Twin Screw Extruder for Engineering Plastic

Main feature: D type gearbox. Capacity can be 800kg/h.

Engineering Plastic Compounding is the main application for twin screw extruder. It is used in a wide variety of application.

 Polymers are blended with their compounding ingredients in extruders. The blending of basic components such as antioxidants and light stabilisers is done either during the last stages of the polymerisation process or soon after in an extrusion step which is followed by granulation. In fact most if not all plastic granules sold in bags, have been subjected to the effects of heat as well as shear, once after their production and another time at the compounders who produce special grades and colours. Compounding extruders are classified in single screw and twin screw extruders. The mixing effect of single screw extruders relies on the shear generated between the walls of the barrel and the screw. The shear mixing of twin screw extruders is more efficient and as a result the polymer is subjected to less thermal history.

Twin screw extruders, are used for bulk polymerisation, processing (pipes, sheet..) or compounding. Bulk polymerisation twin screw extruders are used in the manufacture of TPU, POM etc. Twin screw extruders are also used, like single screw extruders in the manufacture of finished products such as pipes and sheet. This short article will be confined to giving an account on twin screw extruders used in the compounding of engineering thermoplastics. In this case the extrudate takes the form of thin rods which are pelletised. The pellets are injection moulded.

HOW DOES EXTRUSION WORK?

Extrusion processing aims to physico-chemically transform continuously viscous polymeric media and produce high quality structured products thanks to the accurate control of processing conditions.

Our twin screw extruders consist of two intermeshing, co-rotating screws mounted on splined shafts in a closed barrel. Due to a wide range of screw and barrel designs, various screw profiles and process functions can be set up according to process requirements. Hence, a twin screw extruder is able to ensure transporting, compressing, mixing, cooking, shearing, heating, cooling, pumping, shaping, etc. with high level of flexibility. The major advantage of intermeshing co-rotating twin screw extruders is their remarkable mixing capability which confers exceptional characteristics to extruded products and adds significant value to processing units.

In twin screw extrusion processing, the raw materials may be solids (powders, granulates, flours), liquids, slurries, and possibly gases. Extruded products are plastics compounds, chemically modified polymers, textured food and feed products, cellulose pulps, etc.

The Basic Formula

General Engineering Plastic	Advanced Engineering Plastic	Additives
PA6, PA66, PA46, PPA	PBI	Glass fiber
ABS	PAI	Carbon fiber
POM-C, POM-H	PEEK	Flame retardant
PET	PPS	UV stabilizer
PC	PEI	Anti-oxidant
PBT	PSU	Anti-static
PE-(U)HMW	PVDF	Color pigment

 

Applications

• Automotive (Intake manifolds, Header tanks, Filters, Fans and Shrouds, Engine covers, etc.)
• Electrical (Terminal block, Cable ties, Connectors, Power tools, MCCB, relay, light, etc.)
• Blow Moulding (Medical & Cosmetic container, Lubricant & Paint container, etc.)
• Film & Pipe Extrusion (BOPA, BOPET, PET Sheet , Pipe , etc. )
• Fiber (PA fiber, PSF, FDY, PSF, etc.)
• Other Industrial (Construction, House wares, Furniture, Toys, etc.)

 

 

1) Short Fiber Reinforce Compounding

Add the roving shape or chopped glass fiber into twin screw extruder directly and compounding with polymer, you will get 4mm length pellets with glass fiber length 0.2~0.3mm.

a) Roving glass fiber
Locate 20 pieces of roving fiber on the support frame with weighting transducer- Mettler Toledo.
Put the fiber into the venting port on the fifth barrel. They will be taken into the barrel by twin screw.
With weight controller to indicate the feeding rate (kg/h). You can adjust the feeding capacity by increase/decrease the screw speed or number of fiber.

b) Chopped glass fiber (with length 4~6mm)
Feed into the fifth barrel by two stage side feeder:
The top stage is loss-in-weight feeder with output control.
The bottom stage feeder works with fixed speed for force feeding.

 

2) Long-fiber Thermoplastic (LFT)

LFT parts are typically manufactured using one of three different processing methods as the following Figure 1.

 

① Glass Mat Thermoplastics Sheet

Add the roving shape glass fiber into the T-die, then coating by the melting polypropylene PP or PET from extruder. You will get the sheet with width 600~1000mm and the thickness 0.3~1.4mm. It is used for wall panels of Truck/Trailer Vehicles.

• For PP base sheet, single screw extruder is ok.
• For PET base sheet, it should be twin screw extruder with the following advantages:
  • Pre-drying-free technology
  • High efficient vacuum system keeps IV drop minor
  • Optimized screw design avoids yellowing

 

② LFT-G (Long-fiber Thermoplastic Compounding for Indirect Moulding)

Add the roving shape glass fiber into the impregnation mould, then coating by the melting polypropylene (PP) from Twin screw extruder. You will get the 6-16mm pellets with the same length glass fiber inside. The pellets will be used for any product by injection molding machine.

The modulus of LFT-G is double than that of Short fiber reinforce thermoplastic, while impact tension is four times, can keep its function without any change under the temperature of minus -30°C.

 

③ Direct LFT technology

Involves combining the raw materials (fiber and matrix) immediately before entering the molder and avoids the production of an intermediate. This gives the manufacturer the ability to enhance the properties of the resin as required for the application by adding fillers, fire retardants and additives ,while also significantly reducing material costs and only establishing a single heat history of the matrix. This direct process can be separated into two process:

a) LFT-D-IMC (Long-fiber Thermoplastic Injection Molding Compounding Process)
Melts the polymer and mixes it with the fiber, is attached to an injection molding machine.

b) LFT-D-ECM (Long-fiber Thermoplastic Extruder Compression Molding)
Uses two extruders system for the polymer melting and fiber dosing coupled with hydraulic press for compression molding.
There are two primary advantages to LFT-D-ECM over the other two technologies.
First is that the compounding and fiber mixing steps are separated and therefore both extruders can be individually optimized for their specific functions.
Second, the material is not subjected to the same high levels of stress as in injection molding, which helps to maintain fiber length. This is critical because the mechanical properties of fiber reinforced thermoplastics are strongly dependent on the fiber length. Therefore, LFT-D-ECM process gives a maximum degree of freedom in terms of optimizing material choice, polymer modification, and process parameters, which is important for automotive applications.