Injection Molding and Properties of Thermoplastics

I- Purpose/Objective:

The purpose of this laboratory is to enable you to learn about the relationship between processing, microstructure, and properties of selected thermoplastics using a combination of experiments and simulations. We will determine the transition temperatures in polymer resins using thermal analysis. We will conduct a series of injection moldings of some thermoplastics, demonstrate how the process can be simulated with commercial software, and examine the microstructure of molded specimens. We will also determine the tensile properties of polymers that are either predominantly crystalline or glassy. For the thermal analysis, we will use a technique called differential scanning calorimetry (DSC) to determine the transition temperatures in the polymer resins processed by injection molding. For the injection molding of thermoplastics, we plan to use either a reciprocating screw machine or a similar non-screw model system. We will examine the machine conditions in order to optimize the final specimen and compare the process with control charts to test the reproducibility of each group’s processing conditions. For the tensile testing, we will conduct tests on “dog-bone” types of samples, determine the mechanical properties, and examine the effects of processing parameters obtained from samples obtained in various experiments. This lab involves five major tasks, which we have scheduled in a sequence in order to minimize bottlenecks:

  • Simulations of the injection molding process using C-MOLD/MOLDFLOW software either during the lecture period in 2116 Dow or in the Van Vlack laboratory
  • Thermal analysis of polymer resins using the Perkin-Elmer DSC-6 instruments
  • Injection molding using the Arburg Allrounder reciprocating screw machine in Dow 2219 Tensile testing of the dog-bone samples using the Instron in the Van Vlack laboratory
  • Microscopic examination of microstructure of tensile-tested samples, and observation of the crystallization of polypropylene in a hot stage microscope in the Van Vlack laboratory.

II - Experimental Procedure:

A. Molding

Each group will be given one or more of the following resins for molding: semicrystalline isotactic polypropylene [PP], low density polyethylene [LDPE] and high density polyethylene [HDPE]. Polymethyl methacrylate [PMMA] molding will be carried out separately by the instructors outside the laboratory periods to afford other specimens for testing and comparative data.  Each group will be able to determine, based upon thermal analysis, tensile testing, and other observations, which polymer resin they are molding in a particular set of experiments.  Once you are certain which resin is which, consult with your instructor to find the specifications of the manufacturer and other pertinent information for your resin. You will need this to understand the C-MOLD/MOLDFLOW simulations.  Note its type (e.g., PP), brand name, grade number, melt flow index, molecular weight, and other information available from standard sources.1

If molding is done on the Battenfeld machine, we will use a mold with two cavities:  (1) a square plaque of nominal dimensions 2.5 inch wide by 0.1 inch thick, fed from the runner with a fan gate; (2) a bar, fed from an edge gate.  The bar can be cut into a dog-bone tensile specimen and a 2.5 inch by 0.5 inch by 0.125 inch bar. The molds and specimen configurations for the model machine will be introduced in class.

Molding will be conducted on the Arburg Allrounder, a reciprocating screw machine.  The laboratory instructor will demonstrate how to operate the machine.  Each group will search for a combination of machine parameters to achieve good molding of their resin.  For the Battenfeld machine, vary these parameters: (a) injection (packing) pressure and injection (packing) time; (b) hold (screw forward) pressure and hold (screw forward) time; c) screw back pressure (i.e. withdraw speed) and screw back time; d) the temperature of the cylinders and the nozzle.  A comparable list of parameters will be given in class for the model system. Obtain several specimens for each condition and note the quality of the molding.  Select a combination of operating parameters which yields a good part.

Once a set of parameters is found, make approximately 10 moldings for your group’s use and to supply other groups.  For each molding, cut off the plaque from the gate and measure the following: (a) mass of plaque to nearest 0.01 gm; (b) the thickness, transverse width, and longitudinal width of the plaque. Look for distortion in the bar and plaque by noting the flatness.

Use control charts to record the plaque mass and the transverse width of the plaque.  We will compare the control charts of the various lab groups for each resin, to see the extent to which the reproducibility can be held by each group's particular machine conditions.  We will also compare control charts for the other resins.

Take careful note of the molding parameters.  Each individual group will probably have somewhat different molding conditions, so we have an opportunity to compare data.

B. Tensile Properties

Each group should try to conduct tensile tests on approximately six dog-bone samples for each resin. Additional PMMA samples will also be available from the laboratory instructor.  Determine as many as possible of the following parameters for the crystalline polymers:  (a) initial tangent modulus; (b) tangent modulus at 2% strain; (c) apparent yield strength and elongation at yield; (d) ultimate strength; (e) elongation at fracture.  Compare these results for all of your samples.  Take careful note of various phenomena occurring during the test, with particular attention to effects such as necking, crazing, and changes in opacity. Note these carefully for your report; you will be expected to interpret these observations.

C. Microstructure

We will examine tensile-tested samples of each polymer optically in transmitted and reflected light.  For crystalline polymers, examine specimens from the necked region after tensile testing. Observe the optical changes from the deformation, and explain your observations. Compare with brittle PMMA samples, examining crazing. Use the teaching SEM as needed to document your findings for your reports.

We also will examine the crystallization of PP using a hot stage microscope.  Note the melting behavior of spherulites in samples heated above the melting temperature and the nucleation and growth of spherulites during cooling.

