Picking the Correct Thermoforming Plastic by Formability

Most thermoforming resins will make a usable part, but they are nowhere near equal when the goal is accurate, low warp, repeatable components. The differences come down to polymer morphology, shrinkage behavior, melt strength and how wide a processing window each material gives you. Amorphous sheets like ABS, HIPS and PETG are predictable and forgiving, while semi crystalline resins like PP and HDPE fight you with directional shrink and narrow temperature tolerance. Materials like PC, PVC and KYDEX T sit in the middle: they can deliver excellent performance, but only with tighter process control. Understanding where each plastic realistically sits on the formability spectrum helps choose the right sheet before investing in tooling. RapidMade+2RapidMade+2

Comparison chart

Typical values, heavy gauge sheet context, and general vacuum or pressure forming. Numbers are approximate and grade dependent, but they show relative behavior.

*Formability here means: how easily the sheet can be heated to a useful forming window, formed without defects, and then hold size and shape without excessive warp or unpredictable shrink.

What “formability” really means in thermoforming

For this comparison, “formability” is not just how nicely the sheet sags over a mold. It is the combined effect of:

1. Processing window
   How wide the useful forming temperature range is and how tolerant the material is to minor oven variations. Amorphous sheets (ABS, HIPS, PETG, PVC, KYDEX, PC) generally soften over a broad glass transition region and are easier to thermoform than semi crystalline sheets like PP and HDPE which have a tighter usable band. RapidMade+2Curbell Plastics+2

2. Shrinkage and warp
   Amorphous sheets tend to shrink less than 1 percent and mostly isotropically, so parts stay close to the tool. Semi crystalline sheets typically shrink around 1.5 to 3 percent with strong directionality, which means more oversize in CAD and more warp risk. RapidMade+2Covestro AG+2

3. Detail and draw capability
   Materials with adequate melt strength and a decent rubbery plateau capture details without webbing or tearing. PETG and KYDEX are particularly good at deep draws because they combine good flow with enough strength in the melt. Curbell Plastics+2Emco Industrial Plastics+2

4. Process sensitivity
   Drying requirements, stress crazing, environmental stress cracking and sensitivity to cooling rate all matter. PC and semi crystalline polyolefins are more demanding than ABS, HIPS or PETG on this front. RapidMade+3Curbell Plastics+3POLYVANTIS | EN+3

The ranking is built from these criteria, not just “does it vacuum form at all”.

Why HDPE and PP sit at the bottom

High Density Polyethylene (HDPE)

High density polyethylene is semi crystalline with high crystallinity and a relatively high heat of fusion. That means it absorbs and releases a lot of latent heat as crystals melt and re form, and it shrinks aggressively and directionally as it cools. RapidMade+2Victrex+2

Thermoforming references and material selectors list HDPE mold shrinkage around 2.0 to 3.5 percent in sheet forming, compared with around 0.4 to 0.7 percent for ABS, HIPS and rigid PVC. toolcraft.co.uk+2RapidMade+2 A paper from the SPE Thermoforming Division explicitly notes that HDPE thermoforming has historically been avoided due to warp, and shows that success requires temperature controlled aluminum tools and disciplined cooling. SPE Thermoforming Division

Curbell’s thermoforming talk classifies HDPE as semi crystalline and “difficult to thermoform.” Curbell Plastics Combined with the high and directional shrinkage, that makes HDPE the least formable in this group if your metric is accurate, low warp parts.

When it is still worth it
You accept the pain when you need chemical resistance, low density and toughness in wet or aggressive environments, for example chemical tanks and trays. RapidMade

Polypropylene (PP)

Polypropylene is also semi crystalline and behaves similarly to HDPE but with slightly lower density and better hinge life. Shrink in sheet thermoforming averages around 1.5 to 2.0 percent, so better than HDPE but still far higher than the amorphous materials. toolcraft.co.uk+1

Thermoforming guidance from INEOS and other PP producers is blunt: the main deterrent to PP in thermoforming is its higher and narrower forming temperature range. The sheet becomes formable only slightly below the temperature where bubbles and defects start, which makes the usable window small. INEOS+2Ditai Plastic+2 Curbell also lists PP as semi crystalline and “difficult to thermoform.” Curbell Plastics

RapidMade’s moldability ranking places PP near the bottom, with “fair” ease of forming but “poor” dimensional stability, strong snap back and a recommendation for temperature controlled aluminum tooling and generous shrink allowances in CAD. RapidMade+1

PP still wins where chemical resistance, living hinges or high temperature food contact are critical, but in terms of straightforward formability it belongs near HDPE at the bottom.

