What different types of thermoforming processes are appopriate for ACRYLITE® acrylic sheet products?

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What different types of thermoforming processes are appopriate for ACRYLITE® acrylic sheet products?

Forming Processes

These drawings illustrate several thermoforming methods.

Basic Vacuum Forming (Female Mold)

Basic diagram:

ACRYLITE® acrylic sheet is clamped and heated.

Vacuum causes atmospheric pressure to push sheet into mold. Chilled surface of mold cools sheet. Sheet contracts and part is ejected. Areas last drawn are thinnest.

Vacuum Forming (Male Mold)

Basic diagram:

ACRYLITE® acrylic sheet is clamped and heated.

ACRYLITE® acrylic sheet drawn over mold by downward motion of sheet or upward motion of mold.

When seal is created around mold, vacuum is applied beneath the mold-causing sheet to drape over the mold by pressure differential. Formed item has thickest position at base. Rim area is thin.

Plug Assist Vacuum/Pressure Forming (Female Mold)

Basic diagram:

ACRYLITE® extruded (FF) sheet is heated and sealed across mold cavity.

Plug is plunged into plastic sheet and pre-stretches the material. Plug temperature is important. With proper sequence, vacuum is drawn to complete part formation. Pressure enters other side to sharpen definition.

Finished Part.

*These drawings are reprinted through the courtesy of Mr. David R. Zelnick, ZED Industries.

Variations - Wall thickness can be changed by varying design of plug, temperature of plug, vacuum timing, pressure, timing, or any combination of the above.

Drape Forming

Here, ACRYLITE® extruded (FF) sheet is heated and bent over a positive (male) or into a negative (female) mold. To compensate for sheet shrinkage during cooling and sheet "memory", female molds are best. To deter mark-off, cover the molds with rubberized flocking or billiard table felt. For the same reason, set the mold temperature high and forming temperature low.

Cover the surface so the cooling rate is the same on both sides of the mold. Thick cloth or felt blankets make good covers. If the mold and blanks are larger than the finished part, trim off clamping frame marks.

Form compound contoured parts, such as those used for bus rear window and roof glazing, in similar fashion. After heating to the forming temperature, gently position and clamp ACRYLITE® extruded (FF) sheet over a positive mold with the shape of the finished part's external contours.

Free Blowing and Vacuum Molding

Many configurations can be free formed using a shaped clamping ring or vacuum-drawn using a mold box. Items of very high optical quality can be produced by these methods because the surface of the material never touches the mold walls. Thus, no mark-off or local cooling occurs.

Using these techniques, a variety of spherical surface shapes can be molded depending on the geometry of the clamping frame. On the next page, some plan views of selected shapes are shown.

The pressure or vacuum is varied to get the desired height or depth. The height is demarcated with a jig or soft material designed to avoid marking. Pressure or vacuum can also be controlled automatically with optical light sensors. Although this method is expensive, its lack of contact creates an advantage in optical-critical production.

A simple suction or blowing mold consists of a base plate with a clamping frame. Enough mechanical or hydraulic toggle clamps must be provided to maintain frame rigidity and to withstand the forming pressure. Using screw clamps has the disadvantage of being time consuming and may allow the sheet to cool excessively before it is formed. Beaded clamping frames will seal better than flat frames.

For free blowing, use up to 75 psi of air pressure. Provide large air connections so that large parts can be shaped quickly, but be sure that the incoming compressed air doesn't hit the hot panels directly and cause local cooling. The incoming air can be deflected by installing baffles or screens in front of the inlet opening.

Resting the sheet blank on a cold base plate may cause undesired cooling while installing the clamping frame. To reduce this, heat the plate or cover it with thick, non-linting cloth.

Vacuum forming requires the same fundamental conditions as pressure forming except that less clamping force is needed because suction seals the sheet to the vacuum box virtually automatically and the pressure difference is limited to 15 psi or less. If possible, arrange the suction ducts in a ring around the edge of the vacuum box to prevent airflow from cooling only one side of the part. For processing large parts, fit a reservoir (or vacuum tank) in front of the vacuum pump for quick evacuation of large volumes of air.

Vacuum with Plug Assist in Negative Mold

Essentially the same considerations apply here as with blowing except that the smaller available pressure difference - one atmosphere - limits this technique's application to simple moldings without strongly undercut portions.

Vacuum to a Positive Mold with Mechanical Pre-stretching

By comparison with combined processes using negative molds, sucking onto a positive mold has the advantage that the mold becomes the pre-stretching plug. Also, marks appear on only one surface.

Where vacuum provides insufficient force, compressed air may be used. In either case, the mold should be heated and suction or venting holes must be provided at its extreme points.

Plug and Ring Forming

Use this method for trays, sign faces, lighting fixture diffusers, or any part not deep-drawn for which mark-off at the inside corners is acceptable. The mold includes a forming plate, a clamping plate, and a male plug.

The molded part's outside contour conforms to the forming plate's opening. The clamping plate looks like the forming plate, but its opening is large. To provide a sheet's thickness clearance between the male and the female mold parts, the slightly tapered male plug is smaller than the forming plate's inside dimension.

The mold can be set up in an air cylinder press or, for small parts, in an arbor or drill press. Position the heated sheet on the forming plate and hold it using the clamping plate. Toggle clamps or C-clamps can be used to hold the forming and clamping plates together. The male plug is forced through the clamping and forming rings to a predetermined depth.

Vacuum Snap-Back Forming

Free-blowing and vacuum-forming produce natural "surface tension' shapes or bubbles which a forming plate can control only at the base. Vacuum snap-back forming employs a male plug attached to an air cylinder's ram. The plug is lowered into the vacuum-formed bubble. This method combines plug-and-ring and vacuum forming. A bubble is formed y drawing vacuum on a hot sheet. A male plug is positioned inside the bubble. Gradually, vacuum pressure is released and the hot sheet, because of its "elastic memory", snaps back and clings to the male plug.

The plug, frequently of hardwood, should include a 2° to 3° taper (draft) easing removal after cooling and contraction, as well as vent holes to avoid trapping air at the bottom surface. This method enables production of irregular shapes, with the forming plate's cutout and the male plug's shape controlling the contour.

Cooling

After forming, cool the part to below 140°- 160°F (60°- 70°C). Don't just cool the surface - the interior must cool too! Provide uniform cooling on all sides to prevent stress. Completely cover slow cooling thick-walled parts with felt or blankets to block drafts.

There's no rule of thumb for predicting sheet interior's cooling time. Factors include material thickness, ambient air temperature, and airflow to the part. Experience is the best teacher.

As it cools, the sheet shrinks to reverse heat-caused expansion. To prevent stress, allow it to move freely. Shrinkage on the mold can cause stress, so it is best to remove the part as soon as it achieves dimensional stability.

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