by Eric Gregorich, Plastic Assembly Systems
Approximately 15 years ago, a more complex and versatile process evolved in secondary molding technologies. Many of the existing processes (ultrasonics, RF welding, hot air cold upset, impulse, infrared, hot plate welding, and vibration welding) were limited, and manufacturers in secondary assembly needed a process that would accommodate assembling populated circuit boards without damage, assembling plastic components of dissimilar materials, assembling plastic components with fillers (i.e., glass, talc, etc.), assembling plastic components with multiple bosses and/or on multiple plains, inserting metal inserts without noise or particulate, and more. The evolution began when someone took a hot stamping machine, installed custom tools on it, and created the first crude heatstaking machine. Over the past 15 years, there have been major changes in heatstaking machines, from the early days of the time/temperature-only machines to the most recent advancement of servo heatstaking machines. The machines have advanced and so have the tools.
The Heatstaking Process
Heatstaking is a process where precise heat, time, and force are required to reform a plastic boss. The boss protrudes out of one component and will fit through a hole that is in a mating component, such as a PCB, metal plate, another plastic part, etc. The boss that protrudes out past the mating part will be reformed with a heated tool, time, and force to mechanically lock the two components together. The process of heatstaking requires five main components: temperature, time, force/pressure, distance, and post cooling.
Temperature: This is the main component of heatstaking. Most plastics that are used in the heatstaking process have a wide operating temperature. Heating the tooling to the flow temperature of the plastic and then contacting the plastic is what allows the material to reflow/reform.
Time: A certain amount of time is required to reform plastic material. An analogy that I use when explaining the time that is required to heatstake a part is that it’s like baking bread. Bread must be baked at a certain temperature for a certain amount of time. If you try to speed that process up and don’t allow the proper time for baking, the outside of the bread burns and the inside is under-cooked. This is directly related to staking. If the proper amount of time is not allowed during the process, the boss will not be reformed fully and the material can degrade. This, in turn, will weaken the boss.
Force/Pressure: Force is the capacity required to assist in the heatstaking process. Because the plastic material cannot be heated to the actual flow temperature (as used when molding the components), heatstaking requires a certain amount of force to assist in the reforming process. The force/pressure that is required to heatstake a boss is dependent on the material, size, and quantities of the heatstake bosses. For example, if you’re staking one boss that is .250 in diameter, approximately ¼ ton of available force will be sufficient. If you are staking a boss of the same size but increase the quantity to 20, heatstaking will require approximately 2.0 tons of available force.
Distance: Distance is not required for all applications but is a great option when appropriate. This option developed because of a request made by a medical company needing a very high level of precision. Since the product was a medical component that would be used inside the bodies of patients, the company needed to know 100 percent that each part would be manufactured correctly. This option was added to heatstaking machines approximately seven years ago because of that request.
Post Cooling: In the early years, post cooling was not an available option on heatstaking equipment, which greatly reduced the applications that were able to be run successfully. After the boss is reformed, a short blast of ambient air is blown under pressure directly at the interface of the heated tooling and the mating material. This cools the material just enough to destroy the memory in the plastic and ensure the material will not stick to the tooling. Post cooling ensures the stakes are tight, strong, and aesthetically pleasing.
Processes Commonly Performed on Heatstaking Equipment
Heatstaking machines are capable of performing many other processes using the same principles as heatstaking.
Heat Insertion: Typically refers to heating a metal (most commonly brass or steel) insert and pushing the heated insert into a hollow boss. That hollow boss is approximately .010 to .015 smaller in the ID than the insert’s OD. This allows the heated insert to heat the plastic material until it flows around the knurls on the OD of the insert, creating a very strong bond. There are a few very key advantages to heat insertion over other processes: no noise, approximately 30 percent stronger pull out and torque out strength, and no particulate.
Heat Degating: Typically refers to a hot knife that cuts a plastic mold gate/runner.
Heat Swaging: Typically refers to the rolling inwards of or reforming of a wall to retain the mating component.
Heat Sealing: Typically refers to applying pressure and heat to a film or thin plastic material to seal it to another plastic component or to itself. This process also works well with film that has heat sensitive glue.
Heat Embossing: Typically refers to heating steel numbers and/or letters and contacting the plastic component that melts the material and leaves the characters impression.
Heatstaking has become one of the premier processes widely used across a range of industries, including automotive, electronics, medical, food industries, packaging, and telecommunications. With such a broad range of applications, materials, and industries, it’s no wonder heatstaking has become the answer for almost all plastic assembly needs.
Eric Gergorich is vice president of sales and marketing for Plastic Assembly Systems (PAS). For more information, call (866) 882-1701 or visit www.heatstaking.com.