Vacuum Forming Techniques: Prestretching

Vacuum Forming techniques in Lormac’s San Diego facility.

Prestretching

When more uniformity or precise varying of wall thickness is desired, prestretching of the hot sheet before it touches the mold may be required. In using straight Drape Forming, as the hot sheet comes in contact with any portion of the mold, it cools that part of the sheet and decreases the sheet’s ability to flow. This causes most of the stretching to come from the section not yet in contact with the mold.

To reduce premature contact of the plastic sheet on specific mold surface areas that will cause stretching, a prestretching process is applied. There are several prestretching processes used by Lormac when vacuum forming in our San Diego Facility and we will discuss 2 of them in this post:

1. Snap Back
2. Plug Assist

Snap Back 

When prestretching is needed in the vacuum forming process, this method is automatically considered because the tooling is least expensive. A positive mold and a prestretch box box are easier and cheaper to make than a negative mold with plug assist. In this process, the prestretch box is pushed into the heated plastic sheet, causing a seal to form around the periphery. The hot plastic is pulled into the prestretch box by applying vacuum pressure or compressed air to form a bowl shape. To control consistency on the prestretch process, a photo electric eye may be used to automatically shut off the vacuum/compressed air when the stretch hits its mark. A rule of thumb is to prestretch the material approximately 2/3 the depth of the part.

When the plastic material hits the optimal prestretch height, the prestretch vacuum/compressed air is turned off and the plastic sheet is rapidly “snapped” to the detail of the mold surface using vacuum. After the plastic sheet is pushed completely against the mold, the prestretch box is moved away from the part to allow for optimal cooling.

The Snap Back process can be applied in both mold up and mold down formations. Mold Up formation is when the mold is mounted in the vacuum forming machine upside down and at the top of the machine. Better distribution of material is gained when using this formation as it allows you to take advantage of the natural sag of the heated plastic, however, for larger parts, the weight of the tool may prohibit mounting in this formation. Mold down formation is when the mold is mounted right side up at the bottom of the machine. the prestretch box has to work a little harder to billow the heated plastic sheet against its natural sag, but the process works well in keeping uniform wall thickness. At times, blowing carefully controlled compressed air between the sheet and the mold may help in billowing.

Snapback Vacuum Forming

Plug Assist

When using a negative mold and prestretching is required, plug assist is used. In this process, the mold is pushed into the heated plastic sheet creating a seal around the edges. At the same instance the seal is created, the heated plastic sheet is pushed into the mold using a plug assist. As the plug assist enters the the hot sheet, the air between the sheet and the mold is compressed, causing the sheet to billow around the plug. this action prevents the hot sheet from contacting the relatively cold mold as the sheet is stretched into the cavity. the plug stops within 10% of the bottom of the mold when the vacuum is rapidly applied transferring the sheet form the plug to the mold. Using a heated plug will avoid chilling the sheet during the prestretching process.

As with the Snap Back process, a mold up formation is optimal in plug assist for the same reasons, taking advantage of the natural sag and creating more air between the mold and the plastic sheet to increase billowing.

plug assist vacuum forming