What are the important principles of plastic extrusion molds?

Important principles to remember about extrusion. These principles can help you save money, produce high quality products and use your equipment more efficiently.

First Principle: Mechanical Principles

The basic mechanism of an extrusion die is simple - a screw rotates in a barrel and pushes the plastic forward. The screw is actually an incline or ramp that wraps around a center layer. Its purpose is to increase pressure in order to overcome greater resistance. In the case of an extruder, there are three types of resistance to overcome: the friction of the solid particles (feed) against the barrel wall and their mutual friction during the first few revolutions of the screw (the feed zone); the adhesion of the melt to the barrel wall; and the logistic resistance within the melt as it is pushed forward.

Newton once explained that if an object does not move in a given direction, then the forces on that object are balanced in that direction. A screw does not move in an axial direction, although it may rotate laterally and rapidly near the circumference. Therefore, the axial force on the screw is balanced, and if it exerts a large forward thrust on the plastic melt then it also exerts an equal backward thrust on the object. In this case, the thrust it exerts is on the bearing behind the inlet - the thrust bearing.

Most single screws are right-hand threaded, like screws and bolts used in woodworking and machinery. If viewed from the back, they are counter-rotating as they try to screw backward out of the barrel. In some twin-screw extruders, the two screws rotate backwards and cross each other in both barrels, so one must be right-handed and the other left-handed. In other occluded twin screws, the two screws rotate in the same direction and therefore must have the same orientation. However, in either case there are thrust bearings that absorb the backward force and Newton's principle still applies.

The Second Principle: The Thermal Principle

Extrudable plastics are thermoplastics - they melt when heated and solidify again when cooled. Where does the heat for melting plastics come from? Feed preheating and barrel/die heaters may play a role and are important at startup, but motor input energy - the frictional heat generated in the barrel as the motor turns the screw over the resistance of the viscous melt - is the more important source of heat for all plastics. systems, low-speed screws, high melt temperature plastics, and extrusion coating applications.

For all other operations, it is important to recognize that the barrel heater is not a major source of heat in the operation, and thus may play a smaller role in extrusion than we might expect (see Principle 11). Rear barrel temperature may still be important because it affects dentition or the rate of solids transport in the feed. Die and mold temperatures should normally be the desired melt temperature or close to it, unless they are used for a specific purpose such as varnishing, fluid distribution, or pressure control.

The third principle: the principle of deceleration

In most extruders, changes in screw speed are accomplished by adjusting the motor speed. The motor usually turns at a full speed of about 1750 rpm, but this is too fast for an extruder screw. If it rotates at such a high speed, too much frictional heat is generated and the retention time of the plastic is too short to prepare a homogeneous, well-mixed melt. Typical reduction ratios are between 10:1 and 20:1. The first stage can be either a gear or a pulley set, but the second stage is all gears and the screw is positioned in the center of a large gear.

In some slow-running machines (e.g. twin screws for UPVC), there may be three stages of reduction and speeds may be as low as 30 rpm or less (ratios up to 60:1). At the other extreme, some very long twin screws used for mixing can run at 600rpm or faster and therefore require a very low deceleration rate and a lot of deep cooling.

Sometimes the deceleration rate is incorrectly matched to the task - there will be too much energy not available - and it is possible to add a pulley block between the motor and the deceleration stage that changes speed. This either increases the screw speed beyond the previous limit or reduces the speed allowing the system to run at a greater percentage of the speed. This will increase available energy, reduce amperage and avoid motor problems. In both cases, output may increase depending on the material and its cooling needs.

Principle #4: The feed takes on the role of coolant

Extrusion is the transfer of energy from the motor - and sometimes the heater - to the cold plastic, thereby converting it from a solid to a melt. The input feed is cooler than the barrel and screw surfaces in the feed zone. However, the barrel surface in the feed zone is almost always above the plastic melting range. It is cooled by contact with the feed particles, but the heat is maintained by heat transfer from the hot front end to the back end and by controlled heating. It may be necessary to turn on the rear heater even when the front end heat is maintained by viscous friction and no cartridge heat input is required. The more important exception is the slotted feed barrel, which is almost exclusively used for HDPE.

The screw root surface is also cooled by the feed and adiabatic from the barrel wall by the plastic feed particles (and the air between the particles). If the screw suddenly stops, the feed also stops and the screw surface becomes hotter in the feed zone as heat moves backward from the hotter front end. This can cause particles to stick or bridge at the root.

Principle #5: Sticking to the barrel and sliding to the screw in the feed zone

In order for a single-screw extruder to reach the solid pellet capacity in the smooth barrel feed zone, the pellets should stick to the barrel and slide onto the screw. If the pellets stick to the root of the screw, there is nothing to pull them off; the channel volume and the inlet volume of solids are reduced. Another reason for poor adhesion at the root is that the plastic may either thermo-condense here and produce gels and similar contaminating particles, or intermittently adhere and break off with changes in output speed.

Most plastics naturally slide on the root because they are cold when they enter and the friction has not yet heated the root to the point where it is as hot as the barrel wall. Some materials are more likely to adhere than others: highly plasticized PVC, amorphous PET, and certain polyolefin copolymers with adhesive properties that are desired for end use.

For the barrel, it is necessary for the plastic to adhere so that it can be scraped off and pushed forward by the screw threads. There should be a high coefficient of friction between the pellet and the barrel, which in turn is strongly influenced by the temperature of the back barrel. If the particles don't stick, they just turn in place and don't move forward - that's why smooth feeding is bad.

Surface friction is not a factor in feeding. Many particles never touch the barrel or screw root, so there must be friction and mechanical and viscosity interlocking within the granulate.

Slotted barrels are a special case. The slot is in the feed zone, which is thermally insulated from the rest of the barrel and is deeply water-cooled. The threads push the particles into the trough and create a high pressure over a fairly short distance. This increases the amount of occlusion allowed for the same output at lower screw speeds, resulting in less frictional heat generated at the front end and lower melt temperatures. This can mean faster production in cooling-limited blown film lines. Grooves are particularly suitable for HDPE, which is a slippery common plastic except for fluorinated plastics.

Principle #6: Spend more on materials

In some cases, material costs can account for up to 80% of the cost of production - more than all other factors combined - except for a few products where quality and packaging are particularly important, such as medical catheters. This principle naturally leads to two conclusions: processors should reuse as much trim and scrap as possible in lieu of raw materials, and adhere to tolerances as strictly as possible to avoid deviations from target thicknesses and product problems.