acidic species required for the cationic curing mechanism, leading to a complete failure to cure. B. Adhesion Degradation (Physical Barrier) This is the more common issue and applies to all adhesive chemistries. Low Surface Energy: Contaminants like silicone-based mold release agents, oils, or greases create an extremely low-surface-energy layer on the substrate. The adhesive, being a liquid, cannot "wet out" (spread evenly) over this oily layer. Instead, it beads up, dramatically reducing the actual contact area and bond strength. Weak Boundary Layer: Even if the adhesive appears to cure, the bond fails because the adhesive is chemically bonded to the layer of contaminant, not the strong substrate underneath. The contaminant acts as a weak, easily delaminated boundary layer. 2. Solutions and Prevention (Surface Preparation) The cure for substrate inhibition or poor adhesion is rigorous, verifiable surface preparation. A. Cleaning and Degreasing Solvent Wiping: Use appropriate, high-purity solvents like Isopropyl Alcohol (IPA), acetone, or methyl ethyl ketone (MEK). The key is using a two-rag method: the first cloth applies the solvent and removes the contaminant; the second, clean, dry cloth immediately wipes away the residual solvent and dissolved contaminant before it can re-deposit. Aqueous/Detergent Cleaning: For persistent or water-soluble contaminants, a commercial, pH-neutral, surfactant-based cleaning step followed by a thorough rinse is required. Avoid Contaminated Wipes: Ensure the cleaning cloths are lint-free and have not been previously contaminated (e.g., with silicone oil). B. Surface Treatment For stubborn contaminants or low-surface-energy plastics: Abrasion: Lightly abrading or sanding the surface (mechanical roughening) physically removes the contaminated top layer and increases the surface area for bonding. This must be followed by a final solvent wipe to remove dust. Plasma or Corona Treatment: For plastics like polypropylene or polyethylene, which naturally have low surface energy, plasma or corona discharge treatment is used to chemically activate the surface, making it hydrophilic (high surface energy) and receptive to bonding. C. Inspection Water Break Test: A quick field test for a clean surface is the Water Break Test. A properly cleaned surface will hold a continuous, thin film of water for several seconds without the water breaking into droplets. If the water beads up, contaminants are still present. UV Fluorescence: If the contaminant is known to fluoresce under UV light, a black light can be used as a quick visual inspection tool to verify complete removal before applying the adhesive.

Crazing is the formation of micro-cracks or voids within the adhesive bulk or near the interface when the material is subjected to mechanical stress. These internal defects scatter light, causing the material to appear white or foggy. Causes: High Internal Stress: Excessive volumetric shrinkage during the curing process (a common characteristic of highly reactive acrylates) can build up significant internal stress in the bond line, particularly when bonding rigid, inflexible substrates (like glass or ceramics). External Stress: Applying or developing excessive mechanical stress (e.g., thermal expansion mismatch, bending, or impact) on the finished assembly can initiate crazing in a brittle adhesive. Brittle Formulation: Adhesives with a high cross-link density (highly rigid) are more prone to crazing than flexible formulations. Solutions: Choose Flexible Adhesives: Select an adhesive with lower modulus and higher elongation. These materials can absorb stress without fracturing the polymer network. Minimize Cure Shrinkage: Use adhesives that are filled or formulated with higher molecular weight oligomers, as these shrink less upon polymerization. Optimize Cure Cycle: A slower, more complete cure (e.g., using a step-cure profile or a thermal post-cure) can relax internal stresses, making the cured adhesive less brittle. 2. Whitening from Moisture or Chemicals Whitening due to environmental exposure is a sign of material degradation or absorption. A. Moisture Absorption (Hydrolysis) Mechanism: When exposed to high humidity or immersion in water, the adhesive material absorbs moisture. This water uptake can cause two problems: Phase Separation: The absorbed water molecules interfere with the light path, causing scattering and a hazy appearance. Hydrolysis: In some adhesive types (e.g., certain polyesters or epoxies), water can chemically break down the polymer chains (hydrolytic degradation), leading to degradation products that whiten the material. Solution: Use hydrolytically stable adhesives, such as those based on pure polyurethanes or silicones, especially for applications exposed to steam, hot water, or high RH environments. B. Chemical Attack (Solvent Fogging) Mechanism: Exposure to solvents, cleaners, or aggressive chemicals can swell the polymer network. The solvent penetrates the adhesive, causing localized disruption of the polymer structure or leaching out uncured components, which can change the refractive index and cause fogging. Solution: Verify the adhesive's chemical resistance against all expected post-assembly cleaning agents (e.g., IPA, acetone) or operating environment chemicals. Switch to an adhesive that has demonstrated resistance to the specific chemical in question. 3. Fogging (Outgassing on Neighboring Surfaces) While less common, "fogging" can also refer to outgassing where volatile residual components from the adhesive vaporize and condense on nearby surfaces, particularly optical components like lenses or mirrors. Solution: Ensure the adhesive is 100% fully cured (addressing the risk of incomplete cure). For sensitive electronics or optics, use low-outgassing adhesives that meet industry standards like NASA or ESA specifications.

