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Three Major Factors That Affect Screw Life
Greg QuinnVP Sales - Plastics & Rubber Industries
Extreme Coatings
[email protected] | 888-367-2569
Your Presenter Today
Greg Quinn
• Started with Extreme Coatings in
1997.
• Assisted in development of the
Extreme Coatings Technology.
Discussion Topics
• The challenge of processing a wide range of polymers using a
general purpose screw.
• Tips for processing high crystalline polymers and the cost of wear.
• Wear and corrosion resistance options for feedscrews.
Factor #1
Screw Design and Polymers
• Polymers are typically categorized as Amorphous or Crystalline.
• The higher the crystallinity, the more energy required to melt the polymer. Amorphous
polymers require a more gradual melt with minimal sheer to avoid degradation.
• Nylon (high crystallinity) and polycarbonate (amorphous) are two good examples of
polymers that are on opposite ends of the crystallinity spectrum.
• Both have very different melt cycles.
Most molders do not have the luxury of multiple screws with varying designs to
accommodate the range of polymers they must process. Therefore, processing parameters
are a key factor for screw & barrel longevity and consistent quality of parts.
Range of Polymers
Injection Molders typically mold multiple polymers using a
standard General Purpose (GP) feedscrew.
GP Feedscrew Design
20:1 General Purpose Screw 45/35/15
• Compression ratio:
Must account for non-compressable fillers such as fiberglass. A high
compression ratio may lead to severe side loading of the screw to the barrel
resulting in premature adhesive wear.
• Shot size:
Barrel capacity must be large enough to allow the proper residence time for
melting.
Screw Design Factors to Consider
These two factors play a large role in determining feedscrew
& barrel life when processing abrasive fillers.
Result of Severe Side Load
• 80 mm D-2 Tool Steel 60Rc
• Worn .018” in 3 months due to
severe side loading.
Mushroomed Flites
Factor #2
Tips for processing high crystalline
polymers and the cost of wear.
Heat Profiles For Crystalline Polymers
• Setting the appropriate HEAT PROFILE for a particular polymer is a key
factor in prolonging feedscrew and barrel life.
• Polymer manufacturers have “suggested” zone temperature settings which
do not account for screw design, shot size or compression ratio.
• It is often necessary to adjust zone temperatures settings to achieve proper
melt and avoid severe side load.
No hard and fast rules apply. Each project requires a custom solution!
Application Requiring Customization
• Sumitomo 280 ton injection press
• 63 mm 20:1 feedscrew
• Nylon with 33% glass filler
• Machine has a shortened stroke resulting in reduced barrel capacity
• Screw life +/-18 months
Total Shot Weight (in g) 732.00
Total Shot Weight (in oz) 25.77
Machine Rated barrel capacity
(oz) GPPS: 24.00
Specific Gravity of Plastic: 1.17
Shot % of Corrected
Barrel Capacity: 97.30%
Molding Cycle Time (sec): 40
Residence Time (sec): 41.11
Observations• Customer set parameters on this 280 ton machine so that it would produce at the
same rate as a 350 ton machine running the same mold and injection unit.
• Results:
o In order to meet production requirements the screw speed increased
from 145 RPM’s to 175 RPM’s.
o Due to the shortened stroke, residence time was reduced from 52.16
sec. to 41.08 sec.
o Increasing percentage of barrel capacity from 76.69% to 97.30%
o Higher side loading of the screw and reduced service life.
• Recommended residence time for nylon is generally
around 120 seconds.
• Recommended percentage of shot used ranges
between 25%-65%.
Key Processing Parameter Considerations
Options for Corrective Actions
• Best solution is to move the mold to a larger machine.
• If there are no other machine options, a reverse heat profile may increase
production life.
• Increase recovery time to maximum time available.
Barrel Temperature has small effect melt temperature. Steel conducts
heat 100X greater than plastic. Increasing barrel zone temperatures
prevents loss of heat from the plastic to the barrel.
What is a Wear Problem?
• A wear problem occurs when:
• Scrap rate increases.
• Productivity decreases.
• The operator must make adjustments to the machine get consistent output.
#1 Cause of Abrasive Wear: Glass Filled Nylon
Compensating for wear
• Compensating for screw/barrel wear through machine
parameter modifications can cause excessive shear,
burning, and polymer degradation which effects the
quality of the resin.
• Modifications are typically made to maintain out-put rates
or decrease scrap rates.
Quality Resin
• Resin of quality may be defined as resin that is free of
unmelt, was exposed to minimal shear, has seen standard
residence time, and is therefore free of degradation.
• Degradation is a loss of polymeric physical properties such
as compression strength, tensile strength, impact strength,
torsion strength, or other tangible or intangible
engineered properties.
Intangible Cost of Wear
As the screw/barrel gap increases, machine efficiency decreases.
• Polymer and additive degradation & Increased shear - May not be apparent until it’s too late.
• Quality issues unseen in production may become obvious when inspected by the customer.
• Company Reputation!
• Process stability effected - Stable processes produce less scrap.
• Increase scrap - Every scrap part produced is a new part that has to be produced for FREE!
• Lower overall productivity - more scrap equals lower productivity.
• Disposal of scrap parts – Labor, grinding blades, etc.
• Screw replacement – Premature feedscrew wear equals premature feedscrew replacement.
• Increased Barrel Wear – As the screw wears, the gap opens between the screw and barrel. Abrasive polymer back flows over flites and may cause barrel wear.
