Drape forming is similar to straight vacuum forming except that after the sheet is framed and heated, it is mechanically stretched, and a pressure differential is then applied to form the sheet over a male mould. In this case, however, the sheet touching the mould remains close to its original thickness. It is possible to drape-form items with a depth-to-diameter ratio of approximately 4 to 1; however, the technique is more complex than straight vacuum forming. Male moulds are easier to build and generally cost less than female moulds; however, male moulds are more easily damaged. Drape forming can also be used with gravitational force alone. For multi-cavity forming, such as tote trays, female moulds are preferred because they do not require as much spacing as male moulds.
Step 1. The plastic sheet is clamped in a frame and heated. Heating can be timed or electronic sensors a can be use to measure sheet temperature or sheet sag.
Step 2. Drawn over the mold – either by pulling it over the mold and creating a seal to the frame, or by forcing the mold into the sheet and creating a seal. The platen can be driven pneumatically or with electric drive. In some very small machines the platen can be manually moved up or the clamped sheet can be manually pushed over the mold.
Step 3. Then vacuum is applied through the mold, pulling the plastic tight to the mold surface. A fan can be used to decrease sheet cooling time.
Step 4. After the plastic sheet has cooled, the vacuum is turned off and compressed air is sent to the mold to help free it from the plastic. The platen then moves down pulling the mold from the formed part. The formed sheet is unclamped, removed, and a new cycle is ready to start.
Main techniques, differing by the position of the mold during the first stage.
- 1) 1st Method: The sheet (without masking) is placed on top of the mold in its basic, flat state. Both sheet and mold are then slid into a hot-air circulating oven and heated to about 150-155Â°C (300-312Â°F). When the sheet (and mold) reaches the required temperature it sags and drapes over the heated mold. Both are then pulled out of the oven and quickly helped, by gloved hands, to conform more precisely to the mold. It is then allowed to cool down.
- 2) 2nd Method: The sheet is placed into a hot-air circulating oven (without masking), and heated to about 150-155Â°C (300-312Â°F). When the sheet reaches the required temperature it is quickly pulled out of the oven and placed on top of the mold. there the sheet sags, aided quickly by the gloved helping hands, and takes the accurate shape of the mold. For better results we recommend pre-heating the mold to about 80-100Â°C (175-210Â°F) before putting the heated sheet on top. Then it is, likewise, allowed to cool down.
- better part dimensional control on inside of part
- lower mold costs
- ability to grain surface (tubs, showers, counter tops, etc.)
- faster cycle times.
Disadvantage is more scrap due to larger clamps and trim area.
Drape forming is widely used for large panels that require retaining a simple non-flat shape as in a curved display wall. Another useful application of this process is for the construction of wide sections of odd-shaped walls that will still retain overall even material thickness.