Polycarbonate (PC) in the middle of the pack

PC is amorphous and, in principle, easier to thermoform than polyolefins. Curbell explicitly lists polycarbonate alongside ABS and HIPS as examples of amorphous thermoplastics that are easier to thermoform than semi crystalline HDPE and PP. Curbell Plastics+1

However, PC has two big penalties:

1. High forming temperature and operating temperature
   Lexan and similar PC sheet guides list typical sheet temperatures of about 170 to 225 °C for vacuum and pressure forming, with sag starting around 140 to 155 °C. POLYVANTIS | EN+1 A RapidMade operating temperature guide notes that, in general, higher service temperature materials are harder to thermoform, and PC sits at the top of that chart with operating temperatures up to about 125 °C. RapidMade

2. Moisture and stress sensitivity
   PC must be dried before thermoforming to prevent bubbles and splay, and supplier guides warn about internal stress and environmental stress cracking if it is overcooled or exposed to certain chemicals under stress. Hallstar Industrial+4Curbell Plastics+4RapidMade+4

Shrinkage levels are similar to ABS and HIPS (about 0.5 to 0.7 percent) but warp risk is rated moderate to high because residual stress matters more, and tooling typically needs higher mold temperatures and better control. RapidMade+1

So PC forms complex shapes with good accuracy, but it is much more demanding in drying, temperature control and cooling than ABS, HIPS or PETG, which is why it sits below them in the ranking.

PVC and KYDEX T: good, but process sensitive

Rigid PVC

Rigid PVC is amorphous and widely used in thermoforming grade sheet and film for packaging and technical parts. Suppliers that specialize in rigid PVC film for thermoforming describe these grades as engineered for vacuum forming with good rigidity and dimensional stability. HSQY Plastic Group+2Walton Plastics+2

Typical mold shrink values are around 0.4 to 0.7 percent, comparable to ABS and HIPS. toolcraft.co.uk+2RapidMade+2 Forming temperatures for rigid PVC film and sheet are usually in the 160 to 200 °C range, depending on thickness and formulation. Printing Rolls+1

So why is PVC not at the very top? Two reasons:

• It tends to build in internal stress if heating or cooling are uneven, which can read out as warp or long term dimensional drift.

• Some formulations are notch sensitive or brittle at low temperatures, which shows up when you push deep draws or sharp details.

Moldability guides therefore place rigid PVC in the middle of the amorphous pack: very workable, but more sensitive to process discipline than ABS or HIPS. RapidMade+1

KYDEX T (PVC alloy sheet)

KYDEX T is a proprietary PVC based alloy sheet (PVC with ABS or acrylic, depending on grade) designed specifically for thermoforming in applications like aircraft interiors and equipment covers. SEKISUI KYDEX+1

The manufacturer’s technical briefs state that KYDEX sheet:

• Has excellent forming properties, with uniform wall thickness and crisp detail.

• Forms deep draws with low force at the upper end of the forming temperature range.

• Shows unusually high resistance to hot tearing, and generally sags less than many other thermoplastics because of high melt strength. Emco Industrial Plastics+1

Recommended surface temperatures are roughly:

• 165 to 177 °C for thin sheet below about 1.5 mm

• 182 to 196 °C for 1.5 to 3.2 mm

• 196 to 204 °C for thicker gauges

with an upper limit around 204 °C. Emco Industrial Plastics+1 Mold shrink is quoted around 0.4 to 0.7 percent for typical tooling configurations, in line with rigid PVC data. Emco Industrial Plastics+2a.mpcdn.io+2

Curbell’s thermoforming material overview simply calls KYDEX thermoplastics “very good” in thermoforming, and notes they are tougher than HIPS or ABS and have excellent flame resistance. Curbell Plastics

So KYDEX T is clearly easier to form into deep, highly detailed parts than generic rigid PVC sheet, especially in heavy gauge aerospace and transportation interior work. The reason it still sits behind PETG, HIPS and ABS in the ranking is that its shrink and warp behavior are closer to PVC, and it remains quite sensitive to heater tuning and mold temperature for tight tolerance work. RapidMade+2a.mpcdn.io+2

If you care more about deep draws with thick walls, flammability ratings and toughness than absolute tolerance, KYDEX T effectively moves up in practice.

