Few parts of a molding machine are more important than an engine that melts and injects plastic. This combined screw, barrel, and mechanical front end will have a significant impact on the melt quality, the quality of the molded parts, and the economics of operation. How do you solve the basic problems of screw and Injection Molding Screw Barrel handled on these pages.
hardware
Since many screws and barrels are purchased as replacement parts, your supplier must first understand what you are currently installing in the machine. Whose machine is it? What model, serial number and year of production? What is the clamping force and rated injection volume of GP Polystyrene? What about the stroke, maximum screw speed and maximum injection pressure?

Understand wear
Three basic types of wear occur on the screw and barrel. Knowing which type is present on the equipment will help you take corrective actions for existing barrels or screws or when ordering new or replacement parts.
Abrasive wear. As the name suggests, abrasive wear is surface damage caused by fillers such as talc, calcium carbonate, glass fibers, and even titanium dioxide pigments (used in white plastics). These hard particles will scrape a bit of metal from the screw or barrel with each contact. Glass fibers in particular wear the roots of the leading edge of the screw, usually in the transition or compression zone, where the fiber has been exposed due to some melting, and the partially melted particles are extruded onto the screw and barrel. The hard surface materials applied to the screw and barrel help slow down this wear.
Adhesive wear. This is basically abrasion caused by metal-to-metal contact. When excessive friction heating occurs, certain metals will "weld" to each other and pull apart as the screw rotates further. Proper clearance and alignment, compatible materials and proper hardness can prevent this type of wear.
Corrosion wear. When the resin overheats and releases corrosive chemicals, it usually causes the surface of the screw and barrel to be chemically attacked. The most common is the hydrochloric acid released when PVC degrades. Other resins that may release corrosive chemicals include ABS, polycarbonate, cellulose, polysulfone, fiber sizing agents, and flame retardant materials. The best preventive measure for this problem is to avoid overheating; do not leave the machine idle for a long time at operating temperature. Corrosion-resistant screw and barrel materials should be specified.
What is the screw diameter and L/D? What type of screw design is it-metering, mixing, universal, barrier? Single-stage or two-stage? What is the state of the screw? barrel? Is the problem abrasive wear, adhesive wear or corrosion wear (see box on the right)?
Maybe you are considering expanding or reducing the scale of the project-replacing the screw and barrel to obtain better processing conditions. Do you need to expand the size because you can now install molds for large parts on smaller presses? Do you need to reduce the size to avoid the material staying too long in small parts that need to be run in a larger press? Your supplier will want to know the details of that particular part—material, shot size (including runners and runners), and cycle time.

Process conditions
Your supplier also wants to know the exact nature of any problem and the conditions of the problem. Do you have an existing screw recovery time problem? What is the melt temperature you are running? How does this compare to the setting recommended by the material supplier? What is the difference between the barrel temperature set point and the actual temperature of the front, middle and rear zones?

Do you want to improve the current cycle time? Are you experiencing melting decompression? What is the normal screw speed? What about back pressure? What is screw motor hydraulic pressure?

material
What material do you want to run on this machine? It is important to know what additives or reinforcing materials are contained in the resin, especially flame retardants, glass fibers or minerals. Outgassing of volatiles is the main cause of corrosion and wear of the screw and barrel. Is the material refractory? What percentage of waste is reused? Can you calculate the actual residence time of the material?

Mixed problem
How important is the blending function to your operations? Although we don't have enough room to discuss this topic in full here, screw design is usually conducive to distributed mixing or dispersive mixing.

Factors affecting wear
The screw, barrel and drive are aligned.
The straightness of the screw and barrel.
Single Screw Barrel design.
The barrel is heated evenly.
The material being processed.
Abrasive fillers, enhancers, pigments.
The surface of the screw and the lining material of the barrel.
The combination of screw surface and barrel wear.
The barrel is improperly supported.
The load on the discharge end of the barrel is too large.
corrosion.
Excessive back pressure during recovery.
High screw speed.
Distributive mixing is basically the process of separating one component into another. Dispersive mixing is the process of reducing the size of smaller components to improve their distribution in the main components. Generally, distributive mixing is preferred because dispersive mixing involves high shear and the resulting temperature increase.
Understanding how screw design affects mixing is the basis for this choice. If you are using strictly pre-colored materials and do not mix in waste, then melting the plastic is your main goal, not mixing it. However, if you are metering color or additive concentrates on the machine, or the scrap recovery rate is high, the mixing function of the screw is essential to obtain good part quality.

Additives or color concentrates with a large particle size require more mixing than additives or color concentrates with a small particle size. The high concentration level in the color masterbatch means that fewer particles are required, so mixing is more difficult. When the additive is fed near the throat, it usually means that the additive is less dispersed when it enters the barrel.

Finally, the specific resin type will also vary. Crystalline resins are more difficult to mix than amorphous resins because they are less likely to melt. Inorganic additives such as talc or calcium carbonate will agglomerate under pressure and require a lot of shear to decompose again.