An Informational Brief on Polymer Machining

 

Among the many polymer materials we machine at AIP, High Density Polyethylene (HDPE) is a common material choice for commercial polymer applications.  HDPE is part of the Polyethylene (PE) family of thermoplastic polymers with variable crystalline structure.

 

First developed in the 1950s by German and Italian scientists Karl Ziegler and Giulio Natta, PE has become one of the most widely produced plastics in the world.  Polyethylene comes in several compounds each with various applications: Low Density Polyethylene (LDPE), High Density Polyethylene (HDPE) and Ultrahigh Molecular Weight Polyethylene (UHMW) are some of the most well-known.

 

For example, you will find LDPE most likely in the grocery store as plastic wrap or grocery bags.  In contrast, HDPE, due to its high density, is much better suited for construction components like a drain pipe.  And UHMW can be machined into high performance applications for medical devices, bulletproof vests and industrial wear components.

 

In this machining guide, we will discuss what goes into machining HDPE and how its considerations differ from other manufacturing options such as metal machining, injection molding, and 3D printing.

 

A Brief History of Plastic CNC Machining

 

How does AIP approach HDPE and its machining process? To start, let’s explore what plastic machining is, specifically CNC machining.

 

CNC (Computer Numerical Control) machining is a process in the manufacturing sector that involves the use of computers to control machine tools. In the case of plastic machining, this involves the precise removal of layers from a plastic sheet, rod, tube or near net molded blank.

 

Shortly after World War II, the earliest version of CNC technology was developed as a dependable, repeatable way to manufacture more accurate and complex parts for the aircraft industry.  John Parsons is credited with developing numerical control – a method of producing integrally stiffened aircraft skins.

 

While working at the family-owned, Michigan-based business – Parsons Corp., John collaborated on the development of a system for producing helicopter rotor blade templates.  Using an IBM 602A multiplier to calculate airfoil coordinates, and inputting this data to a Swiss jig borer, it was possible to produce templates from data on punched cards.

 

In 1949 Parson’s templates were applied to Air Force research projects at MIT.  Following extensive research and development, an experimental milling machine was constructed at MIT’s Servomechanisms Laboratory.

 

Machining polymers and composites is a precise science that requires strong technical expertise.  For instance, some plastics are brittle, while others melt at a specific temperature.  These diverse mechanical and thermal properties result in varying behaviors when CNC machined.  Thus, it is imperative to understand the polymer structure and qualities of HDPE if you’re machining it.

 

Ever wonder about the differences in cost and process among 3D Printing, Injection Molding or Plastic Machining?

 

Check out our blog:
“Settling the Debate”

 

 

Properties of HDPE

 

HDPE is a high impact, high density crystalline thermoplastic.  It also has a low moisture absorption rate and good chemical and corrosion resistance.  Compared to its sister polymer LDPE, HDPE offers much greater impact resistance and tensile strength.  This polymer has a melt temperature of 266 F (130 C).  Its tensile strength is 20 MPa (2,900 PSI); to put this number into perspective, a slab of concrete may be able to withstand 3,000 PSI.

 

Oftentimes, people use HDPE in everyday home appliances and commercial containers.  Due to its strength and corrosion resistance, it’s a common candidate for garbage bins, laundry detergent cartons and cutting boards.  It is also safe to use for food contact such as milk cartons.

 

PE is available in sheet stock, rods, and even specialty shapes in a multitude of variants (LDPE, HDPE etc.), making it a good candidate for subtractive machining processes on a mill or lathe. However, colors are usually limited to white and black.

 

Machining HDPE

 

Annealing HDPE

Annealing greatly reduces the chance that surface cracks and deformation due to internal stresses will occur from the heat generated during machining HDPE. AIP uses computer controlled annealing ovens for the highest quality precision machining of all thermoplastics.  Talk to our engineers about any questions you have about the annealing of a specific polymer.

 

Machining HDPE

As a crystalline thermoplastic, HDPE can be machined at tight tolerances; remember dimensional stability and strength!  AIP recommends non-aromatic, water-soluble coolants because they are most suitable for ideal surface finishes and close tolerances. Keep in mind however that HDPE has a very low CTLE and therefore will move quite a bit with slight temperature changes.  Some examples are pressurized air and spray mists. Coolants have the additional benefit of extending tool life as well.

 

Some companies machine both metals and plastics, which has detrimental outcomes for clients. Many past experiences have shown parts going to customer without cracks, only to develop cracks over time due to exposure to metal machine shop fluids. Be sure to use a facility like AIP that only machines polymers.

 

Preventing Contamination

 

Contamination is a serious concern when machining polymer components for technically demanding industries such as aerospace sciences. To ensure the highest level of sanitation down to the sub-molecular level, AIP Precision Machining designs, heat-treats, and machines only plastics with any sub-manufactured metalwork processed outside our facility.  This allows us to de-risk the process from metallic cross contamination.