For the transparent PMMA samples, see if you can observe birefringence patterns between crossed polarizers.  Image the molecular orientation, and note this in transverse and longitudinal sections.  In particular, carefully draw or otherwise record the pattern, and relate this to the predictions from your C-MOLD/MOLDFLOW simulation of orientation and residual stress.  Comment on the similarities and differences.  Be sure to note the cause of birefringence patterns and the reason for their linkage to flow.  Specifically, note the existence (if present) of features known as meld lines, weld lines, and air traps.

D. Thermal Analysis

We will examine the transition temperatures of both polymer resins using differential scanning calorimetry (DSC).  You will receive additional data for PMMA from the laboratory instructor. Each group will collect a DSC thermogram for the polymer resins and will identify the transition temperatures which are apparent in the data for the resins.  We will compare these apparent transition temperatures with published values for PP, LDPE, HDPE, and PMMA available in data from the "Handbook of Plastic Materials and Technology" or other sources.


A member of the MSE Tech Staff will conduct a tutorial on the use of C-MOLD/MOLDFLOW, a professional software package for simulating plastic molding. The tutorial will be given during one of the lecture classes or during the laboratory period using the computers in the Van Vlack laboratory. The tutorial will help you become familiar with the use of C-MOLD/MOLDFLOW on these computers.  This will prepare you for your own C-MOLD/MOLDFLOW analyses, which are described in a handout to be distributed.

III - Theory/Background Information:

A. Injection Molding

The available literature on injection molding ranges from very practical to very theoretical.  For background, refer to Donald Rosato and Dominic Rosato, Injection Molding Handbook, 2nd Ed., Chapman Hall (1995), which is available at the Media Union Library, TP 1150, I551 1995.  Also, the following handouts can be obtained:

  • For general information on injection molding, see: “Injection Molding” from Vol. 8 of the Encyclopedia of Polymer Science and Technology,  2nd Ed.
  • For information on shrinkage, stress and warpage, cooling, and mold filling, see the Handbook of Thermoplastic Injection Mould Design, P.S. Cracknell and R. W. Dyson, Blackie (1993).
  • For specific information on polypropylene, LDPE, HDPE, and polymethylmethacrylate, see I. Rubin's Handbook of Plastics Materials and Technology, Wiley (1990).

Additional references will be identified during the lecture and laboratory periods.

B. Thermal Analysis

For a general discussion of thermal analysis techniques often used to characterize polymers, excerpts (pp. 301-326) from D. Campbell and J.R. White, Polymer Characterization: Physical Techniques, Chapman and Hall (1989) can be obtained.  In addition, the ASTM Standard Test Method, "Transition Temperatures of Polymers by Thermal Analysis" can be consulted. A brief tutorial will be given during the lab periods on various thermal analysis techniques used to characterize polymers (and other materials as well).

C. Tensile Properties of Polymers

For a brief discussion of the tensile behavior of polymers, including necking and crazing, excerpts (pp. 356-359, 367-369) from R. J. Young and P.A. Lovell, Introduction to Polymers, 2nd Ed. Chapman Hall (1991) will be useful. Additional introductory material is given in the texts used in MSE 220 and MSE 250. References cited in the chapters on polymeric materials will be useful.

Many other texts can be consulted for information needed to understand your results. Ask the instructor. Some will be brought to each of the laboratory classes.

D. Microstructure

As-molded structure

If a clear injection molded part is placed between two crossed-polarizers, a series of colored bands due to birefringence may be apparent.  The birefringence patterns are easily visualized, and can be quantified (Birefringence is the difference in refractive index in the parallel and perpendicular direction) , using light of one wavelength.  Birefringence patterns in polymers are induced by stressing the polymer below Tg (photoelastic effect) or by orientation in the melt above Tg (flow birefringence).  You may be able to examine some PMMA objects supplied by the instructor and note how the birefringence changes near the gate and at points where the flow converges.
Flow birefringence is also present in crystalline polymers, but is not as easily observed since the materials are not transparent. Crystalline polymers may also have variations in crystallinity across a molded section, since the growth of spherulites depends upon the thermal history. Hence, the amount of crystallinity is greater in the center of thick sections (which cools more slowly) than it is near the wall in thin section (which cools more quickly). 

Microstructure after tensile testing

The semicrystalline PP, LDPE and HDPE are ductile polymers which should support extensive shear deformation accompanied by necking.  Note if there is a difference in the behavior of the PP and the LDPE or HDPE. Cold drawing during necking deforms the spherulites of semicrystalline polymers, as illustrated in several reference texts.3 The orientation can be examined in crossed polarizers from the induced birefringence.  Note the appearance of the neck region in both polymers.

IV - Theory/Background References:

  1. For brand-name data, see M. Ash and I. Ash, Handbook of Plastic Compounds, Elastomers, and Resins, VCH, NY (1992); the yearly Plastics Technology Buyers Guide, or the annual Modern Plastics Encyclopedia

  2. For example, R.W. Hertzberg and J.A. Mason, Fatigue of Engineering Plastics, Academic Press (1980), p. 18

V- Activity Schedule:

VI -Format and Important Questions for Lab Report: