SHS IBDT Guide to Manufacturing Processes

Shaping Processes

Go to Injection Molding
Go to Rotational Molding
Go to Blow Molding
Go to Vacuum Forming
Go to Casting
Go to Extrusion

A. Injection Molding (Plastics)

Injection molding is a method of producing parts with plastic material (thermoplastic and thermosetting resins). This is done by the use of an injection molding machine. The shape which is produced is controlled by what is called a mold. This is a reverse image of the part desired and can be compared to the familiar "Jell-OŽ" mold. The injection molding machine has two basic parts; the injection unit, which melts the plastic and then injects or moves it into the mold, and the clamping unit, which holds the mold. The unit clamps the mold in a closed position during injection, opens the mold after cooling, and ejects the finished part.

Injection molding is the most common plastic manufacturing process. It is used to produce high volumes of parts.

Click  here   to watch a video on the basics of the injection molding process.

Click  here  to see how an injection molding screw operates.

Advantages of injection molding
  • Complex geometry and fine features are easily produced, because very high pressures are possible.
  • Cycle times are relatively low, and many parts can be made from a single mold, making extremely high volumes (millions per year) possible.
  • Relatively low labor cost:
    • Injection molding is commonly automated.
    • Many machines can be run by a single operator.
  • Little or no finishing of parts is required.
  • Able to process a wide variety of materials.
  • Minumum scrap losses.

Picture of injection molded sprue, runner, and gate system for multi-part mold tree.

Disadvantages of injection molding
  • Large undercuts cannot be formed (such as bottles)
  • Mold cost is high, so low part volumes are not recommended (usually less than 1000 parts is considered low; most volumes for injection molded parts are well over 10,000 pieces per year)

Picture of industrial grade injection molding machine

Characteristics of injection molded parts are:
  • The parts are thin-walled (walls greater than 0.125" are not common)
  • Thermoplastic materials are used in this process (thermoplastics can be remelted, as opposed to thermosets)
  • All exterior surfaces of the part touch the mold (as opposed to blow molding, where air forms the inside)
  • Mold cost is high, so low part volumes are not recommended (usually less than 1000 parts is considered low; most volumes for injection molded parts are well over 10,000 pieces per year)

B. Rotational Molding (Plastics)

Rotational molding is a manufacturing process used to produce hollow plastic parts in one piece. This process is utilized by industrial, medical, agricultural and toy companies among others. Rotational molding is a low cost plastic molding process which offers affordable tooling and quick turn around time.

Rotational molded cymbal case is lightweight for ease
of carrying, yet rugged construction protects contents.

In the rotational molding process, heat is used to melt and fuse a plastic resin in a closed mold. Unlike most other plastic processes, no pressure is involved. The three-stage process includes loading the resin in the mold, heating and fusion of the resin and cooling and unloading the mold. After the charged (loaded with resin powder) mold is moved into an oven, the mold is generally rotated on two axes at a low speed in order to evenly distribute the material over the inside wall of the mold. Heat penetrates the mold and the resin adheres to the mold's inner surface until it is completely fused. Air and/or a water spray cool the mold while still rotating to lower the temperature. The mold is then opened and the finished part removed.

Advantages of rotational molding
  • Excellent solution for producing hollow plastic parts, including complex and varied shapes.
  • Economical startup tooling costs as compared to injection or blow molding.
  • Produces parts with uniform wall thickness and no thinning in the extremities of the mold.
  • May be used to mold thermoplastics and thermosetting plastics
  • Ability to mold in inserts for exceptional pull out strength.
  • Ability to produce double wall parts for additional rigidity.
  • Rotomolded parts actually have their thickest wall sections in outside corners where durability is often critical.
    • Strong outside corners with virtually stress-free parts are created.
  • Part wall thickness can be changed easily simply by varying mold charge (amount of plastic in mold.)

Two piece rotational mold open and empty showing polished interior finish.

Disadvantages of rotational molding
  • Relatively long cycle time
  • The number of materials available for rotational molding are limited in comparison to other processes.
  • The process is not suitable for large production runs of smaller parts - for smaller parts, blow molding may be more competitive.
  • Loading of molds and unloading of parts is labor intensive by comparison to other processes, especially for complex parts.

Rotational molded medical specimen case is insulated to protect contents.

Characteristics of rotational molded parts are:
  • Rotational molded parts are seamless parts with little or no waste.
  • Rotational molded products can have wall thicknesses up to 3.8 inches.
  • The wall thickness of rotational molded parts is uniform to approximately 0.015 inches.

For more information on rotational molding, click here.

