How to Minimize Production Scrap from Adhesion Failures

Pierce Geary

In manufacturing it is extremely difficult to eliminate all inefficiencies and production deficits. Setbacks that come in the form of scrap rates and rework resulting from product failures before shipping out or recalls and returns after products have gone to market, are par for the course--but this does not mean the current level needs to be accepted.

It is possible to reduce the amount of scrap being produced and the rework required to fix issues that arise.

This becomes extremely important when a previously acceptable level of scrap or rework needs to be brought down to a lower level. This can be terrifying to manufacturers because if the reasons a 3% scrap rate is occurring aren’t clear, how can they hope to reduce that to 2% or even reduce it at all?

The first thing manufacturers need to do to begin minimizing scrap is understanding what is causing adhesion failure in the first place.

Characterize the Contamination

Historically, cleaning material surfaces has emphasized large particulate removal. Making sure the bond surface is free of debris is a very important aspect of getting the surface to a desirable state for adhesion, but the work cannot end there. 

Cleaning and treatment processes that are only characterized by the removal of particulate do not give a full picture of the requirements for adhesion. A surface needs to be free of chemical contamination because adhesion is a function of molecules interacting at the top few microscopic layers of a surface with a coating, adhesive or other substrate. If the surfaces are not chemically compatible then no amount of removal of particulate will prevent adhesion failures. The scrap rate will be unaffected.

When chemical contamination remains on a surface (which just means the surface has the wrong chemical makeup for the application), often what is being bonded to is this layer of contamination and not the chemically bondable surface that is underneath. When this happens it essentially guarantees that adhesion failure will result.

There are legacy tests that have been used for a in manufacturing to help define the amount of particulate that has been removed (e.g. Millipore test, ROSE test, etc.). These tests are ineffective at offering useful information about the cleanliness of a surface because they are not sensitive to the chemical nature of the surface. Often these tests can indicate that the surface is particulate-free, but adhesion failures continue nonetheless. 

What is needed is a way of quantifying the chemical readiness of a surface for adhesion. Redefining the surface quality as its chemically ideal state and measuring this state as the surface moves throughout the adhesion process creates a highly controlled and manageable system for creating adhesion-ready surfaces.

chemist-studying-the-perodic-table

Non-Destructive Testing

A good amount of scrap comes from conducting tests that require the samples to be disposed of after the testing. This is problematic for a number of reasons.

If the tests (such as dyne ink evaluations, water break tests and others) can’t be done on the actual parts that are going to go through the adhesion process then they can only tell you so much about the readiness of the parts that will be bonded, coated, sealed or otherwise adhered. 

A truly useful test of surface quality is one that is quantitative, can be done directly on the production line on the parts being cleaned and treated, and is non-destructive to the surfaces so an accurate evaluation can be done. 

When the inspections are done directly on the line then more tests can be conducted allowing for the probability of failure to be drastically reduced. The sample set will be more emblematic of the parts being bonded and will give a more true picture of the chemical state of the surfaces.

So, non-destructive tests reduce scrap in two main ways: 

  1. Scrap isn’t created because samples aren’t being destroyed, and
  2. The adhesion process is more closely monitored directly at the points where contamination is occurring, making anomalous changes more easily identifiable

Double Check Difficult-to-Clean Surfaces

Closely examining a surface after an adhesion failure has occurred can tell you so much about why it happened. One thing to look for is consistency in where the failures are occurring. Often, you’ll find, failures occur in small corners, shallow channels where adhesive is applied, sharp edges and unusual geometries. These are conventionally difficult to clean properly and need to be looked at closely and fully evaluated to ensure they are chemically consistent with the rest of the surface.

The non-destructive test used on the line needs to be able to map out the entirety of the material surface so that consistency can be achieved. If one part of a surface is getting cleaned and treated to the desired extent and another part isn’t, then it needs to be understood why that’s happening. It could be due to an inconsistent wash process because of placement of the components inside the racks, something being deposited on the surface during another stage in the adhesion process, or even just using an inappropriate method of cleaning or treatment for the geometry of the surface. 

Awareness of all types of contamination is critical to mitigate adhesion failure. They all need to be identified, monitored and controlled. The methods of identifying, monitoring and controlling this contamination need to be non-destructive themselves so more needless scrap isn’t created. And, lastly, every step of the adhesion process needs to be optimized so that the surfaces are consistent and totally ready for the adhesion application.For more information on getting to the source of adhesion issues and controlling your adhesion process, download the Checklist: Adhesion Failure Root-Cause Analysis for ManufacturerseBook. By the end of the book you will have a rich understanding of how to know for certain where your adhesion problems are deriving from and how to put a stop to them.

Adhesion Failure Root-Cause Analysis for Manufacturers

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