Pigments and fillers cause cure failure via two primary mechanisms: Absorption: Opaque pigments (like carbon black or titanium dioxide) are designed to absorb or scatter light across the visible spectrum, but they also absorb the UV wavelength required by the photoinitiator. The UV light is consumed by the pigment before it can reach the photoinitiator molecules deeper down. Scattering: Inorganic fillers (e.g., glass spheres, silica) increase the opacity of the adhesive. The UV light is scattered and diffused, exponentially reducing the light intensity that reaches the core of the bond. This leads to a cure gradient, where the material closest to the lamp is cured, but the material in the shadowed or bulk regions remains liquid. 2. Mitigation Strategies for Pigmented/Filled Systems Successfully curing an opaque or highly filled UV adhesive requires changing the adhesive chemistry, the light source, or the curing process. A. Change the Adhesive Chemistry (Use Dual Cure) Thermal/UV Dual-Cure: The most robust solution is to switch to a dual-cure adhesive (e.g., UV/Moisture or UV/Heat). The UV light sets the surface layer or exposed edges (tack cure), holding the parts in place. A secondary curing mechanism, usually heat (thermal bake), is then used to complete the cure in the deep, shadowed, or pigmented areas where the UV light could not penetrate. B. Change the Light Source (Increase Penetration) Use Longer Wavelength UV (UV/Visible): Most standard clear UV adhesives cure best at 365 nm. For pigmented systems, use a lamp that emits at 385 nm or 405 nm (Visible Light). Longer wavelengths are scattered and absorbed less efficiently by many pigments, allowing them to penetrate deeper into the material before being fully attenuated. Increase UV Dose: While limited, increasing the UV dose (slowing conveyor speed or increasing lamp intensity) can help push the curing front deeper into the bulk, but this must be done carefully to avoid over-curing and yellowing the exposed surface. C. Change the Dispensing Process Dispense Thinner Layers: Apply the adhesive in the thinnest possible bond line that still meets structural requirements. The shorter the distance the UV light has to travel, the less severe the attenuation effect will be. Use Clear Substrates: If one substrate is opaque and the other is transparent (e.g., metal to glass), ensure the UVlight is directed through the transparent substrate and not through the thick layer of pigmented adhesive.

The risk of flow-out is governed by the relationship between the adhesive's viscosity and the cure time. Viscosity: A low-viscosity (thin) liquid has weak internal cohesive forces and minimal thixotropy (the ability to thicken when at rest). Gravity: On a vertical or overhead joint, gravity exerts a constant shear stress on the uncured material. Result: The adhesive begins to move, resulting in a bond line that is too thin in one area (starved) and too thick in another (overflow/slump), compromising the structural integrity and aesthetics of the bond. 2. Mitigation Strategies for Vertical Joints Addressing flow-out requires either increasing the adhesive's resistance to flow or accelerating the time it takes to solidify. A. Material Selection (Increasing Resistance) Use a Thixotropic Adhesive: Choose an adhesive with high thixotropy. Thixotropic materials have a high viscosity when standing still (to resist gravity) but thin out under shear (when dispensing). Look for adhesives explicitly labeled as gel or high-viscosity formulations. Use Filled Adhesives: Adhesives containing thixotropic fillers (like fumed silica) maintain their shape better on vertical surfaces. Use Higher Molecular Weight Formulations: Adhesives with longer molecular chains (higher viscosity) will inherently resist flow better than low-viscosity materials. B. Process Control (Accelerating Cure) Tack Cure/Pinpoint Cure: Instead of curing the entire bond line at once, use a low-intensity UV spot lamp to immediately "pin" the adhesive in place at the edges or corners of the joint. This quick initial cure creates a solid dam that prevents further flow, allowing the full cure to proceed without sagging. Flash Curing: If using a high-intensity lamp, flash cure the entire joint with a very short burst of UV light. This is just enough time to partially gel the adhesive, increasing its viscosity significantly, but not enough to cause full cure stress or shrinkage. The parts can then be moved to the full curing station. Control Application Temperature: Ensure the adhesive is not being used at temperatures significantly higher than recommended, as increased temperature lowers viscosity and exacerbates flow-out. C. Mechanical/Jigging Solutions Use Fixtures and Jigs: Design custom fixturing to hold the parts in the horizontal plane during the dispensing and initial curing stages, eliminating the gravitational shear stress entirely. Control Bead Size: Dispense a smaller, more controlled adhesive bead or fillet size. A smaller volume of material is less susceptible to sagging than a large, heavy mass.