Determination of the Expansion Coefficients of an Injection Molded Machine Part above and below the Glass Transition Temperature - METTLER TOLEDO

Determination of the Expansion Coefficients of an Injection Molded Machine Part above and below the Glass Transition Temperature

Introduction

Parts that are made by the injection molding of fiberglass reinforced polymers usually have expansion coefficients that are direction dependent. The sample investigated here was a machine component (shaft) made of fiberglass filled polyphenylene sulfide (PPS). For the construction of the machine, it was important to know the the expansion coefficients of the shaft in the axial and radial directions. The information was quickly and easily obtained by TMA measurements.

Sample Preparation

Measurements of expansion coefficients are performed using the least possible sample loading. To improve accuracy, it is advantageous to use samples that are thick in the direction of measurement.

Determination of the Expansion Coefficients of an Injection Molded Machine Part above and below the Glass Transition Temperature
Determination of the Expansion Coefficients of an Injection Molded Machine Part above and below the Glass Transition Temperature

Introduction

Parts that are made by the injection molding of fiberglass reinforced polymers usually have expansion coefficients that are direction dependent. The sample investigated here was a machine component (shaft) made of fiberglass filled polyphenylene sulfide (PPS). For the construction of the machine, it was important to know the expansion coefficients of the shaft in the axial and radial directions. The information was quickly and easily obtained by TMA measurements.

 

Sample Preparation

Measurements of expansion coefficients are performed using the least possible sample loading. To improve accuracy, it is advantageous to use samples that are thick in the direction of measurement.

The upper and lower surfaces of the sample should be flat, smooth and parallel. In this investigation, one sample in the radial direction and two samples in the axial direction were prepared from the shaft. Particular care was taken to minimize any thermal strains when cutting and polishing the samples. The dimensions of the samples were as follows: Sample A (axial), diameter 9.3 mm, height 4.9 mm, Sample B (axial, from the other end of the shaft), diameter 5 mm, height 8.7 mm, Sample C (radial, same end as sample B), height 6.8 mm, area 5 mm x 5 mm.

 

Measurement Parameters

Module: TMA/SDTA840 with a 3.0 mm ball point probe Load: 0.02 N. A 0.5 mm thick quartz glass disk was placed between the probe and the sample in order to distribute the load uniformly over the surface of the sample. Temperature program: 30 °C to 200 °C at 1 K/min Atmosphere: Air, stationary atmosphere.

 

Results

 

Figure 1 shows the results of the dilatation measurements on sample A with and without thermal pretreatment. In the first heating run at 1 K/min, a glass transition can be seen which starts at about 78.3 °C. The effect is rather unclear because of enthalpy relaxation. The second heating run, which was also measured at 1 K/min, shows a much clearer glass transition at 91.2 °C. Figure 2 shows the mean values for the coefficients of expansion and the glass transition temperatures of samples B and C (axial and radial direction of the shaft) measured using the second heating run. 

A comparison of the mean coefficients of expansion of the two samples in the axial direction (sample A and sample B) shows small but nevertheless significant differences. This indicates that the different flow paths also have an effect on the thermal properties of the injection molded part. In other words, one can detect not only direction dependent anisotropic behavior, but also location dependent effects (which are course appreciably smaller). 

Determination of the Expansion Coefficients of an Injection Molded Machine Part above and below the Glass Transition Temperature | Thermal Analysis Application No. UC 107 | Application published in METTLER TOLEDO Thermal Analysis UserCom 10