C. Blow Molding (Plastics)

Blow molding is the process of inflating a hot, hollow, thermoplastic preform or parison inside a closed mold so its shape conforms to that of the mold cavity. A wide variety of hollow parts, including plastic bottles, can be produced from many different plastics using this process.
           Sprite bottle blow mold
Show me a blow molding simulation

Advantages of blow molding
  • Low tool and die costs (compared to other molds)
  • Rapid production rates (Click here  to see how fast blow molding may occur in today's factory!)
  • Able to mold complex shapes in one piece

Blow molded trash can being removed from mold (before final trimming)

Disadvantages of blow molding
  • Limited to the forming of hollow or tubular shapes.

Blow molding cycle

Characteristics of blow molded parts are:
  • Usually have thin walls
  • Range in size and shape from small bottles to automotive fuel tanks

D. Vacuum Forming (Plastics)

Vacuum Forming is the process of creating 3-D objects from a flat plastic sheet by heating the sheet, draping it over a form, and then evacuating air from between the sheet and form. Vacuum forming is especially useful in creating large products. Vacuum forming uses thermoplastic materials.

Taco Bell vacuum formed signs                Candy box dividers              Hot tub shape

Here is a simple simulation of the vacuum forming process:

Advantages of vacuum forming
  • Very low tooling costs (compared to injection or blow molding)
  • Short lead time (able to move from concept to production rapidly)

Disadvantages of vacuum forming
  • Unable to mold parts with negative draft
  • There are limits to the depth of part based on the material's elasticity

E. Casting (Ceramics, Metals, and Plastics)

A casting is commonly defined as a " metal object obtained by allowing molten metal to solidify in a mold ", the shape of the object being determined by the shape of the mold cavity. Casting may also be performed with ceramic and plastic materials. The basic technique of sand casting dates back thousands of years in time. Metal is melted and poured into molds made of sand, an abundant material which is resistant to high temperatures, bound together by water or oil. After the molten metal is cooled, each sand mold is destroyed, and the "casting" removed. A facility that performs metal casting is called a foundry.

Wireframe sketch of a foundry mold

Advantages of casting
  • The most intricate of shapes, both external and internal, may be cast.
    • As a result, many other operations, such as machining, forging, and welding, can be minimized or eliminated.
  • Manufacturing may be simplified.
    • Objects may be cast in a single piece which would otherwise require assembly of several pieces if made by other methods.
    • Rapid production rates
  • Able to mold complex shapes in one piece
  • Cast components are usually stable, rigid, and strong compared with parts made by other processes.
  • The process may be used to form very large shapes economically. Large pump housings, valves, and hydroelectric plant parts weighing up to 200 tons are produced by the casting process.
  • Almost any metal that can be melted can be cast.

Illustration of automotive engine block formed by casting process

Disadvantages of casting
  • Sand-mold castings have irregular, grainy surfaces.
  • Dimensional variations are expected.
  • Impurities in the cast material may result in part defects (and possibly weakness)
  • Cast parts are generally low in toughness and impact strength

This statue of Buddha at Daibutsuden Temple in Nara is reputed to be the
largest casting in the world. The bronze statue weighs 443 tons!

Characteristics of cast parts are:
  • Range in size from jewelry to giant statues!

F. Extrusion (Ceramics, Food, Metals, and Plastics) F. Extrusion (Metals, Ceramics, and Plastics)

Extrusion is used to produce a variety of products ranging from spaghetti and fettuccini to garden hose. It is the most common process used to manufacture long strands of anything. A variety of materials can be extruded including aluminium, clay, rubber, various foods and plastics.

These are extrusion dies used to form the shape of the extrusion.

Plastic pellets are fed and placed into a hopper, from which they drop into a heated barrel. They are moved and kneaded in the barrel by a screw until they reach a molten state (this is then referred to as the melt.) The melt is then extruded or forced out of the end of the barrel through a shaping die as a long, continuous line of shaped plastic. This line is then pulled by a haul-off (puller) through a cooling trough, where it solidifies and then is cut to length or coiled.

An animation of the extrusion process is shown above.

Advantages of extrusion
  • Extrusion is a highly efficient process (high speed process, little waste)
  • Extrusion allows some of the most complex shapes to be formed in high volume
  • Allows for precise cross sectional dimensions to be produced.

A sample of the variety of shapes possible using extrusion.

Disadvantages of extrusion
  • Parts must be cut to finished length
  • The cross-sectional shape of the product must remain the same

Even highly complex shapes are possible using extrusion.

Characteristics of extruded parts are:
  • Continuous cross section
  • Virtually unlimited length is possible!