Concentric extrusion of rubber over a core.
By using a "helicoid" balanced flow control device which replaces a single flow distributor, this article will advise the readers of the benefits attained. Actual in-plant results will be given on one manufacturer who saved time and money by reducing scrap and increasing productivity.
Concentric extrusion of rubber over a core
Manufacturers of rubber covered rolls supply many users with applications ranging from paper handling rolls to copying machines. Within this available market, many manufacturers of rubber covered rollers require an economical process to produce the rollers in a timely and cost efficient manner.
Current methods and results
Companies who currently extrude a rubber layer of material over a core or mandrel, experience a great deal of scrap and lost time. This is attributed to the uneven flow or delivery of material through the crosshead die design. Due to the eccentricity of the extrudate (figure 1) over the core being covered, concentric grinding after vulcanization is required for the finished product to meet the specifications required to be a saleable product.
The necessary grinding equates to additional labor, material usage and time to produce each piece, plus the scrap cost of the grind material that is not salvageable.
Principles of helicoid-balance flow "distribution"
The helicoid design incorporated in the GENCA crosshead provides a balanced flow of the extrudate, which is distributed evenly over the core or mandrel. The uniform distribution is maintained regardless of changes of extruder RPMs. With the aid of a mechanically balanced device, referred to as a helicoid, the user is now given additional control of the process. This allows the user to reduce the amount of excess material to achieve a concentric roll with uniform wall thickness.
The unique design incorporates an even pressure drop within the crosshead die which ensures repeatable performance and control of the extrudate.
Benefits of controlling the extrudate
Many of the benefits of controlling the extrudate are shown in table 1. An additional benefit of the balance flow helicoiders to reduce the extruded wall thickness adder by half, thus saving the final grinding which becomes waste. Rather than over-extruding a .060" (1.52mm) wall to .100" (2.54mm), .070" (1.78min) is now sufficient.
Table 1 - benefits of controlling the extrudate Material usage Ability to run thinner walls. Less scrap Controlling concentricity enables the user to run lower diameter which reduces scrap and waste due to the grinding operation required. Labor Time and cost are decreased in the secondary operation. Less rejections Total control of the operation relating to end product concentricity (figure 2)
The uniformity of the extruded roll has increased from piece to piece. Actual roll concentricity was .005" (.13mm) on a 2.0" (50.8mm) diameter .070 (1.78mm) wall roll. This means the grinding operation of each roll becomes alike and efficiency of the grinding operation has also increased due to less waste on scrap.
The rolls are pushed through the die in a continuous end-to-end fashion. In older-style dies where the spindle portion of the roll caused the diameter to drop off, the nibber would tear off. Usually, this section of compound can be reused later but the unbalanced pressure flow of the compound occasionally caused the rubber to tear back on one side of the roll. This immediately caused a bad product.
A balanced flow device extrudes with a slightly higher pressure than conventional "pipe-in-pipe" dies, the rubber product has far less porosity after cure. This situation of a small hole showing up in the wall during grinding is a big problem with conventional heads.
Higher production rates have resulted when a balanced flow crosshead replaced a conventional head on a 4-1/2" (114.3mm) extruder. Normal production speed was acquired at 60% of the standard extruder RPM, both upstream and downstream equipment was modified to take advantage of manufacturing speed.
With the reliability of a balanced flow crosshead, the manufacturer reduced the number of rolls he originally covered from 2,000 to 1,100 to insure 900 good finished rolls. At present, the production attained is at 93% quality assurance rate. Inherent compounding situations preclude further reduction of reruns.
The uniform pressure of the compound onto the core roll has resulted in uniform adhesion of the material onto the roll. With older-style dies, the adhesion on the start of the roll where the pressure is light resulted in looseness after vulcanization. As the roll diameter enters the compound flow area, it restricts the room for the rubber to come out and a pressure rise will be seen in the extruder barrel. This constant pressure variation in a conventional unbalanced die head causes differences in adhesion and is the main contributor to wall thickness variations. This situation has been improved by placing the rolls on a rod and using spacer pipes between the rolls.
At times, manufacturers have stated that the run duration is limited on specific compounds. If this condition is resulting from the crosshead rather than the extruder, a balanced-flow head has increased these run durations up to five times that of previous runs. In a balanced head, it is essential that all incoming compound leaves the die head on a smooth path to balance the flow of pressure. The principles of hydraulics are used in a crosshead die that employs a helicoid or a distribution system.
With conventional dies, the line must be started at rated or desired RPM and line speed to attain center. This means making scrap at a fast rate until good concentricity is achieved by adjustments. With a balanced flow crosshead die, good concentricity may be achieved at a slow speed. When line speeds are increased, die center stays where you originally put it.
Actual material savings attained using example (figure 4)
Eccentric extrudate as show (figure 1) equates to a .100" (2.54MM) average wall thickness. Concentric extrudate as shown (figure 2) equates to a .070" (1.78mm) average wall thickness. Actual savings is .030" (.76mm) per side or .060 (1.52mm) on diameter. A 32% material savings is gained prior to grinding by using a balanced flow distribution (helicoid) device.
For roll covering of approximately 2" (50.80mm) and under, direct extrusion appears to the most cost-productive, provided you are aware of the operation parameters.
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|Date:||Dec 1, 1993|
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