Three Major Factors That Affect Screw Life

Greg QuinnVP Sales - Plastics & Rubber Industries

Extreme Coatings

GregQ@ExtremeCoatings.net | 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!