1. Compression Molding (Manufacturing Process)
Previous
Definition:
In compression molding, the compound, in powder or preform shape, is loaded directly in to a hot cavity at an average temprature of 340 F.
The required compression molding pressure depents upon: (1) the type of material, (2) the conditioningand preheating of material before molding,(3) temprature of the mold,(4) part design, and (5) mold design. Most thermoplastic materials are not suited for tihs process, so compression molding is used for molding thermosetting resins.
There are four methods of molding by the compression(and transfer) process.These are: (1) hand, (2) semi-hand, (3) seemi-automatic, and (4) automatic. Many different types of presses are made for compression molding.
The requirement of a press is to apply the needed pressure and provide sufficient heat to plasticize and cure (make infusible) the plastic materials. Heat is applied by transferring heat from heated platens, or it can be applied directly to mold.
The basic procedure for compression molding consist of placing the molding compound into the open mold cavity, closing the mold, and the applying heat and pressure through a down-ward-moving force-plıug to the material until it softens and is forced to fill the mold cavity.In closed mold a chemical reaction thet cross-links the polimer chains takes place and the materail hardens in to the required shape.
(Harold V. Johnson,Manufacturing
Process, p.558)
New Definition (Better):
Process description
A measured
quantity of raw, unpolymerized plastic material is introduced into a heated
mold which is subsequently closed under pressure, forcing the material into all
areas of the cavity as it melts. Analogous to closed die forging of metals.
Materials
- Mainly
thermosets, but also some composites, elastomers and a limited number of
thermoplastics.
- Raw
material supplied in either powder or liquid resin form.
Process variations
- Flash-type:
for shallow parts, but more material lost.
- Semi-positive
(partly positive, partly flash): used for closer tolerance work or when the
design involves marked changes in section thickness.
- Positive:
high density parts involving composite Sheet Molding Compounds (SMC), Bulk
Molding Compounds (BMC) or impact-thermosetting materials.
- Cold-molding:
powder or filler is mixed with a binder, compressed in a cold die and cured in
an oven. Strictly for thermosets.
Economic considerations
- Production
rates are from 20 to 140/h.
- Cycle
time is restricted by material handling. Each cavity must be loaded
individually.
- The
greater the thickness of the part, the longer the curing time.
- Multiple
cavity mold increases production rate.
- Mold
maintenance is minimal.
- Certain
amount of automation is possible.
- Time
required for polymerization (curing) depends mainly on the largest cross
section of the product and the type of molding compound.
- Lead
times may be several weeks according to die complexity.
- Material
utilization is high. No sprues or runners.
- Flexibility
is low. Differences in shrinkage properties reduces the capability to change
from one material to another.
- Production
volumes are typically 1000+, but can be as low as 100 for large parts.
- Tooling
costs are moderate to high.
- Equipment
costs are moderate.
- Direct
labor costs are low to moderate.
- Finishing
costs are generally low. Flash removal required.
Typical applications
- Dishes
- Housings
- Automotive
parts
- Panels
- Handles
- Container
caps
- Electrical
components and fittings
Design aspects
- Shape
complexity limited to relatively simple forms. Molding in one plane only.
- Threads,
ribs, inserts, lettering, holes and bosses possible.
- When
molding materials with reinforcing fibers, directionality maintained enabling
high strength to be achieved.
- Thin-walled
parts with minimum warping and dimensional deviation may be molded.
- Placing
of parting line important, i.e. avoid placement across critical dimensions.
- Maximum
section, typically=13 mm.
- Minimum
section=0.8 mm.
- Maximum
dimension, typically=450 mm.
- Minimum
area=3mm2.
- Maximum
area=1.5m2.
- Sizes
ranging from several grams to 16 kg in weight.
Quality issues
- Variation
in raw material charge weight results in variation of part thickness and scrap.
- Air
entrapment is possible.
- Internal
stresses are minimal.
- Dimensions
in the direction of the mold opening and the product density will tend to vary
more than those perpendicular to the mold opening.
- Flash
molds do not require that the quantity of material is controlled.
- Tumbling
may be required as a finishing process to remove flash.
- Surface
detail is good.
- Surface
roughness is a function of the die condition. Typically, 0.8 mm Ra is obtained.
(Swift, K. G., Booker, J.D., Process
Selection From Design To Manufacture, p.70)
2. Cold Rolling (Manufacturing Process):
Previous Definition:
Further flattening of hot-rolled plates and sheets is often accomplished by cold rolling, in order to prepare them for subsequent sheet metal operations. Cold rolling strengthens the metal and permits a tighter tolerance on thickness. In addition, the surface of the cold-rolled sheet is absent of scale and generally superior to the corresponding hot-rolled product. These characteristics make cold-rolled sheets, strips, and coils ideal for stamping, exterior panels, and other parts of products ranging from automobiles to appliances and office furniture.
Further flattening of hot-rolled plates and sheets is often accomplished by cold rolling, in order to prepare them for subsequent sheet metal operations. Cold rolling strengthens the metal and permits a tighter tolerance on thickness. In addition, the surface of the cold-rolled sheet is absent of scale and generally superior to the corresponding hot-rolled product. These characteristics make cold-rolled sheets, strips, and coils ideal for stamping, exterior panels, and other parts of products ranging from automobiles to appliances and office furniture.
(Mikell P. Groover; Fundamentals of Modern
Manufacturing Materials, Processes, and Systems 3rd Edition; pg.392)
New Definition
(Better):
Cold rolling
is a post-hot rolling operation and is used only when the metallurgical and
dimensional properties (such as straightness) of the final product require it.
The process is the same as that used in hot rolling, except that, despite the
limited size reduction per pass possible when working below the
recrystallization temperature of the metal, only four-high mills are used
because the forces imposed on the working rolls are extremely high.
Before cold
rolling can be carried out it is necessary to remove any surface scale left
after hot rolling. This is achieved by immersing the metal in an acid pickling
bath—for steel hydrochloric acid is used. All traces of acid are then washed
off with water, the steel is dried with hot air and finally coated with a thin
film of oil to prevent surface corrosion. It is then ready for cold rolling.
Cold rolling
increases metal toughness and provides it with a degree of surface hardness
because of the work hardening that results from cold working. While these
properties can be advantageous, work hardening severely limits the amount of
reduction that is possible in each rolling pass if metal cracking is to be
avoided. Annealing is therefore normally performed either between rolling
passes, depending upon the number of passes required, or after final rolling.
For steel, annealing is usually carried out in an inert atmosphere to avoid
surface oxidation. The principal steel products that are cold rolled are round,
hexagonal, square and rectangular bar (known as bright bar) and sheet/strip.
When cold
rolling sheet metal, where thickness accuracy and surface finish are critical,
a final post-annealing operation is usually carried out. This is yet another
rolling pass but involves a thickness reduction of only about 1 per cent, and
provides a controlled degree of surface hardness. This is called surface
tempering and produces strip material of exceptional dimensional accuracy and
surface finish. It also ensures the establishment of the metallurgical
properties required for subsequent processing by the end user.
In modern
rolling plants, if the strip material requires surface coatings, the equipment
needed to apply them is frequently incorporated within the flow path through
the mill.
(Waters, F., Fundamentals of Manufacturing for Engineers,
p.49)
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