Injection Molding Reference Guide

Materials, Design, Process Optimization,Troubleshooting and Other Practical Information from the Experts at Routsis Training

 

Plastics is a sophisticated and diverse discipline. To excel, you need a good grasp of a wide range of concepts and data.

 

That’s what this Injection Molding Reference Guide is all about. Designed as both a starting point for entry-level personnel and a refresher reference for experienced professionals, this guide is packed with practical information built on Routsis’s 30-plus years of plastics training and consulting success.

 

• The science of plastics processing

• Materials and properties

• Part and mold design basics

• Process optimization

• Scientific troubleshooting

 

While we hope you find this information useful, it should not be considered a substitute for continuous training. Techniques and technologies are advancing rapidly throughout the industry, which is why top-performing professionals turn to Routsis to keep enhancing their skills.

 

We invite you to further explore the topics covered in this guide through the comprehensive array of online training programs we provide at www.traininteractive.com.

 

DISCLAIMER

This reference guide contains general recommendations intended solely for informational use within the plastics injection molding industry. It is not intended to serve as engineering advice.
The information contained herein is based on published information, knowledge, research, and experience which are presumed to be accurate and complete to the best of our ability.
All information is based on averaged data of commonly available grades of plastics and current industry practices at the time of this printing. Therefore it is the user’s responsibility to review and confirm all design, calculations and processing decisions.
You should always design and process using the recommendations that are provided by your raw material supplier, resin distributer, machine and equipment supplier(s).
Each material, machine, and process has its own set of influencing factors and therefore may, or may not; comply with the information provided in this guide. A. Routsis Associates, Inc., will not accept responsibility or liability for use of the information contained within this guide.

1 Understanding Plastics

 

1.1 General Classification of Polymers

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.

 

1.1.1 Thermoplastics vs. Thermosets

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 thermoplastc materials because they can be reprocessed and recycled.

 

1.1.2 Amorphous vs. Semi-Crystalline

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.

 

1.2 Hygroscopic vs. Non-Hygroscopic

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.

 

1.3 Understanding Variability in Plastics Processing

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 varia-tion 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.

 

1.4 Understanding Viscosity

The viscosity of the polymer is a measure of the material’s resis-tance 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.

 

1.4.1 Capillary Rheometry

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 capil-lary, 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.

 

1.4.2 Melt Flow Index

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 infor-mation 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.

 

1.4.3 Spiral Flow Test

The spiral flow test uses a mold with a long spiral flow channel emanating from the center. Notches are etched along the flow path to help identify the length the polymer has flowed within the mold. The mold can be filled using either a constant velocity (constant shear) or constant pressure (constant strain) to determine the polymer behavior.

 

The behavior of the polymer can be evaluated based on process output data such as flow length, part weight, and pressure at trans-fer. When using the spiral flow test, it is best to use a mold which has a channel thickness similar to the parts actually being molded.

 

1.4.4 In-Mold Rheology

In-mold rheology uses a variety of injection velocities combined with machine data to generate a rheology-curve. This curve plots the ef-fective viscosity of a polymer to help determine when shear-thinning occurs. As the shear rate (or flow rate) of the polymer increases, the viscosity decreases. This rheological behavior is unique to polymers and is called ‘shear thinning’.

 

When graphing this, viscosity is plotted on the vertical, ‘Y axis’ and shear rate is plotted on the horizontal, ‘X axis’

  • The apparent shear rate equals 1/(fill time).
  • The effective viscosity equals (fill time)*(transfer pressure).

 

Shear thinning will appear as a steep decline in the viscosity of the polymer as the shear rate increases. Once most of the shear thinning occurs the polymer’s viscosity starts to level out. After this point, the viscosity will remain relatively consistent – resulting in a more stable process. For this reason, you should process on the right hand side of the curve.

in mold rheology curve for plastics training

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