Plastic products are everywhere and the processes in which to make them are many. Knowing which process to choose for making a plastic part is key to making a quality part, and also to finding a supplier that can help you achieve your production goals.
So, how do you choose? Let’s look at some of the most common processes used to make a plastic part:
1. Blow molding
Typical Use: Blow molding is typically used in the making of hollow parts, like bottles, that have a uniform wall thickness.
Overview of the Process: The first step in blow molding is the creation of a resin parison or a preform. A parison is a tube-like shape of plastic with a hole in one end that allows pressurized air to pass through. The parison is then clamped into the machine and air pressure is used to inflate the material which fills up the mold and creates the desired shape.
Variations of the Technology: Within the technology there are three types; extrusion blow molding, injection blow molding, and injection stretch molding.
Typical Use: Within thermoforming there are two categories: Thin-gauge and thick-gauge. Thin-gauge thermoforming requires sheets less than .060in thick (1.5mm) and thick-gauge, the sheets are greater than .120in thick (3mm). Trays and packaging for medical, food, and retail are created using thin-gauge and larger items like plastic pallets, bumpers, refrigerator lines are made using thick-gauge thermoforming.
Overview of the Process: A plastics sheet is heated until it is pliable and placed over or between a steel mold of the shape to form customized plastic products. Typically thermoforming is done in a continuous, high-speed process where thousands of parts are made each hour.
Variations of the Technology: Vacuum forming, compression molding, pressure forming
3. Rotational plastic molding or roto molding
Typical Use: Rotational molding is typically used when making hollow parts that require uniform wall thickness such as tanks or kayak bodies.
Overview of the Process: A mold is filled with a polymer resin – typically in powder form; the resin is then heated to a molten state while the mold rotates bi-axially so that the resin coats the inside of the mold cavity in a uniform fashion. Once cooled the part is removed from the mold.
Variations of the Technology: Much of the variation in rotational molding lies in the production equipment itself. There are a variety of methods used to actually rotate the molds: Rock and Roll, Clamshell, Carousel, Vertical, Shuttle and Swing Arm machines to name a few. Rotational molding often times is confused with rotational or spin casting which have slight variations that make them different than rotational molding.
Typical Use: Extrusion is used to make products that have linear and fixed cross-sectional profiles such as pipe, hose, and fenestration products; and is the reason why extrusion is often times referred to as profile extrusion. Extrusion is also one of the most common processes used to make compounded plastic pellets for extrusion or injection molding.
Overview of the Process: Either plastic compounded pellets or a dry blend of chemicals are placed into the material hopper and then loaded into the barrel of the extrusion machine where they are heated and worked along a screw to the end of a machine where they exit through a die. The die shape dictates the ultimate dimensions of the profile coming out of the machine. The shape or profile is then cooled and cut to the desired length. Because of the length requirements, often times extrusion equipment can take up an extensive amount of space on a shop floor.
Variations of the Technology: Often the screw does the majority of the work to extrude the product through the die, but in highly filled polymers such as fiber-reinforced profiles sometimes a method of pulltrusion is employed where the extrudate is pulled through a long die.
5. Injection molding
Typical Use: Injection molding is the most common method of manufacturing plastic parts and is ideal when production of a single part is of high volume. Injection molding allows for a fast rate of production, the ability to have many textures, finished, colors and complex parts.
Overview of the Process: Similar to extrusion, plastic compounded pellets are loaded into the barrel of a machine where the material is melted and worked down the length of a screw. Unlike extrusion, however, instead of exiting the machine through a die, the material is pushed through a runner system into a closed mold made of steel in the shape of the desired part. The mold goes through a heating and cooling cycle and once the desired temperatures and time settings are achieved the mold opens and the part can be removed.
Variations of the Technology: Much of the variation that exists in injection molding has to do with the way in which the injection molding machine itself is positioned (vertical or horizontal) and also the way the tool is designed. Tooling and the machine are always dictated by the complexity of the part and the volumes that need to be achieved while optimizing manufacturing efficiencies.
Now that you know the processes let’s refer to a chart for advantages and disadvantages of each as it relates to potential applications.
Types of Parts
Disposable containers for packaging liquid consumer goods (soda bottles)
Produce a one-piece hollow part
Best suited for mass production of small containers
Higher productivity than rotational molding
Limited to thermoplastics
Limited to hollow-forms
Wall thickness hard to control
Tables, trays, liners, bumpers, packaging
Make parts quickly
Large and small parts can be made
Material is higher in quality and durability
Tool costs are less than other processes
Material costs can be as much as 50% higher than other methods
Uses more plastic than other methods
Tanks and other large, hollow parts
Very little material wasted
Best suited for making large hollow parts
Not fast-moving process
Material costs are high
Drinking straws, pipes, tubes, hoses, optical fibers, fenestration products, deck boards
Low initial setup cost
Low production costs
Restricted to only parts with a uniform cross section
High volume, complex shapes
Many types of resins & additives
Low labor costs
High initial tooling cost
Part design restrictions
Accurate Part design required at initial stages
Hopefully this information will help you when you have to decide what process to use when making a plastic part!