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13. Screw Design
13.1 Background - the technical and business context
Mixing plays a critical role in plastics processing. At low stress, distributive mixing occurs. When more stress is applied, dispersive (intensive) mixing occurs. Dispersive mixing ruptures the cohesive forces that cause components to agglomerate into droplets or particles and thus gives more uniform distribution of the components.
The use of all polymers requires the addition of materials that confer the required performance properties. These can include thermal and UV stabilizers, plasticizers, organic or inorganic pigments, regrind or even physical blowing agents to produce evenly distributed uniform cell structures. To be as effective as possible, these additives need to be dispersed as evenly as possible within the polymer matrix. The technological challenge of uniform pigment and additive dispersion can be addressed by systematically modifying screw designs.
Design of a screw for a specific application will depend on the requirements, which will vary according to the composition of the compound, the equipment and the end application. A host of technological uncertainties may arise during this design process. For example if we need to introduce new mixing elements into a screw designed for an injection machine, special challenges can arise because the plasticating unit -(the plastic melting and conveying section) of an injection machine is relatively short. Moreover, the new screw design may fail to produce the expected dispersion, to the detriment of the end product.
Screw designs must provide the best combination of distributive and dispersive mixing in order to produce the required properties in the product. The screw of an injection-moulding machine is typically a single stage and single flighted conveying screw. Studies have shown that the shear flow generated by screws of this type is inefficient for dispersive mixing. Instead, generation of elongational flow, in which particles such as filler agglomerates undergo a stretching type of deformation without rotational movement, helps in efficient dispersive mixing.
The company is using injection moulding to manufacture a product for the construction market. If a product can be made with a higher impact resistance, then a new application would be possible, which would result in a new product line and increased sales.
13.2 Project description
13.2.1 Section A: Purpose and project description
The development of an improved screw design that will allow us to increase the impact resistance of the product and at the same time increase productivity without increasing production costs.
13.2.2 Section B: Technological advancements Sought
A new screw design that will offer:
* Improved dispersion, without changing the process conditions.
* Consistently improved impact properties.
* Shot repeatability, with reduced variations in part weight.
* The ability to include a higher percentage of regrind materials without sacrificing performance properties.
* Enhanced particulate dispersion without irregularities of colour.
13.2.3 Section C: Technological uncertainty
* Our new product line demands parts with better impact resistance. This means that a new ingredient must be mixed efficiently. For this injection moulding application, we would like to insert the mixing elements into the screw of the injection moulding machine. Although this new design might improve mixing, the process conditions could thereby be changed, giving a process window that might not allow cycle time adjustments. The use of regrind use might be restricted and costs might rise.
* The current process produces a commercially viable product, but variations in part weight and inconsistent impact properties have been reducing productivity. Mixing elements are available that can improve the relative amounts of dispersive and distributive mixing. The available technology allows us to make directional predictions, but the science is not available to make quantitative predictions. Thus experimental development work is required to improve the performance of the screw in this application.
13.2.4 Section D: Summary of work carried out
* The output of the injection screw was found to be much less than its theoretical (calculated) output. Possible reasons for this discrepancy included:
a. A feed problem
b. Wear of the screw or barrel, or
c. Insufficient melting of the ingredients at high screw speeds.
* Measurements of the barrel and screw did not uncover any wear related issues. Changing the barrel temperature settings did not improve the output or the variability of the parts.
* Different chemical measurements (viscosity, solvent extractions and thermo-gravimetric analysis) and measurements of surface properties on different sample parts showed that the dispersion of solids was not uniform. Un-melted resin pellets were even observed occasionally.
* Based on the above investigations, two strategies were undertaken to advance the technology:
1. The design of the non-return valve (NRV) was changed to improve mixing. This was achieved by adding a CRD7 mixing element to an injection screw as follows: mixing pins were placed inside the slide ring of the NRV. The pins were elongated in the axial direction in order to accelerate of the melt fluid as it passed between two pins. This gave elongational flow, which resulted in effective dispersive mixing. Adding similar pins to the outside of the stop subjected the mixture a second time to elongational flow. The large number of pins induced many splitting and reorientation events and the result was efficient mixing.
2. Instead of ground calcium carbonate (GCC), precipitated calcium carbonate (PCC) was used in the composition. Although PCC is costlier than GCC, PCC's particles are much finer and their high specific area and controlled structures make them more effective nucleating agents.
* A set of experiments was designed and carried out, in which the percentages of PCC and GCC were varied in a systematic way to discern their effects on product properties.
* After incorporating the CRD mixer onto the NRV, several runs were carried out and the resulting impact properties were measured. Using more pins allowed the proportion of regrind to be doubled and surface gloss improved considerably.
* To check weatherability, specimens were placed under QUV panel and their impact properties determined after various exposure times. Samples containing GCC showed marked loss in impact properties while samples containing PCC did not.
13.2.5 Section E: Project status
As a result of this work we were able to improve the impact resistance of parts. In addition we produced less scrap and regrind content was doubled from 5 to 10%. Since the work has taught us that mixing can be improved, we anticipate continuing this work in the next year to further increase the regrind cycled content.
13.2.6 Section F: Documentation
* Screw design
* Test results from experimentation
* Literature
13.3 Comments
The description shows that work was carried out in a systematic manner to advance both screw design technology and to improve the technology available to use regrind in our processes.
The work meets the criteria for Experimental Development.
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7 Chris Rauwendaal Device
