The Difference Between Set Time and Open Time

While hot melt adhesives date back to the 1940s, the market for these hot glues has grown at a phenomenal pace over the past few years. The global hot melt adhesives market size was valued at $12.25 billion in 2021 and is projected to expand at a 7% CAGR (compounded annual growth rate) between 2022 and 2028. The market is driven by the many applications of hot melt adhesives in sectors ranging from case and carton sealing, paper converting, folding carton

and bookbinding , to food and can labelling. These adhesives are available in many forms, including pillows, pellets, chubs, and chips. In some sectors, the adhesives you chose must meet the most stringent regulatory and safety requirements. When using hot melt adhesives, it’s important to know about open time and set time. Here’s a look at these two important concepts.


Know the Specs When Choosing Adhesives

Both open time and set time are specified and should be considered when choosing adhesives, to achieve the desired adhesion. The chose depends largely on the application for which the adhesive is being selected.


Open Time: This refers to the amount of time you have before the adhesive solidifies. In other words, open time is the maximum time between applying the adhesive to one surface and ensuring it comes into contact with the other surface. If you apply the adhesive on one surface and leave it unattended longer than the open time, proper adhesion may not take place. Not having enough open time will lead to problems like pre-set.


Set Time: Is the time required for proper adhesion to take place after the application of the adhesive and the two surfaces coming together to form a bond. If the bonds are pulled apart before the adhesives sets it can lead to cohesive failures.



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What Impacts the Set Time and Open Time of Adhesives?

While each type of adhesive has an optimum open time and set time, there are other factors that impact these timelines. Among these, the five primary factors that determine the set time and open time are:

Type of Substrate: The substrates to be bonded will have an impact on the open time and set time.

Type of Adhesive: Hot melt adhesives can be formulated to have shorter or longer open times and set times. The amount of adhesive used will also have an impact on the open time and set time.


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Temperature: As a general rule, lower temperatures lead to a shorter open time and set time. Higher temperatures lead to longer open-time and set time. Keep in mind that conventional hot melts run at 350F, and cool melts run at 250F. You can adjust temperatures to suit your application needs as long as you are within the recommended running temperatures.

Application Bead Size: How much glue is applied will impact your open and set times. More adhesive means longer open and set times. Less adhesive means shorter open and set times. You can adjust your bead size by increasing or decreasing your air pressure.


After taking these factors into account, one needs to ensure that the bonding process does not occur too slowly or quickly, to ensure optimal bonding.