Thermoformed Plastics Design Requirements: Specifications and Metrics

 Designing thermoformed plastic components requires careful consideration of geometric design, material properties, and thermal conditions. Following these specifications helps ensure that parts can be manufactured efficiently while delivering the required performance and durability.

I. Geometric Design for Thermoforming (DFT)

Design Element	Specification/Metric	Constraint & Rationale
Internal Corner Radii	Minimum radius: 1.5 mm.	Larger radii, such as a minimum of 0.125″ for structural components, help reduce stress concentrations and minimize material thinning.
Tool/Material Radius Ratio	The minimum radius should be equal to or greater than the original material thickness.	A radius smaller than the starting material thickness can make the forming process difficult or even impossible.
General Draft Angles	Generally recommended: >2°.	Adequate draft allows the finished part to be removed from the mold easily and helps prevent surface defects.
Mold Draft Angles	Negative molds: 1.5°–2° standard. Positive molds: 4°–6° recommended.	Appropriate draft angles support clean tool release and improve manufacturing consistency.
Rib Draft Angle	Minimum draft: 3°.	A sufficient draft angle is necessary for easy part removal and consistent material distribution.
Rib Base Radii	Minimum: 25% of the material thickness.	For example, a 0.250″ gauge material should have a minimum rib base radius of 0.0625″.
Negative Forming Ratio	Depth-to-width ratio should not exceed 1.5:1.	Ratios above this level can cause substantial material thinning and increase the possibility of rupture near the bottom edges.
Wall Thickness	The original sheet thickness represents the maximum wall dimension.	Areas subjected to the deepest draw will naturally become the thinnest sections of the part.
Typical Tolerance	General formed features: ±0.060″.	Tighter tolerances, such as ±0.010″, generally require additional and more expensive manufacturing operations.
Polycarbonate (PC) Shrinkage	Mold shrinkage: 0.005–0.007″ per inch (0.13–0.18 mm).	For 1.80–2.30 mm PC sheet, shrinkage in the machine direction (MD) is typically 6–7%, while transverse direction (TD) shrinkage is approximately 0.5%.

For more detailed information on thermoformed plastic design requirements, specifications, and important metrics, see the Om Raj Tech guide on thermoformed plastics design requirements.

II. Processing and Temperature Specifications — Polycarbonate Example

Process Step	Temperature/Time Specification	Critical Requirement
Pre-Drying of PC Sheet	250°F (121°C) in an air-circulating oven.	Drying must be completed before thermoforming to prevent moisture from vaporizing during heating, which can create air bubbles or voids.
Drying Time — PC Example	A 0.236″ (6 mm) gauge sheet requires 24 hours at 250°F (121°C).	Sheets stacked together without sufficient air spacing will not dry properly.
PC Softening Point	Glass transition temperature: 298°F (148°C).	Softening begins rapidly once the temperature rises above 311°F (155°C).
PC Sheet Forming Temperature	Target range: 340°F–415°F (171°C–213°C). Optimum range: 350°F–375°F (177°C–191°C).	Polycarbonate has a comparatively narrow temperature range for effective forming.
PC Mold Temperature	Recommended range: 210°F–250°F (99°C–121°C).	A heated mold promotes better shaping, more controlled cooling, and lower levels of induced stress.

III. Material and Compliance Requirements

Selecting the correct material is essential for thermoformed parts, particularly when components will be exposed to UV radiation, heat, chemicals, fire, or mechanical stress.

Requirement	Specification/Metric	Relevant Materials & Example
UV Resistance	Required for exterior components such as consoles and housings.	ASA provides excellent UV stability. ABS is highly sensitive to UV exposure and requires a UV-resistant cap layer, such as ASA, for outdoor applications.
Low Heat Tolerance	An inadequate heat distortion temperature (HDT) can result in permanent deformation.	PVC/Acrylic blends have a relatively low heat distortion point of approximately 71°C (160°F). HDPE typically has a melting point of 120°C–135°C.
Flammability Rating	Minimum requirement for enclosed interiors and electrical housings: UL 94 V-0.	Under V-0, burning must stop within 10 seconds on a vertical specimen, with no flaming drips. V-2, in comparison, specifically allows flaming drips.
Fire Testing	The component is exposed to a flame for 2½ minutes.	During testing, the temperature within one inch of the component must reach at least 648°C.
Environmental Stress Crack Resistance (ESCR)	Must be specified for structural polyolefins such as HDPE when used near chemicals.	Standard HDPE grades can be vulnerable to brittle failure when they are under stress and exposed to surface-active substances such as fuels and cleaners.
Chemical Encasing — Tanks	Cellular plastic must not change volume by more than 5% or dissolve after 24 hours at 29°C in reference liquids.	Non-polyurethane encasing plastic must provide a minimum compressive strength of 60 pounds per square inch at 10% deflection.
Thermal Expansion	The thermal expansion rate of PC is approximately four times higher than that of metal.	When PC components are fastened to metal, slotted holes and controlled torque are required to prevent failures caused by thermal strain.

Conclusion

Successful thermoforming depends on maintaining the right balance between part geometry, material selection, forming temperatures, mold conditions, tolerances, and compliance requirements. Proper attention to radii, draft angles, forming ratios, wall thickness, drying, temperature control, UV resistance, fire ratings, chemical resistance, and thermal expansion can improve manufacturability and long-term part performance.

For additional guidance on thermoformed plastic design requirements, specifications, and metrics, refer to the Om Raj Tech resource linked above.