HIP Products

Products for the Plastic Industry

Authors’ Note: CPM and 10V are registered trademarks of
Crucible Materials Corporation. The material contained in
this paper is intended for general information only and
should not be used in relation to any specific application
without independent study and determination of its applica-
bility for the intended application. Anyone making use of
this material or relying thereon assumes all risk and liability
arising therefrom.

The production of plastics and plastics
components requires equipment that can
withstand severe wear and, in a high per-
centage of cases, wear and corrosion envi-
ronments. There are two basic elements of
plastic extrusion equipment: the barrels and
the screws. Both must manifest similar prop-
erties, but since screw elements are less costly
and easier to replace, they are usually de-
signed to wear out first. Due to the high cost
of wear/corrosion-resistance materials, the
industry used clad (i.e., bimetallic) compo-
nents. Barrel sections and screw segments
are both produced as hot-isostatic press clad
components using similar processes. There
are any number of material combinations
that are used and that are possible for the
right application.
INTRODUCTION
The United States entered the plastic
age in 1983 when the consumption (i.e.,
dollar volume) of plastic products ex-
ceeded that of metals. By 2000, the
amount of plastic usage is expected to be
three to four times that of all metals. In
the early days of plastic processing,
screws and barrels for injection-mold-
ing machines/extruder machines were
made with low- to medium-alloy steels.
A step up from this was to have these
screws and barrels nitrided or given
other surface treatments to combat the
wear and occasional corrosion problems
that were encountered. This helped, but
the industry’s need for better perform-
HIP-Clad Products for the
Plastics Industry
Morley F. Bishop and Clinton K. Nickel
ing plastics or compounds demanded
that more additives, such as glass fiber,
fire retardants, color, etc., be incorpo-
rated. All of these fillers plus the newer
types of resin compounds increased the
wear and corrosion on the screws and
barrels. The machines were also running
faster, with higher output per hour and
more overall production. Down time was
a costly problem.
The initial material upgrade was to
use high-grade tool steels heat treated
for better wear. Now, the trend is toward
bimetallic screws and barrels. Most of
these bimetallic components are made
by the powder-metallurgy (P/M) pro-
cess of hot isostatic press (HIP) cladding.
In this way, they can be tailored to the
needs of the plastic producers and the
materials they are making.
The old rule of thumb in the industry
was 90 percent of the problems with
screws and barrels were wear related,
and ten percent were corrosion related.
This is changing because the newer ad-
ditives, faster production, and higher
heat generation are all conditions that
require both wear and corrosion resis-
tance in the same application.
PLASTIC EXTRUDERS
Plastic-extrusion processing is defined
as converting granular plastic powder
into a continuous, uniform melt (viscous
material) and forcing it through a die to
yield the final or desired shape. The
melted material must then be cooled to
its solid form where it is held and used.
End products made by extrusion are
• Custom profiles: auto trim, house
siding, molding
• Sheet: vacuum-formed products,
cups, plates
• Pipe: water and soil pipe
• Film: trash bags, laminates
• Blow molding: bottles, auto gas
tanks
• Pellets/granules: for further pro-
cessing by injection molding
There are two basic types of extrusion
machines—the single screw and the twin
screw. Both work on the principle of
screw-and-barrel combination.
Single-Screw Extruders
Single-screw extruders are very com-
mon and versatile. They have the ability
Figure 1. A typical single-screw extruder.
Figure 2. A single-screw HIP-clad barrel.
to process a wide range of materials and
common production sizes range from
38 mm to 203 mm in diameter; some are
as large as 450 mm in diameter. The
barrel size is designated by its inner
diameter, and the barrel length is con-
sidered the axial length of the material
enclosed in the barrel and in the forward
edge of the feed section (Figure 1).
The single-screw extruder accepts the
plastic granules or powder in the feed
hopper and then into the feed section of
the screw/barrel, where the material is
conveyed along by the action of the screw
and heated while moving inside the bar-
rel. The goal is to have the molten mate-
rial reach the die section at the end of the
barrel in a totally liquid state ready to be
pumped out at a consistent rate. Screw
rotation is constant and normally turns
between 20–200 rpm. The barrel gener-
ally has a hard, wear-resistant liner to
protect its bore from the wear encoun-
tered during extrusion (Figure 2). The
screw-flight tips sometimes have only a
0.127–0.380 cm total diametrical clear-
ance inside the barrel. In practicality, the
clearance is questionable, and some rub-
bing between the screw and the barrel
exists. The barrel is heated by jackets
surrounding its outer wall, which aid in
melting the plastic during the process.
The screw is the heart of the extrusion
process and usually the first component
to be replaced. Screw designs have var-
ied over the years, and many screw build-
ers in the market place offer many differ-
ent alloys, but they always try to match
the screw material to that of the barrel to
force the screws to wear out first. Screws
are far less expensive than barrels and
are easier to change.
The majority of the bimetallic barrels

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