Considering investing in the production of plastic parts with a plastics molding manufacturer? Purchasing custom plastics molding from manufactures can be a challenging process depending on your background and experience. You may be:

  • a materials scientist fully knowledgeable of the plastic polymers
  • a mechanical engineer competent in hydraulic, gears, motors and leverage
  • a new purchasing agent trying to get your mind around the terminology

Regardless of your background, we hope to provide articles that engender insight and intrigue, articles that motivate creativity in plastics. We will provide enough definition to keep the novice reading while not boring the more advanced readers. We are starting this series of articles with an overview of the types of plastic for molding and general descriptions. From there we will delve more deeply into various aspects of the plastic molding process.

When working with a plastic you may need to know something about it. Or at least be able to understand a conversation with the manufacture who has selected the polymer to satisfy your manufacturing needs.

Through a series of questions, a manufacturer has determined the color, shape, size and functional aspects of your final product. They have now recommended a polymer by DuPontTM. Their choice is Zytel® 151 NC010. You have heard of the name DuPont a well-known chemical company, but that is the last recognizable word. There are numerous manufactures of polymers; including many, you do not know. You are a little confident and a lot lost.

The company continues to explain the melt temperature of Zytel® 151 NC010 is 250°C with a range of 230-290°C. Well you understand it is Celsius; the other temperature scale you learned in high school science and quickly forgot. You somehow remember Fahrenheit = (Celsius x 1.8)+ 32 yielding a melt temperature of 482°F with a low side melting temperature of 446°F. Wait that makes some sense, you told them, “The part must be able to sustain a temperature 30% greater than the highest potential temperature of 300°F.” It is certainly above 330°F, but, is that the temperature the material begins to sag or the temperature needed to make it liquid for the mold. The material is a nylon resin, which should mean something to you, but other than being nylon means nothing!

The manufacturer says there is an up charge for a pre-dryer to ensure less than .15% resin moisture, drying at 80C with a drying time of 2-4 hours. In calculating processing they mention 4-5s/mm wall thickness with typical hold pressures of 35-140MPa(Million Parts per cubic foot). Huh? Okay, lost is lost. You heard what they said, but it might as well have been Greek.

Before you can trust what someone is telling you, you at least need a hint of what they are saying. Let’s build a little background in what constitutes a plastic polymer. First, we need to get a good understanding of the raw materials in plastic.

Plastics are a series of Polymers (poly means many and mers means parts) in varying grades to adapt to a wide range of functionality. Functionality can mean a variety of different things related to the structural integrity of the final product to the polymers performance while being molded. Melt temperature, temperature range and humidity considerations are important to the manufacturing process, but can also have implications to the environmental conditions the final product can handle.

Polyethylene is many ethylene particles that are bound together through a technique called polymerization where the Monomers (single molecules) are linked /joined together.

Modification of existing polymers leads to additional variations. Existing polymers are typically modified through a change in the molecular structure by high-energy ionization radiation and the chemical impact of peroxide. Varying the intensity and duration of the radiation and chemical impact of peroxide derive a multitude of properties in the polymers. Polymer solutions that may be rejected today as unwanted results may be the grounds for new products when demands for their properties arise. A resin with a low tolerance for heat of any kind can be a safety devise for temperature sensitive components. A resin that shatters readily into small pieces with rounded edges may function well as a breakable barrier under emergency applications.

There is a clear distinction between two major classes of polymers defined by how they respond to heat or the shift between Glassy (solid) and Melt (liquid) state. All polymers have three states Glassy, Viscoelastic (Rubbery) and Melt. In the Glassy state (TG), they are brittle. The Viscoelsatic state is a transition state between being rigid and soft. The melt state is where the custom manufactures of polymers are most interested, where the plastic can be formed into various shapes. But they must be cognizant of all three states and their behavior.

The material arrives at their plant in the Glassy state most often pellet form and is feed into a hopper above the feed screw. The feed screw drives the material toward the mold. As the polymer travels the distance down the screw it is heated through the viscoelastic state to its melting temperature so that when it arrives at the mold it is ready to be formed. Too much moisture, too high a temperature or too low a temperature can be the start of trouble Temperature and moisture are just a few of measures that must be continuously monitored. Released from the mold the only change that has taken place is the shape of the polymer, with proper care in molding it retains all of it’s valuable properties.

