Effect Of Material And Processing Parameters

Published Date: 02 Nov 2017

Injection Moulding

Polymer Engineering MPC012

Scott Hammond

Executive Summary

Taking part in a laboratory session involved the use of a Negri-Bossi NB62 injection moulding machine, with Dimi EL2 control, where the material of choice was general purpose polystyrene 251L. Altering some process variables such as injection speed, hold-on pressure and melt temperature produced different results when it came to analysing the ‘dog-bone’ structures that were created in the mould.

Initial investigation identified some visible defects that occurred when changing the variables such as voids, surface roughness and mould flashing.

Further analysis included measuring the length and weighing the parts after they had been formed, then slowly annealed by holding them in an oven just below the glass transition temperature to anneal the structures before again measuring the length and weight of the parts.

From this analysis combined with material taught in lectures, a correlation could be drawn up to show the shrinkage of the parts after annealing and suggestions as to why this has happened due to the variations of the process controls.

From these investigations a number of conclusions could be drawn up to determine that increasing the hold-on pressure increases the mass of the components, and increases the amount of shrinkage that occurs after annealing. As the injection speed increases, this also increases the mass of the component as well as increasing the amount of flash around the part, but reduces the amount of shrinkage during the annealing process. Finally, increasing the melt temperature reduces the amount of shrinkage during annealing.

Contents

1.0 Objectives of the experiment

The purpose of this laboratory experiment is to:

Introduce the plastics injection moulding conversion process

Examine the influence of some variables on the process efficiency :

Effects of Holding Pressure

Effects of Injection Speed

Effects of melt temperature

Examine some properties of moulded Polystyrene (Grade 251L):

Weight

Length

Visible defects

Examine the effect of further processing variables have on the mechanical properties on the injection moulded part.

Figure - Injection moulding process [5]

Method of experiment

The experiment carried out closely followed the laboratory sheet handed out at the beginning of the session and can be found in appendix 1. But mainly consisted of using the Negri-Bossi NB62 injection moulding machine with general purpose unmodified polystyrene, grade 251L.

Then, by varying the process variables listed above, the aim was to determine how the product varied in terms of the polymer properties and establish why the effects happen from a molecular level.

2.0 Results

After collating all of the information from the experiment, and considering the process variables in combination with the length and weight of the dog bone components before and after annealing, a number of results and relationships could be determined.

An annealing temperature of 110°C was chosen, this was because it is just above the glass transition temperature (Tg) of this particular polymer, and by placing the components into a fan oven at this temperature reduces the residual stresses in the parts produced during injection, and hence the tensile testing bars reduced in length to reveal just how much stress was induced in the parts.

The graph in figure 2 below demonstrates the shrinkage ratio with the test pieces which were subject to the effect of holding pressure. The shrinkage ratio for all the samples was analysed to reveal the influences of each of the moulding conditions. From the graphs below it can be seen that the optimum holding pressure was at 30 bar, as this had the smallest shrinkage ratio, with the next best holding pressure coming out to be 10 bar, and the 60 bar holding pressure coming out as the least effective.

The shrinkage ratio can be determined using equation (1) below:

(1)

The next process variable was the effect of injection speed on the shrinkage ratio, and from the graph in figure 3 it can be seen that altering the injection speed also has an effect on the shrinkage of the component once it has been annealed, with the 90%/5% having the lower shrinkage ratio.

Figure - Injection speed

Figure - Holding pressure

The final variable was the melt temperature, increasing this to 210°C, to determine whether this also had an effect on the properties of moulded polystyrene. For this variable, an injection speed of 60% and a pressure of 30 bar was used as this was the average setting for the previous variables, the results of which can be found in table 1.

Table - Shrinkage ratio

3.0 Discussion

How do the variables affect the residual stresses within the moulded component? The three variables considered were; Melt temperature, Injection speed and hold-on pressure. From the results it could be seen that by altering these parameters, the structure of the finished component could be largely distorted.

Shrinkage in all components is inevitable, as was seen by altering the process variables and was measured in order to identify stresses that occur whilst moulding. The first shrinkage occurs straight after the part has left the machine and cools very quickly, not allowing the molecules much time to recoil and ensuring that stresses remain in the part therefore minimising the distortion of the shape of the product

The next stage of shrinkage occurred when the dog bone components were annealed in the oven just above the glass transition temperature. This allowed the molecules to relax and recoil and ultimately reduce the residual stresses within the component.

Therefore, by measuring the component both straight after moulding, and after annealing reveals the stress induced and gives an insight as to the molecular structure of the moulding technique.

Since Polystyrene 251L is an amorphous thermoplastic, the orientation of the long chain molecules is usually random in an unstressed melt and relaxes to an isotropic distribution, and generally wishes to conform to the ‘random-coil’ state. However, if there is any shear or tensile stresses present at the time of moulding, then by varying the process controls, this can have a large effect on the orientation of the molecules after moulding.

