5-6: Forming Processes

Before #

So far in the class, we have seen a wide range of additive and subtractive processes which are capable of creating components in metals, alloys, polymeric materials, ceramics, and composites. The one thing that these processes all have in common is that they are serial — meaning that one feature is produced after another in a predefined sequence. This serial nature inherently limits productivity and may require considerable operator skill. High operator skill, which leads to high overhead costs, is especially needed when a tool such as a lathe is manually operated, but may be needed even if the tool is computer numerically controlled, because work still has to be properly mounted and the machine correctly aligned to it. Subtractive processes have been used for centuries, both in the mass-production of precision components, and in one-off custom and prototyping jobs. Additive manufacturing has until recently been seen as a purely prototyping tool, although increasingly is being adopted for short-to-medium production runs.

Casting, and injection molding, on the other hand, form all features in a component almost simultaneously by forcing molten material into a mold, and potentially offer a faster, more affordable route to mass production. Injection molding is suitable only for mass production because of the high expense of the molds needed, and production runs of >10,000 are usually needed for economic operation. Casting comes in many flavors, from high pressure die-casting — which is really the equivalent of injection molding for processing metals — to gravity-driven sand-casting, which is used both for long production runs and for one-off fabrication jobs because of its ability to create very large components (> 1 m in some cases) with reasonable simplicity. The higher throughput of casting comes at the expense of lower achievable tolerances than machining (considerable fractions of a millimeter, as opposed to tolerances of a few micrometers). Forming processes such as casting are thus often followed by subtractive secondary processing to bring key features within tolerance. In this module we will highlight some of the attributes of casting and injection molding.

Forming of Polymers (Molding Processes) #

Injection molding #

An injection molding machine consists of a mechanism to melt and extrude the polymer, and a molding unit. Injection molding is the workhorse of plastics manufacturing and can achieve cycle times of under a minute. Most plastics that are injection molded are thermoplastics, meaning that they can be reversibly changed between solid and fluid states by heating and cooling. Injection molding, however, can also be used to inject materials that covalently crosslink when heated (thermosetting plastics), making components that are more heat-resistant than thermoplastics. Injection molding has even been adapted to mold metal powders which are then sintered inside the mold (powder injection molding).

Polymer Melting #

In conventional injection molding, the material to be molded enters the machine as solid pellets, which are drawn into a rotating, heated screw whose diameter decreases along its length to compress the material, drive out air voids, and turn it into a continuous molten flow. This flow is then extruded from a nozzle into a clamped mold, which is typically machined from tool steel but may contain inserts of other materials for specific applications.

  • Very thick material (viscosity) isn’t an issue due to large pressures used (Pressures typ. > 100 MPa)
  • Pellets go into hopper where they’re carried along and melted
    • The inside diameter of the screw gets larger so the pellets get compressed as they’re pushed along the screw until all air is driven out and it’s a constant stream
    • Wire mesh ensures only ’liquid’ passes (removes any debris/dust/funky filament)

Machinery #

  • The core is the part that is inserted into the mold (surrounding case)
    • These two components constrain geometry
    • Must have a very tight fit due to very high pressures (hydrolics is commonly used)

Process Stages #

  1. Heat up and screw the molten plastic to the tip of the core
  2. Plunge the core into the mold and inject the molten material
  3. Hold for a few seconds to let the material to solidify
  4. Retract the core and bring back the mold to release the new piece
  • Cycle times approx. 10 s to 1 minute

The mold is held at a temperature that is high enough to allow the polymer to fill the mold before solidifying, but cool enough that cycle time is not excessively long. The molded material needs to fall to below its glass transition temperature, below which the material becomes rigid, before it is ejected from the mold. Ejection is usually automated, with mechanical ejector pins emerging from the mold to push the molded component into a collection bin.

Features #

Connectors are placed as to minimize material used and heat; fractals can be used to optimize these patterns

Hot Runners #

Overmolding and insert molding #

  • Multiple polymers
    • Rigid and rubbery
      • Can make hinges between rubber and rigid material
      • Enables tooth brush bristles to be held in
      • You need a precise temperature in the range of the material so it deforms enough to bind with another surface while not permanently deforming
    • Different colors
  • Encase metallic objects – object is molded around for..
    • Rigidity
    • Strength
    • Abrasion resistance
    • Way to circumvent having to 3D printed threads

