Using good-quality, consistent sheet is the most important key to getting good thermoformed body parts using paint film, according to a researcher from Ford Motor Co.
Kedzie Fernholz, a Ford engineer in materials research and advanced engineering, described the automaker's research into what variables affect the quality of rocker panels molded with paint films.
Fernholz said even a small difference in sheet quality makes a big difference in waviness of the finished part. ``You've got to control that incoming sheet quality - it's very critical. The only way to reach these [quality] targets is to start with good sheet,'' she said in her presentation Sept. 17 during the Society of Plastics Engineers' Thermoforming Conference in Cincinnati.
While the need to start with good sheet may seem obvious, Fernholz did toss out a few surprises. One other conclusion from the research: the thermoforming practice of applying chilled air to a paint-film part does not improve appearance.
``Chilled air, it can certainly improve your cycle time, but on this particular type of film, it's not going to impact the gloss,'' Fernholz said.
She described how Ford researchers experimented to measure the impact of chilled air. They built a pressure box to greatly increase the amount of chilled air on the relatively large rocker-panel part. Later, they even used outside air - on a 12° F day in February.
They also studied the sheet temperature and found that controlling that variable is important.
Research into surface quality on high-gloss parts harkens back to Henry Ford's early days: any color, as long as it's black. ``Any time we're using surface variations, we always use black,'' because any defects show up very clearly, she said.
Fernholz said the tests were done on a Geiss thermoforming machine with halogen-bulb ovens. More research needs to be done to confirm the findings for other types of heating, she said.
But the bottom line is pretty simple: ``If you don't have good sheet, you won't have good parts.''