What is draw ratio and why does it matter?
Draw ratio is the ratio of part depth to the smallest plan dimension (width or length). It is the single most important parameter for predicting wall thickness distribution in a thermoformed part.
// Draw ratio formula
draw_ratio = part_depth / smallest_plan_dimension
// Example: 200mm deep × 500mm wide × 800mm long
draw_ratio = 200 / 500 = 0.4:1
As draw ratio increases, the material must stretch more to fill the mould. This stretching is not uniform — corners and the base of deep features thin disproportionately compared to sidewalls. Understanding this distribution is essential for selecting the right starting sheet thickness.
Wall thickness distribution by draw ratio
The table below shows typical wall thickness as a percentage of original sheet thickness at different locations in the part, for a rectangular part without plug assist:
| Draw Ratio | Flange / Top | Upper Sidewall | Lower Sidewall | Corner | Base Centre |
|---|---|---|---|---|---|
| 0.25:1 | 100% | 85% | 75% | 65% | 70% |
| 0.5:1 | 100% | 75% | 60% | 45% | 55% |
| 0.75:1 | 100% | 65% | 50% | 35% | 45% |
| 1.0:1 | 100% | 55% | 40% | 28% | 38% |
| 1.5:1 | 100% | 45% | 30% | 18% | 28% |
Values are approximate for rectangular parts without plug assist. Actual values depend on material, mould geometry, and forming temperature.
How to select starting sheet thickness
Work backwards from your minimum required wall thickness at the thinnest point (usually the corner):
Sheet thickness formula
required_sheet_thickness = minimum_wall_thickness / thinning_factor
Example: minimum corner wall = 2mm, draw ratio = 0.5:1, corner thinning = 45%
sheet_thickness = 2mm / 0.45 = 4.4mm → use 5mm sheet
Always round up to the next available sheet gauge. Add a 10–15% safety margin for process variation. If the required sheet thickness becomes impractically large (e.g., >8mm for a 1:1 draw ratio), consider plug assist or redesigning the part to reduce draw ratio.
When to use plug assist
Plug assist is a mechanical pre-stretching device that pushes the heated sheet partially into the mould cavity before vacuum is applied. It redistributes material from the flange area (which would otherwise remain thick) into the sidewalls and base (which would otherwise thin excessively).
| Draw Ratio | Without Plug Assist | With Plug Assist | Recommendation |
|---|---|---|---|
| < 0.5:1 | Acceptable uniformity | Excellent uniformity | Optional — only if very tight thickness tolerance needed |
| 0.5–0.7:1 | Moderate thinning at corners | Good uniformity | Recommended for structural parts |
| 0.7–1.0:1 | Significant corner thinning (30–45%) | Acceptable uniformity | Required for most applications |
| > 1.0:1 | Severe thinning — part likely fails | Moderate thinning — may need multiple plugs | Required — consider redesign if > 1.5:1 |
Machinecraft's PF1-X and PF1-C series machines include servo-driven plug assist as standard. The ATF series is specifically designed for deep-draw applications with draw ratios up to 1.5:1.
Frequently asked questions
What is draw ratio in thermoforming?
Draw ratio in thermoforming is the ratio of part depth to the smallest plan dimension (width or length). A 200mm deep part with a 400mm base width has a draw ratio of 200/400 = 0.5:1. Draw ratios above 0.5:1 cause significant material thinning at corners and the base. Ratios above 0.7:1 typically require plug assist to maintain acceptable wall thickness.
How much does wall thickness decrease in thermoforming?
Wall thickness reduction depends on draw ratio and part geometry. For a 0.5:1 draw ratio, sidewall thickness is typically 60–75% of the original sheet thickness. At corners, thickness can drop to 40–50%. For a 1:1 draw ratio without plug assist, corner thickness can fall below 30% of original sheet thickness. Starting with a thicker sheet or using plug assist compensates for this thinning.
When is plug assist required in thermoforming?
Plug assist is required when the draw ratio exceeds approximately 0.7:1 and uniform wall thickness is important. The plug pre-stretches the heated sheet into the mould cavity before vacuum is applied, distributing material more evenly and preventing excessive thinning at the base and corners. Without plug assist at high draw ratios, base and corner walls can be 50–70% thinner than the sidewalls.