Each of the process variables has an effect on the degree of shrinkage of the components. The hold-on pressure can have a large impact on the orientation of the molecules, with a larger pressure increasing the orientation, but from the results it can be seen that the minimum shrinkage ratio occurred at the pressure of 30 bar, and the maximum shrinkage ratio at a pressure of 60 bar. However, this may be due to other effects that cooling had on the part. Bending may well have taken place on the component at 30 bar, which would not be discovered as it was the axial length being recorded. With the highest shrinkage being the 60 bar pressure, this was much expected since the most orientation occurs at the higher hold-on pressure, and as the material is an amorphous polymer, this will incur the most movement of the molecules to return to their random state when being annealed and relieving the residual stress. Further analysis of the components included the weight of the components, the lowest mass was the 10 bar hold-on pressure (10.065g), followed by the 30 bar pressure (10.125g), then the largest pressure (10.515g), this due to the fact that as the pressure increases, it is simply forcing more material into the cavity and consequently increasing the mass. As for visible defects, it was noted that as the hold-on pressure increased, flash became apparent and more prominent, which may have also added to the mass of the components.

The injection speed of the machine also has an effect on the properties of the component. It is expected that as the speed increases, the residual orientation in the moulded part decreases. Therefore as the flow increases, the polymer molecules are less orientated, and remain distant from alignment. As with the results from the experiment, this confirms the initial theory since the tests with the higher injection speed had a lower shrinkage ratio (0.111) than the lower injection speed (0.131) once they had been annealed. The mass of the components didn’t differ much between the variables, with the faster speed averaging at 10.12g, and the slower at 10.07g, however, the slightly larger mass may be from the screw quickly forcing the polymer into the mould before any freezing takes place, allowing slightly more material to be injected than the slower rate. The faster injection also showed signs of flash where the material had been inserted into the cavity quickly and forced the worn die slightly apart.

By increasing the melt temperature, the melt viscosity increases and orientation decreases, and less stress is induced when the material flows through the barrel. Therefore since there is little residual stress in the component, and little orientation of the molecules, it can be predicted that there would be little shrinkage in a component with increased melt temperature when the other settings are reasonable. This is exactly what was achieved during the experiment, with the shrinkage ratio averaging at 0.0846.

Effect of material and processing parameters on the occurrence of visible defects

Throughout the laboratory experiment there were very few major defects visible; however, on all of the samples there was flash evident at the top of the parts. This occurred on all of the samples, and in fact had nothing to do with the parameter settings, rather it indicates that the mould is worn and of poor quality, or that the clamping force is not great enough.

When the injection speed was reduced to the (30%, 5%) setting, flow marks could be identified where the melt had flowed into the cavity. This is due to the flow being relatively slow, and therefore can be cooled more in relation to the distance travelled. From this it can be said that as the injection speed is slower, the melt will cool as it runs through the runners and gates and become much more viscous than desired as it enters the cavity, and can seen in the form of visible flow marks, and can become a problem where the aesthetics of the product are paramount.

Although no other defects were evident in the experiment, there were a few defects which may have occurred should the experiment have included more samples. Sink marks and voids are usually observed at lower holding pressures where the outer most part of the melt begins to freeze against the mould cavity wall, and the inner molecules begin to be pulled outwards towards the frozen skin. This can be countered with increasing the holding pressure.

Although not visible in the clear polystyrene samples, sink marks were evident within the polyethylene samples. Since polyethylene is a semi-crystalline, it has a higher shrinkage than the amorphous polystyrene.

Effect of processing parameters and part shape on the mechanical properties of an injection moulded part

The processing conditions must be carefully considered when approaching the manufacture of components in industry since there can be significant alterations to the mechanical properties of the part. Since this experiment was producing ‘dog-bone’ tensile testing components, it is beneficial to have the molecules aligned in the direction of how the bars are to be loaded, since this provides the greatest tensile strength. Another aspect may be the stress in the part, with the component performing best when there is no residual stress since this can act as a stress raiser, particularly when the part is subject to large tensile stresses.

If for example, the manufactured part was to be a commercial product such as a CD case, then the mechanical properties are far less important, since it will be subject to much less wear than a testing component. However, the visual properties will be important, therefore it is vital that the part free from visual defects. Therefore a balance must be struck to balance cycle time and aesthetic part quality.

The shape of the part also has a large effect on the mechanical properties of the final product, as mentioned earlier, if the tensile testing bars are taken as an example, these are relatively long and narrow, meaning that large shear forces can be expected inside the cavity of the mould, whereas taking the CD example, this would result in very uniform properties throughout the part.

4.0 Conclusion

This experiment has identified a number of issues that manufacturers face on a daily basis, to set an injection moulding process which is as efficient as possible, whilst maintaining a reputation to produce high quality products, and striking a balance between these factors.

Throughout the experiment and further analysis of results, a number of conclusions could be drawn up relating the process variables and the ultimate quality of the final product after production.

From the experiment the following conclusions could be made about each variable:

Holding pressure

Increasing the hold-on pressure increases the mass of the component and resulting in defects such as flash.

Increasing the hold-on pressure increases the amount of shrinkage after annealing.

Injection Speed

Increasing the injection speed increases the mass of the moulded component.

Increasing injection speed increases the amount of flash produced on the part.

Increasing injection speed reduces the amount of shrinkage that takes place during annealing.

Melt Temperature

Increasing the melt temperature has no significant effect on the mass of the component.

Increasing the melt temperature reduces the amount of shrinkage that occurs during annealing.