The problem with physical blends

A MOLDING MACHINE IS NOT DESIGNED TO MIX.  The molding machine is designed to plasticate (in Laymen’s terms, melt) and convey the molten material into a mold, it is not designed to mix.

Making a physical blend of, let’s say, 50% by weight 45% glass reinforced nylon 6,6 with 50% by weight 15% glass reinforced nylon 6,6 to obtain 30% glass reinforced nylon 6,6 does not work. By “does not work”, we mean finished molded parts with consistent 30% glass content shot to shot. The consistency shot to shot will not be there when molding this blend.

In this cited case nylon 6,6 is hygroscopic. This means that nylon 6,6 absorbs moisture. In the case of nylon 6,6, the moisture is not easily removed once absorbed by the nylon 6,6. Even though nylon 6,6 will not flow at above 509^o F, the recommended drying temperature for nylon 6,6 is 185^o F.  Why not dry at 250^o F, or 300^o F, or even 450^o F if the nylon 6,6 does not flow until 509^o F? Nylon 6,6 and its counterpart nylon 6 exposed to temperatures above 185^o F oxidize in the presence of oxygen. Oxidization is another word for burning.  Nylon 6,6 pellets will turn yellow at drying temperatures @ greater than 185^o F.  Keep in mind that the larger the pellet the more difficult it is to dry relative to a smaller pellet. Smaller pellets have a greater surface area per unit volume than do large ones. Smaller pellets will dry easier than large ones. Smaller pellets will dry faster than larger ones.

As a side note:  How does the particulate size of “regrind” form/pellet size affect the drying of regrind?

The injection molding process and the resultant parts produced from injecting molding is about consistency. If a molder sets up a particular job (mold), and he has a procured a truckload of 27 boxes (say 1,500 pounds/box) of a given material to run this molding job, he sets the molding parameters and doesn’t want to go near the machine until the job is complete.  Box (1) one in this lot through to box (27) twenty-seven must be consistent. This means that every shot has the same viscosity.

When you boil down what injection molding companies do, I believe they rent machine time. The more consistent the raw material is, the less time a technician has to tend to the machine and process. The less tending, the more money the molder makes as the machines are producing good parts.

This physical blend must be dried. If both pellets are the same size, then one would assume since glass doesn’t absorb moisture the 45% glass reinforced nylon 6,6 pellet would yield its moisture more readily than the 15% glass reinforced nylon 6,6 pellet. The pellet containing 45% glass has 55% nylon in it.  The pellet containing 15% glass fiber has 85% nylon in it. The potential for more moisture to be present in the 15% glass reinforced pellet versus the 45% glass reinforced pellet exiting the dryer prior to the physical blend entering the throat of the molding machine is there. If both pellets are of different sizes, then the potential of more moisture being present in the larger pellets is apparent. The moisture content is inconsistent. Moisture content inconsistencies entering the throat of a molding machine, molding “wet” nylon 6,6 can be nightmarish. The nozzle of the molding machine tends to “freeze off” and then “drool” intermittently when moisture is present in the melt.

It has been my experience that unless you (rhetorical) specify the size of the pellets when ordering compounds, you get what you get. I don’t believe I’ve seen a purchase order for glass reinforced nylon or other thermoplastic materials where the pellet size was spelled out on the Purchase Order. It doesn’t happen.

Both nylon 6 and nylon 6,6 are peculiar compared to other hygroscopic thermoplastics such as polyesters, polycarbonate (PC) and Polybutylene Terephthalate (PBT). Polyesters such as PC and PBT need to be dried very well. The term “bone dry” can apply to the level of dryness for both these thermoplastics entering the throat of the molding machine. Both PC and PBT require a moisture content of < 0.03% moisture content by weight entering the throat of the molding machine. Higher moisture content than 0.03% by weight may result in moldings made from polymer chains that have undergone “chain scission”. Cut molecules of polymer.  Chain scission is a reduction in molecular weight. A reduction in molecular length is perhaps a better way for the Layman to understand. Shorter molecules than intended, yielding poor toughness properties of the finish products produced. The moisture present reacts chemically with the polymer chains of PC and PBT in an adverse way. The chemical reaction is random.  The chain length is not consistent. The flow of polyesters that have been processed with water is inconsistent. Shorter molecules flow easier than long molecules.

