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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.
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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. |
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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 150-155°C
(300-312°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 150-155°C (300-312°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 80-100°C (175-210°F) before putting the heated sheet on top.
Then it is, likewise, allowed to cool down.
Advantages
- 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.
Applications:
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.
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