Bonding Manifolds, Defects To Look For, & How To Avoid Them

In the previous section, we covered different aspects about both drilling for single-layer manifolds (SLMs) and factors for consideration when designing bonded/multilayer manifolds. In this section, we detail more about the bonding process and how it affects the finished manifold.

We'll start by reviewing the many different ways that exist to bond two pieces of plastic together: thermal, solvent, laser, and vibration. While these remain most popular, several other methods exist that engineers use when the occasion calls for it. We will then move onto how to look for defects and how to avoid those defects should any appear in the future.

About The Different Bonding Process Types

Selecting the right bonding process can become a challenge quickly as it largely depends on a few factors such as the type of plastic used, production volume, precision requirements, and cost considerations. Most manufacturers can assist in evaluating these factors in this decision that would align with the product's purpose. Here's a brief overview of those types of plastic bonding as we progress into the next sections.

  • Thermal

    This type of bonding involves heating the mating surfaces of the two plastics until they soften. Upon softening, a machine presses them together to forge a bond as it cools. While it offers strong and durable bonds with no other materials needed for bonding, manufacturers limit this process to thermoplastics. It also harbors the potential for uneven heating that can lead to inconsistent bonds. 

  • Solvent

    Solvent bonding involves applying a solvent that softens plastic surfaces, much like what we see in chemical polishing methods. Once the solvent softens that surface, a machine presses the two components together. After, the solvent evaporates and creates a bond. This process suits a wide range of plastic materials and produces strong bonds without heating the plastic. However, using these requires a capable hand and careful control to apply properly and evenly. Some of these solvents are hazardous and one must take proper safety precautions before starting this process. 

  • Laser

    Like thermal bonding, laser bonding uses heat to melt and fuse surfaces together. However, the laser makes the heat more concentrated and targeted rather than over a whole surface, which many consider an advantage over other processes that require that precision and less risk of contamination. Yet manufacturers limit this process as well to certain types of plastic that can handle the heat without degradation. Also, this process can become rather costly as this requires specialized tools to perform. 

  • Vibrational or Ultrasonic

    Much like what we saw considering threaded holes and staking methods, this method also uses high-frequency vibrations to generate heat at the surface and forge a bond when cooled. Many praise this method for its quickness and efficiency as well as its wide applicable range on many types of plastics. However, like laser bonding, this caters more to localized areas of a component and not generally used on large or complex parts. This also requires specialized tool design for specific applications. 

Few manufacturers have a proprietary diffusion (thermal) bonding process like we do. This method utilizes heat, time, and pressure to melt the two manifold layers together into a seamless union. We highly recommend diffusion bonding as the best choice for just about any industry that requires sensitive operations, but particularly for those in life science instrumentation. We know this process for its scalability for physical size and efficiency for production in small and large quantities.

Known Defects in Bonded Manifolds & Their Causes

Bubbles & Foreign Objects

During bonding, bubbles can form and small foreign objects or debris can be caught between the plastic. Bubbles are only a concern when they connect to channels allowing for cross talk. Incidental bubbles along channel edges do not affect performance.

Foreign objects (like dust or other small particles) typically have no effect on the integrity of the part. While these defects may not be aesthetically desirable, ultimately the functionality of the part is most important.


This happens when manifold layers begin to separate. When bonded properly, delamination should not occur no matter which process the manufacturer used. This becomes a big problem that leads to assembly failure. If anyone notices this happening, it implies something went wrong in the bonding process that needs addressing immediately.

Stress Cracking

We've already covered stress-cracking in other sections throughout this guide, however to reiterate, amorphous plastics are susceptible to cracking when subject to mechanical, chemical, and thermal stress beyond the material’s capabilities. Cracking often occurs due to two or more of these stresses in combination. To reduce the possibility of stress cracking, all manifolds have a final stress relieving process. This reduces the possibility of stress-cracking from any combination of these influences.

Two clear acrylic tubes where one has no signs of stress cracking while the other does at the top of the component.
Design Tip

To Prevent Further Stress Cracking

Loctite products should be avoided for fasteners retention as well as anaerobics as they are generally not used with most plastics. They will cause stress cracking.

Types of Stress-Cracking & Their Causes

  • Chemical

    Chemical stress is caused by exposing a material to a chemical or a concentration of a chemical that degrades the plastic. Excess chemical stress should be avoided by choosing the right material for the application.