The drying and processing of both nylon 6 and nylon 6,6 do not require either of them to be dried “bone dry” entering the throat of the molding machine, rather it has been scientifically proven that both nylon 6 and nylon 6,6 require a percentage of water in the nylon portion of the compound to obtain ideal moldings. Moisture present in the compound acts as a plasticizer. This moisture present in the molten state increases the flow length of the nylon into the mold. The science says 0.08 – 0.2% moisture present in the nylon portion of the compound will yield the best moldings from nylon 6 and/or nylon 6,6. This moisture content cannot be measured in a blend of different glass contents or should we say different nylon contented pellets.

Typically, I’ve observed blends such as this (45% and 15%) that the higher glass content pellets are larger than the lower glass content pellets. Larger pellets will not dry at the same rate as smaller pellets. This is due to larger surface area per unit volume of the small pellets as compared to the large pellets.

This physical blend of two different types of pellets must be conveyed from a box into the molding machine. Conveying plastic pellets causes agitation. The pellets are vibrating as they are being moved. Injection molding machines vibrate. They jostle with every opening and closing of the mold.

Granular convection is a phenomenon where granular material such as plastic pellets subjected to shaking or vibration will exhibit circulation patterns similar to types of circulation patterns in fluid convection. It is sometimes described as the “Brazil nut effect”.

The “Brazil nut effect” is when the largest particles of a physical blend end up on the surface of a granular material containing a mixture of variously sized objects upon agitation; this derives from the example of a typical container of mixed nuts, where the largest nuts are Brazil nuts. The phenomenon is also known as the Muesli effect since it is seen in Muesli breakfast cereal. The particles of different sizes as muesli mix separate.

Note that sand and gravel are separated using the “Brazil nut effect”.

Under experimental conditions, granular convection of variously sized particles has been observed forming convection cells like cells observed in fluid motion. The convection of granular flows is becoming a well-understood phenomenon.

The effect is of serious interest for some manufacturing operations; once a homogeneous mixture of granular materials has been produced, it is undesirable for the different particle types to segregate.

Processing thermoplastics we want to insure we do not break, cut or shear the molecules.  Thermoplastic molecules have a very high aspect ratio.  Aspect ratio can be defined as the length over diameter. A thermoplastic molecule can have an aspect ratio of 100,000:1, for example.  The molecule’s length is 100,000 times greater than its diameter.  Long thermoplastic molecules in a finished part impart toughness. When molecules of thermoplastic are exposed to too much heat or to too much “shear” they break. This is a random event and therefore the length of the molecules is inconsistent. Inconsistent length molecules affect flow adversely.

An extruder is designed to mix. It is designed to mix with minimum shear. Shear can be defined as “breaking”. In the plastic industry we use the term shear or shear heat to describe frictional heat. It is this heat caused by friction (shear) that causes the polymer chains to be cut. Thermoplastics are insulators and localized heating will cause chain scission. A 30% glass reinforced nylon 6,6 made on a compounding extruder will be homogeneous, can be packaged into hermetically sealed containers to prevent moisture absorption, will not suffer the “Brazil nut effect” when conveyed from package to dryer hopper as the pellets are uniform in size and composition. The extruder screw set up is designed for the specific compound to be run. A compounded 30% glass reinforced nylon 6,6 is much different in quality than a physical blend of different glass contented nylon 6,6 materials to yield a 30% glass reinforced nylon 6,6.

Shear stress: the Greek letter Tau (τ) is used as the symbol for shear stress.  Shear stress is applied force over cross sectional area. Formula (Tau) τ = F/A. F= force and “A” = cross sectional area.

Twin-screw extruders offer higher process productivity as compared to single-screw extruders. The plasticizing capacity of a twin-screw extruder is better and faster. In single-screw extruders, the granular materials stay longer in the extruder slowing down production time. A thermoplastic compound is a mixture of ingredients that includes a “base resin” and additives.  Base resin could also be known as base polymer.  The word polymer is a Greek word.  “Poly” meaning many and “Mer” meaning unit.  A polymer chain is many units.