PETG: very formable, with some caveats

PETG is a glycol modified PET that suppresses crystalline regions and behaves as an amorphous sheet. desuplastic.com+1 Material data and supplier guides consistently describe PETG sheet as easy to thermoform:

• Forms in a relatively low temperature band, around 120 to 160 °C. bayplastics.co.uk+1

• Typically does not require pre drying for standard thicknesses, unlike PC and ABS in many cases. bayplastics.co.uk+2Curbell Plastics+2

• Has low shrinkage in thermoforming, around 0.2 to 0.5 percent for common Eastar type grades. RapidMade+1

Curbell calls PETG “great for deep draw parts” and emphasizes its transparency and toughness compared with acrylic. Curbell Plastics+2Ditai Plastic+2

The downsides:

• Poor UV stability unless specially stabilized, limiting outdoor use. Curbell Plastics+1

• Lower heat deflection temperature than ABS or PC. bayplastics.co.uk+2Uplastech+2

• If over stretched and then cooled too quickly, especially at edges or sharp corners, it can become brittle or show whitening and stress. Ditai Plastic+1

Thanks to its low forming temperature, lack of drying in many cases, low shrink and good detail capture, PETG ends up slightly ahead of PVC and KYDEX T in the formability ranking, but below HIPS and ABS which are even more forgiving on large structural parts.

HIPS and ABS: the workhorses at the top

HIPS (High Impact Polystyrene)

Multiple thermoforming design guides single out HIPS as one of the easiest materials to vacuum form. Universal Plastics states that HIPS “forms extremely well” and is one of the most easily processed materials, and a Sirius Plastics data sheet calls HIPS the “most versatile” sheet for trouble free processing. universalplastics.com+2siriusplastics.com+2

Shrinkage is similar to ABS and rigid PVC at about 0.4 to 0.7 percent. toolcraft.co.uk+2RapidMade+2 Curbell’s material guide simply says HIPS is “easy to thermoform,” highlighting its low cost and ease of printing, gluing and painting. Curbell Plastics

Thermoforming temperature is around 160 to 190 °C for typical grades. zetarvac.com+1 Warp risk is low and the material is forgiving in large area parts, which is why HIPS dominates cost sensitive packaging, trays and interior covers that do not need high heat resistance. RapidMade+2RapidMade+2

The compromise is mechanical performance: lower stiffness, limited high temperature capability and moderate impact compared with ABS and PC. But purely on “can I easily form accurate, stable parts,” HIPS scores very high.

ABS

ABS sits at the top largely because it combines:

• Amorphous structure and wide forming window

• Low and predictable shrinkage similar to HIPS and rigid PVC, roughly 0.4 to 0.7 percent in thermoforming. toolcraft.co.uk+2RapidMade+2

• Good melt strength and sag behavior for heavy gauge forming.

Curbell’s material page describes ABS as low cost with “outstanding impact resistance, machinability and thermoforming characteristics,” and their thermoforming presentation explicitly calls ABS “easy to thermoform.” Curbell Plastics+1 Heavy gauge thermoforming guides and online FAQs typically quote forming temperatures near 150 to 190 °C, matching supplier data. toolcraft.co.uk+3EngineerFix+3s-polytec.com+3

RapidMade’s moldability spectrum ranks ABS first among common thermoforming plastics for dimensional stability, with low warp risk and the best tolerance capability, with HIPS very close behind. RapidMade+1

The main practical issues are:

• Thick gauge ABS can absorb moisture and should be dried to avoid blisters. s-polytec.com+2Select Plastics+2

• Gloss or orange peel can vary with heating profile and mold surface.

For precise housings, enclosures and cosmetic panels where you need tight CNC trimmed tolerances with minimal fighting over warp, ABS is usually the easiest path.

How solid is this ranking and when does it flip

There is no universal industry “formability scorecard” for all eight materials, so any ranking is built from multiple data points:

• Shrink and warp data from thermoforming specific guides and vacuum forming material spec tables. RapidMade+2RapidMade+2

• Amorphous vs semi crystalline behavior from polymer and thermoforming references. Multifab Manufacturing+3Curbell Plastics+3RapidMade+3

• Supplier forming guides and process notes for each resin. China MDE plastics Company+6POLYVANTIS | EN+6bayplastics.co.uk+6

The relative positions are robust at the extremes:

• Bottom two for dimensional control and ease of forming are clearly HDPE and PP because of their shrinkage, warp and processing demands.

• Top three for straightforward, accurate forming are ABS, HIPS and PETG, with ABS and HIPS usually winning where tight tolerances and cosmetics are important and PETG dominating clear deep draw work.

In the middle, PC, PVC and KYDEX T can trade places depending on what you weight:

• If you prioritize high temperature service, PC rises despite being harder to process. RapidMade+2Curbell Plastics+2

• If you prioritize deep draws with thick walls and flammability, KYDEX T can behave “more formable” than PETG or even ABS for that specific geometry, but you give up some dimensional stability. RapidMade+3Emco Industrial Plastics+3a.mpcdn.io+3

So treat the table as an engineering starting point, not a law of nature. For serious work you would still:

• Pull the sheet supplier’s thermoforming guide for the exact grade and thickness.

• Build directional shrink into CAD for PP and HDPE from day one.

• Run first article shrink and warp studies before locking tolerances, especially on semi crystalline materials. RapidMade+1