Physical limitations on geometry #

  • As soon as coming in contact with surface, the polymer looses heat
    • Thus, penetration depth is limited by how quickly the polymer ‘freezes’
    • You can increase pressure and temperature (thus viscosity) to circumvent this – but there are of course tradeoffs (longer heat up time, ‘maximum heat’ material can withstand)
  • For very tiny surface the surface geometry starts to matter too (e.x. income-angle may matter, as does mold material and how it interacts with the polymer)

Defects #

  • Flash – extra material squeezed out around edges, typically not a big deal
  • Flow lines – may matter for certain applications
  • Short shot – not enough material injected
    • Typically because extruder doesn’t output enough or because mold is pulled away too soon
  • Misrun – material doesn’t make it to end of material
  • Warping – uneven section thickness, causing non-constant heat density and thus warping
    • The more rigid geometry, the more differential thermal contraction can happen before warping occurs
    • Why ribs are used commonly used – small regions of strips of material is, from a structural POV, better than evenly distributing it across the surface
  • Trapped air – solved by adding very tiny venting channels for air to escape
    • Can fail if clogged with tiny debris, causing flash
  • Silver streak – polymer flowing over surface and solidifying too early
  • Sink marks – mass below surface pulls downwards, resulting in tiny dip
    • Caused by ribs (supports) in particular!
    • Visible in polished surfaces – thus you can avoid it by giving the surface a finish

Design of Molds #

  • Cost of molds drives economic batch size
    • ≳ 10,000 parts
  • Design considerations
    • Draft angle: typically 1 − 2°
    • Air vents
  • Advanced features
    • Multi-core molds
    • Active water cooling
    • Materials with very high thermal conductivity e.g. Cu-Be alloys

      For large moldings where achieving rapid enough cooling of the component is a challenge, high-thermal-conductivity materials such as copper–beryllium alloys may be used for part of the mold.

  • Material shrinkage needs to be compensated for in the sizing of the mold
  • Examples of typical linear contractions between solidification and room temperature:
MaterialApprox. linear shrinkage (%)
ABS0.6
Nylon2.0 – crystaline
PC0.7 – amorphic
PE2.5
PS0.4
PVC0.5

Watercooling system around mold

Surface texturing methods #

  • Polishing
  • Sanding
  • Grinding
  • Blasting – for more complex geometry
    • Sand or glass beads
  • EDM
  • Chemical etching
    • Ferric chloride, nitric acid

Nanoscale Features #

Nanopatterned hard mold coatings such as chromium nitride have recently been introduced to enable injection molding of nanostructures giving butterfly winglike structural color – structural color

Other Polymer Forming Processes #

Reaction Injection Molding #

  • For if you aren’t making many copies (so creating a mold isn’t worth while)
  • Materials are reactive when they come in contact – think of epoxy
  • Enables thermosets, not just thermoplastics, to be molded

Compression Molding #

A lower-throughput but simpler and more affordable polymer molding technique is compression molding, in which a charge of the material to be formed is placed directly into the mold, and the mold halves are then brought together. The simplicity of the technique lends it to modest-sized production runs, and it is particularly suitable for thermosetting polymers, which crosslink when heated and can be removed from the mold at the molding temperature, because they do not need to be brought below a glass transition to acquire rigidity

  • Much simpler apparatus; tens of MPa (rather than hundreds)
  • Relies on a thermosetting material to solidify part
    • Flash is common

Extrusion #

  • Wide range of length-scales possible
    • FDM printing (~0.1 mm)
    • Large pipes and construction sections (> 1 m possible)
    • “Keeping mass as far away from center is a good way to optimize usage”
  • Before leaving die, materials which are extruded outwards slightly because the high pressures cause the extrusion to shrink down
  • Hollow sections possible with specially designed mandrels
  • E.x aluminum alloys (80-20), PVC are common extrusion materials

Blow Molding #

Blow molding is very widely used to produce components with thin side walls, such as plastic bottles. An extruded tube of softened thermoplastic polymer is clamped between two halves of a mold and air pressure inside the tube is increased. This expands the material until it conforms to the mold, stretching the material and making it thinner as it does so.

