This is a research project between DTU and two danish companies to make injection molding companies less demanding to highly accredited laboratory to measure the part at the standard condition which is 20͒ C 50%RH. was between DTU and injection molding and a metrology. You can find this research here. The accomplished task are as follow:
- Thermo-mechanical modeling of polymer parts after injection molding (FEM).
- Identifying the deformation due to measurement with dial/clock gauge
- Effect of moisture on dimension of polymer parts
- Measurement of shrinkage in a polymer parts after 60 days of production
- Uncertainty budget for measurement
Dynamic Length metrology in simple language
Polymer parts are very unstable materials. Their dimensions are varying even at room temperature. There are four parameters which affect polymer part dimensions
Thermal deformation is caused by temperature change in a part. This thermal deformation is more significant for polymer parts than metal parts. Polymer part even shows thermal deformation even at room temperature. Temperature and thermal deformation are related to each other by coefficient of thermal expansion (CTE). In this research the a polymer part is heated up an its length dimension is measured during it cools down to 20͒ C. This experiment is compared with a FEM thermo-mechanical model to calculate a CTE for this polymer part.
Force of measurement
The probe force for measurement of length deforms the polymer part. It is important to select a location for measurement that has minimum deformation. The edges and surfaces which are not supported and bending occurs as well are not a good choice for measurement location. The Hertzian contact theory and contact simulation with Abaqus are used to find the amount of deformation during measurement. This amount is compensated from total measurement.
Injected mold parts are dry and they absorb moisture after production. They usually expand as the water molecules create bonds inside the polymer part. Moisture is related to the deformation of polymer parts by coefficient of moisture expansion (CME). In this research polymer parts after injection molding are put in a humidity chamber and their length and weight are measured. The measurements are compensated for by the effect of temperature and probe force. The result is compared with hygro-mechanical simulation in order to define CME value for this polymer part.
Shrinkage or creep
Both phenomena are considered as the creep.The length dimension of polymer parts are measured by a CMM in different time steps for two months after injection molding. The effect of temperature, probe force and moisture are compensated from measurement. The remaining deformation is due to creep. A Kelvin-Voigt model is used to find a mathematical model for experimental results.
The compensation equation is validated two times, first time with 10 parts and second time with 40 parts from different cavities and times of production. 1D compensation equation is able to predict the final dimension which is obtained by measurement the same parts after two months and at temperature of 20͒ C.
How can the industry benefit from this research?
First, it shows that it is possible to transfer a complex 3D hygro-thermo-mechanical problem to a simple 1D equation by a structural plan for measurement of a simple mathematical model.
Second, this research covers uncertainty budgeting for measurement of polymer parts. It covers all important parameters and deep into details which can be used by other companies and laboratories. Moreover the two dimensional uncertainty budget is a handy tool to find uncertainty of measurement in different time and humidity.
How Industry 4.0 can help this research?
In this research the measurement and mathematical modelling is used to obtain CTE, CME, effect of force and creep model, since there is no history of length evaluation of this part after production. This research costs money and time which the industry is not willing to invest on. Industry 4.0 provides a foundation to store data of injection molding machines and measurement devices. Data is substituted to the measurement performed in this sturdy and makes feasible to reach a prediction formula for more polymer parts.