FREQUENTLY ASKED QUESTIONS:
Question 1: What type of plastic is this? Is it contaminated?

Question 2: Is my material degraded from molding? Is regrind present?
Question 3: Why is my part cracking?
Question 4: How do I know if I have good incoming plastic raw material?
Question 5: Is my plastic compatible with a specific chemical?





Question 1
Q:What type of plastic is this? Is it contaminated?

A:
The easiest way to determine the identity of a plastic is by Fourier Transform Infrared Analysis (FTIR). Light in the infrared spectrum is passed through a small amount of the sample and the wavelengths where the light is absorbed or transmitted are measured and automatically entered into a computer. The resultant spectrum looks like a series of peaks and valleys and acts like a fingerprint for each material. The computer compares the fingerprint to known spectra to identify the generic type of material, i.e. polycarbonate, acetal, polypropylene etc. Further thermal analysis, such as Differential Scanning Calorimetry (DSC) may be required to refine the identification of some materials (i.e. nylon type, or homopolymer vs. copolymer). Additionally, computer subtraction of one spectrum from another can be used to identify contaminants which are present in sufficient quantity.


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Question 2
Q:Is my material degraded from molding? Is regrind present?

A:
Nearly all molding processing and thermal histories including the use of regrind, cause some polymer degradation. One of the most common ways to determine the extent of this degradation is to look for changes in the molecular weight (i.e. polymer chain length) that occur as a material breaks down. One test that correlates to molecular weight is Melt Flow Index, where the amount of melted plastic that flows in a given time through a controlled orifice at a specific temperature and pressure is measured. Other techniques use viscosity tests like Intrinsic Viscosity or Relative Viscosity where the flow properties of the dissolved polymer are determined. Comparison of the molded sample to the original resin can be used as a measure of the amount of degradation. It is virtually impossible to tell if regrind is present from degradation studies since it is impossible to distinguish between the effects of regrind, excessive heat, improper shot to barrel ratio, wet resin or any of the other common causes of molecular weight reduction.


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Question 3
Q:Why is my part cracking?

A:
There are many reasons plastic parts crack. Some of the most common are stress related due to molded in stress or poor part design, chemical attack which can be accentuated by stress, cross-polymer contamination which can also cause delamination, and poor processing which has resulted in degraded, brittle parts. Failure analysis of a cracked part often entails evaluating for most likely of these causes after a careful visual inspection of the part.


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Question 4
Q:How do I know if I have good incoming plastic raw material?

A:
Most resin suppliers today are anxious to work closely with you to assure that you have good incoming raw material and are willing to provide Certification Sheets on each lot of plastic. A good Certification Sheet should provide actual and pertinent test data like Melt Flow Index, Heat Deflection Temperature or Impact Resistance, as appropriate, on the exact lot along with specific information on the test methods employed. This specific lot information is often of more value in quality assurance vs. Data Sheet information which can reflect typical values that fluctuate in a range. An internal quality program where these Certifications are carefully analyzed and retains of each lot are archived can be of great assistance. Actual testing of the incoming material using standardized test methods (i.e. ASTM, ISO, etc.) can be employed on an occasional or routine basis as required.


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Question 5
Q:Is my plastic compatible with a specific chemical?

A:
Chemical compatibility testing of plastics is a complex and not always exact science. Many techniques and test procedures have been developed most of which employ exposure of test coupons or parts to a chemical under specified conditions. Stress and temperature are often employed to accelerate the chemical attack. Observations of weight gain, appearance, cracking, crazing and changes in mechanical properties like Tensile Strength or Elongation are often used to infer the relative resistance of the plastic to the chemical environment being evaluated.


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