L/D RATIO:
The L/D ratio is commonly known as the flighted length of the screw.L/D ratio is the distance from the start of the feed pocket to the register face at the end of the metering section. Most screws have a flight lead (pitch) equal to the flight O.D. To determine the L/D ratio,divide the flighted length by the screw's O.D.
COMPRESSION RATIO:
Compression ratio refers to the difference between the depth of the metering root diameter and the depth of the feed root diameter. The most common compression ratios are 2:1 to 3:1. Typically referred to as general purpose compression ratios.Most resin manufacturers can provide you with information that will allow you to select the correct compression ratio.
SCREW PROFILE:
Screw profile refers to the number of flights that make up each section of a screw: feed, transition, and metering. Screw profiles can be changed to achieve specific results for the resins being processed.
It is not uncommon to change one or all three areas of the screw geometry to improve the screws processing capabilities on many resins.
SCREW MATERIAL SELECTIONS
ALLOY STEELS:
PPE provides many alloy steels for general purpose screw applications. AISI 4140, AISI 4340, and nitralloy 135M are typical examples.
Many different surface treatments are used with these alloy steels to improve their performance in more demanding applications. The alloy steels provide a very good torsional strength and average or below average wear characteristics without some kind of surface treatment.
TOOL STEELS:
PPE manufactures screws from many different tool steels:H-13, D-2, and CPM9V. These materials can offer excellent wear characteristics on the flight O.D. and root diameters when running high percentage filled resins. Typical drawbacks with tool steel screws are they cannot be repaired and they lack torsional strength.
STAINLESS STEELS:
Many different grades of stainless steel materials are available from PPE to suit many applications. These include corrosive and combination corrosive-abrasive resistance materials depending on the resin and additives being processed.
SPECIALTY MATERIALS:
For fluoropolymers, we offer high nickel materials such as Hastelloy, Monel, or Duranickel to combat highly corrosive resins.
SURFACE TREATMENT SEL ECTIONS
CHROME PLATING:
Chrome plating is a common surface treatment for thermoplastic screws. It offers a low coefficient of friction and has a natural hardness of approximately 70 Rc and offers some corrosio n protection. Double and triple chrome plating is common for PVC applications.
NICKEL PLATING:
Electroless Nickel is another plating option for corrosive applications.It I has a deposited hardness of 46 Rc.
NITRIDING:
This is a cost effective way to increase the life of screws made from alloy st teels such as 135M andAISI4140.This will give you a case hardness of approximately 60 to 70 Rc and approxin nate case depth of about .020 to .025".
HARD SURFACING SELECTIONS
STELLITE #6:
Stellite #6 is a cobalt based material. This is one of the most common wear resistant alloys used on screw flights. The typical hardness of Stellite #6 is 37 to 42 Rc.Very good for general purpose and slightly filled resins. Other Stellite alloys are also available.
Stellite #12, average hardness 40 to 47 Rc. Stellite #1, average hardness 46 to 54 Rc.
COLMONOY #56:
Colmonoy #56 is a nickel based material. This seems to be the material of choice by many molders of glass filled resins. This is a harder more wear resistant alloy.The typical hardness of Colmonoy #56 is 50 to 55 Rc.
Other Colmonoy alloys are also available.
Colmonoy #5, average hardness 45 to 50 Rc. Colmonoy #6, average hardness 56 to 61 Rc.
ADHESIVE WEAR:
Many factors contribute to this condition of metal to metal contact, causing wear to the barrel and screw.
*Barrel straightness
*Materials being processed
*Screw straightness
*Excessive back pressure
*Drive end alignment of both barrel and screw
*High barrel temperature variation
*Contamination, unwanted metals
*Size and design of the screw and barrel
ABRASIVE WEAR:
Abrasive wear is caused by the fillers and additives in the resins being processed.
* Glass fiber reinforcements
* Silica
* Calcium Carbonate
* Alumina Trihydrate
* Talc
* Calcium Silicate
* Barium Ferrite
* Ceramic fibers
* Titanium Dioxide
* Kaolin (clay)
* Mica
The above fillers are typical of the hard and fine particles that cause abrasive wear in the barrel and on the screw.
CORROSIVE WEAR:
Corrosive wear is the chemical attack of the barrel and screw surfaces. Many of the resins, additives, flame retardants and coupling agents release corrosive chemicals.Hydrochloric Acid, Hydrofluoric Acid, Formic Acid, Bromic Acid, and Sulfuric Acid are typical examples. Temperature, moisture in the resin, and operating procedures are all key elements in controlling corrosion.