Simple Mold Process

In plastics, you will find two basic types amorphous and crystalline

ABS, PC and PS are examples of amorphous polymers, polymers which have a random crystalline structure. This random structure causes them to gradually melt over a wider range of temperatures. Such polymers do not reflect a distinct temperature range for the viscoelastic state or Melt Temperature (TM). A broader melting point allows for more variance in working with the material to form products. That is, temperatures are less critical. All polymers shrink when cooling, amorphous polymers have a lower percentage of shrinkage to be accounted for when developing the mold. The mechanical properties (resilience, endurance, ability to handle higher temperatures) are also lower. ABS( Acylonitrile Butadiene Styrene), PC(Polycarbonate), PS(Polystyrene)

Nylons, Acetal and PET are crystalline polymers, polymers that are an ordered crystalline structure exhibiting a narrow Melt Temperature (TM)range with a clearly delineated viscoelastic state. This set of polymers require close attention to temperature ranges. Mold tolerances between the Glassy state and Melt state are much closer with greater shrinkage. Changes in the Viscoelastic (rubber) state exhibit radical drops between Glassy to Viscoelastic and Viscoelastic to Melt. More tricky to work with, these polymers have higher mechanical properties (resilience, endurance, and ability to handle higher temperatures). PET(Polyethylene Terephthlate)

Now our manufacture could have recommend a PET (Polyethylene terephthalate) like DuPont’s Rynite 555 NC010A a Thermoplastic Polyester resin. It far exceeds our first polymer with a bottom end melting temperature of 280°C, but what about the cost for the material and production. Where the hold time of 4 s/mm wall thickness is similar, the production cost is higher due to a higher drying temperature and a higher molding temperature. The moisture content must be less than .02% a much more stringent requirement.

We can see the manufacture has a lot invested in the type of polymer selected, especially if they are interested in satisfying their client (you) and your customers. Selecting a polymer is more than just color. Knowing the polymer; will impact the equipment used, the press type and size along with variances in mold design and environmental controls.

Selecting a resin has three major ingredients.

  1. Application function (temperature range, stress, appearance of the final product)
  2. Cost for quality available raw goods from a reliable source (minor defects can mean faulty parts)
  3. Cost to manufacture (additional time, power consumed and steps increase cost)

As you read about polymers for production you will find there are two additional ways to classify them. Engineered plastics are those polymers that can stand mechanical stress or endure chemical and physical conditions over a wide range of temperatures. They are defined as General Purpose Engineering Polymers and Special Engineering Polymers.

General-purpose engineering polymers are most popular:

  1. Polyamide (nylon, PA)
  2. Polycarbonate (PC)
  3. Polyoxymethylene (POM)
  4. Polyester (primary PBT)
  5. Polyphenyl ether (PPQ)

Special engineering polymers are not as widely used:

  1. Polyphenyl thioether (PPS)
  2. Polyimide (PI)
  3. Polysulfone (PSF)
  4. Polyetherketone (PEK)
  5. Liquid crystal polymer (LCP)

Talking terminology, here are 25 types of plastics with abbreviations. Within each type of plastic there may be numerous versions of that polymer that have wider or narrower temperature ranges for melting points, higher or lower hold times, or may require less or more attention to humidity.

Polyacrylic acid (PAA) Polycarbonate (PC)
Cross-linked polyethylene (PEX or XLPE) Polytetrafluoroethylene (PTFE)
Polyethylene (PE) Polystyrene (PS)
Polyethylene terephthalate (PET or PETE) Polyurethane (PU)
Polyphenyl ether (PPE) Polyester (PEs)
Polyvinyl chloride (PVC) Acrylonitrile butadiene styrene (ABS)
Polyvinylidene chloride (PVDC) Polymethyl methacrylate (PMMA)
Polylactic acid (PLA) Polyoxymethylene (POM)
Polypropylene (PP) Polysulfone (PES)
Polybutylene (PB) Styrene-acrylonitrile (SAN)
Polybutylene terephthalate (PBT) Ethylene vinyl acetate (EVA)
Polyamide (PA) Styrene maleic anhydride (SMA)
Polyimide (PI)  

Looking for insight, below is a table of typical plastics with their identification codes. Does your plastic need to be recyclable? Look at the center column and you get a general idea of the properties for each plastic product. The third column offers some potential uses. Look at the symbol on a product and you can back track the type of polymer, its uses and properties in general terms. The product may have properties you desire or wish to avoid in your final product yet to be developed.

