Thermoformed Plastics Design Requirements: Specifications and Metrics

 Designing thermoformed plastic parts requires careful attention to geometric, material, and thermal specifications to ensure proper manufacturability and reliable performance.

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For more information about thermoformed plastics design requirements, specifications, and important design metrics, visit the complete OmRajTech guide:


Thermoformed Plastics Design Requirements: Specifications and Metrics:

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Geometric Design for Thermoforming (DFT)

Design ElementSpecification/MetricConstraint & Rationale
Radii (Internal Corners)Minimum radius: 1.5 mm.Larger radii (e.g., 0.125″ minimum for structural parts) are important for minimizing stress concentrations and material thinning.
Tool/Material Radius RatioRadius minimum should be equal to or greater than the initial material thickness.When the radius is smaller than the starting thickness, forming becomes difficult or impossible.
Draft Angles (General)Generally recommended: >2°.Helps ensure easy demolding without causing surface defects.
Draft Angles (Molds)Negative molds: 1.5° – 2° standard. Positive molds: 4° – 6° recommended.Supports clean tool release and improves reproducibility.
Rib Draft AngleMinimum draft: 3°.Necessary for easy part removal and uniform material distribution.
Rib Base RadiiMinimum: 25% of the material’s thickness.Example: For a 0.250″ gauge, apply a minimum base radius of 0.0625″.
Forming Ratio (Negative)Depth-to-Width Ratio should not exceed 1.5:1.Higher ratios can cause significant thinning, increasing the possibility of rupture at the bottom edges.
Wall ThicknessThe original sheet thickness is the maximum wall dimension.The deepest drawn areas will naturally have the thinnest material.
Tolerance (Typical)General formed features: +/- 0.060″.Tighter tolerances (e.g., +/- 0.010″) require additional and more costly operations.
Shrinkage (PC)Polycarbonate mold shrinkage: 0.005 – 0.007″ per inch (0.13-0.18 mm).Shrinkage in the extrusion direction (MD) for 1.80-2.30 mm sheet is typically 6-7%, while transverse direction (TD) shrinkage is 0.5%.

Processing and Temperature Specifications (Polycarbonate Example)

Process StepTemperature/Time SpecificationCritical Requirement
Pre-Drying (PC Sheet)250°F (121°C) in an air circulating oven.Must be completed before thermoforming to prevent moisture vaporization (air bubbles/voids).
Drying Time (PC Example)0.236″ (6mm) gauge sheet requires 24 hours at 250°F (121°C).Sheets stacked without air spacing will not dry.
PC Softening PointGlass transition temperature is 298°F (148°C).Softening begins rapidly above 311°F (155°C).
Sheet Forming Temp (PC)Target range: 340°F to 415°F (171°C−213°C) Optimum: 350°F−375°F (177°C−191°C)Polycarbonate has a relatively narrow forming temperature range.
Mold Temperature (PC)Recommended mold temperature range: 210°F−250°F (99°C−121°C)A heated mold provides better shaping, more gradual cooling, and reduced induced stress.

III. Material and Compliance Requirements

RequirementSpecification/MetricRelevant Materials & Example
UV ResistanceRequired for exterior parts (e.g., consoles, housings).ASA provides excellent UV stability. ABS is highly UV sensitive and requires a UV cap layer (e.g., ASA) for outdoor service.
Heat Tolerance (Low)Insufficient HDT leads to permanent deformation.PVC/Acrylic blends have a low heat distortion point, around 71°C (160°F). HDPE melting point is typically 120°C to 135°C.
Flammability RatingMinimum for enclosed interiors/electrical housings: UL 94 V-0.V-0 requires burning to stop within 10 seconds on a vertical specimen, with no flaming drips. V-2 explicitly permits flaming drips.
Fire TestingComponent subjected to flame for 2 1/2 minutes.Temperature within one inch of the component must reach at least 648°C during the test.
Environmental Stress Crack Resistance (ESCR)Must be specified for structural polyolefins (HDPE) near chemicals.HDPE standard grades are susceptible to brittle failure when stressed and exposed to surface-active agents (fuels, 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 have compressive strength of at least 60 pounds per square inch at 10% deflection.
Thermal Expansion





PC thermal expansion rate is approximately four times higher than metal.


For fastening to metal, slotted holes and controlled torque are mandatory to prevent thermal strain failure.










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