Forming Makrolon ® GP Polycarbonate sheets.

Brake Bending
MAKROLON® Polycarbonate Sheet can be brake bent to 90° angles in gauges = .100?. Attempts at bending heavier gauges can result in cracks or breakage at the time of bending or later due to high levels of stress in the bend area. For this reason strip heat bending is recommended on all gauges = .118?.

Cold Bending
MAKROLON® GP Polycarbonate Sheets may be cold-bent with the minimum radius based upon the sheet thickness. As a guideline, the material can be radiused to 100 times the thickness: (R=T x100).

Cold Bend Radii

Sheet Thickness (T)

Minimum Radius (R)

1/8”

12.5”

3/16”

18.7”

1/4"

25.0”

Drying Time Hours

Thickness

250° F

180° F

.093”

4

8

.118”

6

14

.150”

8

20

.177”

12

30

.236”

24

50

Thermoforming
MAKROLON Polycarbonate Sheet can be thermoformed on standard equipment. Vacuum forming, free blown forming, and line bending are the most extensively used processes.
While most standard forming techniques can be used, critical process modifications specific to polycarbonate are necessary to ensure uniform and repeatable formed parts.

Predrying MAKROLON® Polycarbonate Sheet

MAKROLON Polycarbonate Sheet must be dried before thermoforming because polycarbonate absorbs moisture at a high rate. Trapped moisture forms vapor above 250°F, and the vapor expansion creates bubbles in the sheet. Sheets of MAKROLON Polycarbonate sheet should be

placed in a dehumidifying air circulating oven for predrying. Temperature should be 250°F and monitored with controls. Recommended predrying time periods are shown below. Be cautious. Polycarbonate sheet begins absorbing moisture immediately upon removal from the predrying

oven. The rate of absorption is dependent upon the ambient dew point. For this reason, it is crucial to transfer the sheet directly to the forming machine as quickly as possible.

 

Forming Equipment

The thermoforming machine should be capable of generating and maintaining a vacuum of 20 in. Hg pressure throughout the thermoforming cycle. Vacuum forming machines with infrared heating elements perform well for MAKROLON Polycarbonate Sheet forming. Rotary and shuttle designs with automatic or semi-automatic controls are most suitable. Key features of this type for equipment are timer control accuracy, uniform heating sources, and sufficient vacuum power. Single-sided heating has proven effective for MAKROLON Polycarbonate Sheet in gauges up to .177?. For thicknesses above .177?, it is recommended that dual-sided heating ovens be used for

effective radiation penetration.

 

Heaters

Infrared cal rod, coiled nichrome or ceramic heating elements provide the best heating sources. Gas-fired heaters or convection ovens are not normally used with polycarbonate. Uniform heating of the sheet is critical. Radiation absorption graphs for polycarbonate are available. These graphs help match the emissivity of the heating element with the sheet for the most efficient heat penetration.

 

Heating Cycle

Heating MAKROLON® Polycarbonate Sheet for vacuum forming requires heat penetration to achieve a 350°F to 360°F. The heat required will be higher for more complex shapes. When MAKROLON Polycarbonate Sheet reaches forming temperature, uniform “sag” occurs. The amount of sag depends on the size and thickness of the sheet. A 12? x 12? x .060? sheet will sag approximately 1?. A 36? x 36? x .177? sheet may sag 4?- 6? at the center. Once uniform temperature has been achieved, timers can accurately reproduce the condition, and part-to-part

consistency can be maintained.

Procedure:

- Sheet thicknesses up to .177? gauge can be heated from one side. Above .177? gauge, two-sided heating is normally required to significantly enhance productivity.

-Heat source is removed and heated sheet is forced over or into mold where vacuum is applied.

Helpful Hints:

- MAKROLON Polycarbonate Sheet “sets up” very quickly compared to other thermoplastics and can be removed from the mold in a short period of time.

Caution:

MAKROLON Polycarbonate Sheet remains quite hot during this cycle and care must be observed when handling finished parts.

- Throughout the vacuum forming process, it is imperative that dust and dirt be controlled. MAKROLON Polycarbonate Sheet has a static charge that attracts foreign particles which can create surface imperfections. Molds also attract dust particles and should be cleaned to avoid creating surface defects.

Shading and Screening

Shading is often used to balance out hot spots in an oven for uniform temperature. Shading may also be used to control the sag of MAKROLON Polycarbonate Sheet during heating.

Procedure:

- Use heavy-duty metal screening to shade the major portion of the clamped sheet, leaving several inches along the edges unshaded to compensate for cooler areas.

- Screens can be installed permanently or placed loosely above the sheet, depending on how much shading is required.

Helpful Hints:

- Use slow heating. This is particularly important with heavier gauges in order to prevent gradient heating.

- Allow heat to reach uniformity at the center of the sheet.

- The heating rate may be reduced by lowering the heat intensity or by moving the sheet farther away from the heaters.

Thermoforming Troubleshooting

Problems

Possible Causes

Suggested Solutions

Bubbles or blisters

Too much moisture in sheet

- Predry sheet for longer time period

- Check predry oven temperature

Pinholes or surface marks

Vacuum holes too large

- Use smaller diameter vacuum holes

Mark off

Mold surface too smooth

 

Mold surface too rough

- Vapor hone or use extra fine emery cloth to lightly abrade mold surface

-Preheat mold

Sheet pulls out of damping

frame during forming

Sheet too cold to form

- Heat sheet for longer time period

Non-uniform sag

Uneven heating

- Check heaters

- Screen “hot” areas

Incomplete part formation detail

Sheet too cold -

Inadequate vacuum

- Heat sheet for longer time period

- Check vacuum system for leakage

- Increase vacuum

Webbing

Sheet too hot

Improper mold spacing

- Reduce heating cycle

- Redesign mold

- Use mechanical assist/bridging

Article Title:
Forming Makrolon ® GP Polycarbonate sheets.
Article Summary:
Bending and Thermoforming information
Article Date:
6/5/2008
Article ID:
446
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