  • Better than injection molding for very thin walls
  • Extrusion followed by inflation
    • Or, can use injection molded preform (e.x. plastic bottle threading)

Forming of metals and alloys (Casting processes) #

Reasons to use casting #

  1. You have an established component design which needs to be mass-produced: casting can create many parts in less time than machining or additive manufacturing.
  2. Geometric versatility: with appropriate mold design, sophisticated re-entrant geometries can be produced in about the same time as a simple geometry. Processing time does not increase strongly with geometric complexity, as it does in machining.
  3. The need to process high-melting-point (“refractory”) alloys: melting point is correlated with hardness, making machining increasingly difficult as melting temperature increases. A highprecision casting technique such as investment casting can be a good option for achieving demanding tolerances with refractory alloys.
  4. A desire to minimize material wastage: in casting, very little material is used beyond that required for the actual component, and any surplus (e.g. for risers and runners, as discussed below) can be re-melted and re-used. In contrast, in subtractive manufacturing much of the stock may be machined away as chips which are laborious and may be somewhat energy-intensive to recycle.
  • Variables in casting
    • Material insertion: gravity- or pressure-driven?
    • Mold material: expendable or permanent?
    • Pattern material: expendable or permanent?

Sand Casting #

  • Gravity-fed
    • Heat loss and shrinkage is a concern, so extra risers are included (which also serve as vents)
  • Manual process that creates large objects
  • Few-hundred runs a year, takes a few hours per run
  • If the mold halves do not mate perfectly, molten metal may travel along their interface, leading to flash, which is usually thin enough to be easily broken off or abraded/machined away
  1. In sand casting, a single-use mold is produced by compacting a sand mixture around a pattern, which has the same shape as the final required component. The pattern might be made by machining timber or metal, or by additive manufacturing. The mold is prepared in two halves, the cope (top) and drag (bottom), with the sand being contained within a metal flask that has two interlocking parts. The pattern is removed from the compacted sand mixture before the mold is closed up. If re-entrant or hollow components are to be cast, one or more cores are introduced into the mold before the cope and drag are brought together. These cores are also single-use and may be made from sand that is bound with organic material such as gelatin.
  2. The mold needs to have several features in addition to the cavity that will contain the actual cast component. First we need a downsprue, or sprue, into which the molten metal can be poured. Second we need runners which are horizontal channels that carry the molten metal from the sprue to the mold cavity.
  3. Thirdly we need one or more risers, which allow air to escape from the mold as the metal enters, and are also designed to be the last part of the metal to solidify. Risers can thus provide a small amount of surplus material to compensate for the volume shrinkage that occurs when the metal solidifies. Risers need to be strategically positioned around the mold cavity to supply this material where it is needed. The design of sprues, runners and risers requires skill and experience, and is one of the key competitive advantages of successful foundries.
Photos

Materials #

Sand, or silica, is used as a mold material because of its high melting point (~1600 ˚C) relative to that of the material being cast (e.g. 660 ˚C for aluminum). Sand on its own, however, will not hold the shape of the mold. Green sand is a sand, clay, and water mixture which holds together via the capillary forces between particles and by particle jamming between the larger sand particles and the much smaller clay particles. Green sand has the advantage that it can simply be shaken off the cast component after solidification, and recycled many times.

If a mold cavity has particularly large unsupported regions, a stronger mold material may be needed. In this case the clay and water may be replaced by a stronger binder, which may thermally or chemically cure to make a solid mold material. Such molds are more robust but are not reusable.

An alternative approach is to use loose sand and compact it under vacuum between plastic sheets to make it solid. The plastic is vaporized as soon as the molten metal hits it.

  • Options for sand-based molds:
    • Green sand
      • Typically SiO2, particles a ≥ 10µm in diameter
      • Mixed with 7-10% clay (e.g. kaolinite, 2SiO2.Al2O3.2H2O). Typically < 2 µm diameter
      • Bound with 2-4% water
      • Clay particles promote mechanical jamming of sand particles and with the water bind sand together
      • Shake off and sieve to re-use
    • Dry or chemically bound sand
      • Mix sand with an organic binder
      • May require heat to cure: 200 − 320 ℃
      • Or may cure via a chemical reaction
      • Enables larger unsupported mold cavities than green sand

      • But sand harder to re-use: needs grinding down
  • Why use sand?
    • It is refractory: i.e. high melting point – higher than the metal/alloy being poured.
    • e.g. silica: melts at 1600 ℃; pure aluminum: melts at 660 ℃

Gravity casting: pouring #

As molten metal is poured by hand from the crucible in which it has been heated, it accelerates under gravity and flows into the mold cavity. The height of the sprue is crucial in determining the speed with which the metal enters the cavity, and hence how long it takes to fill. If filling is insufficiently fast, there is a risk that metal will solidify before filling is complete; if, however, it is too fast, the momentum of the molten metal may dislodge sand from the walls of the mold, leading to the inclusion of sand impurities in the cast component and weakening the casting.

