Molding and Testing of Complex Materials

I- Purpose/Objective:


II - Experimental Procedure:

A. Analysis of samples prepared in lab by hot stretching and compression molding.

  1. All three compression molded PVC samples, and PMMA and/or PP/ PE:
    1. Obtain a DSC trace of each going  up at 20o/min. and down at 20o/min.
    2. Obtain an FTIR scan of each sample during the DSC runs( takes only seconds for an FTIR run)
  2. Compression molded PE, PP, PMMA, and all 3 PVC samples:
    1. Cut out dogbones and obtain stress-strain plots for each. It may be difficult to get a sample from the pure PVC and PMMA without it shattering but try
    2. X-ray diffraction patterns of each
    3. Evaluate the squares between crossed polarizers and position the material squares in various orientations to see if a difference in light transmitted is apparent.  (This will be compared to the oriented hot drawn films similarly  evaluated.)
  3. Hot drawn films
    1. X-ray diffraction of films drawn various amounts noting draw direction
    2. Tensile properties in draw direction, cut out dig  bone samples with cutter
    3. Visually evaluate films between crossed polarizers, either between the polarizing sheets or on the polarizing microscope with the polarizers crossed and the films oriented 45 degrees to either polarizer.

 

B. Instabilities in complex materials undergoing deformation

  1. Add polyethylene pellets to the barrel bore (as demonstrated) in increments of about 1⁄2 teaspoon and QUICKLY pack them into the bottom of the bore with the brass ramrod. WHEN FULL, MATERIALS MUST COME NO CLOSER THAN 2 INCHES FROM THE TOP OF THE BARREL BORE AS INDICATED BY THE GROOVES ON THE BRASS RAMROD.
  2. Put plunger into the bore making sure that it is centered and falls snuggly into the bore
  3. Lower head rapidly at first using 10 in/min. button until top of plunger is centered into the hole on the head,  then manually crank the head down with the crank in the door until the head engages the plunger, as indicated by an increase in pressure on the gauge, and material begins to come out of the bottom of the bore.
  4. Push 0.03 in/min button and collect about a 2 in. sample
  5. Continue successively with 0.1, 0.3, 1, 3, and 10 in/min. collecting a sample at each speed.
  6. Measure the diameter of the samples in at least 4 places as best as possible and tape and label the samples onto a sheet of paper.
  7. Repeat with the two other materials, polypropylene and PMMA in that order.

 

C. MSE365 w08 Lab2  A.  COMPRESSION MOLDING OF MATERIALS

  1. The press is at a temp. about 200C, which should  work for all materials.  It takes too long to change the platen temperature.
  2. Take a piece of aluminum foil as wide as the square mold and twice as long.  Fold it in half and put the mold onto one half of the foil so that the second half folds over the mold. 

    FOR PELLETS
  3. Put about 10 ml. of pellets in a pile in the center of the mold and fold the Al foil over it.
  4. Carefully slide this between the hot press platens, close the door, and raise the platens so they are together but no pressure shows on the gauge.  Let it preheat for about a minute, raising the platens slightly as the plastic matl. Melts.
  5. Raise the pressure slowly to allow the material to flow up to about 10,000 psi for about 1 min.
  6. Release pressure, lower platen so can take mold out, slide it out, if necessary use a screwdriver or spatula to break it loose, place it on the counter top and allow to cool.
    FOR PVC AND PLASTICIZED PVC
  7. Put about 10 ml. of PVC powder into the mold as described above and press it as described above.  The sample will be a dark reddish brown but it should be OK.  If you prefer, make another one keeping it in the press for half the time.
  8. In a plastic cup,  weight out 10 gms. of PVC powder and put 10 gms. of plasticizer into it.  Mix this thoroughly and put about  10 cc into the mold as described above and mold it as above.
  9. Repeat #8 using 5 gms. of PVC and 10 gms of plasticizer. Mold as above.

SAVE ALL SAMPLES FOR LATER ANALYSIS.

 

D. Deformation processing: Hot stretching

  1. Produce undrawn samples as a function of screw speed.
  2. Produce drawn films by varying screw speed and take up roll speed



III - Theory/Background Information:

A. Analysis of samples prepared in lab by hot stretching and compression molding.

Below is what oriented fibers/films would look like in three dimensions.  We only have the capability of obtaining patterns along one direction so the info below is not applicable.

 

 

 

Assume degree of orientation is proportional to cos Q where Q is angle the arc subtends with lines drawn from the center of the pattern.

B. Instabilities in complex materials undergoing deformation
When complex materials are deformed, unlike simple materials they have the ability to store elastic energy within them due to the imposed stimuli (tensile, compressive, and shear stresses) and this energy being elastic must  come out. But as opposed to being truly elastic, it is really viscoelastic, the relevance is that the recovery is time dependent. It therefore can couple in  with the rate of deformation of the material.  Depending upon many aspects of the deformation regime, it can present itself in  many unexpected, unusual, and unpredictable ways.  One catch all term  used to describe them are "flow instabilities" or deformation instabilities.  Although they occur within ceramic slurries and molten polymers and well as glass, etc., it is more convenient  to observe these using molten polymers because the temperatures are are not as high as for ceramics and variations are more easy to  obtain. We will be observing these and studying them in a qualitative fashion for one reason in that there are no theories to predict what is going on.

 

D. Deformation processing: Hot stretching

By virtue of the shearing conditions in nearly all processing operations some molecular orientations is realized in a polymer product because of the long chain nature of the molecules  and even in metals due to grain distortion and orientation.  Flow along the screw of the extruder, through the barrel and orifice of an injection molder, thru the cavities of the die, and even flow into the mold cavity will result in partial molecular orientation of the polymer melt.  Certain post melt forming operations such as drawing or hot stretching the melt when it is in it's rubbery state just above Tg to the product final dimension, will induce even more orientation.  This orientation if retained in the end product  will markedly affect its properties and performance.  Therefore it is necessary to determine the degree of molecular orientation and to consider the effects of such orientation on properties.  It is extremely important in final properties, eg. fibers are products with a high degree of orientation in the fiber direction and films have a high degree of orientation in the plane of the film.  Metal wires are similar except grains are oriented vs. molecules or crystals.  
A quantitative way of measuring orientation is that of birefringence; that is measuring the difference of refractive index between the direction of maximum orientation and that of minimum orientation.  Because we do not have such a device,  one of which is called a Babinet Compensator,  we cannot determine what is called and "orientation factor".  However we do have a polarizing microscope with which we can view the sample with various orientations between crossed polarizers, these being in the draw direction and perpendicular to it.  
The first part of the lab. is using the extruder and take off  line and chill roll, to draw  polymer melts to various draw ratio, i.e. (Ao-Af)/Ao , Ao = original cross section and Af =   final cross section, the orig. cross section being the   width and length of the orifice of the sheet die.


IV - Theory/Background References:


V- Activity Schedule:


VI -Format and Important Questions for Lab Report:

B. Instabilities in complex materials undergoing deformatio:

  1. Make a table of die swell vs. speed of extrusion for the  materials:
    DIE SWELL = DIAMETER OF PRODUCT/ ( 0.05 in.)  DIAMETER OF ORIFICE
  2. Describe the appearance of the samples as  function of the materials and rate of extrusion out of the machine.

 

D. Deformation processing: Hot stretching

  1. Draw ratio vs. qualitative evaluation of birefringence
  2. Get relative orientation from x-ray orientational patterns
  3. Tensile strength, strain to fracture, modulus, vs. draw ratio
  4. Other methods of evaluation including x-ray, etc.