Acrylic sheet can be thermoformed using several types of equipment such as vacuum, pressure, or stretching equipment and a variety of heating methods including coiled nichrome wire, metal (cal) rod, hot air ovens, ceramic elements, and quartz tube (nichrome filament and tungsten filament). These heat sources derive average life spans ranging from 1, 500 to 20,000 hours at varied efficiency levels.

The following technical information is offered as a resource for thermoforming equipment and procedures. Users should undertake sufficient verification and testing to determine the suitability for their own particular purpose.

Heating methods

Vertical Ovens
Forced circulating-air ovens heat uniformly at consistent temperatures and are commercially available. These ovens consist of an outer and inner shell separated by a space containing fiber or rockwool insulation. Inside, the ovens have thermostatically
controlled heaters. Baffles and electric fans can also be used to ensure even heat distribution. Because of its relatively low molecular weight, acrylic sheet cannot be hung from a single edge. A clamping frame should be used for support and to facilitate sheet transfer to the forming station. Before heating with an air oven, be sure that you can reliably control temperature thermostatically within 10(F ( 5(C) between 340-350( F; air velocities across the sheet range between 200-1000 feet per minute (1-5 m/s); temperatures throughout the oven are uniform; and the clamping frame exerts constant uniform pressure on all sides as the sheet becomes soft.

Horizontal Heaters
Using horizontal infrared heaters (ceramic elements for instance) instead of hot air ovens are faster and less labor intensive. Horizontal units are also more flexible because either the heater or the tooling can be moved. Working clearances can often be improved by moving the heaters, and systems can be designed to handle custom made blanks.

When designing a horizontal heating system, independently controlled zones can be set up to improve control and flexibility.

Thermoforming Machines
Thermoforming machines are ideal when large volumes of the same shape are being produced. However, tooling costs are usually high.

Multiple shapes can be formed at the same time from a single sheet. Post machining will be needed to separate and finish the parts.

Vacuum and Pressure-Forming Equipment
Many types and sizes of forming equipment are commercially available. Trade publications dealing with plastics list many equipment manufacturers.

How to Make a Vacuum Chamber
An airtight vacuum chamber can be fabricated from welded steel plates. Steel should not be used for the forming plate since it may "chill" the hot sheet. A hardboard forming plate about 1/2 inch in thickness is recommended.

The detachable forming plate can be sealed to a flange on the top of the vacuum box using a gasket. The sheet blank is clamped in place over the forming plate using a clamping or holding ring and several toggle clamps.

The shape of the formed part's base will be determined by the shape of the cutout in the forming plate. Regulating the vacuum between the vacuum chamber and a vacuum storage tank will control the height or depth of the part. Required equipment: a high speed vacuum pump (10 cfm minimum), a vacuum storage tank, a 1" gate valve, a vent valve for releasing vacuum after the part is formed, and a vacuum gauge.

Free-Blowing Equipment
The required equipment for free blowing includes a plywood board with an air hose attached to its underside and a forming plate for controlling the piece's contour at its base. To evenly distribute incoming air, baffle the air intake with foam, felt, or cardboard. Cover the plywood board with flannel or polyurethane foam to prevent mark-off. The forming plate or ring should be made from approximately 1 1/2" thick hardboard. quick-acting toggle clamps may be used to attach the heated sheet and the ring to the plywood base.

Plug-and-Ring Forming Equipment
Rings or plates can be made from hardboard, plywood, or metal; plugs are usually made from hardwood; and the equipment should be coated with flocked rubber sheet to minimize mark-off.

For producing large volumes of parts, durable aluminum rings and plugs can be used.

Procedures

Be sure to follow manufacturers' safety recommendations for equipment and material as various acrylic sheet products react differently when heated to forming temperatures.

Shrinkage
Because of the orientation imparted during manufacture, acrylic sheet shrinks slightly when heated to thermoforming temperatures. Manufacturing direction can be determined from the sheet label or print on the masking. The lines of print are perpendicular to the direction of manufacture.

Original dimensions won't change in fabrication operations not requiring heat. However, sheet heated to thermoforming temperature changes dimensionally by about 3 percent maximum shrinkage in the manufacturing direction and approximately 0.5 percent maximum width increase (transverse direction).

Measure the shrinkage in a preliminary test, if acrylic sheet isn't held in a retaining frame. Then, determine the size of material required to compensate for shrinkage before cutting any blanks.

Predrying
Predrying acrylic sheet is rarely necessary. Keep the sheet wrapped until used. To prevent blistering, dry high-water-content sheet in a forced-circulation drying or vacuum oven before heating. Drying time depends on water content and material thickness. 24 hours at 176(F (80(C) dries most sheets.

To reduce the length of the forming cycle, pre-dry the sheet in a spare oven and transfer it directly into the forming oven at 176(F (80(C) after the drying period.

Heating
To avoid blistering or distortion, heat the sheet to the low end of the forming temperature range using convection, conduction, or radiation heating.

The standard procedure for a vacuum forming machine is to clamp the cold sheet in a frame and heat it by infrared radiation. Stresses may arise due to the sheet becoming hot while the clamped edges stay cold. Tearing, edge distortion, and asymmetrical shapes may also occur. To avoid these problems, heat the clamping frame to 140-180(F (60-82(C). Clamp the sheet and continue heating.

When using high-intensity quartz tube heating panels, a short soak cycle should be included after the heating cycle and before forming. This results in more even heat distribution across the sheet's thickness.

Forming Temperature
The forming range for acrylic sheet is 290-320(F (142-160(C). Even temperature distribution throughout the sheet's thickness is recommended. Before forming, the sheet's temperature must be higher than the desired temperature to allow for cooling that will occur prior to the start of forming.

Specification of adequate oven temperature makes for easy adjustment and control. In cases like infrared heating, where temperature specification is impossible, find the approximate temperature by using a pyrometer calibrated for plastics. Thermometer papers that show the sheet's surface temperature by a color change can also be used.

Temperature requirements depend on forming conditions - the degree of shaping (stretching) and the forming rate. To prevent pimples, blisters, shading changes, and other damage, avoid higher-than-necessary temperatures. A template or mold can mark overheated material.

Heating Time
Heating time depends on material thickness and heating method. Conditions during heating like air velocity in the oven, panel-to-heater distance, etc. also affect heating time. Final product and surroundings are other factors.

These variables are too numerous to predict mathematically. Minimum heating times should be determined by running test cycles. With some experience, cycles that result in evenly and thoroughly heated sheets can be developed.

Forming Rate
The maximum forming rate of a sheet is limited to the speed at which it will stretch without exceeding its strength and fracturing. The minimum forming rate must be fast enough to prevent the sheet from cooling appreciably.

A highly pigmented sheet should be formed slower than a colorless or transparent material.

Excessive fast forming rates will impart high stresses and cause low craze resistance. To minimize stresses, use moderate forming rates and ensure a uniform temperature distribution over the surface of the sheet and across its thickness.

Higher forming temperatures are needed to achieve greater "draws" or increased definition. For a "slow" forming operation it may be necessary to continue with infrared heating while the part is being formed.


Drape Forming
Here, acrylic sheet is heated and bent over a positive (male) or into a negative (female) mold. Female molds are best because they compensate for sheet shrinkage during cooling and sheet "memory." To deter mark-off, cover the molds with rubberized flocking or billiard table felt. For the same reason, set 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.