|
|
|
There a numerous manufacturing processes to transform the raw material into designed products.
The process of extrusion is when plastic resin is converted from solid to liquid. This process, known as screw extrusion, is the
first step for several manufacturing processes (processes to follow). It can be
described as a large continuously rotating screw inside a heated tube. Plastic pellets, powders, and/or liquids enter one end of
the tube and are conveyed, melted, mixed, and pumped out in to a molding
process, which form plastic products used in the material handling industry.
The process of low pressure injection is when a mold cavity is filled with "expanding" foam resin under low pressure. This process
is commonly used to make material handling products and is similar to injection molding. The major distinguishing difference
between foam molding and injection molding is that in foam molding a foaming agent expands the resin and produces a product which
has a generally smooth outer skin and a cellular core. This greatly enhances the stiffness to weight ratio, however
dramatically reduces the base material to withstand the impact found in common material handling systems.
The family of products typically developed through the process of blow molding are: pallets, containers and barrels, which all
share a similar design needs; hollow sections. The blow molding machine consists of one to five extruder/injector heads feeding a
die. It drops (by way of gravity) a round or oval molten tube (parison) of material between two open mold halves. The parison walls may vary in thickness and can consist of several different layers of material. When the parison is in position, the mold halves are closed upon it, capturing and sealing off a section of parison, which is expanded and
held against the mold walls by air pressure until cooled. The part is then removed and trimmed. This process has an advantage over
twin-sheet (TTF). With blow molding, the resin pellets are processed at the machine and thereby eliminates handling and heat costs
associated with producing sheets for
thermoforming. The disadvantage of blow molding is it does not distribute the material in a finished part as efficiently as
twin-sheet thermoforming nor can substrates be easily encapsulated for ultimate stiffness.
The profile extrusion process is used to manufacture plastic lumber used in some forms of plastic pallet construction. The
thickness to height ratio distinguishes it from sheet extrusion. This process uses an extruder to force molten resin through a die
the shape of the cross section required. As - or before - the material leaves the die, the cooling process is initiated and
continues until the extrusion is rigid enough to cut in to lengthwise strips. The large extruded cross-section allows for the use
of difficult-to-process recycled consumer plastic products, however the pallet constructed of this material can weigh many times
that of a wood pallet of like size and constructions.
The injection molding process is used to make single-piece products or components used in the construction of products, such as;
pallets, bins, and totes. A multiple piece mold, containing voids in
the shape of the desired part, is held shut by an hydraulic press. It is then filled with molten material by means of an extruder
and high pressure injector system. When the part has sufficiently solidified, the mold opens to eject the formed part. A wide
variety of materials can be processed using this method and few, if any, secondary operations are required to finish the part
after ejection. Typically, this
process requires the greatest initial tooling mold investment.
This process forms flat sheets used for load separators, slip sheets, and sheet stock required in vacuum forming. It is a
continuous process using one or more extruders feeding a single die, which
forms a wide sheet of one or more layers of different materials. The die dispenses the molten sheet onto a series of rolls. This
controls the thickness of the sheet and begins the cooling process, which
solidifies the material. Rolls can also be used to impart patterned textures onto the surface of the sheet. Other materials, such
as stripes and printed substrate, can be combined with the surface of a molten sheet as it is fed through the rolls, forming an
excellent bond. The sheet is cut to required size or wound onto a roll without interrupting the flow. The sheet extrusion process
can efficiently combine several materials, laminates, and textures into a single highly engineered sheet.
This process is when a single extruded sheet is heated and positioned over a horizontally oriented mold and sealed at its perimeter.
The air between the mold and heated sheet is evacuated and
atmospheric pressure forces the sheet to conform to the surface of the mold. Plugs, which help stretch the material to create
vertical wall thickness, are used to optimize material distribution to critical
areas. Material handling industry products, such as; pallets, totes, trays, containers, and components are manufactured using this
process. Other advantages of this process include, low cost prototype parts, low cost tooling, partially encapsulated substrates,
and the full range of options associated with extruded sheet.
Twin-sheet thermoforming (TTF) heats two individually engineered sheets, on two individual molds, positioned horizontally; one
above the other. Before the two sheets are brought together to form a
single welded part, a stiffening substrate can be positioned between the sheets, which then is encapsulated by the welding step.
The large size of TTF machinery, and the high cooling capacity of twin molds, provide relatively high processing volumes. The
combination of high material distribution efficiency and the ability to weld two individually engineered sheets together provide
high performing hollow light weight structures. Low cost custom prototype parts and moderate cost molds are additional benefits
unique to TTF. Pallets and containers are two major families of material handling products manufactured with the TTF process.
A soft "blob" of material, known as a billet, is placed on a two-piece horizontal mold surface, which is mounted on a press. The
mold is closed, forcing the billet to conform to the mold surface. Once the conformed billet has cooled, it is removed from the
mold. The "flow" that is created due to this molding
process allows for a variety of difficult materials and structural fillers to be produced with various wall thicknesses and
minimum distortions.
| Process |
Description | Considerations |
| Sheet Extrusion | Sheets are formed in a continuous process and stored in rolls or flat sheets | Low cost |
| Thermo Forming | Heated sheets are formed over molds. Twin sheet welds two sheets together during the process. | Low cost tooling and prototyping parts
High production rates
Can totally encapsulate substrates |
| Injection Molding | A hollow mold is filled under high pressure. | High tool costs |
| Structural Foam Molding | A hollow mold is partially filled with plastic and foaming agent: the mix expands to fill the mold. | High tool costs
Stiffer but more brittle parts |
| Blow Molding | A sealed tube of plastic is blown like a bubble against the walls of a hollow mold. | High tool cost |
| Roto Molding | Melting pellets line the inside of a mold which tumbles on three axes. | Low cost prototypes
Slow process |
| Compression Molding | A molten billet of plastic is squashed between the closing mold halves. | Can use very stiff materials |
|
|
Site is best viewed with Internet Explorer
4.0 or higher.
|
 |
|
|
|
|
|
