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This informative blog allows plastics professionals to discuss plastics training and technology. Brought to you by Routsis Training: the plastic industry's premiere training provider.

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Making Significant Changes

When making a change to a plastics process, it should always be large enough to make a noticable change. It is often difficult or impossible to differentiate between normal variation and the effects of a small change.
Question: Why do you advise against making small gradual adjustments?
My Response: Plastics materials introduce an inherent degree of variability to the process. Additionally, aspects such as temperature, humidity, and equipment wear are always fluctuating over time. For this reason, you will see always see some amount of shot to shot variation in the process as well as process fluctuations. For example, if you increase the mold temperature 2° to adjust a dimension, any change you might see could easily be the result of normal process variation in a different parameter such as material viscosity. If the temperature was adjusted by 10° and the dimension did not change significantly, then you know with certainty that the mold temperature does or does not significantly affect this dimension. 
-Andy

Melting Semi-Crystalline Polymers

Semi-crystallinity in polymers is a commonly misunderstood concept since it cannot be demonstrated or easily visualized. Semi-crystalline polymers have all the softening characteristics of an amorphous polymer combined with the melting characteristics associated with semi-crystalline behavior.
Question: What is different between melting amorphous and semi-crystalline polymers during recovery.
note: I have addressed this in two blog posts.
Semi-Crystalline Polymers: Like amorphous polymers, these polymers get much of their strength through chain entanglement and inter-molecular attraction. The difference is that semi-crystalline polymers have small portions of the long polymer chains that align into neatly packed and arranged semi-crystalline sites known as semi-crystalline regions. These small compact sites are points where the polymer has higher strength and increased rigidity contributing to the overall strength of the part. When melting semi-crystalline polymer, these semi-crystalline sites remain in tact until the polymer reaches a specific ‘melt’ temperature where the semi-crystalline sites dissolve. If not properly heated, the semi-crystalline sites remain in tact and the polymer chains will actually rip apart resulting in polymer chain degradation. For this reason, semi-crystalline polymers prefer to receive a lot of heat first and a small amount of shear in the end to compete the melting.
-Andy

Melting Amorphous Polymers

The melting of amorphous polymers is not like traditional liquids such as water. There is no specific temperature here the material tuns from solid form to liquid form since there is always some amount of polymer chain entanglement and inter-molecular attraction present to provide strength.
Question: What is different between melting amorphous and semi-crystalline polymers during recovery.
note: I will address this in two blog posts.
Amorphous Polymers: All polymers have polymer chain entanglement and some amount of inter-molecular attraction. When heated, the polymer chain mobility increases and the inter-molecular attraction will decrease, causing the polymer to ‘soften’. In amorphous polymers, a constant shearing and heating of the polymer chains works best to prepare the polymer for processing. Screws designed for melting amorphous polymers tend to have a short feed zone with long gradual transition zone to provide consistent shear to help melt the polymer.
-Andy

Circumferential Screw Speed

The circumferential screw speed from one machine does not directly relate to that of another machine, but it is a good starting point. This is because the material melt, shear, and flow characteristics within the screw channels will be different for each screw, so it is a machine-dependent parameter.
Question: Is circumferential screw speed a machine-independent parameter?
My Response: Circumferential screw speed is a neat parameter option for many molding machines as it measures the rotational speed of the outside of the screw. In theory, I like this parameter better than RPM when comparing different diameter screws, but it is not a fully-transferable machine parameter due to the differences from one screw to another when the diameter of the screw changes. Essentially, this data is a helpful reference when establishing the initial process. It is much more important to match the back pressure and recovery time as these are fully-transferable machine-independent parameters.
-Andy

Test Sample Conditioning

The purpose of conditioning is to ensure the polymer chains are in a consistent state to ensure the testing is objective. The orientation, crystallinity, strength, and impact resistance of polymers can be easily influenced through conditioning.

Question: My company deals with polymer compounding and we have an testing lab. According to ISO 291, we are required to condition the multipurpose testing bar for 88 hours. Wish to ask what should we do if during the 88 hours, the condition went of the specification. Thank you.
My Response: The standards do provide allowance for deviation and minimum times for compliant conditioning. For example, most materials require the atmospheric pressure must be within spec for 88 hours, but many material classes require less than 88 hours for temperature conditioning, especially when drying or moisture removal is needed. For this reason, I recommend you verify the specific requirements for your material class. If the conditioning does go out of specification, you must begin measuring the conditioning time at the last point when conditions are in specification.
Additional Thoughts: Keep in mind, many companies will test samples under non ISO conditions for practical or comparative purposes, yet the results cannot be considered ISO or ASTM compliant and should not be reported as such. As a compounder, if your intention is to derive objective data for the customer, then you must follow the standards. You should also have a data recorder which records the temperature, relative humidity, and atmospheric pressure in case your results are disputed. For this reason, you should have a standard methodology for processing the test samples and keep accurate records of this as well. 
-Andy

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