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Injection Molding Screws and Extrusion Screws
The basic function of either a injection molding or extrusion screw is to simply convey the material to the mold or die in an even mixture of resin and pressure to properly fill the mold cavities in order to generate a proper plastic part. With that being said there are certain circumstances that require a different design screw. Certain things need to change when the material becomes harder to melt, the addition of certain colors, or the need for more output without generating high heat that will degrade the material. The screw in itself is broken up into 3 different sections. First you have the Feed Section where the plastic is introduced to the screw and barrel and very little to no heat is generated as the plastic slides on the inside surface of the barrel. The second section is the transition section where the material is under more compression and heat and where the material is almost completely molten before heading to the third section. Lastly the metering section is where the screw takes the molten material and mixes it before it goes to the tip assembly and then into the mold.
Screw Tips Designs
There are many different designs for screw tips, 4 piece, 3 piece, Locking , Castel design, non- locking, free flow, standard flow and so on. Every manufacturer that builds for the OEM’s all have their own design.
The most common is the 3 piece Free flow with a non- locking design. This will work with almost any material and is the most forgiving in the process. These tips will generate less heat without shearing and give good flow to the mold. The free flow design will also clean easy for material change overs and cut the time and purging material costs it takes to go from light to dark material. If you run many different types of material this would be the most flexible tip for a plant that has a lot of changes.
The 4 piece tip design will help with wear issues you may have if you run high wear material. The design will give you a front seat and rear seat that are designed to wear and then you just replace the seat and not he retainer or ring. Good Idea but it can shorten the flow path of the material to make room for the second seat and generate more heat in the front zone. Because they are not free flowing they can be harder to clear of materials during change overs.
Locking designs are designed for a more consistent and quicker shut off. The ring turns with the screw and makes sure the ring is in the correct position to shut off every time you rotate the screw. Again the screw tips can be more consistent but there are some draw backs. First because you are turning the ring with the screw it will generate a lot more shear heat in the front end of the screw. Second because you are turning the ring with the screw and you have to make the clearance between the OD of the ring and the wall of the barrel much larger. If not you can have the ring because of the heat annealling itself to the barrel wall and damage the Tip, screw and even barrel. So by having to open up the clearance you have open the tolerances for the material to flow between the barrel and the ring. Clean up on the locking or castel style tips is harder. With the ring tabs to clean as well it just takes more time and purging to get it clean.
Free flowing designs are easier to work with because of the open channels of flow to the end of the tips. Easier to clean and generate less heat in the front zone. Allows for better flow and less degradation of the material. For most any material this is recommended just for the faster process and the ease of use.
Material for tips are three basic materials H13, D2 and CPM9V. There are more but this covers 95% of the tips made. H13 is a very forgiving material. If you don’t run glass filled or talc filled materials or Clear materials this will work very well and is very cost effective to buy. D2 is a material that also is cost effective but will run better on clear materials such as Poly Carbonate. It will run well with most other unfilled materials but H13 is the standard of the industry because of its cost point. CPM9V is very good and the standard for high ware application. It works very well and will last about three times as long in glassed filled applications and H13. Well worth the cost difference. The issue is that a lot of suppliers will send D2 in place of CPM9V because it is cheaper to use. If you see a D2 tip it will be polished to a nice shine, CPM will look cloudy and dull even after polishing. This is a clear way to tell the difference.