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Which plastic processing method should you choose?

by Administrator17. March 2016 09:23

 

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.

 

2. Thermoforming

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. 

 

4. Extrusion

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.

 

Technology

Types of Parts

Advantages

Disadvantages

Blow molding

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

Thermoforming

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

Rotational Molding


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

Extrusion

Drinking straws, pipes, tubes, hoses, optical fibers, fenestration products, deck boards

Low initial setup cost

Low production costs 

Limited precision

Restricted to only parts with a uniform cross section

Injection Molding

High volume, complex shapes

Most versatile

Many types of resins & additives

Fast production

Low labor costs

Design flexibility

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! 

 

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Manufacturing | Plastics

Comments (1) -

Michael Verrett
Michael VerrettUnited States
6/16/2016 10:32:50 AM #

Thanks for posting this! I learned a lot especially those methods used in processing plastic materials.
www.premierplasticsnj.com/

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