How to Prevent Marks in Thermoformed Plastic

Thermoforming is an efficient, cost-effective way to produce large, molded plastics like vehicle trunk tubs as well as run short orders of plastic parts, products, and components. While the design and tool development is less involved than injection molding, the manufacturing itself still requires care and precision to avoid marks that can affect the structural integrity and the aesthetics of the finished product. To help you know what to look for, our thermoforming company in North Carolina is sharing what chill marks, mark-offs, and flow lines are and how to avoid them.

Understanding Thermoforming

Thermoforming is a method of plastic manufacturing in which a sheet of plastic is heated to the point where it's soft, but not liquid. It's then clamped over a custom mold that is designed to press the needed shape into the plastic. Vacuums and/or pressurized air then press the plastic more tightly against the mold to get a more accurate design and shape, and once the desired outcome is achieved, the plastic is cooled, removed, and trimmed.

Preventing Flaws and Marks in Thermoforming

There are multiple variables when thermoforming plastic that can affect the outcome and cause flaws in the completed product. This includes, but is not limited to:

• Mold design, such as draft angles and vent location;
• The plastic temperature when fitted to the mold;
• Ambient air temperature;
• Rate of cooling the plastic;
• Amount and location of vacuum and air pressure;

When one or more of these factors are not in alignment, imperfections occur. Let's look at the three most common types.

Flow Lines

Flow lines and webbing look like waves around the exterior perimeter of the plastic part of the component and are often slightly off-color. These are most often caused by using clamps around the perimeter of the sheet of plastic, so when the plastic is pulled tightly against the mold, excess material can be drawn into the cavity.

While these don't affect the structural integrity of the part, it's important to avoid this issue. By using individual clamps for each cavity of the mold, you can guarantee more even material distribution within each cavity and even sheet distribution across multiple channels.

Chill Marks

Chill marks look like white streaks or spots on colored plastic or opaque areas on transparent materials. This is caused by something cold coming into contact with the sheet after it's been heated and during the forming process. Because the cold areas will not stretch as efficiently as they will where the sheet is hotter, color differences can occur.

Something as small as a plug, flanges, or cold spots in the tooling can cause chill marks, so it's important to measure sheet temperature, oven temperature, and the temperature of the tooling and parts to ensure consistency and avoid chill marks.

Mark Offs

A mark-off is a generic term to refer to basic flaws and imperfections on the formed plastic. The most common cause of these bumps and marks is a build-up of excess plastic residue or chips, nicks, or other flaws on the surface of the mold or plugs. Again, these aren't going to affect the performance of the plastic, but the presence of flaws can have your client concerned or questioning the integrity of the plastic components.

By cleaning and maintaining all tooling, clamps, and plugs, the presence of mark-offs and other flaws can be almost completely eliminated.

Contact Us for Custom Thermoforming Today

Since 1988, Advanced Plastiform, Inc. has been a leading supplier of thermoformed and injection molded plastics in North Carolina. We have a team of experienced designers, engineers, and manufacturers who take pride in delivering high-quality, durable custom plastics with a short lead time and low per-unit cost. We supply plastics across the Southeast and Mid-Atlantic states including North Carolina, South Carolina, Pennsylvania, Maryland,  Tennessee, Georgia, and Virginia. To get started, call us at  919-404-2080 or contact us by filling out this form.