Few days back a friend called me, from Rajkot regarding to machinery. He is already in plastic industry since 5 years and currently planning for installing Plastic waste reprocessing unit and plastic film extrusion unit. From our discussion one can conclude that he is very much confused about which supplier to trust and what major aspects to consider before purchasing machine. And this I think is a biggest concern for those who are completely fresh in the business.
There are major two reasons for these confusions, Many often it happens that buyer is not that much technically sound being new comer in the industry and another reason is sometimes sales people of machine create confusion in between what is and what is not PERFECT for buyer in term of equipment / technology.
So, there are few things for everyone should be knowing before making investment in machinery because it is a long term decision and you don’t want to spend more after purchasing machine.
Since last 20 years or so, plastic industry has grown manifold with speed of light. In terms of quantum and in terms of quality as well. Constant and continuous inventions in the industry led to sophistication in design of screw barrel compatible to particular resins. Just by altering screw design with technical considerations, one can get 2 to 3 times more output than 25 years ago on the same screw size.
An Ideal screw, which we can call STANDARD SCREW is a single stage with single flight. By altering one(or more) of following we can perfectly compatible screw with comparing resin or end product
- L/D Ratio
- Channel Depth
- Compression Ratio
- Helix Angle
It is the ratio of the flighted length of the screw to its outside diameter. The ratio is calculated by dividing the flight length of the screw by its nominal diameter. By increasing L/D ratio following changes are observed
- More shear rate can be uniformaly generated in the plastic without degradation
- Generate opportunity for mixing resulting in better homogeneity of the melt.
- Greater the residence time of the plastic in the barrel possible permitting faster cycle of larger shots.
The ratio of the channel volume in the feed zone to channel volume in the meter zone is refereed as the compression ratio. It typically ranges from 1.5 : 1 to 4.5 : 1 for most of the thermoplastic resins.
The higher the compression ratio the greater the resulting and….
- Shear heat imparted to resin
- Heat uniformity to melt
- Potential of creating stresses in some resins.
It is angle of a screw flight relative to a plain perpendicular to the screw axis. Although the helix angle is not commonly altered from the standard square pitch such a change can have majority impact on processing.
A reduced pitch is very common in screws that have short L/D ratio, Two stage screw design or developing mixing section.
A change to a smaller helix angle, hence more flight turns per diameter
- Increases the axial melting length
- Conveys stiffer resins with greater ease and requires less troque
- Reduces the rate at which resin is conveyed
Channel depth in the meter zone of the screw is determined according to the resin to be processed. The channel depths of Feed and Transition zones are depended upon the selected compression ratio and screw profile.
It influences the degree of shear heat developed by the screw and the throughput of the screw. For ex. A Shallow Screw Channel……..
- Increase exposure of plastic to heated barrel.
- Increases the shear heat imported to the resin structure
- Reduces the potential throughput of screw
Deep screw channel would have just the opposite effect helping reduce the shear heat and potentially increasing the throughput of screw.
Normally screws are made of Nitrated Steel. But in easily processed material like LDPE / PP screw can perform well and durable while for engineering material such as Nylon-Glass Filled it might be opposite results. So considering resin / durability it can be hardened in many ways. It can be chrome plated after polishing or It can be Bi-metalised or it can be flame hardened etc.. Definitely the cost is also varied depending upon post treatment is selected.
So Final selection should be made only on the basis of direct practical experience with with the design in the question or on the basis of manufacturer’s guarantee of performance
(PART 2 on 30/12/2017)