Which route curved glass takes is decided by volume before anything else. For a few dozen to a few hundred pieces of sampling, CNC milling the profile is the simplest option: no tooling, lead time counted in days. Once monthly volume reaches the thousand-piece range, the tooling cost of heat bending starts to amortise. And for genuinely large curvature with optical-grade surfaces, precision hot pressing is the only way. There is no fixed cut-off point — the larger the glass, the more complex the curvature and the tighter the tolerance, the higher that point moves. The worst mistake I have seen is quoting a batch order using sampling-scale CNC unit pricing; the customer only discovers that every piece loses money after signing the contract.
The one-line conclusion: CNC cold processing needs no tooling and holds tight tolerances but costs a lot per piece, so it suits sampling and small batches. Heat bending forms against a mould, giving a low unit cost but high tooling cost and long lead times, so it suits volume curved parts. Precision hot pressing (3D moulding) delivers the best surface-form accuracy, at the price of short mould life and expensive equipment, which is why it is reserved for optical and high-end cosmetic parts. The three are not substitutes for one another; plenty of projects run heat bending, then CNC trimming, then chemical strengthening in sequence.
Heat bending: the money goes into the mould first, the unit cost drops later
The principle is plain: heat the glass close to its softening point, let it deform under its own weight or under a mould, then cool it slowly along an annealing curve. Ordinary soda-lime glass starts to soften around 550°C, while high-alumina silicate glass needs to go considerably higher, with a softening point near 800°C depending on the grade. A tunnel-type bending furnace typically has a dozen or more temperature zones, controlling preheating, forming, annealing and cooling separately. Get one segment of that curve wrong and the part carries high internal stress, which later shows up as spontaneous cracking during edge trimming or a drop test.
Three mould materials are common: graphite, heat-resistant stainless steel and ceramic. Graphite machines easily and is cheap but wears quickly and sheds powder, and powder landing on the glass surface becomes pitting. Stainless steel is durable but conducts heat fast, making temperature control difficult. Ceramic sits between the two. Mould cost follows size and curvature complexity — tens of thousands of yuan for small parts, several hundred thousand for large or multi-cavity moulds. Tooling lead time starts at a few weeks and reaches one to two months for complex shapes. That money and that time are the wall between heat bending and small-batch sampling.
What heat bending cannot do is equally clear. Profile tolerance generally lands in the ±0.1 to ±0.3 mm band, and deteriorates as the part grows. The formed edge is a rough blank, so it almost always needs a CNC pass afterwards, which means the drawing has to leave a few tenths of a millimetre of machining allowance for the downstream step. Then there is springback: once the glass leaves the mould the curvature contracts slightly and the actual angle is smaller than the mould, so the mould design needs compensation. The compensation amount is tuned by trial forming, and it is normal for the first attempt to miss.
CNC: no tooling, but every piece burns machine time
CNC cold processing mills flat glass directly. A diamond burr mounted on a high-speed spindle, usually running above twenty or thirty thousand rpm, mills the profile first, then drills holes, then grinds and polishes the edges. The advantage is that no mould is required — changing the drawing means changing the program — and samples arrive in three to five days. Dimensional accuracy can hold within ±0.05 mm, an order of magnitude better than heat bending, which is why many projects rely on CNC parts during the validation stage to check assembly.
The price is machine time. For a piece of glass the size of a phone cover, milling the profile plus drilling plus polishing takes anywhere from a few minutes to over ten. Larger parts and more holes mean slower work. CNC output must be chemically strengthened, meaning the glass is immersed in molten salt for ion exchange at around 400°C for several hours. Before strengthening, the edge must not have chipping — a chip is a stress concentration point, and after strengthening the part cracks along that chip the first time it is dropped. The other ceiling for CNC is shape: it can produce 2D profiles and 2.5D chamfers and arc edges, but not a true three-dimensional large curvature. Using CNC to chase a surface with several millimetres of drop is not economical in cost or yield.
How to split by volume: for sampling and anything under a few hundred pieces, prefer CNC and skip the tooling cost and trial-forming time. From the thousand-piece level upwards, with a curved shape, go to heat bending. Only when surface-form accuracy must reach the micron level and transmitted imaging cannot distort should precision hot pressing be considered. It is entirely normal for a project to span two stages — validate assembly with CNC, switch to heat bending for production — but the structure must leave clearance according to heat-bending tolerances from the start.
