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Functions of a Plasticating Screw
Injection Molding Screws
and Extrusion Screws
Functions of a Plasticating Screw
Whether the molding application requires that we process a LCP material with a cycle time of 30 seconds or a Polypropylene with a cycle time of 95 seconds, every plasticating Screw has to perform certain functions in order to process polymeric material effectively and produce a high quality molded part.
The functions are:
1. Feed the resin consistently
2. Melt the resin uniformly
3. Pump the fluid steadily
4. Mix the resin homogeneously
Feed Screw Design
Processing most materials on General Purpose feed Screws is not a problem, using the machines control parameters like screw speed, back pressure and temperature. Material suppliers recommend very little change in Screw design from one family of materials to another. Molders often address screw design to overcome processing problems like color distribution or issues arising from improving overall process cycle time.
Overall process controllability and constancy being the goal, concerns for a Screw section lengths and flight depths are of more concern before any type of mixer. Other factors like flight pitch, length to diameter ratio and radius can play a part in improving a Screw’s function.
Processing PP with talc and color requires the use of a mixing section to allow for both good Screw recovery (output) and to meet your mixing requirements. We would recommend a design of mixer that is considered a full volume mixing action. Unlike the type you that would split the flow into two channels, this type of action splits the melt into small segments and blends only in small segments. These types of mixers also count on increased pressure by reducing the flow volume (area) of flow, thus building up the pressure before going thru the mixing section. This higher pressure reduces the output of the Injection Molding Screw.
Our HTS - 100 mixer provides a very effective mixing reaction without any increase in pressure, change in flow direction or dead zones to trap materials. Using a kneading action that transfers the full volume of material flow into and over it self in our application 6 to 8 times, with 18 parameters that can be changed or tailored to provide the most effective fit for a customers needs.
The base parameters for each section of the injection Molding Screw and Extrusion Screw are described below.
Feed Section
Lengths should be to a minimum, based on projected shot size. Excessive length is wasted processing space. Front or push side flight radius very small for proper feeding, back or trailing side radius should be larger to increase strength and can be used to control input volume. Heat Tech Systems Inc. has been supplying Plasticizing components for Injection Molding, Extrusion and Blow Molding Industries for over 25 Years. Our Staff has the back ground to help you with your GP Product needs or developing a special design for your application.
Transition Section
Very long compression length impart the shear or drive energy more uniformly, allow more electrical energy into the process and increase the length and effect of any back pressure applied.
Meter Mix Section
Shorter length can be adequate when using a type of distributive mixer to interrupt the problems associated with lamina flow, in the screw, nozzle body and hot runner systems. Our Products are made out of the highest quality materials in the Industry. We use material specifically designed to help increase our customer's productivity.
Compression Ratios
Much higher compressions that range from 3.2 – 3.8:1 can be used, the shear impact is more that overcome with a long transition length. The required compression should be lower for longer L/D screws and higher for lower L/D Screws.
Flight Depth
Without making major reductions in the screws output or recovery times, depths should be kept low. Less material in the screw improves overall control over process and increase the window of materials that can be processed.
Once a polymer is molten, the flow model is a straight flight section keeps the material in sections. Braking up the flow can improve the thermal uniformity, thus reducing problems like flow lines and voids in the part. The right type of mixer will also allow the screw to purge or change material faster.
The General Purpose Screw, if designed properly, can perform these functions satisfactorily.
Cycle time, shot size pellet geometry, back pressure and screw geometry determine residence time. When the cycle time/shot size relationship is outside of the melt curve, the General Purpose Screw design has a difficult time performing the melt and mix function. When the application is on the low side of the residence time it is also true that the design will not have enough melting capacity and/or mixing ability to properly prepare the melt.
Conversely, when there is too much residence time, the screw is most likely to be too deep in the metering section to
Ensure a good melting and mixing and a good melting strategy play a major role in achieving an adequate blend and ensure good melting and mixing. A good melting strategy plays a major role in achieving an adequate process. Our Screws and Tips are Designed to meet all OEM Specifications and improve our customer's productivity.
Difficult Task
In defense of the General purpose Screw, the injection molding process requires that the plasticating Injection molding Screw be very versatile. Not only does it have to perform the above mentioned functions, but it has to reciprocate the equivalent of five diameters in length. Effective melting length is changed and the solids bed is disrupted. Cycle time generally vary from 4 to 120 seconds, which affects the residence time of the polymer. The RPM reaches 1000 mm/sec in circumference speed to facilitate an extremely short cycle time and the hydraulic back pressure control can reach 5000 psi of plastic pressure. This affects the pumping ability and the pressure flow of the drag flow equation.
If the application were similar to and extrusion process where most of the variables are known and the Extruder Screw is in fixed positions, it would be easier to study the process and design a Injection Molding Screw to meet each objective. This explains why all of the math models and melt simulation packages are supplied for the extrusion process. In an Injection Molding application there are more variables involved and more importance put on adopting a melting strategy that requires good decisions by the technician. Why not use a barrier design? It is likely other screw suppliers will recommend the use of some type of barrier flight. Barrier screws provide more efficient melting providing increase output or screw recovery times. This advantage is offset by the loss of control; barriers impart more drive energy by using both upward root compression and cross compression, which can be unstable with any ambient changes in the process. There is also a risk of degrading, deep flight stagnant areas in which there is reduction in the flow of material tend to hole and allow for degrading, this can result in the degrading material flaking off and cause black specking and increased scrap.
Mixing Nozzles
