KommerlingUSA
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KOMADUR
Komadur

Fabrication: Forming

KOMADUR forming chart


KOMADUR PVC sheets can be formed, (i.e. worked or shaped without cutting), by means of a number of different processes: folding, bending, compression molding, upsetting, stretch forming and thermoforming.

The accompanying diagram shows that the forming properties of KOMADUR PVC sheets are dependent on the temperature. These properties are of practical importance with regard to workability and use of the sheets.

The modulus of elasticity and the tensile strength indicate the deformation resistance, the elongation at tear, and the deformability of the material at the respective temperature. Usually, the sheets are formed during the elastic-plastic phase of the material, in which maximum degrees of expansion/elongation are reached and the forces required for forming can be kept low.

In order to prevent damage to their structure, KOMADUR sheets must not be formed at and below the softening temperature, (i.e. at temperatures below 90 °C). The most favorable temperature range for each of the various hot-working processes depends on the sheet material and application (120-140° C for folding, bending, compression molding and thermoforming; 135-180° C with restrictions). If the material is heated above 180° C, it initially leads to discoloration and then to thermal damage. Avoid forming rigid foam sheets at temperatures below 120° C to avoid damaging the cell structure.

Good forming is possible only when the sheet material is thoroughly and homogeneously heated. The sheets are preferably heated using infrared heaters, but heating ovens or circulating-air ovens can also be used. During folding processes, for example, the sheets require localized heating, for which heating elements can be used.

Furthermore, we recommend sheets 3 mm thick and more be heated on both sides in order to avoid damage to the one surface through overheating and to shorten the heating time.

Thermoforming and stretch forming
KOMADUR PVC sheets can be thermoformed or stretch-formed on all commercially available forming machines. The only stipulation is that the machines are protected against draughts on all sides.

Fast working cycle times can be achieved if the tools used for these forming processes are cooled. In the case of vacuum forming, sand blasted surfaces have the advantage that the air can be completely extracted without leaving any air pockets. The diameter of the vacuum bores should not exceed 0.8 mm in order to prevent deformation of the drill holes. The edges should generally be rounded with a radius that is no more than between one and three times the thickness of the sheet.

In the case of KOMADUR, male molds must be sufficiently conical: approx. 5°. Female molds do not need to be conical, as the thermoformed parts separate from the mold as they cool down.

Cooling is generally carried out using compressed air. Compressed air combined with sprayed water, however, must not be used until the surfaces have hardened.

Of the various thermo and stretch-forming processes, particular mention should be made of the air-slip processes in which the heated sheets are pre-stretched pneumatically, (i.e. by air). These processes are the ones that generally produce thermoformed parts with the most uniform material thickness.

In this connection, it is important to mention that the forming process of the material stops wherever the PVC sheet touches something (the mold).

Thermoformed parts will for the most part not warp if the forming temperature is high, the parts are cooled down slowly, the molds are opened at rather low temperatures, and the rims are trimmed immediately after the parts have been removed from the molds. For the forming process itself and the design of the molds, the shrinkage of sheets has to be taken into account. KOMADUR sheets can shrink by up to 0.5%, particularly in the direction of extrusion.

Folding, bending, compression molding
Folding and bending of KOMADUR PVC sheets must only be performed on suitable equipment. The bending radius must never be smaller than two to three times the thickness of the sheet. The heated zone should have a width of at least five times the thickness of the sheet.

After cooling, linear heating of the material will lead to stresses developing within the PVC, which cause warping when folding short lengths.

Sides with a length 20 times the thickness of the sheet will not warp. For this reason, we recommend you heat up the entire section in order to prevent short sides from warping.

Folds with a relatively small radius can be made by notching (V-notch) the inner side.

Hot-folding of sheets
To prevent the cell structure from being stretched to too great a degree, a minimum bending radius of approximately twice the thickness of the sheet must be observed. In the case of PVC sheets 8 mm or thicker, please remove the excess material by milling a V-shaped groove along the inside of the bending edge before bending.

You can then carefully heat up the outside of the bending edge, bend up a fold and bond the joint created. However, a remaining thickness of 1 to 2 mm must be left in the rigid foam sheets. When milling, make sure that the V-shaped groove is 1° larger than the desired bending angle, i.e. 91° in the case of a 90° angle.

Cold-bending of sheets
The minimum bending radius when cold-bending rigid foam sheets is roughly 100 times the thickness of the sheet (i.e. 200 mm for a 2 mm-thick sheet). To cold bend thick sheets, use a bench saw to saw around ten parallel grooves into the sheet with a clearance between them the width of the saw blade, leaving a remaining thickness of 1 to 2 mm. You can also use this method to bend thick sheets to any desired angle without heating them.

Embossing
Impressions such as letters, characters, numbers and ciphers are embossed with tools familiar from sheet-metal working and the cardboard and leather industries. The tools must be preheated (100–130 °C are recommended).

The actual embossing process generally does not require the sheet material to be heated. Further information and useful advice on forming PVC-U sheets can be found in VDI specification 2008, pages 1 to 3.

Hot-working processes
In order to prevent damage to their structure, KOMADUR sheets must not be formed at and below the softening temperature, (i.e. at temperatures below 90 °C). The most favorable temperature range for each of the various hot-working processes depends on the sheet material and application (120-140° C for folding, bending, compression molding and thermoforming; 135-180° C with restrictions). If the material is heated above 180° C, it initially leads to discoloration and then to thermal damage. Avoid forming rigid foam sheets at temperatures below 120° C to avoid damaging the cell structure.

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