Thermoforming is a forgiving process in some ways and unforgiving in others. Engineers who come to it from injection molding or metal stamping often carry assumptions that don't transfer cleanly, and the early design review is where that shows up.
None of the mistakes below are fatal. Every one of them is fixable. The cost of fixing them depends almost entirely on when in the process they get caught. Catching a draft angle problem in the CAD model costs nothing. Catching it after first-article tooling is cut costs time and money that nobody had budgeted for.
These are the six issues PCI's engineering team encounters most consistently when reviewing designs from engineers who are new to thermoforming or converting from another process. Some of them are counterintuitive. All of them are worth knowing before the solid model goes to the quote request.
Injection molding commonly requires one to three degrees of draft on vertical walls to allow the part to eject from the steel mold. Engineers who know this rule often apply it to thermoforming designs automatically. The problem is that thermoforming typically needs more.
In thermoforming, the plastic sheet cools directly against the tool surface and can grip tenaciously on negative draft if the geometry isn't right. A general rule is to target three to five degrees of draft for female tools and somewhat less for male tools where the part releases away from the mold. Complex parts with tall, steep walls or fine ribs need careful attention. Three degrees often works; zero degrees often doesn't, and the failure mode is a part that tears or distorts during demolding.
The Fix
Review vertical wall angles at the DFM stage before tooling, not after. PCI's engineering team evaluates draft in the solid model review and can identify problem areas before the mold is machined. Undersize draft is almost always cheaper to address in CAD than in tool steel.
Thermoforming starts with a flat sheet of uniform thickness. The forming process stretches that sheet as it conforms to the mold. The more a section of the sheet has to stretch to reach a deep draw area, the thinner the wall becomes in that area. This is called material thinning, and it's a physical consequence of the process that doesn't have a close analog in injection molding or stamping.
A part with a nominal 0.180-inch starting sheet may have walls that thin to 0.120 inches or less in areas of high draw ratio, typically the corners of deep pockets, the bases of tall features, and any area where the sheet has to travel a significant distance from the clamp frame. Designing as if finished wall thickness will be uniform throughout a complex thermoformed part leads to structural calculations that don't match production reality.
The Fix
Share structural load requirements with the thermoformer early. An experienced team can model expected thinning patterns for a given geometry and starting gauge, identify areas of concern, and recommend tool modifications or starting material gauge adjustments that ensure finished wall thickness meets requirements in critical areas. Starting thicker is sometimes the answer; repositioning a deep feature or adding a radius is often a better one.
Undercuts are features that prevent a part from releasing from the mold in a straight pull direction, basically any protrusion, lip, or pocket that faces opposite the direction the tool opens. In injection molding, these are handled with side-action slides and lifters built into the closed mold. In thermoforming, the approach is different.
Simple undercuts in thermoforming can sometimes be handled by designing the part to flex slightly on release. More significant undercuts may require split tooling, where the mold is constructed in sections that move apart to release the part. Both approaches are workable, but they require design intent to be communicated clearly. An undercut that isn't flagged in the design review can lead to tooling that produces parts that won't release without damage, which is a problem nobody wants to solve after the tool is built.
The Fix
Mark any intentional undercut features in the solid model and include a note in the quote package. The thermoforming engineer can confirm whether the undercut is achievable as designed, suggest minor modifications that preserve the function, or propose a split-tool approach with a cost and lead time impact that the project can evaluate before committing.
Materials that thermoform well are not identical to materials that injection-mold well, and vice versa. Some materials that perform excellently in injection molding have narrow thermoforming processing windows, poor surface quality when formed, or significant draw ratio limitations. Engineers who build a material specification around injection molding data and then try to apply it to thermoforming can end up with a material that's technically correct but practically difficult to run.
ABS is a thermoforming workhorse with excellent formability, good impact resistance, and broad environmental resistance. Polycarbonate and PC/ABS blends form well and offer high impact strength. HDPE, HMWPE, and TPO all thermoform reliably in the right configurations. KYDEX is a preferred choice for medical and antimicrobial applications. Nylon, on the other hand, is harder to thermoform and typically not the first choice when it can be substituted. Acetal is generally not thermoformable. Knowing which materials behave which way in the forming process changes the specification conversation significantly.
The Fix
Bring the material requirement to the thermoformer as a functional specification, not a specific material callout, when possible. State the environmental exposure, structural load case, regulatory requirements, and cosmetic expectations. An experienced thermoformer can recommend materials that meet all of those requirements and are appropriate for the forming process, which may include grades or formulations the design team hadn't considered.
Sharp internal corners are stress concentrators in any plastic part. In thermoforming, they're also problem areas for material thinning because the plastic sheet has to conform to a tight radius while simultaneously being drawn down into the mold. The tighter the internal radius, the more material thinning and the higher the stress concentration in service.
A common rule of thumb is to specify internal radii of at least 25% of the adjacent wall thickness, and generous radii wherever the geometry permits. External corners need less attention since the material wraps around them with less stretching, but internal corners of deep features, pockets, and ribs deserve explicit thought at the design stage. This is one area where the thermoforming design rules are actually more conservative than injection molding, where gate pressure can fill tight corners more reliably.
The Fix
Build corner radii into the design intent rather than leaving them to the manufacturing interpretation. If the application has cosmetic or structural requirements that limit corner radius options, flag those constraints specifically during the design review so the engineering team can identify mitigation approaches early.
This one comes up in metal-to-plastic conversions more than anywhere else. An engineer sends over a legacy metal part, sometimes an actual physical sample, and the expectation is that thermoforming will replicate it dimension for dimension. The intent is understandable, but it misses an important opportunity and sometimes creates real problems.
Metal parts are designed around metal manufacturing constraints: bend radii, weld joint locations, fastener patterns, and structural geometries that make sense for steel but add unnecessary cost or complexity when translated directly to plastic. A direct copy of a metal guard in ABS may be technically achievable but won't take advantage of thermoforming's ability to integrate features, simplify assembly, eliminate secondary operations, or optimize for the material's actual properties.
Conversely, some metal features don't translate to thermoforming without modification. Wall sections that are structurally adequate in 12-gauge steel may not be adequate in a thermoformed thermoplastic at the same dimension. The structural logic of the original part needs to be re-evaluated for the new material, not assumed to be equivalent.
The Fix
Share the functional requirements alongside the existing part. The thermoforming engineer needs to understand what the part does, the loads it carries, the environment it operates in, and the assembly interfaces it connects to, not just what it looks like. PCI's team regularly converts metal and fiberglass parts and routinely identifies design improvements that reduce cost or improve performance during the conversion process. That value only gets captured if the conversation starts with requirements, not just geometry.
The earlier, the better. PCI's engineering team reviews solid models, prints, and existing parts at the quoting stage as a standard part of the process. A design review conversation before tooling is an investment of an hour or two that can prevent weeks of delay and tens of thousands of dollars in tool modifications later.
If you're designing a new part with thermoforming in mind, or evaluating a conversion from metal, fiberglass, or injection molding, the right time to engage a thermoformer's engineering team is during concept design, not after the model is complete. The earlier the process constraints are part of the design conversation, the more efficiently the final tool and part can be optimized for cost, performance, and lead time.
PCI accepts SolidWorks native files, STEP, IGES, and Parasolid formats for design review. Physical parts and existing tooling are also evaluated when available. The engineering review is included in the quoting process at no additional charge.
PCI's team will evaluate your solid model for thermoforming feasibility, flag any DFM concerns, and provide a comprehensive quote that covers prototyping and production. No charge for the review.