Thermoforming: Vaccum-Forming

A. General Guidelines
Vacuum forming is the most versatile and widely used thermoforming process. The equipment required is simpler and less costly to operate than most pressure or mechanical thermoforming techniques. Good results are quite easily attained with pre-dried sheets and decent vacuum-forming equipment.
Reduction in Thickness
The Vacuum forming procedure generally stretches parts of the processed sheet to smaller thickness at varied areas (according to the system chosen). Always take this into consideration when choosing the primary sheet thickness.
Molds
Prototype or Limited Production Molds can be prepared of smoothed plaster, hardwood, reinforced epoxy or polyester resins (or a combination of them).
High quality finish or full production run molds for yielding quality results or quick release operation tools have to be heated to working temperatures of 120 - 130 °C. Higher mold temperature (within the limits) gets better product’s finish. Polished aluminum or steel are preferable materials.
Mold Corners
Design the mold with rounded “sharp” corners, with radii at least as large as the processed sheet thickness, to avoid excess thinning or webbing during forming.
Mold Release
Good release of the molding can be achieved by designing the mold with a draft angle of at least 4 to 6 degrees on the upright walls. Allow for a molding shrinkage of about 1%.
Air Evacuation Through the Mold
Vacuum forming operates on suction principle, creating vacuum underneath the processed sheet. Suction is obtained through small holes put into the mold face. To prevent marking the molding, holes diameter on the exterior should not surpass 0.8 mm (0.031 in.). On the interior side of the mold the hole could be enlarged, to speed up air evacuation.
Male (positive) or Female (negative) Molds
Vacuum forming can be performed on male or female molds, using different equipment and technique, with different results.
Male Mold Forming
A heated sheet is lowered over a protruding mold and stretched down to the bottom, then air is evacuated through the mold and creates vacuum, which “sucks” the stretched sheet until it clings to the mold face.(Actually similar to vacuum assisted drape forming).
Female Mold Forming: A heated sheet is placed over the cavity of the negative mold, then air is sucked through the mold. The vacuum “sucks” the sheet until it clings to the inner face of the mold.
Male Mold
Its use results in thicker bottom and thinner walls. The internal finish of the final product is better. This type of mold is suited for deeper drawing (up to 4:1 depth to diameter ratio). It usually has a single protrusion, in a simple or more elaborately shaped forming. This is a relatively complex and slow technique needs a longer production cycle.
Female Mold
Its use results in thinner bottom and thicker walls and edges. The exterior finish of the final product is better. It can be used in a single cavity (simple or elaborate), especially suitable for multi-cavity, smaller spacing moldings. Its use results in edges thinning during deep draw, thus most suitable for simple, shallow, quick release designs, with a relatively fast production cycle.
Automatic Vacuum-Forming Machines
These are preferable in use, gripping the worked-on sheet on all sides during the process. It is notably important when working on thin [1 or 2 mm (0.04 or 0.08 in.)] thick sheets.Thin sheets tend to shrink up to 5% during the thermal processing and the cooling period, and must be firmly held in a fixed size frame.
When a sheet is inserted into the vacuum-forming machine, verify that the UV protected side is properly oriented to suite the finished product. Vacuum forming usually requires pre-drying of the sheets. It can also be carried out without pre-drying, in shallow molds and careful treatment. In that case the sheet temperature should not exceed 160 °C (320° F). Uneven heating, resulting in localized hot spots, over 160-165 °C (320-330° F), may cause bubbles to appear at the overheated zone.

B. Various Vacuum Forming Techniques
1) Straight Vacuum Forming: In straight vacuum forming, the PALSUN sheet is clamped in a frame and heated until it reaches an elastic state.
It is then placed over the female mold cavity, and air is then sucked out of the cavity by vacuum. The atmospheric pressure forces the hot sheet against the contours of the mold. When the PALSUN sheet has cooled down sufficiently, the formed part can be removed from the mold.
Thinning at the upper edges of the part usually occurs with relatively deep female molds. The hot sheet being drawn first to the mold center causes thinning. The sheet area at the edges of the mold stretch the most, thus becomes the thinnest section of the formed item. Straight vacuum forming is normally used for simple, shallow designs. See the figures below.

Other Thermoforming Techniques: Other, more specialized, thermoforming techniques use vacuum along with other power assistance, and others depend on different mechanical principles.
These techniques were developed for typical applications or to achieve certain results.

2) Matched Mold Forming: A heated sheet is placed between two matching male/female heated molds, which are then pressed to each other. Trapped air pockets are vented by vacuum through holes in the molds.
System enables very good detailing of surfaces, but is relatively costly due to need of accurate tooling and closer tolerance.

3) Trapped-Sheet Contact-Heat Pressure Forming: The process is based on similar principles as vacuum forming, except that both air pressure and a vacuum assist are used to force the PALSUN sheet into a female mold. It uses lower working temperature, and has a quicker production cycle. Other benefits are better dimensional control and finer finish.
The figures below depict the steps in the process.

4) Plug Assisted forming: Available in varied pressure/vacuum systems for deeper drawings and better control over wall thickness.
5) Pressure-Bubble Plug-Assist Vacuum Forming: The pressure-bubble plug-assist vacuum forming technique is used when a sheet is to be formed into deep articles that must have good thickness uniformity. The framed sheet is heated, then controlled air pressure is used to create a bubble. (See the figures below.) When the bubble is stretched to a predetermined height, the male plug-assist (normally heated) is lowered to force the stretched sheet into the cavity. Plug speed and shape can be varied for improved material distribution.

6) High air-pressure blows-up a heated sheet through a shaped aperture, resulting in Free Forming, a smooth, free-form bubble shape, without touching mold’s face. Both interior and exterior surfaces remain smooth and blemish-free.

7) Mechanical Forming: This method uses no molds, only mechanical means like bending, stretching or holding to form parts of relatively simple design.