Two Ways to Get Plastic Onto a Mold — With Very Different Results
Drape forming and vacuum forming both produce plastic parts by heating a thermoplastic sheet and pressing it against a mold. The difference is in what does the pressing. In drape forming, gravity and the weight of the heated plastic sheet do the work. In vacuum forming, atmospheric pressure — roughly 14.7 psi — is applied to the sheet by evacuating air from the forming cavity.
That difference in forming force produces different results in mold detail reproduction, wall thickness distribution, achievable draw depth, and the range of applications each process can serve. Understanding where drape forming is appropriate and where it falls short clarifies both the decision to use vacuum forming and the machine requirements that follow from it.
The BV C-Class series is commonly used for drape-form-equivalent applications — manual sheet-fed equipment where the primary forming action is gravity-assisted mold contact, supplemented by vacuum where required.
What Is Drape Forming?
Drape forming — sometimes called free forming or gravity forming — heats a thermoplastic sheet to forming temperature and then lowers it over a male mold. The heated sheet, softened by temperature and responding to gravity, conforms to the mold surface under its own weight. No vacuum, no pressure differential, no mechanical assistance beyond the platen bringing the sheet into contact with the mold.
The process is simple and requires minimal equipment: a heater capable of bringing the sheet to forming temperature, a means of supporting the sheet during heating, and a mold for the sheet to drape over. It can be performed on basic forming equipment or even on improvised apparatus for prototype and low-volume applications.
The limitation is forming force. Gravity produces a forming pressure proportional to the weight of the heated sheet — typically 0.003 to 0.01 psi for most production sheet thicknesses. Against the roughly 14.7 psi available from atmospheric pressure in vacuum forming, this is three orders of magnitude less. Drape forming cannot reproduce fine mold detail, cannot achieve deep draws, and cannot force material into sharp corners or complex geometry.
What Can Drape Forming Achieve?
Drape forming produces acceptable results in a specific and well-defined range of applications:
- Simple convex geometry on large format parts: A gently curved panel, a dome shape, or a broadly radiused surface. The heated sheet conforms to convex geometry under gravity without requiring pressure to force it into concave detail.
- Thick material over male molds with generous radii: Heavy gauge sheet that sags significantly at forming temperature can drape convincingly over large male forms. Acrylic skylights and bath surrounds have historically been produced by drape forming on male molds.
- Applications where one surface is non-cosmetic: The mold-contact surface conforms to the mold geometry (within gravity’s limits). The opposite surface is entirely free and uncontrolled — making drape forming unsuitable for applications requiring cosmetic finish on the non-mold face.
- Prototype and low-volume production where equipment investment is not justified: For a single prototype or a handful of units, the simplicity of drape forming avoids the capital and setup requirements of vacuum equipment.
Where Does Drape Forming Fall Short?
The applications where drape forming fails are defined by its forming force limitation:
- Concave mold geometry (female molds): Gravity cannot press the sheet into a cavity. Drape forming is inherently limited to convex (male) geometry, or at best shallow bowls where sagging under gravity partially conforms the sheet to gentle concave forms.
- Fine mold detail: Texture, lettering, sharp radii, and crisp edges require the forming pressure of vacuum to force the sheet against the mold surface tightly enough to reproduce them. Drape-formed parts have softened edges and reduced texture reproduction relative to equivalent vacuum-formed parts on the same mold.
- Deep draw geometry: A sheet draped over a male mold at draw ratio above approximately 0.3:1 begins thinning dramatically at the deepest sections. Gravity cannot provide the sustained forming force required to draw material into deep geometry uniformly.
- Tight dimensional tolerances: Because forming pressure is low and variable (depending on sheet weight, temperature distribution, and mold support geometry), drape-formed parts show more dimensional variation cycle to cycle than vacuum-formed equivalents.
- Production volume: Drape forming cycle times are longer than vacuum forming because the limited forming force slows conformity to the mold. At any meaningful production volume, the throughput difference favors vacuum forming.
How Does Vacuum Forming Address Drape Forming Limitations?
Vacuum forming adds atmospheric pressure to the forming event — evacuating the space between the sheet and the mold so that 14.7 psi of atmospheric force presses the sheet against the mold surface. This pressure:
- Forces the sheet into concave geometry that gravity cannot reach
- Reproduces mold surface detail with high fidelity — texture, radii, lettering
- Enables draw ratios up to 1:1 and beyond with plug assist tooling
- Provides consistent and repeatable forming force across every cycle, independent of sheet weight
The machine that delivers this capability is a vacuum forming machine — not a drape forming table. The practical difference is a vacuum system capable of evacuating the forming cavity to the residual pressures required for complete sheet conformity. For vacuum system sizing requirements, see Vacuum Pump Sizing for Thermoforming Equipment.
When Is Each Process Actually Used?
| Factor | Drape Forming | Vacuum Forming |
|---|---|---|
| Mold type | Male (convex) only | Male or female |
| Mold detail reproduction | Low — soft edges, reduced texture | High — sharp radii, texture, fine detail |
| Maximum draw ratio | Approximately 0.3:1 | 1:1+ standard; 2:1+ with plug assist |
| Dimensional consistency | Low — variable forming force | High — consistent atmospheric pressure |
| Equipment required | Heater + platen + mold | Heater + platen + mold + vacuum system |
| Part geometry range | Simple convex | Moderate to complex |
| Production volume | Prototype to low volume | Low to high volume |
| Surface finish control | One surface only (mold contact) | One surface controlled; pressure forming controls both |
| Material gauge range | Any gauge — thick preferred | Any gauge |
| Capital cost | Low | Moderate to high |
The practical crossover: when part geometry is sufficiently complex, detail reproduction is required, or draw depth exceeds what gravity can produce, vacuum forming is the correct process. Drape forming remains appropriate for simple prototype work, very large shallow geometry where vacuum tooling cost is disproportionate to volume, and applications where the non-mold surface is not cosmetically significant.
What Is the Relationship Between Drape Forming and Machine Selection?
A vacuum forming machine can perform both drape forming (by heating and applying the sheet to the mold without activating vacuum) and standard vacuum forming (by adding vacuum to the forming event). The machine does not need to choose — the forming event parameters determine which process occurs.
This flexibility makes vacuum forming equipment the more versatile investment even for operations that currently drape form. Adding vacuum capability to a drape forming operation that will grow in volume or part complexity costs less than replacing drape forming tooling and process infrastructure with vacuum forming equipment later.
For operations evaluating their first plastic forming equipment, see How to Choose a Vacuum Forming Machine for the full selection framework. The pressure forming page covers the third option in the forming pressure spectrum — pressure forming, which adds positive pressure above atmospheric to achieve even higher forming forces than standard vacuum forming.
The Plastics Industry Association publishes industry standards and technical resources covering thermoforming process selection criteria that complement machine-specific guidance.
Belovac: Vacuum Forming Equipment for the Full Application Range
Belovac manufactures vacuum forming machines for the range of applications that extend beyond drape forming capability — complex geometry, production volume, fine surface detail, and deep draw parts that require consistent atmospheric forming pressure.
Contact Belovac to discuss whether your current drape forming application would benefit from vacuum forming capability and what machine configuration serves the geometry and production volume you are running. Request a quote to begin the conversation.