The speed can be estimated by considering the amount of gravitational potential energy that is converted to kinetic energy during the pouring process. The other important concept here is continuity, where we assume that the molten metal is incompressible (constant volume). So molten metal will flow at a higher velocity along narrower parts of a runner, for example.

Bernouilli’s Law (conservation of energy): #

  • We can usually neglect head losses and pressure changes:

Continuity (conservation of mass): #

  • Continuity of mass and energy together explain why the downsprue is sometimes tapered: to prevent a pressure drop and aspiration of air (or sand) from the sidewalls into the molten metal.
  • Mold filling time:

Solidification #

Solidification occurs when sufficient heat has been conducted out of the metal into the surrounding mold (or from the top of a sprue or riser to the air). The outside of the component will usually solidify earliest because of its proximity to the sand, leading smaller grains or crystals of metal to form near the surface of the component and larger, columnar grains to be directed radially towards the center of the component. This inhomogeneous grain structure may give undesirable mechanical properties which may need to be corrected by subsequent heat treatment or by machining back the surface.

A widely used model for solification time is Chvorinov’s Rule, in which the solidification time is expressed as the product of a mold constant, .$C_M$, and a geometrical term, .$(V/A)^2$. The mold constant is a purely material-dependent constant depending on the properties of the alloy being cast and the mold material. The geometrical term is the square of the .$V$olume to surface .$A$rea ratio of the component being cast. The reason the exponent is taken to equal 2 is that most of the heat leaves the cast material by conduction (rather than convection or radiation), so heat is transported by diffusion. The relevant timescale thus increases as the square of the relevant linear dimension for heat transport.

  • Total solidification time .$T_{TS}$ is composed of the sum of:
    • Pouring time, .$T_{MF} = V/Q$
    • Solidification time, .$T_S$
      • .$T_{TS} = T_{MF} + T_S;\ \ T_S \gg T_{MF}$ (heat)
  • Chvorinov’s rule: $$T_{TS} \approx C_m \left(\frac{V}{A}\right)^n$$
    • .$V$: Mold Volume
    • .$A$: Surface Area
    • .$VA$ is geometry dependent only
    • .$C_m$: Mold constant (material property)
    • .$n \approx 2$: heat transport from molten metal is largely by conduction; so is governed by diffusion of atomic vibrations – consider Fick’s laws of diffusion
  • Solidification begins from the outside of the mold cavity
    • Small “chill crystals” form (which can be machined away)
    • Columnar grains grow inwards
    • Equiaxed grains near center

Shrinkage #

Two types of shrinkage are relevant in casting. The first is solidification shrinkage, which is a volumetric reduction associated with the phase change from liquid (amorphous) to solid (ordered, crystalline). A good question to ask about solidification shrinkage is whether the material is effectively solid or liquid while this shrinkage is happening, as its state during shrinkage will determine the shape of a component after the shrinkage occurs. To answer this question, we can envision the liquid-to-solid phase change happening through the nucleation and growth of solid metal crystals within the cooling molten metal. Thus, up until the moment when solidification is complete, we can reasonably think of the material as still being fluid, so that gravity will cause it to fill the walls of whatever container it is in. Thus, solidification shrinkage will not be uniform in all directions, but rather will predominantly be in the vertical direction (see the shrinkage question in the homework on casting).

The second form of shrinkage is thermal contraction, a type of shrinkage that you have probably already encountered in other contexts. Here, the solid cast component reduces in dimensions linearly with temperature, as its temperature falls from the melting point down to the temperature at which it is extracted from the mold.

  • Cast components exhibit shrinkage:
    • Solidification shrinkage (due to liquid-to-solid phase change: disordered liquid becomes a denser, more ordered lattice structure). May lead to unwanted porosity if additional molten metal cannot enter the region vacated by the shrinking metal.

    • Thermal contraction (reduction in solid volume as component cools from melting point to room temperature). May lead to hot tearing/hot cracking since the material is relatively soft just below its melting point.

Possible defects and remedies #

DefectDescription/causePossible remedy/ies
PorositySmall voids in casting; caused by solidification shrinkage .$^1$Appropriately placed risers to supply material during solidification; chills
MisrunMetal solidifies before reaching the extremities of the mold cavityAvoid high-aspect-ratio features in design; pour metal at higher temperature; use heated mold (e.g. cast iron); redesign sprue/runner/gate and/or add additional sprue locations
Hot cracking/tearingDifferential thermal contraction of solidified material: material which has just solidified, but is still soft because of its highChills to promote faster cooling of thicker sections so molten regions are not trapped inside the casting
Pin holesAir entrapped in molten metal flowMore careful pouring; appropriate sprue design
FlashMetal escapes along parting line between two mold halvesMore careful packing of sand; post-processing of casting to remove flash
Cold shutMetal flows through multiple paths in mold and solidifies before the metal from the different paths meetsSame remedies as for misrun
Cold shotSplattered, solidified metal entrained in flowSlower pouring
Sand washErosion of mold surface in metal flowReduce pouring velocity/height
Sand blowEmbedded sand particles in castingCareful design of sprue taper

