THE WORLD OF MATERIALS

When considering plastic injection molding materials for your product there are so many properties and tradeoffs to consider. We find that material selection is one of the heaviest and most painful decisions when developing a new part.  Our goal is to guide you the best we can with this.  understanding the material families is a great place to start the investigation.  After we understand what family candidates we will consider then we break it down a bit further into; commodity, engineered, and specialty performance grade options.  after understanding more about these costs and availability tradeoffs we help you make the best decision for your business model.  For example, if a cost is the number one driver of your part design, you may be inclined to select a reprocessed or even recycled material. but if material availability is a concern you may need to qualify a virgin to resign as a backup.  furthermore, depending on your part geometry tolerances, some materials can be tested during the T1 trials.  additives and processing aids can also be introduced for testing at this time.  It is important to note that your final material selection can most times be done after your mold is built.

Polyethylene can be classified into three ranges of density. Low density polyethylene (aka LDPE) has a density of .910 to .925 g/cc; whereas the medium density grade has a density of .926 to .940 g/cc. The high density grade (HDPE) has a density of .941 to .960 g/cc. As a general rule, the higher the density, the harder, the more rigid and more heat resistant the plastic. Low and medium density polyethylene’s, for example, have melting points of about 230 -240°F whereas the high density material has a melting point about 25°F higher. Higher density resins show improved tensile and flexural strength, chemical resistance, surface hardness and abrasion resistance but poorer elongation and brittleness at low temperatures. Various polyethylene copolymers are also available that show much different characteristics than the parent monomer.

PE

Poly Pro

CTFE

PTFE

PS

TPUR

Polyethylene can be classified into three ranges of density. Low density polyethylene (aka LDPE) has a density of .910 to .925 g/cc; whereas the medium density grade has a density of .926 to .940 g/cc. The high density grade (HDPE) has a density of .941 to .960 g/cc. As a general rule, the higher the density, the harder, the more rigid and more heat resistant the plastic. Low and medium density polyethylene’s, for example, have melting points of about 230 -240°F whereas the high density material has a melting point about 25°F higher. Higher density resins show improved tensile and flexural strength, chemical resistance, surface hardness and abrasion resistance but poorer elongation and brittleness at low temperatures. Various polyethylene copolymers are also available that show much different characteristics than the parent monomer.

PE

Poly Pro

CTFE

PTFE

PS

TPUR

PHYSICAL MATERIAL PROPERTIES

Property used to define the hardness or softness of a plastic

ASTM D2240  Test Method based on indentation of specimen when force is applied.

Property used to define the how heavy or light a plastic is based on volume

ASTM D792  Test Method based on the ratio of mass of a given volume of material usually reported relative to water

Property used to define the plastics ability to withstand tensile stress while being pulled or stretched without failure

ASTM D638  Method based on pulling a dog bone specimen and reading the rate at which the material yields or breaks

Property used to define the materials ability to resist a force applied in an instantaneous implementation of a load

ASTM D2240  Method based on swing pendulum with a defined weight. Testing can be performed as Notched Izod with and without low temp definitions.

Property used to define the ability to stretch the material of a plastic

ASTM D638  Method based on indentation of specimen when force is applied.

Property used to define the length of recovery of a material after it has been stretched

ASTM D790  Method is based on measuring % of a specimen original length

ASTM D395  Method based on geometric recovery of specimen after force is applied for 24 hours.

Method based on measuring deformation at time intervals from 1 to 1000 hours

Property used to define the amount of growth or shrink a specimen exhibits during temperature change

ASTM D696  Method based on measuring the % coefficient of linear change in length of a material relative to each degree of temperature change

Property used to define a slow deformation phenomena while under constant strain (also known as “Cold Flow”)

ASTM D2990  Method based on measuring deformation at time intervals from 1 to 1000 hours

ISO27 Property used to define a material deflection perpendicular to force applied

Method based on indentation of specimen when force is applied.

ENVIRONMENTAL MATERIAL PROPERTIES

Property used to define the hardness or softness of a plastic

ASTM D2240  Test Method based on indentation of specimen when force is applied.

