Thermoforming tolerances are one part of an entire manufacturing operation that matters to the long-term health of both the finished product and the production program. Unlike machining or injection molding, thermoforming tolerance strategy must account for heated-sheet variation, material stretching, and other parts of the process.
Allied Plastics makes notes of thermoforming tolerances that OEMs should expect in scalable programs.
Why Dimensions in Thermoforming Operations Move
Whether it’s your first thermoforming operation or merely your first of the quarter, it’s important to note the process. A thermoformed part starts out as a flat sheet, which is heated and then stretched over or into a tool. It’s then cooled, released, trimmed, and can feature secondary fabrication.
During every one of these steps, dimensions – and thermoforming tolerances – can shift. The catalysts of tolerance for any thermoforming program are sheet thickness and gauge variation, draw depth and geometry, shrinkage and thermal recovery, tool construction and process, and if part of the program, secondary operations.
Starting with sheet thickness, it more so relates to material thickness as that varies depending on the selection. Thickness reduction is not a uniform metric after forming. Deep draws, vertical wall, sharp corners, abrupt section changes, and localized details cause uneven material distribution which impacts thermoforming tolerances.
As the material cools, it also contracts, but the amount varies depending again on material selection as well as orientation, forming temperature, and cooling rate. Selecting materials that are both manufacturable and fit the product’s intended application matters most here. And with consideration to tool construction, whether it’s vacuum forming, pressure forming, or twin-sheet forming, each of them provide different levels of definition and repeatability. To better influence consistency throughout a program, control venting, clamp-frame, cooling, and automation.
Design for Scale
Addressing thermoforming tolerances starts with design considerations and it is important to remember to not place a tolerance every dimension at a precision level. Rather, use categories for placing tolerances.
Critical to function dimensions is a strong starting point for defining tolerance in a part. This includes seal lands, mating interfaces, mounting hole patterns, latch engagement, display openings, fluid paths, electrical clearances, and safety-related features.
Next would be critical to assembly dimensions. This merely means features that locate the thermoformed part to metal brackets, molded subcomponents, chassis rails, or another feature.
From there, consider the dimensions and tolerances of appearance. Such as visible gaps, flushness, surface waviness, and symmetry. And lastly, reference dimensions including envelope sizes, cosmetic edge locations, and non-mating profile.
Considering Material Flow
A separate consideration in the design process where thermoforming tolerances are considered is material flow. Thermoforming rewards gradual changes in geometry rather than sudden ones.
Incorporate usable draft so parts release predictably and consistently, include generous radii at corner and wall to floor transitions, minimize abrupt depth changes, avoid placing tight tolerance interfaces in the deepest or farthest stretched areas, an specify a minimum finished wall requirement in high-load, fastening, sealing, impact, or insert-bearing areas.
These considerations will help achieve more consistent tolerances throughout.
Qualification at Production
Prototyping is a strong way to qualify multiple parts of a manufacturing program, but thermoforming tolerances is not one. Between the materials used, hand trimming, and low-volume processing, prototyping doesn’t present an accurate tolerance measurement.
A more comprehensive and accurate way to qualify tolerance starts with freezing material specifications. This includes naming the resin family and grade, sheet thickness, color, texture, and any approved alternates.
From there, run production-intent tooling that validates on the actual tool while defining inspection datums and fixturing. Thermoformed components can flex under their own weight, so it’s critical to measure it with an agreed and repeatable restraint condition that represents assembly.
Another vital inspection is across the sheet and the part itself. Measure beyond the perimeter dimensions and also include deep-draw regions, wall thickness at critical locations, hole to hole locations, flatness, and cosmetic areas.
And lastly, control change management. This should require notification and requalification rules for resin supplier changes, sheet extrusion changes, tool repairs, trim-program revisions, processing window changes, or relocation of production if necessary.
What Drives Costs
Achieving tight thermoforming tolerances means approaching a few different areas of the entire manufacturing process carefully. Through a combination of refined tooling, controlled processing, dynamic fixturing, restrictive processing windows, and more inspection, desire thermoforming tolerances become more achievable.
When there’s more attention to detail, however, it can lead to increased costs for the OEM when partnering with a contract manufacturer. Those costs appear in some of the following areas.
- Tight tolerances over a large footprint
- Precision relationships between deep formed features
- Complex 3D profiles that lack acceptable draft
- High cosmetic standards
- Tight hole to hole positions
- Dimensional control across bonded assemblies
- Tight specs without a clear measurement method or datum scheme
OEMs: Experiencing Difficulties Achieving Desired Thermoforming Tolerances? Partner with Allied Plastics
Allied Plastics, located in Twin Lakes, Wisconsin, is one of the custom thermoforming manufacturers that OEMs, supplier programs, and other customer bases rely on. Since 1995, we’ve created the custom, heavy-gauge thermoforming solutions that numerous industries use daily.
We specialize in heavy-gauge thermoforming solutions with vacuum forming, twin sheeting and pressure forming custom parts in sizes up to 9 ft x 14 ft x 60” depth of draw. Starting sheet thicknesses can range from .060” – .550”. Our team has experience in vacuum forming a wide variety of materials, such as HMWPE, HDPE, TPO, ABS, PP, and PS.
Our engineering capabilities allow us to help your team reach the thermoforming tolerances necessary with thorough DFM reviews, prototyping, mold design, material selection, and much more.
Contact Allied Plastics today for more information on how we can help you reach the thermoforming tolerances your team needs and let’s create something special together.