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Ram Extrusion of GUR® Ultrahigh Molecular Weight Polyethylene
In this process, resin is fed from a hopper and packed into a cylinder in repeated increments by a reciprocating plunger. The frequency and amplitude of the plunger stroke is controlled by an oil hydraulic system. The compressed material moves through a heated zone where it is fused into a profile matching the cross section of the barrel or die. The output rate is proportional to the length and frequency of the ram strokes. Die length, electrical heater capacity, hydraulic system power and maximum force, and the strength of the construction materials determine equipment capability. Typical extrusion rates are 10 to 20 lbs./hr.
The force on the piston must overcome friction of the dry powder against the die wall as the powder is compressed, and viscous drag of the fused polymer on the wall downstream. Movement of material is incremental (plug flow) and a stick-slip condition exists. As the ram contacts the material in the die, an extremely high force is required to overcome the static friction. The force then decays as dynamic friction becomes controlling.
The largest profile to be extruded will determine the size of the hydraulic unit. The rating for hydraulic systems commonly falls between 10 and 100 tons, with some production machines as large as 400 tons. The hydraulic unit usually consists of a double acting piston, an oil pump with circulating system, and controls for varying the length and frequency of the piston action. The holding plates and spacers are of heavy construction steel to withstand the large forces required.
The die, normally honed to a fine finish, is fastened to the forward mounting plate and is of very heavy construction steel to withstand the high internal stresses that are generated. The die is temperature controlled by electrical heaters, usually divided into three zones. Die lengths of up to 10-13 ft may be required. A slot is machined into the die for feeding the resin. The plunger is attached to the hydraulic unit and can be considered an extension of it. In most cases, it is the same shape as the extruded profile. Dies of different sizes and shapes can be attached to the same hydraulic unit. In order to produce tubes, a fixed or floating design mandrel must be installed.
Thin wall cross sections are more difficult to make than heavy wall types because the pressure required on the ram increases markedly. The minimum wall thickness achievable is limited by the maximum available pressure and the maximum strength of the steel. The expected dimensional shrinkage after leaving the die is 5.5-6.5%.
The main function of the take-off equipment is to support the extruded profile as it cools with a minimum of drag. Tubes and rods can be extruded onto a guide rail arrangement.
The die should have 80% of its surface area covered by electric heaters. The heaters should be arranged in three zones, the first and last zone being shorter than the middle zone. The thermocouple is located in the middle of each zone. The first zone should be controlled at 320-360°F; the second zone 425°F and the last zone 320-360°F. The power output should be designed for 1 KW per 2.2 lbs./hr. For example, if the output is 22 lbs./hr, the die should be designed for 10 KW electrical power. The heated length starts at 4” downstream of the feed opening on the die. This section around the feed throat should be provided with cooling channels in order to cool the feed section. All the exposed wiring connections should be covered to protect against electrical shocks. The use of an insulation jacket for the die is desirable for personnel safety and energy conservation. Ticona recommends using Ampco 18 aluminum bronze for the ram. The ram clearance should be 0.004” maximum. The die barrel should have an inside 16 microfinish minimum and be made of tool or hardened steel.
With a three-shift operation running five days a week, each extruder less than 2” diameter should be producing at least 50,000 lbs./year; above that diameter 100,000 lbs./year.
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