We all know about the variables involved in thermoforming: sheet quality, machine configuration, ambient air temperature, mold design, plug material & geometry, sheet temperature, etc. And we know that there is science behind each of these items. So even after checking everything and documenting everything, why do we still see parts that have to be rejected?
Process control and repeatability are two challenges faced by the thermoformer. WIth new data-driven tools such as ToolVu and Intelligent Control, there is a movement underway to bring more rigor to the thermoforming process and reduce or eliminate errors. Still, this technology has not yet been universally adopted, so what can we do in the meantime?
We often get questions about mark-off or lines on parts. What causes these imperfections? Well, as it turns out, not all marks are created equally. Let’s take a look at a few different types of marks:
Flow lines are usually caused by inadequate clamping of the sheet. They tend to appear as ‘waves’ around the perimeter of the part. When the tool closes, excess material can be pulled / pushed into the cavity. Perimeter (of the tool) clamping is not as effective as individual cavity clamping. By using individual clamps, you can ensure that the area of the sheet being formed is the same for each cavity. This also helps ensure more even material distribution across the tool.
Chill marks are caused when anything colder than the heated sheet comes into contact with the sheet. This could be a plug, a male mold or flanges of a female mold. They can sometimes look white or opaque compared to the rest of the material. These differences in temperature can leave marks on some materials and they will also impact the material thickness at the point of contact. Even a 3 degree fluctuation in temperature can result in problems. There will be less stretch in the colder areas and more stretch in the hotter areas. In some cases, such as with pre-printed sheet and associated distortion, it is preferable to have colder areas. Measuring sheet temperature (not just oven temperature) as well as tool/plug temperature can help identify potential trouble spots.
This is a bit of a catch-all term to identify imperfections on the formed part. Causes of mark-off can be traced to plastic residue building up on the plug or the presence of small nicks/voids/chips on the plug surface. Taking care to machine and polish plugs effectively can reduce or eliminate the causes of mark-off.
In the next two weeks, we are going to release our newest whitepaper (summarized here) which is a comprehensive guide to designing, machining and modifying plugs. It is the result of many conversations and empirical data gathered over the years by CMT. Stay tuned for more details.