Introduction to advanced and flexible sheet forming Flashcards
What are the trends that move towards new manufacturing solutions?
Demographic, regulatory, competitive
What are the different common materials for sheet metal?
Traditional steel, hot-formed steel, aluminum, magnesium, composites
Advantages of traditional steel metal sheet?
-Proven material, in use for decades
- Good forming capabilities
- Good availability
Disadvantages of traditional steel metal sheet?
- High specific weight
- Large number of process steps required (incl. tooling)
Adavantages of hot-formed steel sheet?
- very high strength with good formability
- attractive cost-benefit ratio for weight saving
- variable strength through sophisticated processing (eg. tailored tempering)
Disadvantages of hot-formed steel
- Higher investment, sourcing and operation costs
- Use limited to structural BIW components
Rank the materials according to the specific stiffness (MN.m/kg against price)
From Lowest to highest price: steels, aluminum alloys, mg alloys and CFRP (Carbon fiber reinforced polymer)
Steels, al alloy and mg alloys have relatively the same specific stiffness (except for some Al alloys which have slightly higher specific stiffness). While the CFRP has potentially more than 3 times the specific stiffness of the steels.
Why do we need advanced and flexible forming processes as an alternative to conventional?
Conventional rigid tools are expensive as they require a long production lead time and are meant to medium to large production batches.
It is difficult to make minor changes to the part design.
Heavy, difficult to handle manually.
Rigid tools may also induce aesthetical defects.
Difficult to work on materials with low ductility (HSS) and with complex geometries.
What are the pricnipal defects in sheet metal forming?
-Excessive thinning/necking/fracture
-Excessive thickening/buckling/wrinkling
Biaxial streching state of stress and biaxial stretching state of stress will fall in which quadrant of the FLD diagram?
1st quadrant
Conventional deep drawing will fall in which quadrant of the FLD diagram?
second quadrant. On this quadrant, larger deformations are possible before fracture.
In which condition do we see the lowest amount of plastic deformation at fracture?
Plane strain condition
By looking at the FLD diagram, we know that our process will not have defects if?
Our map falls below the FLD limit (red line)
How can the FLD be obtained?
- Experimentally, by etching a regular grid on the sheet before deformation and then measuring the deformation of each square of the grid.
- by FEM
Elastic springback is higher for:
- Smaller values of Young’s modulus E
-Higher values of the stress hardening coefficient K - larger values of the ratio p (ro) / t
Advanced sheet forming can be categorized in two “big” categories.
High temperature forming and Flexible forming.
Advanced sheet metal forming subcategory “High temperature forming”, can be divided in turn in two subcategories:
- Hot forming and Warm forming
What is the difference between hot and warm forming?
Both hot and warm forming improve formability, while Hot forming reduces forces and warm forming reduce springback
Flexible forming can be divided in 3 categories:
- Rapid tools
- Flexible media
- CNC forming
Define semi-rigid rapid tools
Rapid or “semi-rigid” tools are made with hard plastics (generally thermosetting if the are machined or thermoplastics if 3D printed).
They do not require thermal or surface tratments nor any grinding operation.
Define flexible tools
The tool is compliant, as in rubber pad forming or fluid forming (e.g. hydroforming)
One of the tools, either the die or the punch is replaced by a flexible medium (rubber or water) and the shaping action is provided by a conventional rigid tool
Define CNC forming
Dieless forming (air forming, air bending).
Part of the tooling setup is not present as in air bending.
- Single point incremental, explosive, electro-magnetic are other processes that do not have significant industrial applications.
Define punch
Convex tool that penetrates the material
Define die
Concave tool that determines the final shape of the part
Advantages of rigid plastic tools over conventional hard metal
Less expensive, produce no scratch as it is less hard that the sheet metal
Disadvantage of rigid plastic tools
The deformation of the tool must be taken into account when designing the forming process (FEM).
Tool wear.
Describe how compliant tools work
A traditional metal tool (typically the punch) determines the shape and the flexible rubber die pad is the opponent tool.
This allows changin only the punch to obtain different curvature radii.
Flexible tools must not be stiff but must be incompressible (Poisson’s coefficient = 0.5).
Why can water be considered as the perfect flexible media?
When used in hydroforming, water can be seen as nearly uncompressible flexible media.
What is the disadvantage of using water?
It is liquid and therefore the part will get wet, or a special “membrane” should be placed to contain the water
Typical rubber forming material?
Rubber Polyurethane
When is teflon used?
In anti-friction tooling applications
Good aesthetics can be accomplished with what kind of die material?
Nylon is used for soft punches and dies for aesthetical purposes
Teflon is measured with what durometer scale? What is the meaning?
Shore D or Rockwell R. Teflon is in the middle of the hard/soft polymer spectrum, with a hardness similar to a golf ball.
Classify from soft to hard the following: PU, Teflon, Nylon PC and Ultem
PU - ranging from Shore A 20 to Shore D 80
Teflon 65 Shore D
Nylon 110 Rockwell R
Ultem: 125 Rockwell R
PC (polycarbonate): 120 Rockwell R
Ultem is the commercial name for?
Polytheremide, PEI
Advantages of rapid tools compared to steel tools?
- lower manufacturing costs
-shorter manufaturing times - softer on the sheet material, no aesthetical defects
Disadvantages of the rapid tools compared to steel tools?
-Low durability
-Lower precision and repeatability
-Higher springback
-Higher forming forces
Advantages of flexible media over steel tools
- Lower manufacturing costs
- Shorter manufacturing times
- Softer on the sheet material, no aesthetical defects
- General purpose tool, many shapes are possible with the same medium
Disadvantages of Flexible media over steel tools
-Medium durability
-Lower precision and repeatability
- Higher springback
- Higher forming forces
-Difficult to form small radii
Shaping processes are divided in cold shaping and hot shaping. Which are the cold shaping processes?
Sheet metal forming, divided into deep drawing and bending
Shaping processes can be divided into cold shaping and hot shaping. Which are the hot shaping processes?
- Polymer and short fiber composite forming -> injection moulding, compression moulding, thermoforming, blow moulding
-Long fiber composite forming: pre-preg sheet lay up
What is prepreg?
Dry fabric with a semi-cured resin impregnated. The layers are put one on top of the other on top of the mold, cured with the resin and then vacuumed while the resin is curated
What is the meaning of EAM on rapid tooling
Extrusion-based Additive Manufacturing
SLA stands for?
Stereolithography
Differences between nominal geometry of the part and the actual shape after forming are mainly due to:
-Tool wear
-Geometrical errors while manufacturing the tools
- Uncompensated springback (elastic deformation of the tools make it worse)
On the semi-rigid tool, where is the maximum stress located?
At the corners and the radii (generally compressive stress)
Encapsulating the semi-rigid tools into metal casings allows to
reduce the need for springback compensation, stiffen the tools and prolong their life
When the fluid acts as a fluid punch (convex), the pressure required is:
large pressure is required
When the fluid acts as a fluid die (concave), pressure required is:
lower pressure required
Pressure of the fluid can be