• Increased power consumption – Increase RPMs, heater band temp adjustments and scrap all lead to increase power consumption.
• Down Time – Non productive employees and machines down for repair cost money.
Tangible Cost of Wear
True Cost of Your Feedscrew
• Calculate over the entire life of the screw the total cost
of scrap, regrind and/or disposal of scrap, the cost of
parts made FOR FREE to replace scrap parts, power
consumption, labor, etc.
• Add that cost to the original price of the screw to get
the REAL cost of the screw.
Factor #3
Wear and corrosion resistance
options for feedscrews.
Commonly Used Feedscrews
• Nitride – typically supplied with new machines
• 4140 steel with hardfacing (C-56) inlay on the flites and
Chrome Plating – Most common
• Powdered Metal Tool Steel – Commercial name CPM9V
• Tungsten Carbide Encapsulation – Most wear resistant
The Key To Increased Productivity and Barrel & Screw longevity
• Maintain the OEM tolerances between the Screw O.D. and Barrel I.D.
Adhesive Wear
high points interact and adhere
or weld to each other
• A load (135kg) is applied to the block against the
spinning ring (300 RPM) for a fixed number of
revolutions (30,000). Each sample coupon is
weighted before and after testing and the
volume loss in mm3 calculated.
ASTM G77 Block on Ring Adhesive Wear Test
G-77 Adhesive Wear Test Data
0.000 0.250 0.500 0.750 1.000 1.250 1.500
Carbide / WC Cermet
Carbide / CPM-9V
Bimetallic / WC Cermet
Bimetallic / Col 56 Volume Loss 4.700 mm3
Blue - Fixed Block
Red - Rotating Ring
G-77 Adhesive Wear Test Data
0.000 0.025 0.050 0.075 0.100
Nitride /Nitride
Carbide / Nitride
Blue - Fixed Block
Red - Rotating Ring
Volume Loss 1.19 mm3
Wear Resistance Testing
ASTM G65 (Dry Sand)
• 6000 revolutions
• 12 Kg Load
• 23 cm rubber wheel
• Graduated dry sand abradent
• Volume loss in mm³
G-65 Wear Resistance Test Data
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Tungsten Carbide
Cermet
Powder Metal (9V) D2 Tool Steel Nitrided steel
(EN41B)
Chrome Plating
Comparagraph Relative Abrasion Resistance
ASTM G65 A 6000 revolution
We
ar F
acto
r
Why is Carbide Most Wear Resistant
• Hardness DOES NOT always equate to wear resistance!
• The volume of carbides in the alloy or composite determines the
resistance to wear.
Treatment Description Hard. RC Vol. % Carb. Type Carb. Rel. Wear Resist
Nitrided Steel Aluminum Alloyed steel heat treated 68 0% AIN 1.0
Chrome Plate Chromium electro deposit surface treatment 70 0% Pure Cr 2.3
Bi-metallic Weld deposit of Ni or Co alloy on flight OD 48 25% CrC / WC 2.3
Tool Steel Specialty steels heat treated to through-harden 55 20% VC / CrC 3.5
Tungsten Carbide Thermal spray surface coating high carbide content 68 80% WC 11.7
TAFA JP-8000 Thermal Spray Gun
Temperature 6000°F (3300°C) melt powder particles. (Cobalt or Nickel)
Particle velocities > 3300 feet/sec (1000 m/sec)
Kinetic energy contributes additional heat and promotes bonding, high density, and
high hardness
Substrate temperature is maintained below 300°F (150°C)
Application Details
• 26 mm CPM9V screw with a compression ratio of 3.2:1
• Processor running 30% GF LCP on 25 machines.
• Has process issues at .006” OD wear. Approximately 750k shots or 6-8
months.
• wear was isolated to the O.D. on the last 50% of screw.
• Inconsistent processing forces removal of the screw from service.
Empirical Evidence
Solution
• Compression ratio was reduced to 2.8:1
• Last 50% of screw (worn section) was coated on the flite lands with
Tungsten Carbide via the HVOF process.
• After 5 months and 650k shots this screw has a maximum of .001” of
O.D. wear.
Empirical Evidence
Design change and increased wear resistant O.D. changed the
dynamic. 3.0 – 3.5 M shots are now possible.
O.D. Surface Treatment
Tungsten Carbide Overlay
Empirical EvidenceApplication Details
• 50 mm CPM9V Screw
• 60% GF Nylon
• 3-5 months life due to almost 90% of the barrel volume used with each
cycle
• Flight side erosion was observed on all screws.
• Due to the high barrel usage, residence time was reduced which
resulted in accelerated wear.
Empirical Evidence
Solution• The system was upsized to a 57 mm diameter feedscrew which
was encapsulated with Tungsten Carbide.
• Reduced barrel usage to 70% of the total, still high but
manageable.
• Reverse heat profiles implemented.
A combination of reverse temperature profile, extended recovery
(plasticizing) time plus surface treatment of Tungsten Carbide improved
overall screw life to 30+ months. (previously 3-5 months) The surface
treatment completely eliminated flight side erosion.
Empirical Evidence
Tungsten Carbide Overlay
Conclusions
When planning a project it is important to consider the three
major factors that effect feedscrew life and performance.
• Feedscrew design
• Processing parameters
• Wear and corrosion protection
Thank you!