Spot Welding

Questions and Answers

One should always attempt to make a resistance weld under steady state conditions. If the desired pressure, current and time were applied consistently each weld, one can expect every weld to be the same weld after weld. If one of those three input varies then one could expect that the welds might vary. Rising pressure, loose cables, electrode issues, misalignment, faulty cooling or many other issues can and do affect the welding results.

  Pressure vs Contact Resistance


If one has a robust process that is running well but you are pushing the limits on the pressure build up, remember to keep a close watch on the weld results. You may be taking advantage of the lag in the weld gun reaction and initiation speed. The machine does not react instantly and takes several more cycles before the electrodes are actually closed. You should always attempt to reach 90% of desired force before the weld is initiated.

However:

If one has something that works

It is a Robust process

It is monitored closely

NEVER ARGUE WITH SUCCESS!

Maintaining the proper pressure during a welding operation is very important to obtaining good welds and a long running operation.

Many articles have been written in this blog on the importance of pressure and force. They include:

Why is the control of pressure important in resistance welding?

How are force and pressure related?

How does pressure affect resistance?

HOW DO FORCE AND PRESSURE CONTAIN EXPULSION?

 

Reference:  RWMA - Resistance Welding Manual 4th Edition

The subject of too much pressure/force has been addressed in several articles on the blog. Increasing the pressure reduces the contact resistance between the electrode and the part being welded. Less heat is generated on the part surfaces which effectively cools the part. Conversely less pressure creates more contact resistance and heat at the surfaces. The part will heat more.

  Pressure vs Contact Resistance                                      Pressure vs Contact Resistance 1

INCREASING PRESSURE DECREASES RESISTANCE           DECREASEING PRESSURE INCREASES RESISTANCE

Read more ...

A dent free spot weld can be defined as a “Cosmetic Weld”. This is a spot weld where one side of the weld would show little or no electrode indentation. The middle of an automobile hood would be a location where an indentation would not be desired. Medical instruments are another example. By the nature of the resistance welding process metal fusion and indentation are inherent features of the process. Control of the amount of heat and where it is applied is the path to producing a cosmetic weld.

Read more ...

Other articles in this blog describe:

WHAT IS UPSLOPE IN RESISTANCE WELDING?

IS UPSLOPE HELPFUL IN REDUCING SPATTER OR EXPULSION IN RESISTANCE WELDING?

To summarize here what these articles contain, upslope is used to slowly heat up the part as the heat initiates before full current application. This give the parts a chance to realign, relax/anneal, adjust position, melt coatings and contaminants and improve the part to part contact before full current is initiated. The end goal is for better contact and less expulsion.

 TIME DIAGRAM ILLUSTRATES COMPLETE FUNCTIONS

Upslope is shown in the first section of the red current line - as it increases. This period is often used on galvanized materials to allow the zinc to melt and be pushed out of the joint. In other cases it is a chance for the components to anneal or relax and get all material into full contact before full power is applied.

Reference: RWMA – Resistance Welding Manual 4th Edition

That is an interesting question. Stainless is highly resistive and won’t conduct heat too far or fast but the plastic is only 3 mm away.

My gut says without some sort of extra cooling the plastic is likely to see some heat.  I trial run is needed.

Is water an issue? Put a mist of water in the weld area as the part welds, similar to a seam welder just less water. Initiate the mister with the weld cycle.

Make sure the electrodes are cooled well to reduce time at heat. Minimized the size of the weld and weld nugget. Keep the electrodes in place after the weld to pull heat out.

GOOD LUCK!

Have a Question?

Do you have a question that is not covered in our knowledgebase? Do you have questions regarding the above article? Click here to ask the professor.