Twin-sheet thermoforming is a promising but challenging process for making
hollow or double-walled industrial parts. The process is increasingly being
adopted to make vending-machine chutes, equipment frames, pallets, bed-liners,
air ducts, and both structural chassis parts and surface panels for medical,
printing, and electronic equipment.
The most widely used form of twin-sheet forming clamps two sheets horizontally
in a rotary or in-line shuttle machine by means of a double clamp frame with
spacers. After heating, the upper and lower sheets are simultaneously vacuum
drawn into top and bottom female molds. The molds are subsequently brought together, and the hot sheets are compressed at joining areas designed into the mold. The two sheets are thus thermally welded into a single part without use of adhesives or fasteners.
Combining two sheets into a unified part is far more difficult than forming
a single sheet. Thermal bonding has to be precise, making proper alignment of
tools critical. Control of conditions in the area between joined sheets is also
essential, requiring the incorporation of vacuum holes and appropriate cooling
devices (e.g., blowing needles) in the mold. And the inability to use plug assist
imposes limits on the draw ratio.
Challenge #1: Precise mold alignment
Twin-sheet forming uses two molds that often differ widely in terms of design
intricacy and depth of draw (see diagrams). Yet the molds must be perfectly
matched in terms of the “press points” where the two sheets are compressed
together. The two molds must also align perfectly at the exact moment when platens
and molds begin pressing the sheets together—with no “wiggle room”
allowed.
|
|
| 1. Thickness difference between the two sheets must be minimized or the thinner sheet may sag or warp too much.
2. Remember to consider the degree of compression in the weld areas, which
can vary somewhat from cycle to cycle.
3. Hot air trapped between the sheets must be vented and cool air introduced
to prevent collapse of the hollow sections.
|
|
Challenge #2: Compensate for compression
Twin-sheet forming involves 40% to 50% compression of the sheets where they
are thermally bonded. In one actual case, twin-sheet forming of a pair of 0.125-in.
sheets yielded a finished part with only about 0.125-in. thickness in the bonding
area. In calculating final wall thickness and mechanical properties of the part,
it is necessary to allow for slight variations in compression that can occur.
Challenge #3: Cooling & venting
Twin-sheet processing traps a pocket of heated air between the sheets. Upon
cooling, it could collapse the panel. To prevent this, the mold is designed
with vent holes that allow cool air inside the panel. Holes should be sized
to provide enough cooling and positioned to avoid creating an eyesore.
Hole placement is most critical with appearance parts such as panels for office
equipment. One solution is to position holes on the inside face of the panel
so they remain invisible to users. When hinges, brackets or other attachments
are to be joined to a panel, it is possible to position holes so they are hidden
by the attachment. Vent holes can also be designed into the perimeter of parts
to disguise their presence.
Vent holes permit a natural flow of cooling air into the panel. But at times
it is desirable to positively control cooling. An ideal way is to pump cool
air into one side of the hot part and vent it through another hole. That can
reduce cycle time and part cost.
Challenge #4: Draw-ratio limits
In single-sheet forming, it is possible to design parts with 3:1 draw ratios
and deep undercuts. What often makes this feasible is plug assist, which permits
material to be mechanically pushed into the mold to reduce wall thinning. But
plug assist is not an option in twin-sheet forming. This means that twin-sheet
panels should have generous turning radii at corners and indents and should
avoid draw ratios exceeding 2:1.
Draw ratios are computed with the following formula: D = 1 + 2H (L + W) divided
by LW, where D is nominal draw ratio and H, L, and W are part height, length,
and width respectively. Consider a square box designed to be 10 x 10 in. and
6-in. deep. The draw ratio (2.41:1) exceeds the 2:1 suggested maximum. It would
be advisable to redesign the part to reduce D to around 2:1.
|
The almost indiscernible bond between sheets in this twin-sheet X-ray handle comes from precise mold alignment and careful compensation for compression of top and bottom sheets.
|
Challenge #5: Eliminate shadowing
“Shadowing,” or visible variation in surface finish, often occurs
at areas adjacent to where the sheets are pressed together. If the panel is
to be painted or is a non-appearance part, consequences are slight. But if unpainted
surface finish is important, shadowing becomes a critical issue. One solution
is to bead-blast the mold so that the entire panel has a textured finish that
masks the shadowing. Another answer is to disguise press points as much as possible
(e.g. locate them beneath an attachment).
Challenge #6: Sheet- thickness compatibility
One of the two sheets is often assigned the load-bearing role, the other being
intended for cosmetic purposes or to contain insulating foam that will be pumped
into the cavity. Yet it is generally best to keep the difference in thickness
between top and bottom sheets to 0.0625 in. or less. Greater thickness differentials
tend to put twin-sheet forming out of sync: The thinner sheet would be subjected
to excessive heating and cooling (because the thicker sheet takes longer), potentially
causing excessive sag and warpage.
Kintz Plastics in Howes Cave, N.Y., is a custom heavy-gauge and twin-sheet
thermoformer. Michael Righi is engineering manager, and Roger Cusano is a manufacturing
engineer. (518) 296-8513, www.kintz.com.
