Process of Thermoforming

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Thermoforming is a manufacturing process that heats plastic sheets and forms them over or into molds for cost-effective production of detailed parts. It requires less tooling investment than injection molding and suits various applications.

PROCESS OVERVIEW

Thermoforming involves heating an extruded plastic sheet to a pliable temperature, forming it against a mold using vacuum, pressure, or gravity, and then trimming excess material. Common types include vacuum forming, which uses vacuum to pull the sheet to the mold; pressure forming, applying air pressure alongside vacuum for sharper details; drape forming, relying on gravity; membrane press forming, a vacuum-assisted drape variant; and free forming, avoiding mold contact for optical clarity.sinotech+3

Draw Ratio

Draw ratio measures surface area of the formed part divided by the base sheet footprint, guiding initial sheet thickness selection to avoid thinning. Stretching during forming thins material more in deeper areas, so higher ratios demand thicker starting gauges—for example, a ratio of 2.62 with target 0.120" thickness requires at least 0.314" sheet.rayplastics+1

Sharp Angles

Sharp corners weaken parts due to stress concentration and thinning, but techniques mitigate this. Chamfering breaks a corner into milder angles; material choice matters, with ABS holding sharper radii than polycarbonate; pressure forming outperforms vacuum for precision; deeper parts need larger radii to compensate for stretch.thermoform+1

Technique Benefit
Chamfer Distributes stress via multiple milder corners [thermoform]​Material (e.g., ABS)Retains sharper corners than polycarbonate Pressure Forming Enables finer details than vacuum Larger Radii
Prevents excessive thinning in deep draws

Undercuts

Undercuts enhance strength, add snap fits, fastening points, and hide edges, often cheaper via pressure forming than injection molding. They typically need movable mold sections, though small ones may allow shrinkage release; include in draw ratio math, step back at corners, plan parting lines, and match process capabilities.thermoform+1

Draft Angles

Draft angles ease part release from molds. Standards are 1.5°-2° for female vertical walls, 4°-6° for male; zero or negative drafts need special tooling like tool tilting. For ribs, louvers, or vents, match depth to spacing to avoid thinning; add PVC components post-forming for strength or mounting.thermoform+1

Feature
TypeDraft AngleFemale vertical1.5°-2°
​Male vertical4°-6°
Zero/negativePossible with accommodations [thermoform]​

Tolerances (mm)

Dimensions measured from molded side using temperature-controlled tooling.

Forming:

• ±0.02 for first 12 mm, +0.001 per additional mm

• 0.06 mm surface profile

• ±2° angles

Trim:

• ±0.005 hole/slots up to Ø1 mm, +0.001 per 0.125 mm

• ±0.02 molded-to-trim first 12 mm, +0.001/additional

• ±0.015 edge-to-hole/slot

• ±0.010 hole patterns

• ±0.015 other features

• 0.125 mm min inside radii

Rib/Post Assembly: ±0.03 first 12 mm, +0.0015/additional.

Tolerances (Inches)

Equivalent imperial specs align closely with metric for global use.

Forming:

• ±0.5 for first 305 mm (~12"), +0.03 per additional 25 mm (~1")

• 1.53 mm (~0.06") surface profile

• ±2° angles

Trim:

• ±0.13 holes/slots up to Ø25 mm (~1"), +0.03/additional 3 mm (~0.125")

• ±0.5 molded-to-trim first 305 mm, +0.03/additional

• ±0.38 edge-to-hole/slot

• ±0.25 hole patterns

• ±0.38 other features

• 3.18 mm (~0.125") min inside radii

Rib/Post Assembly: ±0.75 first 305 mm, +0.038/additional.