A prototype shop forming ABS instrument housings from 3 mm sheet produced a run where every fourth part had a thin white patch on the deepest corner. The mould was right, the sheet was right, and the vacuum was arriving late: the sheet had cooled below forming temperature before the draw completed.
ABS sheet vacuum forming: the forming window
ABS absorbs little moisture but enough to matter: two hours at 80 degrees Celsius before forming is normal, and it prevents the fine surface bubbles that look like contamination.
The forming window is 160 to 175 degrees Celsius. Below 150 the sheet is too stiff and the part springs back; above 185 the surface loses gloss and the sheet sags into the mould before the vacuum applies.
- Starting sheet 2 mm: 160 to 168 degrees Celsius, draw ratio 1.5:1
- 3 mm: 163 to 172 degrees Celsius, draw ratio 2:1
- 4 to 5 mm: 168 to 175 degrees Celsius, draw ratio 2.5:1
- 6 mm and above: 175 degrees Celsius, and expect longer cycle time
The window is narrow compared with polycarbonate, which is why ABS forming rewards a controlled oven over a bank of infrared heaters.
Step two: the mould, and the draw ratio that limits it
Draw ratio is the depth of the part divided by its width. At 2:1 with 3 mm sheet, the deepest wall thins to about 1.6 mm. At 3:1 the same sheet thins below 1.2 mm, which is where parts start failing drop tests.
That arithmetic, not the mould material, decides whether a part is formable in ABS. If the design needs a deeper draw, the answer is a thicker starting sheet or a different process.
Mould detail
A female mould with a 3 to 5 degree draft, 2 mm radius on internal corners and vent holes at every low point. Without vents, trapped air becomes a witness mark on the part and a rejected part in inspection.
Aluminium moulds at 60 to 80 degrees Celsius give the best surface finish in ABS. Cold moulds chill the sheet before the draw completes, which is exactly the white patch in the opening example.
Step three: cooling under vacuum
The part must be held under vacuum until it drops below about 80 degrees Celsius, then cooled with air on both faces. Releasing vacuum early lets the part spring back at the corners, and the distortion is permanent.
A 3 mm part typically needs 20 to 40 seconds of vacuum hold. Shorter cycles exist and they produce parts that look correct and measure wrong.
Common failures and their cause
- Webbing between close features: spacing below 1.5 times wall thickness
- Thin white corners: vacuum late or mould too cold
- Surface deglossing: sheet overheated above 185 degrees Celsius
- Spring-back at release: vacuum removed above 80 degrees Celsius
- Cracks at trim: trimming while the part is still hot
Each of these is a parameter, not a material fault, and each is visible at the machine rather than at the customer.
For the starting sheet, the ABS weight table gives the weights per thickness that correspond to each draw ratio. Prototype enclosures built this way are covered in the ABS manufacturer note.
Forming a new part? Send the drawing and annual volume to oversales@rpplexiglass.com for sheet grade and thickness advice.
Frequently Asked Questions
At what temperature does ABS form?
Between 160 and 175 degrees Celsius for sheet, with 165 the usual set point. Below 150 the part springs back; above 185 the surface deglosses and the sheet sags.
What draw ratio can ABS take?
About 2:1 for a simple female mould at 3 mm starting sheet, and less as thickness drops. Beyond that the walls thin below the design minimum.
Why does ABS web between mould features?
Webbing comes from the sheet bridging two features that are too close together, or from uneven heating. Spacing features at least 1.5 times the wall thickness apart, and heating the sheet evenly, remove most of it.