The three sets of data design must hand to the factory
The first set is the 3D curved surface data. Handing over a 2D drawing with a few R-angle callouts is not enough: the mould shop will extrapolate the surface itself, and the surface it derives is often not the surface the ID team wanted. We learned this the hard way. On a curved panel, the drawing carried the outline dimensions and a few radii but no 3D surface; the mould came back and the edge sat a few tenths of a millimetre off the mid-frame when assembled, with uneven gap width, and the only fix was to rework the mould. Since then we hand over STEP files for every curved part, together with the mating surfaces of the mid-frame, so the factory extrapolates against the real assembly relationship.
The second set is fit clearance and the tolerance chain. Glass is brittle and cannot be forced to act as a structural member. Between the glass and a plastic mid-frame, leave 0.1 to 0.15 mm per side, with foam or dispensed adhesive in between so an elastic element absorbs the tolerance. Too little clearance and the glass is loaded during assembly, so a corner drop cracks it; too much and the gap is visible and dust gets in. The tolerance chain has three layers: the glass body, the position of printing or coating, and assembly. Curved printing uses pad printing or heat transfer, one grade worse in positional accuracy than flat screen printing, so artwork needs generous margin from the edge — otherwise a slight print shift lands on the curved transition zone, where adhesion is worst.
The third set is the downstream process list. Trimming, strengthening, printing, coating and bonding each change dimensions or affect yield, and the factory needs the list up front to sequence the work. Wrong order causes real trouble: strengthen before drilling and the micro-cracks at the hole edge destroy the strengthened layer; print before strengthening and the ink discolours in the hot molten salt. There is also the minimum bend radius. Do not draw an aggressive small radius on the drawing and expect the factory to rescue it — the smaller the radius, the thinner the outer surface is stretched, and the more obvious the optical distortion and stress concentration. Curved panels with a viewing window deserve particular care: looking through them at the LEDs behind often produces ghosting or rainbow banding, and that class of problem can only be solved by enlarging the radius or making the area flat. It cannot be tuned out afterwards.
The item design most often forgets: annealing and stress information. Curved glass must be annealed along a curve after demoulding. Parts whose internal stress has not been fully released can shatter during subsequent CNC trimming or a drop, and the shatter location is usually not the load point. Asking for a few extra samples for drop and thermal-shock testing during the sampling stage is more useful than editing tolerances on the drawing.
Questions we get most often
Q: At the sampling stage, choose CNC or open a heat-bending mould directly?
A: Prefer CNC for sampling, because there is no tooling, changing the drawing means changing the program, and parts arrive in days, which suits validating assembly and appearance. Unless the curvature drop is so large that CNC simply cannot produce the shape, spending tens of thousands on tooling at the sampling stage carries high risk — a heat-bending trial takes weeks, and changing the curvature in between means reworking the mould, so the time cost far exceeds the extra per-piece price of CNC. The sound approach is two rounds of assembly validation on CNC, then tooling once the 3D surface data is locked.
Q: What dimensional tolerance can heat-bent glass hold?
A: Profile tolerance on conventional heat-bent parts lands between ±0.1 and ±0.3 mm, depending on glass size, curvature complexity and mould condition: the larger the part and the more drastic the curvature change, the harder the tolerance is to tighten. For higher accuracy the outline can be brought within ±0.05 mm by downstream CNC trimming, but the surface-form accuracy of the curvature itself is difficult to improve by trimming — that is decided at the forming stage. Where the glass mates with the mid-frame, do not expect the glass to hold the dimension itself; leave a few tenths of a millimetre and absorb the tolerance with foam or dispensed adhesive.
Q: Why does curved glass show optical distortion?
A: Distortion comes from thickness variation and residual stress in the bent region: the tighter the bend, the thinner the stretched outer surface and the more compressed the inner one, so light refracts in inconsistent directions and the image warps or shows rainbow banding. The tighter the bend and the smaller the radius, the more obvious it is; incomplete annealing leaves internal stress that makes it worse. For products with a viewing window looking through at LEDs, we generally recommend enlarging the bend radius, or making the window area flat and keeping curvature in the non-transparent region. It cannot be tuned out afterwards.
Q: How does precision hot pressing of 3D glass differ from heat bending?
A: Heat bending mainly relies on self-weight or a soft mould, so surface-form accuracy is ordinary. Precision hot pressing uses a pair of high-accuracy upper and lower moulds to press softened glass, transferring the mould surface directly onto the glass, with surface-form accuracy reaching the micron level. The cost is that the moulds use hard alloy or special graphite, are extremely demanding to machine, and have short life under high temperature and pressure, so equipment and tooling investment is far higher than heat bending. Only applications such as optical lenses and aspherical elements justify it; ordinary cosmetic parts rarely take this route for surface-form accuracy.
Originally published by the Hezi Industrial Design team, based on our industrial design project practice in Dongguan. Learn more: Industrial design services
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