.$^1$: Voids can appear in the more massive regions of a casting which solidify later than the surrounding material, or the surface of a casting sinks, producing aVdefect known as a pipe. Some of the effects of solidification shrinkage can be counteracted by introducing chills, which are more thermally massive and conductive regions of a mold, made, e.g., of steel and embedded in the mold in close proximity to the more massive regions of a cast component to accelerate cooling.

Other Casting Processes #

Vacuum molding: a sand-casting variant #

  • Sand held in place under compressive stress
  • No need for binders – sand is easy to recover
  • Plastic burns away quickly when molten metal poured

Low-pressure casting #

Rather than pouring the molten metal directly into the mold, it is held in a ladle beneath the mold, and pneumatic pressure is used to drive the metal through a tube into the mold. This has the advantage that it can be automated, so that filling time can be made highly repeatable. It also helps to prevent the oxides that form at the surface of the molten metal from being injected to the mold — in gravity casting, some of these oxides risk entering the mold during pouring and weakening the cast component. The mold in low-pressure casting is typically metallic (e.g. cast iron) and reusable.

  • Advantages over sand casting:
    • Metal enters cavity at a more controlled velocity
    • Mold is reusable: better surface finish
    • Molten metal is taken from center of the bath – scum (oxides) from surface does not enter the mold

Hot Chamber #

  1. Suitable for metals with lower melting points (e.g. Zn, Sn, Pb, in some cases Mg)
  2. Plunger system exposed to high temperatures: the metal being cast must not chemically attack this mechanism
  3. Pressures around 7-35 MPa
  4. Production rates up to 500 parts per hour

High-Pressure Die Casting (Cold Chamber) #

Die casting comes in two main types: hot chamber and cold chamber. The choice of machine design depends on the melting point of the metal to be cast, with the hot chamber approach more suited to lower-melting-point materials for which it is feasible to build a plunger assembly that can be immersed in the molten material. The pressures in die casting range from several MPa to over 100 MPa, which is comparable with plastic injection molding and enables molten metal to be driven rapidly into the fine geometries in a mold. Casting dies are made of alloys with higher melting points than the metal to be injected, and require highly precise machining.

  1. Suitable for metals with higher melting points (e.g. Al and alloys, brass, Mg alloys) since plunger mechanism is not immersed in the melt
  2. Pressures around 14-140 MPa are needed because the molten metal cools more rapidly
  3. These high pressures can cause flash
  4. Not as fast as hot-chamber die casting because of the ladling process

Rubber Plaster Molding #

  • Rubber-plaster molds at General Foundry
  • “Chills”

Investment casting #

Involves coating a single-use wax pattern with a slurry which is a suspension of ceramic particles in liquid. The slurry is baked to solidify it, the pattern is melted out, the metal cast and the mold then broken off after solidification. The pattern can be produced to extremely fine tolerances (e.g. by 3D printing or machining), and the slurry particles are far finer than sand particles, so the surface finish achievable in this way is far superior to that of sand casting.

  1. Prepare wax pattern
  2. Several components may be connected on a “tree”
  3. Coat in fine-particle refractory material (ceramic slurry)
  4. Build up and solidify refractory coating
  5. Melt out wax
  6. Pour molten metal and solidify
  7. Break mold away from component

Other forming processes #

Drawing #

  • Pull solid material through a die, instead of pushing as in extrusion
  • Strain-hardens the material as well as reducing diameter

Deep drawing #

  • Rapid way of forming thin-walled, 3D metal shapes
    • Sinks
    • Drinks cans

Pressing #

Deformation of sheet metal between tools/dies #

  • Vehicle body panels
  • Introduce stiffening features, curvature

Forging #

  • Deforming metallic materials at significant fraction of melting temperature
  • Pressing nearly molten metal with a die to form a shape
    • 60-70% melting point
  • Can be open- or closed-die (left and right below, respectively)

Rolling #

  • Reduce thickness of metallic sheet between stacked rollers
  • Can be a continuous process
  • Material is generally near but below melting point
  • Cold rolling possible for small thickness reductions: smooth finish