There’s a great meme floating around out there with the title “engineering flowchart”. It’s been around in several versions for many years so I assume it’s public domain. It sort of memorializes engineers’ love for duct tape as a fix-all.
Yes, it’s funny, but I can’t recall ever specifying duct tape on any bill of material in my career. That being said, I do keep duct tape handy, and I do regularly specify specialty tapes in my designs.
What to call it
First some nomenclature. There are lots of types of adhesives. We all know elmer’s glue since we were in kindergarten, it’s a type of adhesive. So is liquid nails, super glue, and that gummy stuff on post-it notes. What we usually think of as tape is really a pressure-sensitive adhesive (PSA) usually with some other film or substrate to give it some tensile strength. If the adhesive has no substrate and is double sided, it’s often called transfer tape, and it may be used to stick together layers – for example on a label. There are as many substrates as you can imagine, and sometimes we even create our own tape by combining some specialty substrate with a transfer tape. Those are usually used in places where we are trying to provide some protection, a low friction substrate, or hold a rubber gasket in place. Another application uses low-tack PSA and clear film to protect surfaces during manufacture and shipping. We’ve all seen a piece of equipment that still has this protecting tape in place because it was not obvious enough to the end user that it should be peeled away.
For assembly though, one type of tape stands out. The most popular brand is 3M’s VHB (stands for Very High Bond). There are certainly many competitors, but I’m most familiar with 3M’s lineup. The uses for VHB in my experience are typically for holding parts together, and for creating water and air-tight seals. The products are not one-size-fits-all though so you need to be careful in the selection process. I’m going to use 3M’s “VHB” as a catch-all name for any double sided PSA.
The steps in selecting VHB for assembly are as follows:
- Determine if VHB is a good mechanical solution
- Decide if it it needs to be disassembled and by whom
- Determine if it can be assembled
- Figure out which PSA family for what’s being bonded
- Select a thickness
- Figure out the form factor (2” wide rolls, 24” wide roll, sheets etc)
- How will you cut it to shape?
- Where to source it?
Determine if PSA is a good mechanical solution
First things first – you need to figure out if tape can stand up to your needs or if things will fall apart. If you are using VHB just to create a seal, you might not need it to hold much mechanically. But if it’s the only or primary means for keeping two parts together, you need to consider the mechanical limitations. Good candidates are parts with large surface area, at least one semi flexible substrate, and part that need to stand up to physical shock. The manufacturer provides some strength data, but prototyping and testing is essential. A couple things to remember, read the application notes (cleanliness counts!), consider surface primer, and it takes time to cure to full strength. Clean surfaces are critical to adhesion – this should be obvious, you’ll need to do some homework to find out how you should clean your substrates. Isopropyl alcohol is common, but we’ve found certain applications where vinegar was better and some where ultrasonic cleaning was the way to go. Consider both tensile strength and also whether your design could be prone to peeling. Tapes will peel away at a much lower force than they will pull apart in tension.
Decide if it it needs to be disassembled and by whom
VHB is for one-time application. So it should be obvious that it would be a poor choice to seal a battery door that the end user needs to open regularly. Just because it’s a one time application, doesn’t mean it’s not serviceable. It’s just a replacement gasket is needed, and the service technician will need to remove the old gasket and clean the surfaces. Of course VHB is great for applications where you’ll never need disassembly, but it can also be a good choice if you want to avoid unauthorized service. Disassembly can be aided by heat and solvent, you’ll need to figure out what solvents and heat your design can tolerate.
Determine if it can be assembled
With VHB, once your parts touch, it’s kind of game over. There are some tricks to sliding around on adhesive I’m going to keep to myself, but pretty much your parts should have some alignment features so they can’t come together in the wrong position, and they should not have to slide on each other to fit. Also, if you’re using a very narrow gasket width and a large part, the gasket can be very difficult to handle, especially once the release liner is peeled off the first side. There are also tricks for handling ungainly gaskets which I won’t go into, but if you went through 10 VHB gaskets trying to put together 1 prototype, you might be a candidate for some fixturing. That’s beyond the scope of this article, but there are some really clever ways to get the VHB gasket on your part.
Figure out which PSA family for what’s being bonded
3M has lots of data on their website, I like a their converters catalog as it has lots of information in one place. Your choices will primarily revolve around what materials are being bonded (you’re going to need to understand surface energy), temperature range and strength. There are good applications engineers out there both with 3M and working for the distributors. If in doubt you can get some help. Review temperature ranges and chemical compatibility to make sure the adhesive family is going to do what you want in the long term. Of course very high or very low temperatures can be a problem as can aggressive solvents.
Select a thickness
There are usually several thicknesses available in each class of VHB, and I’ve even stuck together multiple layers if I needed something special. Thickness is important in two ways – if you need to have some give for uneven surfaces, or if you have a set dimension you need to bridge.
Figure out the form factor (2” wide rolls, 24” wide roll, sheets etc)
Form factor is really a matter of preference depending on how you’re cutting the material to shape and what leaves behind the least waste.
How will you cut it to shape?
VHB is easy to cut with steel rule dies and for prototypes, laser cutting works great. If you’re cutting really narrow widths of VHB (⅛” or less) the VHB can want to stick in between the knives. There are some tricks to getting around this – consult your die maker or use a laser. Keep in mind that VHB can be pretty soft. About the only thing holding together a narrow gasket is the release liner (the peel away backing). So consider handling, and storage of the gaskets – even more so if they need to be shipped somewhere before assembly. Most VHB comes on rolls with release liner on one side only. That’s fine if you don’t need to cut it to shape, but if you’re die cutting, you’ll need a second layer of release liner. The converters catalog can help you select one. If you buy sheets of VHB, there will be release liner on both sides. Keep in mind that different liners will peel off easier. That’s no mistake. If trying to peel off one side but keep the second side in place, it is a MUST that one of the release liner layers stick a little more that the other. It also means you might need to consider which side is up when die cutting.
Where to source it?
3M sells through distributors. The big ones I’ve used are are Ellsworth and APDMRO who mostly sell production quantities . For small prototype quantities, it can be costly to to buy. Many of VHB products have a minimum order quantity over $2000 and some are made to order so the lead time can be several weeks. That’s important to plan for in the prototyping process or you could find yourself with the perfect solution for your prototype and no way to implement it. Smaller quantities can sometimes be gotten through McMaster.com, Uline.com, iTapeStore.com and even Digikey.com.
We like using VHB so much that we have our own in-house cutting press, a relationship with a great steel rule die maker, and also a laser cutter. You should consider adding VHB tapes to your assembly arsenal.