The additives change the physical properties of the base resin. If a higher degree of impact resistance is required in the finished part, rubber might be added. If higher strength is desired, then glass could be one of the options for inclusion in the compound. Often additives are incorporated to protect the base resin from heat, ultraviolet light and potentially chemicals. Additives can be pigments as well. Additives can be foreign polymers that are used to enhance a particular physical property. One could almost say that the difference between a polymer and a plastic is that a plastic has been compounded.

If a single-screw extruder is used to make a compound, the L/D could be 28:1 or 30:1. What is L/D?  L/D is the length over diameter.  Generally, the diameter of the screw at the top of the flight is equal to the distance between the flights. Doing the math, the L/D is a way of saying how many fights are on the screw. More flights on the screw more mixing is obtained. Extruder screws, depending upon what one wants to accomplish as a result, can also incorporated mixing sections, variations in compression ratios, as well as other components for the desired end results.

Flights in our twin-screws can be adjusted to accomplish more mixing or less mixing.  We are cognizant of the heat generation as mixing creates friction and how heat can cause chain scission.  Twin-screw extruders have two screws, and each could have an L/D of 28:1. Twin-screw extruders are designed to mix thoroughly without imparting excess heat. They are designed to thoroughly mix gently without generation of excessive heat.

Molding machine screws don’t have mixing sections. These screws are not designed to mix. A typical injection molding machine has a screw with an L/D of 17 or 18.  This means when you (rhetorical) do the math the molding machine screw is very much shorter than the screw in a compounding extruder. For example, seventeen (17) flights on the screw for an injection molding machine compared to twenty-eight (28) flights on the screw of an extruder. The result of molding a physical blend is that the poor mixing makes a poor molded part. The parts do not contain the intended amount of nylon nor the intended number of additives nor the intended amount of glass. Due to the fluctuation of the moisture content in the material entering the throat of the molding machine the moldings are inconsistent shot to shot. Due to the variation of the glass content shot to shot, the moldings are inconsistent. The parts are inconsistent. This inconsistency can lead to failure of parts in the field. The described physical blend above mathematically yielding 30% glass reinforcement would not necessarily yield a molded part with 30% glass reinforcement. Shot to shot shooting a blend; one part may have 30% but others will have a range in glass content. The resultant moldings could be 25% or even 35% glass when the parts have an intended glass content of 30%.  This is way out of range for quality parts. This is inconsistent.

When a physical blend is made as in the case of mixing, 45% and 15% glass reinforced nylon 6,6 together. The 45% and the 15% have not exited an extruder in a timely manner. Timely manner means that the 45% glass nylon 6,6 could have been made two weeks ago, and so on. Unless special procedures are in place the two components are wet. Special procedures would be to package each component coming off their given extruder in hermetically sealed containers to the exact moisture content for the physical blend. The physical blending needs to take place in a moisture free environment. Each component’s pellets need to be the same size and shape as the other. Is this possible?

The physical blend entering the throat of the molding machine may at a given time be 30% glass.  At another time the physical blend may be 26% glass and at another time the physical blend may be 34% glass. Glass fibers do not melt.  They are solid fibers of glass.  If in these two scenarios 26% glass and 34% glass; their molten viscosity result will be different. The 26% glass material will flow easier than the 34% glass material. The 26% will have a far different viscosity than the 34% glass. Parts produced from the 26% will be weaker than ones produced from 34%. The 26% parts will have less glass reinforcement. Parts produced from the 34% will be heavier than the parts produced from 26%. The machine is set up to run 30% glass and does not adjust the process to compensate for the variation in glass content shot to shot. The parts made from the lower glass content portion of the physical blend will be more “packed out” relative to parts made from a higher glass content portion of the physical blend. The process is “out of control”.

Physical blends are made, I believe, due to haste and cost.  Physical blends can be made relatively quicker than a fully compounded compound.  Physical blends of smaller quantity can be made as well and this reduces the cost. Physical blends are not consistent. Physical blends will yield molded parts from a process that is out of control. You get what you pay for.

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