  • Thermal

    Manifolds can also stress crack from thermal shock where a hot plastic is quickly subject to very cold temperatures (or vice versa). The uneven cooling from outside to inside creates mechanical stresses degrading the plastic.

  • Mechanical

    Many ways exist to apply mechanical stress to a manifold that will cause it to stress crack. We've gone over those in detail here

  • Thermal & Chemical Combination

    When a chemical is heated, it becomes more reactive. Most chemical resistances are measured at room temperature or to a defined temperature. Consider operating temperature of the reagent when choosing the manifold material.

  • Other Combined Stressors

    Often it is the combined input stress from multiple sources that cause stress cracking. For example: A manifold under a mechanical tension is heated while a mildly aggressive reagent is utilized.

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Stress Relieving Plastic Manifolds

Stress relieving plastics is an important part of precision plastic part machining. Manufacturers should consider stress levels of the component from the start of the machining process to post conditioning, especially for the amorphous plastics. If the plastics experience squeezing or another form of stress on them, the general process involves gentle warming to loosen the plastic's tension from processing. Many other methods exist, yet this one stands as being the most popular and pertinent to complete production.

However, even after the stress-relieving process, this does not mean a part cannot stress-crack. Overtightening a fastener or using an incompatible chemical can and will damage the manifold. We recommend reading the next section on further caring for the manifolds to ensure optimal performance and longevity.

Bond Lines & Deburring

Manufacturer's QA/QC departments should have rigorous processes implemented where they examine products for fidelity and quality. That's where certifications like ISO come into the picture. We covered those in more detail in this previous section about colors and certifications. QA/QC departments should notice any defects before proceeding further, and we work hard to ensure that every part meets or exceeds expectations before shipping to the customer. However, we always recommend to the customer to inspect the components themselves to ensure satisfaction.

We explain what to look for when it comes to bond lines and other potential defects to inspect in plastic manifolds.

How to Inspect & Test a Bond Line on Bonded Manifolds

While companies generally have their own processes when it comes to inspection and testing, we have a few processes outlined to guide others on assessing their product's quality.

  • Visual Inspection

    We visually inspect all of our bonds before further processing. If our QA/QC crew notice a defective bond bond, they tend to spot it via a thorough visual inspection. If they see any of the bond plane, they label it as an improperly bonded manifold not fit for shipment. If the team notices a single bubble, many small bubbles, or small impurities/foreign objects, they do not classify that manifold as defective as such blemishes are aesthetic in nature only.

  • Bond Strength Tests

    Manufacturers perform bond strength tests at the start of all new projects. A bond strength test is simple. It consists of applying a shear force to the bond until failure. If the bond breaks cleanly at the bond plane, it fails. If the part breaks irregularly through the plane of the bond, it passes.

    Once the team has defined the entire process and tested bond strength with the first few products, we typically perform visual inspections only on the proceeding products. As the bonding process is quite reliable and precise once calibrated to specifications, usually the manufacturer needs only to visually inspect bonds before more processing.  

  • Decay Tests

    If a customer is concerned with manifold integrity, manufacturers do offer pressure or decay tests to alleviate those concerns. A Decay Test stands as one of the most effective tests for manifold integrity. QA/QC team members perform this test by blocking off target channels and pressurizing them. They then monitor the pressure over time to verify channels do not exceed leak rate.

  • Pressure Tests

    Pressure Tests involve sealing a manifold and pressurizing it as well. However, this differs from the decay tests in that it requires the component be placed in a bucket of water where QA/QC checks for bubble generation fail points. As opaque parts create barriers to visual inspection (unlike clear parts), we recommend that customers ask for a decay or pressure test to ensure manifold integrity. 

  • Oxygen Service

    In applications where oxygen is used, manifold cleanliness is critical. As we label this as a specialized process, we ask that anyone interested to contact engineering to discuss your application. We have successfully produced manifolds for oxygen service. 


The best way to handle burrs in plastic machining is to not make them in the first place. Using plastic-specific tooling with constant maintenance aids in avoiding burr creation. Although little bumps and bits of extra plastic can happen and roughen the exterior, any manufacturer should be able to smooth those down to a refined surface without incident, depending on the design and the material.

Image of plastic manifold interior before deburring Image of plastic manifold interior after deburring