In the industry, plastics are often referred to as polymers, and the actual plastic pellets are commonly referred to as resin or raw mate- rial. A polymer is classified using different criteria and is considered to be either; natural or synthetic, thermoset or thermoplastic, and amorphous or semi-crystalline. Natural polymers are those found in nature, such as rubber, cotton, and silk. Injection molding calls for the use of man-made synthetic polymers such as polyethylene,
ABS, and nylon.
Polymers get their strength from a process called polymerization. During polymerization, small molecules called monomers combine to form long polymer chains. Thermosets are polymerized during processing while thermoplastics are polymerized before being processed. During processing, the polymer chains in thermosets fuse together, or cross-link. Once these polymers cross-link, they undergo a chemical change which prevents them from being melted and reprocessed. An egg is an example of a natural polymer which thermosets. Once the egg is heated, it solidifies and cannot be melted again.
Thermoplastics are long polymer chains that are fully polymerized when shipped by the resin manufacturer. Thermoplastics can be
re-ground, melted and re-processed while retaining most of their original properties. An example of a natural thermoplastic material is wax. It can be melted and formed. Once cooled, the hardened wax can be melted and formed again. Unlike thermosets, most plastics companies prefer thermoplastic materials because they can be reprocessed and recycled.
Thermoplastic polymers can be categorized into two types; amor- phous and semi-crystalline. Amorphous polymers melt gradually when heated. During cooling, amorphous polymer chains solidify slowly in a random orientation. By the end of the cooling phase, they shrink about one half of a percent. Common amorphous polymers include ABS, polystyrene, polycarbonate, and PVC.
Semi-crystalline polymers melt quickly, once heated to their melt- ing temperature. The rapidly melting polymer is easy to process
compared to amorphous polymers. As a semi-crystalline material cools, portions of the polymer chains remain in a random state while portions orient into compact structures called crystalline sites. These crystalline sites increase the strength and rigidity of the poly- mer. During cooling, semi-crystalline polymers shrink up to three
percent – much more than amorphous polymers. Semi-crystalline
polymers include nylon, polyester, polyethylene, and polypropylene.
Thermoplastic polymers processed in the plastics industry are either hygroscopic; meaning they absorb moisture from the air, or non-
hygroscopic; meaning they do not tend to absorb moisture from the air. Many low-cost commodity polymers, such as polypropylene,
polyethylene, and polystyrene are non-hygroscopic polymers, which do not absorb moisture from the air. Any non-hygroscopic polymer can still get wet when exposed to water, or attract surface moisture in high humidity environments – such as outdoor silos, storage tanks, and overseas shipping containers.
Most engineering and specialty resins such as nylon, acetal, and polycarbonate are hygroscopic polymers, which absorb moisture from the air. These polymers have a natural attraction between the resin and water molecules. This creates a chemical bond, causing the polymer to retain water when it is exposed to moisture. In most cases, hygroscopic polymers require air which is both heated and dried to ensure proper material drying. This air must have the mois- ture removed through a dehumidifying process, such as desiccant or vacuum dryers.
Too much moisture in a hygroscopic polymer will interfere with the molding process due to hydrolysis. Hydrolysis is the breakdown of a water molecule when heated. Once broken down into hydrogen and oxygen, these molecules will chemically react with the polymer chains, causing them to break. Visual defects such as splay, poor surface finish, bubbles, or delamination can occur as a result of moisture in hygroscopic polymers. Hydrolysis can also cause a sig- nificant change in the physical properties of the polymer including: reduced strength, increased brittleness, dimensional stability, poor heat resistance, and tendency to warp.
The development of a robust injection molding process is highly dependent on the injection molder’s ability to cope with variability. This variability can be introduced by many aspects of the process including: the material, the mold, the machine, the operator, and the process.
A material can vary greatly from grade to grade and lot to lot. Changes in additives, colorants, molecular weight, molecular weight distribution, moisture level, and percentage of regrind can result in a variation in the ability to process a given material. Aspects such as ambient conditions, hydraulic fluid quality, equipment wear, and water supply can all result in variation in the molding process. Many steps such as material qualification, preventative maintenance, and scientific molding are used to minimize the influence of this variation on the quality of the final molding product. The goal of a good molder is to develop a system and process which is best able to compensate the variation which is always going to occur.
The viscosity of the polymer is a measure of the material’s resistance to flow. A material which flows easily has a low viscosity, while a material with a higher viscosity does not flow as easily. Most polymers are available in different grades; each grade having its own flow characteristics. Typically, materials with lower viscosity have lower molecular weight. These materials are easier to process, but typically have lower mechanical strength than the same polymer with a higher viscosity.
The viscosity of the polymer can be used to compare the flow characteristics of different polymers, or different grades of the same polymer. Viscosity data can also be used to qualify a new material or compare a newer lot of material to a previously used batch of material.
Rheology, as defined by Merriam Webster, is ‘a science dealing with the deformation and flow of matter’. A polymer’s resistance to flow is known as its viscosity, and the rate at which the polymer flows is referred to as its shear rate.
The capillary rheometer melts the polymer inside a small barrel, and then a plunger forces the polymer melt through a small capillary. The rheometer measures the amount of force required to push the polymer through the capillary. The shear stress on the melt equals the force divided by the surface area of the plunger. The shear rate is a measure of how fast the material is being tested.
The shear rate is determined by the rate of flow through the capillary, and the die geometry. The viscosity of the material is equal to the shear stress divided by the shear rate. In capillary rheometry, the viscosity is usually determined at different temperatures and shear rates. When the viscosity data is graphed, it provides a good representation of how the material behaves during processing. If capillary rheometry data can be obtained, it is a good method of comparing
the flow characteristics of different resins. Always compare capillary rheometer data from similar shear rates and temperatures.
Melt flow indexing is the most popular, and yet least accurate way to determine material viscosity. This method uses a standard test- ing apparatus with a standard capillary to measure the flow of the material. The melt flow indexer tests the polymeric material at a single shear stress and melt temperature. The melt flow index is the measure of how many grams of polymer pass through the capillary over 10 minutes.
A higher melt flow index indicates a lower material viscosity. This means that a material with a melt flow index of 20 flows easier than a material with a melt flow index of 5. The value obtained through the melt flow index test is a single data point. Melt flow index information from different materials and material grades may be used for a rough comparison of flow characteristics for different materials. The melt flow index value is given for each material by virtually all material suppliers.