Property used to define the how heavy or light a plastic is based on volume

ASTM D792  Test Method based on the ratio of mass of a given volume of material usually reported relative to water

Property used to define the plastics ability to withstand tensile stress while being pulled or stretched without failure

ASTM D638  Method based on pulling a dog bone specimen and reading the rate at which the material yields or breaks

Property used to define the plastics ability to withstand tensile stress while being pulled or stretched without failure

ASTM D638  Method based on pulling a dog bone specimen and reading the rate at which the material yields or breaks

Property used to define the plastics ability to withstand tensile stress while being pulled or stretched without failure

ASTM D638  Method based on pulling a dog bone specimen and reading the rate at which the material yields or breaks

Material Tonnage (per in²) Shrink Values (in.) Vent Depth (in.)
Acryl Butadiene Styrene (ABS) 2.5 – 3.5 .004 – .008 .0010 – .0020
Acetal (POM) 3.0 – 4.0 .004 – .007 .0005 – .0015
Acrylic (PMMA) 3.0 – 4.0 .002 – .010 .0015 – .0020
Ethylene Vinyl Acetate (EVA) 2.0 – 3.0 .010 – .030 .0005 – .0007
High Density Polyethylene (HDPE) 2.5 – 3.5 .015 – .030 .0008 – .0010
Low Density Polyethylene (LDPE) 2.0 – 3.0 .015 – .035 .0005 – .0007
Polyamide – Nylon (PA) Filled 4.0 – 5.0 .005 – .010 0003 – .0010
Polyamide – Nylon (PA) Unfilled 3.0 – 4.0 .007 – .025 0005 – .0020
Polybutylene Terephthalate (PBT) 3.0 – 4.0 .008 – .010 .0005 – .0015
Polycarbonate (PC) 4.0 – 5.0 .005 – .007 .0010 – .0030
Polyetheretherketone (PEEK) 4.0 – 5.0 .010 – .020 .0005 – .0007
Polyetherimide (PEI) 3.0 – 4.0 .005 – .007 0010 – .0015
Polyphenylene Sulfide (PPS) 3.5 – 4.5 .002 – .005 .0005 – .0010
Polyphthalamide (PPA) 3.5 – 4.5 .005 – .007 .0005 – .0020
Polypropylene (PP) 2.5 – 3.5 .010 – .030 .0005 – .0020
Polystyrene (PS) 2.0 – 2.5 .002 – .008 .0015 – .0020
Polyurethane (PUR) 2.5 – 3.5 .010 – .020 .0004 – .0010
Polyvinyl Chloride (PVC) 2.5 – 3.5 .002 – .030 .0005 – .0020
Thermoplastic Elastomer (TPE) 2.5 – 3.5 .005 – .020 .0008 – .0010

MATERIAL IDENTIFICATION CHART:

This chart is used to reverse engineer any material. Although it is not a perfect science, you can learn a lot about your material with a few simple procedures.

Virgin vs Wide spec vs Reprocessed vs Recycled vs Regrind:

Let’s clear up the confusion around these different types of plastic materials. By understanding these four categories, you’ll have a better sense of the risks involved with your product:

  1. Prime Virgin: This is plastic that has only been processed once by converters. It comes with precise specifications and certifications, ensuring high quality and consistency.
  2. Wide Spec Virgin (Off Spec): This is a blend of virgin polymers that didn’t meet the strict spec tolerances. Converters often use this material and blend it into a pelletized compound within a specific range, making it a cost-effective option with slightly more variation.  UV, pigment, colorant, glass, talc, and other additives can be added.  The typical spec ranges are +/- 15% on melt flow which most injection molders can adjust to.
  3. Reprocessed/Recycled (Repo): This material is divided into two categories: post-consumer and post-industrial. Post-consumer recycled plastics tend to have more variation, while post-industrial materials are more consistent, having less variability.
  4. Regrind: This material comes from the grinding of plastic waste parts. It can be uniform or a mix of colors, depending on the source. While most regrind suppliers filter for metals, it’s often unclear how many heat cycles the material has gone through. After 6-7 heat cycles, certain polymers start to lose their properties, which could affect your product’s performance.

In the future we plan to dive deeper into this topic with videos and collaboration with material partners.