Common Defects in Roll Formed Products & How to Prevent Them
Ryan Hernden
Learn the most common defects in roll forming & how to prevent them. MMC ensures quality results through precision engineering & strict quality control.
Learn the most common defects in roll forming & how to prevent them. MMC ensures quality results through precision engineering & strict quality control.
As roll forming is a continuous process, it’s likely that defects may occur throughout the run and affect every piece produced. Some of these defects include marks on the part’s surface, distortion that puts features outside drawing limits, and cracks.
This blog explores these defects in more detail and how manufacturers like MMC Roll Form prevent them from happening.
Surface defects won’t usually affect how the roll formed product performs in service, but they are unsightly. Surface defects could include scratches and abrasions, variations in surface finish, and imprinted slivers of material. Oil or grease may occasionally be transferred from rollers and bearings to the strip.
Surface defects are primarily caused by:
Prevention consists of following good housekeeping practices and applying effective planned and preventive maintenance. Both are reinforced through documentation in Standard Operating Procedures (SOPs).
“Housekeeping” refers to keeping the roll forming equipment clean and free from debris. Time is allocated for cleaning, materials are made available, and equipment condition is checked periodically.
Maintenance involves inspecting rolls, bearings, guides, and other components and replacing them as needed, as well as removing excess lubricant to prevent it from transferring to the strip.
Springback refers to the tendency of metal to return to its initial shape after being deformed. It’s a function of elastic modulus and yield strength (material properties), leading to geometric (particularly angular) errors in the parts being roll formed. It can also potentially cause strip to jam as it’s fed through the machine.
Springback occurs more with some metals, notably steels than others, such as nonferrous alloys. It is usually predictable and only becomes a problem when it occurs differently than expected. Factors causing this include:
Springback is anticipated during roll design and addressed by overbending the strip to a defined amount. Issues arise when the material responds with an unexpected amount of springback, either too much or too little.
Roll formers prevent this in three ways:
Roll formed products tend to open up at the cut ends, resulting in an inconsistent profile along the length, which may put the ends outside drawing limits and lead to problems at assembly.
End flare can also appear where there are deep cutouts on the sides of the strip. If it is different from what was allowed for during tooling design, it can cause the strip to jam at the next set of rolls.
End flare defects result from the tensile and compressive residual stresses put into the strip during roll forming.
Roll design dictates the levels of residual stress put into the profile and, hence, the degree of end flare that will occur. Two techniques for reducing end flare distortion are:
A third, equally important action is to only purchase high-quality strip where dimensions and residual stresses induced by rolling to thickness have been carefully controlled.
Cracks and wavy edges are the two primary types of structural defects in roll formed products.
Cracks can take two forms: 1) longitudinal along a bend and 2) short lateral cracks at the strip edges. Cracks affect the integrity of the roll formed profile.
Edges can also develop a wavy form, known as “wavy edge,” and flat regions can take on a waviness referred to as “oil canning.” These defects are more aesthetic than functional but are still undesirable.
Longitudinal cracking is caused by:
Wavy edges and edge cracking result from stress along the outer edges of the strip. This, in turn, is a function of the amount of deformation applied and the uniformity of the strip thickness.
Oil canning is also evidence of uneven residual stress within the strip.
Longitudinal cracks are avoided by using the largest possible bend radii in conjunction with more ductile alloys. Reducing deformation speed (spreading the bend out over more roll stands) also helps.
Wavy edges, edge cracking, and oil canning are all avoided by looking closely at roll and roller bearing conditions, roller parallelism, and strip alignment. The uniformity of the input strip is also important, as a wedge profile can cause uneven stresses along the edges and at the center. Additionally, thicker material is less prone to oil canning.
MMC Roll Form prevents defects through comprehensive project planning and stringent quality control procedures. We have over 50 years of experience planning projects, use an ISO-9001-compliant quality management system, and follow TS 16949 requirements for automotive projects.
Please visit our website to learn more about our process, or contact us today to request a project analysis.