 

HDPE (High Density Polyethylene) Machining Guide: Supportive Information

 

Miscellaneous Materials Guide

ISO 13485:2016 Certification

ISO 9001:2015 Certification

Learn more about HDPE and its applications in other industries

 

Discover what HDPE can do

 

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Learn about the re-useable capabilities of precision plastics

 

In the world of recycling, plastic tends to have a bad reputation or it gets whispered like a dirty word.  Indeed, according to the UN Environment Programm, one million plastic drinking bottles are purchased every minute.  This is certainly a disturbing statistic, and we are tasked with addressing the consequences of this waste.  However, it is important to distinguish the type of plastics causing severe pollution.  Plastic bottles and plastic bags are single-use, disposable plastics.  These are the ones that are clogging up the environment.

 

What people don’t discuss often is plastics that are re-usable and recyclable.  At AIP, the plastics that we precision machine are high grade, quality polymers made for durability and continuous use in the following industries: Aerospace and Defense; Medical and Life Sciences; Power and Energy; Specialized Industrial.  That means they are evergreen materials that will not only last, but could be repurposed for a different application altogether.  Read on to find out about some of the high-performance polymers we work with, what they are used for and how they can be recycled.

 

Everyday Sustainable Precision Plastics
PolymerPropertiesAIP’s Machined Applications
PPSBroadest chemical resistance; zero moisture absorption; dimensional stability; ultra-low wear factors and structural strength

*available in several grades

Case Study: High-quality PPS wheel bushings for a theme park water ride.

  • Reduced ride downtime
  • Saved on maintenance and inventory costs
  • Lower energy cost
  • Efficient design
  • Low-wear
TORLONHighest performing, melt-processible plastic; maintains strength and stiffness up to 500 F; chemical, thermal and stress resistance

*available in several grades

Ideal for critical mechanical and structural components for severe levels of temperature and stress

  • Jet Engine Components
  • High Temperature Electrical Connectors
  • Automotive Transmission components
  • Wear Rings in Oil Recovery
  • Valve Seats
PEEKBiocompatible; abrasion and chemical resistant; low moisture absorption; very low smoke and toxic gas emission

*available in several grades

Case Study: PEEK Dynamic Telescopic Craniotomy (skull plate for brain traumas

  • Reduced ride downtime
  • Saved on maintenance and inventory costs
  • Lower energy cost
  • Efficient design
  • Low-wear
RADELImpact resistance; hydrolytic stability; excellent toughness; chemical resistance; heat deflection temperature of 405 F (207 C)
ULTEMExcellent heat and flame resistance; high rigidity and strength; low thermal conductivity; highest dielectric strength

*available in several grades

Used as structural components in several industries

  • High-voltage circuit-breaker housings
  • High-temperature bobbins, coils, fuse blocks and wire coatings
  • Jet-engine components
  • Aircraft interior and electrical hardware parts
  • Microwave applications
  • Replaces glass in medical lamps

 

Thermoplastics – The Green Plastic

 

There are two types of polymers – thermoplastics and thermosets.  The plastics that we work with primarily at AIP are thermoplastics.  So, what’s a thermoplastic and how is it re-usable or recyclable?

 

It’s all about how the polymer reacts to chemicals and temperature.  Thermoplastics soften when heated and become more fluid, which makes them a very flexible polymer.  For this reason, these plastics can be remolded and recycled without losing their mechanical properties or dimensional stability.  Let’s go in depth on some of the common thermoplastics we use for evergreen applications.

 

The AIP case study focusing on the use of PPS for the log flume ride bushing component is an excellent example of a thermoplastic built and machined for continuous use.  The bushing made from PPS could be used over and over again without wear.  Furthermore, it could be immersed in water and other chemicals without losing dimensionality or durability.

 

PEEK and ULTEM are both common polymers we machine at AIP.  With PEEK’s high chemical resistance and biocompatibility, it is ideal for surgical applications such as the Dynamic Telescopic Craniotomy Case Study.  This polymer can withstand the internal temperatures and fluids of the body for extended use.

 

ULTEM is known for its strength and rigidity in extreme environments and temperatures.  This polymer is often used for re-useable medical instruments, since it reacts well to autoclave sterilizations.  Additionally, it’s flammability rating and dimensional stability make it ideal as a weight-saving aerospace component.

 

As the plastics industry continues to innovate, the next generation of research will turn towards more sustainable and environmentally conscious materials.  Thermoplastics are one of the pioneers of this industry – leading plastics into the future as a material that can be reused and recycled.

 

Unrivaled Expertise. Unparalleled Results

 

With 36+ years of experience in the industry, our dedicated craftsmen and ties to leading plastic manufacturers allow us to provide you with unrivaled knowledge and consulting in material selection, sizing, manufacturing techniques and beyond to best meet your project needs.

 

AIP offers a unique combination of CNC machining, raw material distribution, and consultancy as a reliable source for engineering information for materials such as PEEK, TORLON, ULTEM and more.

 

We are AS 9100D compliant; certified and registered with ISO 13485 and ISO 9001 and standards in our commitment to machining quality custom plastic components for specialized industrial sectors. Quality assurance is included as an integral part of our process and is addressed at every step of your project, from concept to completion.  Unrivaled Expertise.  Unparalleled Results.

 

 

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