Annealing Batch

8/2/2019 Annealing Batch

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Annealing

Annealing is the heating a molded plastic part to just below its glass

transition temperature

Annealing is used to reduce inherent stresses that may be introduced

during the molding and cooling processes

High stress levels generally make molded plastic parts more susceptible tostress cracking

Stress reduction can provide an extra measure of safety from aggressive

chemical environments that may come in contact with the molded part.

Applications such as medical devices, automotive lenses, and institutional

/ commercial food ware are areas where stress-cracking failures can be

critical.


2/17

MAnnealing is not a cure-all!

First consider proper mold design,

processing parameters, resin selection,application requirements and your resin

supplier!


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Annealing

Three different types of annealingprocesses are typically used in the

plastics industry:

Batch Hot Oven Annealing

(CFHA) Conveyorized Forced Hot Air

Annealing

(IR) Infrared Annealing


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Annealing

Batch Annealing consists of placing parts, usually onracks, in a hot air convection oven that is set at a certaintemperature for a specific time period

Annealing cycles can take from thirty minutes to as long a

2 hours or more at temperatures ranging from 230F to

270F

Long cycles are sometimes necessary due to restrictedairflow caused by parts being packed on top of or placedvery close to one another

Parts closest to the airflow see hotter temperatures for alonger period of time. This can lead to inconsistentannealing throughout the population of parts in the oven.


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Batch Hot Air Annealing


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IR Infrared annealing is a process that is best suited for in-line implementation. It is a continuous annealing process much like CFHA.

However, unlike CFHA, heat is generated in the molded part by absorption of electromagnetic energy transmitted from IR emitters. Direct

energy absorption by the molded part generates the required heat for annealing. This method is a very fast and efficient way of

generating heat. Comparatively, the hot air annealing methods generate part heat by first heating the surrounding air which in turn heats

the molded part. Typical in-line IR annealing cycles range from about 60 seconds to as long as six minutes, depending on the size and

wall thickness of the part.


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with cycle times ranging from 60 seconds to 5 minutes

Although IR annealing is generally the fastest of the three methods, it also has a few drawbacks. Off-line

implementation is possible, but not generally recommended. Also, part orientation sometimes is critical for

successful IR annealing.


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Recommendations

In-line implementation only!Use a robot to place parts on the conveyor belt

Cycle time must be consistent part to part

Parts should reach the IR oven within 30

seconds after de-moldingOven parameters generally change with various

part geometry's


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Conveyorized Forced Hot AirCFHA Annealing uses forced hot air set at a certain velocity and temperature as the heat source. Unlike the batch process, this process is

conveyorized and is well suited for implementation as an in-line annealing process. Parts can be set on the conveyor belt immediately after

molding which works especially well with robotic part pickers. This method allows for each part to be heated identically, which yields consistent

annealing results. Typical annealing times using CFHA can range from as little as four minutes to as long as fifteen minutes or more with

temperatures ranging from 250F to 275F, depending on part design, size, and thickness. This process can also be implemented as an off-

line process where parts could be annealed some time after molding if necessary.


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cycle times ranging from approximately 7 minutes to 1 hour.


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Advantages

Small oven footprint

Short annealing cycles

In-line processing

Disadvantages

Difficult part geometry's and

thick parts may be challenging

Smaller processing windows

Advantages

Larger processing window

In-line or off-line processing

Difficult part geometrys ok

Disadvantages

Larger oven footprint

Longer annealing cycles

Annealing Methods for Reducing Stress in Molded Plastic Parts

Bayer Corporation recommends annealing only after

mold design and processing parameters are

optimized!


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Injection molded thermoplastic parts generally develop both compressive and tensile stresses during molding and subsequent

cooling. High residual stresses in the form of surface tensile stresses are generally responsible for premature part failure.


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ENVIRONMENTAL STRESS CRACKING

(ESC)

What is it?

Exposure to Stress

Cracking Agent

Localized cleaving of portions of intermingled molecular chains in an area of concentrated stress

Localized high stress area Stress Crack (refracting light)


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Annealing Equipment

BatchDespatch Industries Inc.

P.O. Box 1320

Minneapolis, MN 55440

1-800-328-5476

CFHA

Despatch Industries

Despatch Industries Inc.

P.O. Box 1320

Minneapolis, MN 55440

1-800-328-5476

IR

Hennecke Machinery

Mayview Rd, & Park Dr.

Lawrence, PA 15055

412-777-2000

Annealing Batch

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