Injection Screw Barrel

Barrel
The barrel is a metallic tube that surrounds the screw. It forms the outer boundary of the screw channel as in an extruder. For injection molding, one can assume that the major portion of the heat that must be applied to the plastic is supplied by the barrel.

Non-return Valve
A non-return valve is a component at the foremost section of the screw that prevents the backflow of the plasticated material during the injection and holding pressure stages. It can accomplish this duty best if it produces a high-pressure loss of small free cross-sections that can be closed rapidly. illustrates the principle of the ring-type non-return valve, which is the most common.

Screw Tips
The highest pressure occurs at the screw tip, which is the front end of the screw. Therefore it makes sense from a processing viewpoint to prevent the backflow of the material into the rear flights using closing elements. This is especially important during the injection and holding pressure stages. It is advisable to use a screwed-on tip with a larger diameter than that of the screw root at the foremost end of the screw. The narrow gap with the barrel causes a pressure rise that restrains the backflow.

Melting and Mixing
The screw barrel is responsible for melting and mixing the raw plastic material. The screw, with its helical design, applies mechanical and thermal energy to the plastic material, causing it to melt and mix with other additives or colorants.

Conveying
The screw barrel also functions as a pump, pushing the molten plastic material through the extruder. The rotation of the screw creates pressure, which helps in the continuous movement of the material.

Plastic Extrusion
The assembled extrusion screw barrel is an integral part of extrusion machines used for manufacturing various plastic products. It is used in processes like film extrusion, sheet extrusion, pipe extrusion, profile extrusion, and extrusion coating.

Temperature Control
The screw barrel may have heating or cooling elements embedded in it to control the temperature of the molten plastic material as required during the extrusion process.

Melting And Homogenization Of Plastic Materials
The primary function of the injection molding barrel is to melt and homogenize the plastic materials. As the plasticizing screw rotates within the barrel, it generates heat through friction, melting the solid plastic pellets or granules. The barrel’s internal heating elements provide additional heat as needed, ensuring complete melting and homogenization of the plastic material and eliminating any inconsistencies or unmelted particles.

Abrasion And Corrosion Resistance
The plasticizing screw inside the barrel is subjected to significant wear due to the abrasive nature of plastic materials. To counteract this, manufacturers use durable, wear-resistant materials to construct the barrel, enabling it to withstand repeated friction and contact with the plasticizing screw.

Pre-Forming Preparation
The main preparations before the injection molding cycle are inspection of the appearance and process performance of raw materials, preheating and drying; cleaning or replacement of the barrel of the injection machine. For mold release difficulties, a reasonable choice of release agent; preheating the insert, some molds also need to preheat. 

Injection Process
Feeding
Adding granular or powdered plastic to the hopper of the plastic injection molding machines.

Plasticizing
The plastic material in the screw is heated by the heating device of the injection molding machine to melt and become a plastic melt with good plasticity.

Injection
The plasticized plastic melt is pushed by the plunger or screw of the injection machine, and passes through the nozzle and the pouring system of the mold at a certain pressure and speed to enter and fill the mold cavity.

Pressure-Holding And Shrinkage Replenishment
After the melt fills the cavity, the melt still keeps the pressure for replenishment under the push of the plunger or screw of the injection machine, so that the melt in the barrel continues to enter the cavity to replenish the shrinkage needs of the plastic in the cavity, and it can prevent the melt from backflowing.

Cooling After Gate Freezing
After some time make the molten plastic in the cavity solidify into a solid, to ensure that the plastic part has enough rigidity to not warp or deform when the mold is released.

Demoulding
The molded part is cooled to a certain temperature, and the molded part is pushed out of the mold by the push-out mechanism.

Post-Treatment Of The Molded Part
Annealing Treatment
Annealing treatment is to make the plastic parts in a fixed temperature heating liquid medium (such as hot water, hot mineral oil, liquid paraffin, etc.) or hot air circulation oven for some time, and then slowly cool. Its purpose is to eliminate the internal stress of the plastic parts and stabilize the size.

Humidity Adjustment Treatment
A post-treatment method to put the plastic parts that have just been remolded into the heating medium (such as boiling water, or potassium acetate solution) to accelerate the moisture absorption equilibrium speed. (Mainly used for plastics with strong hygroscopicity and easy oxidation, such as PA)

Types
Adhesive Wear
This occurs when metal surfaces (screw flight land and interior surface of the barrel) come in contact. Extremely high stresses occur when the surfaces briefly come in contact and weld together, then fracture. Evidence of adhesive wear can be a burr or "rolling over" of the trailing edge of the flight land or where hard-facing material is either scored or missing from the tops of the flights. The barrel should also be checked for scoring or galling of the I.D. at the point where the damage on the screw was located in the barrel.

Abrasive Wear
This may be the most common type of wear in plastic machinery. It can be minimal in cases where unfilled or low-viscosity polymers are being processed. As pellets are transported down the screw and compressed against the barrel wall, the shearing action will cause some degree of wear on the screw flight and the barrel wall. Abrasive wear can become significant when processing resins with fillers, additives, or contaminants in the regrind, or even when processing high-viscosity polymers. Hard materials tend to remove material from softer surfaces. For this reason, the flight lands are usually hard-faced with abrasion-resistant alloys. Glass- and mineral-filled resins can also cause wear to the root of the screw, usually in the rear of the channel, before the additive wetting and mixing with melted plastic.

Corrosive Wear
Corrosion is the gradual destruction of materials by chemical attack. Most of the atoms on the same metal surface are oxidized, damaging the entire surface. The breakdown of the metal surface allows the surfaces to be vulnerable to the mechanism of mechanical wear. Corrosion is most commonly experienced when processing PVC or polymers in the fluoropolymer family and will aggressively attack components that have a high iron matrix.

Causes
Straightness, Concentricity, and Alignment
When preparing to install a new or rebuilt screw, the screw should be carefully checked for straightness and concentricity. If the old screw is available, it’s a good idea to compare the two for overall length, shank, and drive configuration. Next, if the screw is to be installed in a new barrel (or the existing barrel is available and clean), slide the screw into the barrel and rotate it by hand, if possible, to assure straightness and concentricity.

Poor Screw Design
When the capacity of the melting section of a screw is less than the amount of unmelted material being delivered by the feed section, it can cause the channel to plug with unmelted material. The pressure generated forces the screw against the barrel on the opposite side, causing galling and damage to the screw flight as well as the barrel I.D. surface. This condition is often referred to as solids wedging. Solid wedging often occurs when a screw that was originally designed for processing a specific resin is run on a different polymer, with a different melting rate.

The Effects of Poor Screw Design in Plastics
When the capacity of the melting section of a screw is less than the amount of unmelted material being delivered by the feed section, it can cause the channel to plug with unmelted material. The pressure generated forces the screw against the barrel on the opposite side, causing galling and damage to the screw flight as well as the barrel I.D. surface. This condition is often referred to as solids wedging.

Abrasive Fillers, Additives, and Foreign Materials
As applications for polymers and plastics become more specialized and more demanding, abrasive fillers and additives have necessitated improved protection from wear and corrosion. Resins with glass fibers, minerals, flame retardants, stabilizers, and other additives require consideration of their impact on abrasive and sometimes corrosive wear in extrusion or molding with these materials. Some of the most aggressive fillers can reduce screw/barrel useful life by 50% or more. If screws and barrels are manufactured with proper protection, wear life can be extended and abrasive wear reduced considerably.