Handy hints and tips for forming plastics

COLD BENDING
HEAT BENDING
THERMOFORMING

BRAKE FORMING AND COLD BENDING
Brake forming and cold-bending operations can be used to make simple bends and curved areas with some plastics. Brake forming can be done on standard sheet metal brakes. Do not attempt to bend gauges over 3.0-mm, because stress levels are too high and failure can occur. Some plastics may be cold formed into circular shapes by observing the rule that the radius of curvature must be at least 100 times the material thickness. Example:2.0 mm gauge = 200 mm radius(400 mm diameter circle).

STRIP HEATING PROCEDURE:
1. Remove protective masking from area to be bent.
2. Regulate heat source to allow material to reach 110 – 150C
3. Place sheet over heat source at bend area.
4. Allow heat to soften material; time depends on gauge. 3.0 mm normally requires 2 minutes.
5. Remove sheet and make desired bend, and place in a jig to cool.
6. Allow bent part to cool in fixture.

HELPFUL HINTS:
Try to reproduce the suggested steps accurately from part to part. Avoid rooms with a draft, which can cause uneven heating and cooling. Be sure to cover forming fixtures with soft fabric to avoid scratching the material. Overheating can cause bubbles along bend area. Bending when it is too cold results in a highly stressed, weakened part. Thicker gauges (over 3.0 mm) may require heating on both sides by turning the sheet over periodically during the heating cycle. Always bend the sheet with heated side forming the outside radius.

STRIP HEATING TROUBLESHOOTING
PROBLEM: Bubbles in bent area
POSSIBLE CAUSE: too much heat.
Suggested Solution:
1. Reduce temperature.

PROBLEM: Warpage
POSSIBLE CAUSE: Part too wide for strip heating. Heating not uniform.
Suggested Solution:
1. Check for drafts.
2. Check heat source for uniformity.

PROBLEM: Warpage
POSSIBLE CAUSE: Cooling not uniform.
Suggested Solution:
1. Check for drafts.
2. Cooling fixtures may be removing too much heat in an area.

PROBLEM: Mark off
POSSIBLE CAUSE: Heater is contacting plastic.
Suggested Solution:
1. Increase gap between heater and sheet.

PROBLEM: Mark off
POSSIBLE CAUSE: Masking not removed in wide enough area.
Suggested Solution:
1. Remove masking further away from heated area.

THERMOFORMING
The most extensively used processes are vacuum forming, free blown forming and line bending. In general, most sheets normally do not require pre-drying. However, with some materials pre-drying is recommended for thicker gauges (2.0 mm or greater). Be careful not to exceed a pre-drying temperature of 60C. Heating cycles should be adequately controlled for uniform product quality.

FORMING EQUIPMENT
The thermoforming machine should be capable of generating and maintaining sufficient vacuum pressure throughout the thermoforming cycle. A minimum of 20 in. Hg. throughout the entire vacuum cycle is required to retain part integrity. Most commonly used vacuum forming machines with infrared heating elements perform well. Single-sided heating has proven effective for sheet in gauges up to 4.5 mm For thicknesses above 4.5 mm dual-sided heating ovens can be used for faster radiation penetration and quicker cycle times.

HEATERS
Infrared cal rod, coiled nichrome, or ceramic heating elements provide the best heating sources. Uniform heating of the sheet is critical.

Thermoforming, troubleshooting

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