Extrusion Guidelines

Building a Custom Profile Extrusion

Wall Balance

Maintaining uniform wall thickness is essential to achieving a quality profile extrusion as material will fluctuate between thick and thin sections if walls are unbalanced. An unbalanced profile may require an extra cooling phase to prevent misshaping or bowing, which causes production to slow down, and costs to increase.

Balanced walls provide greater control of production costs through efficient production with better control of tolerances. You will also find that with balanced walls, you will have increased options in configuration and more material and surface finish alternatives.

Hollow Sections

The use of hollow sections is generally discouraged unless it aids in the achievement of wall balance.

The die for a hollow is more costly because of additional requirements to help maintain the shape as the part cools, such as air pressure, internal mandrels, and vacuum sizing. However, you will still save on total setup costs versus a die with unbalanced walls.

The following precautions may help minimize problems when a hollow is necessary:

Maintaining close tolerances is difficult on internal projections or legs inside a hollow, so try to avoid them when possible.
When an internal leg is necessary, ensure that it projects into the hollow no more than the thickness of the wall.
Another challenging design to avoid is a small hollow inside a larger one. Close tolerances are difficult to maintain, and it is costly to produce.

Sink Marks

Sink marks can be found opposite adjoining projections, caused by shrinkage of the material. To hide the marks, try raising or indenting the surface at the sink marks, or adding a surface design.

Corners and Radii

It is preferred that the inside radius, as well as outside corners, be a minimum of 1/64-inch. This will help reduce instances of breakage at a sharp corner, which is doubly important with rigid materials. There is less stress at the corners and less warpage when a larger corner is used.

Selecting Materials

Aluminum or Thermoplastic

Thermoplastic extrusions can be a better choice over aluminum for a variety of applications, as there are many benefits. For example, plastics can offer corrosion-resistance and out-perform aluminum with impact dents. The ability to perform secondary treatments in line can also make plastic a better choice, since aluminum may require adjustments after extrusion. The below chart highlights just some of the differences between these two materials.

FEATURE	PLASTIC	ALUMINUM
Energy Consumption	Less energy needed to produce and process extrusions in the 250° to 600° range	More energy needed for mining and production as well as extruding at temperatures exceeding 1000°F
Strength	Tensile yield to 10,200 psi for polysulfone, 9,600 psi for modified polyphenylene oxide	Tensile yield to 24,000 psi for 1000 series, to 36,000 psi for 3000 series, to 91,000 psi for 7000 series
Impact Resistance	Excellent impact resistance in rigid plastics	Aluminum alloys offer minimal denting resistance
Corrosion Resistance	Excellent, even without coatings	Resists corrosion-weakening; however, surface treatment is required for appearance
Thermal Expansion Coefficient	Varies but higher for thermoplastics	Varies with alloy
Color	Uniform throughout the part	Must be painted or anodized; easily chipped and scratched
Multi-Function Extrusions	Dual and tri-extrusions and other chemical/mechanical bonding of dissimilar materials is possible; saves cost of mechanical fastening	Separately produced extrusions are later assembled with extrusions of other materials or with flexible sealing members, often plastic
Secondary Operations	Productivity advantages with automated in-line fabrication to produce ready-to-install parts	Generally, requires heat treatment, straightening, and other post-extrusion operations in addition to fabrication processes

Co- and Tri- Extrusions

Carefully choosing two or three materials can result in an exceptional part for you. The below chart demonstrates the bonding characteristics of many plastics that we can use to make co- and tri-extrusions.

Shore Scale

The Shore scales are used to measure the hardness of plastic materials, with A scale plastics being flexible, C scales, semi-rigid, and D scales, rigid. There is some overlap in the Shore scales as exhibited in the below graphic.

Chemical Resistance

When a part will be exposed to chemicals, attention should be given to selecting the proper materials, as chemicals can affect the strength, flexibility, color, and surface of plastics. There are three types of interactions that can occur when plastics are exposed to chemicals: Stress cracking, softening or swelling from absorption of the chemicals, or changes in the physical properties of the plastics. Additionally, other factors can affect a material’s reaction to chemicals, such as temperature, pressure, grade of the materials, and length of exposure to the chemical. The below chart can assist with general guidelines only.

MATERIALS

HPDE – High-density polyethylene
LDPE – Low-density polyethylene
NYL – Nylon (polyamide)
PA – Polyallomer
PC – Polycarbonate
PETG – Polyethylene terephthainte copolyester
PP – Polypropylene
PS – Polystyrene
PUR – Polyurethane
PVC – Polyvinyl chloride
TPE – Thermoplastic elastomer

CHEMICAL RESISTANCE

E – Excellent, no effects on physical properties
G – Good, no dramatic effects
F – Fair, some effects
N – Not recommended

Calculating Thermal Expansion

The Shore scales are used to measure the hardness of plastic materials, with A scale plastics being flexible, C scales, semi-rigid, and D scales, rigid. There is some overlap in the Shore scales as exhibited in the below graphic.

Material	COE (amount of movement per inch per 1°F)	Formula for Calculating Movement* (COE x 10¯⁵ x length in inches x temperature range)		Amount of Movement
Polystyrene	4.2	.000042x72x100	=	.302 inches
Acetal	6.1	.000061x72x100	=	.439 inches
Nylon	8	.000080x72x100	=	.576 inches
Glass filled nylon	2.3	.000023x72x100	=	.166 inches
ASA	5.9	.000059x72x100	=	.425 inches
Polyethylene	6.1	.000061x72x100	=	.439 inches
Rigid PVC	3.4	.000034x72x100	=	.245 inches
Aluminum	1.2	.000012x72x100	=	.086 inches
*Based on 6 feet (72 inches) of material exposed to a temperature range of 100°F.

Performance vs. Cost of Common Thermoplastic Materials

Fire Properties of PVC

HEAT BUILD-UP

HEAT BUILD-UP
Heat build-up is an important factor when determining the stability of a thermoplastic intended for outdoor use.
COLOR		°F	°C
Black	288	74	41
Green	530	50	28
Blue	460	56	31
Gray	240	49	27
Ivory	035	45	25
White	138	45	25
White	145	45	25
Brown	382	52	29
Brown	372	50	28
Red	730	49	27
Tan	360	45	25
Gold	070	50	28
Olive	080	50	28
Yellow	650	36	20
For a vertical* wall, calculate heat build-up by adding the maximum temperature for the region to the above figures. Example: If the air temperature can reach 105°, Black 288 can rise to 179°F. *Heat build-up for an incline or horizontal position would be about 20% higher.