Codes Descriptions Properties Packaging Applications Recycled Products
100px-Plastic-recyc-01_svg Polyethylene Terephthalate (PET, PETE). PET is clear, tough, and has good gas and moisture barrier properties. Commonly used in soft drink bottles and many injection molded consumer product containers. Other applications include strapping and both food and non-food containers. Cleaned, recycled PET flakes and pellets are in great demand for spinning fiber for carpet yarns, producing fiberfill and geo-textiles. Nickname: Polyester. Clarity, strength, toughness, barrier to gas and moisture, resistance to heat. Plastic soft drink, water, sports drink, beer, mouthwash, catsup and salad dressing bottles. Peanut butter, pickle, jelly and jam jars. Ovenable film and ovenable prepared food trays. Fiber, tote bags, clothing, film and sheet, food and beverage containers, carpet, strapping, fleece wear, luggage and bottles.

100px-Plastic-recyc-02_svg High Density Polyethylene (HDPE). HDPE is used to make bottles for milk, juice, water and laundry products. Unpigmented bottles are translucent, have good barrier properties and stiffness, and are well suited to packaging products with a short shelf life such as milk. Because HDPE has good chemical resistance, it is used for packaging many household and industrial chemicals such as detergents and bleach. Pigmented HDPE bottles have better stress crack resistance than unpigmented HDPE bottles. Stiffness, strength, toughness, resistance to chemicals and moisture, permeability to gas, ease of processing, and ease of forming. Milk, water, juice, cosmetic, shampoo, dish and laundry detergent bottles; yogurt and margarine tubs; cereal box liners; grocery, trash and retail bags. Liquid laundry detergent, shampoo, conditioner and motor oil bottles; pipe, buckets, crates, flower pots, garden edging, film and sheet, recycling bins, benches, dog houses, plastic lumber, floor tiles, picnic tables, fencing.

100px-Plastic-recyc-03_svg Vinyl (Polyvinyl Chloride or PVC). In addition to its stable physical properties, PVC has excellent chemical resistance, good weatherability, flow characteristics and stable electrical properties. The diverse slate of vinyl products can be broadly divided into rigid and flexible materials. Bottles and packaging sheet are major rigid markets, but it is also widely used in the construction market for such applications as pipes and fittings, siding, carpet backing and windows. Flexible vinyl is used in wire and cable insulation, film and sheet, floor coverings synthetic leather products, coatings, blood bags, medical tubing and many other applications. Versatility, clarity, ease of blending, strength, toughness, resistance to grease, oil and chemicals. Clear food and non-food packaging, medical tubing, wire and cable insulation, film and sheet, construction products such as pipes, fittings, siding, floor tiles, carpet backing and window frames. Packaging, loose-leaf binders, decking, paneling, gutters, mud flaps, film and sheet, floor tiles and mats, resilient flooring, cassette trays, electrical boxes, cables, traffic cones, garden hose, mobile home skirting.

100px-Plastic-recyc-04_svg Low Density Polyethylene (LDPE). Used predominately in film applications due to its toughness, flexibility and relative transparency, making it popular for use in applications where heat sealing is necessary. LDPE is also used to manufacture some flexible lids and bottles and it is used in wire and cable applications. Ease of processing, strength, toughness, flexibility, ease of sealing, barrier to moisture. Dry cleaning, bread and frozen food bags, squeezable bottles, e.g. honey, mustard. Shipping envelopes, garbage can liners, floor tile, furniture, film and sheet, compost bins, paneling, trash cans, landscape timber, lumber

100px-Plastic-recyc-05_svg Polypropylene (PP). Polypropylene has good chemical resistance, is strong, and has a high melting point making it good for hot-fill liquids. PP is found in flexible and rigid packaging to fibers and large molded parts for automotive and consumer products. Strength, toughness, resistance to heat, chemicals, grease and oil, versatile, barrier to moisture. Catsup bottles, yogurt containers and margarine tubs, medicine bottles. Automobile battery cases, signal lights, battery cables, brooms, brushes, ice scrapers, oil funnels, bicycle racks, rakes, bins, pallets, sheeting, trays.

100px-Plastic-recyc-06_svg Polystyrene (PS). Polystyrene is a versatile plastic that can be rigid or foamed. General purpose polystyrene is clear, hard and brittle. It has a relatively low melting point. Typical applications include protective packaging, containers, lids, cups, bottles and trays. Versatility, insulation, clarity, easily formed Compact disc jackets, food service applications, grocery store meat trays, egg cartons, aspirin bottles, cups, plates, cutlery. Thermometers, light switch plates, thermal insulation, egg cartons, vents, desk trays, rulers, license plate frames, foam packing, foam plates, cups, utensils

100px-Plastic-recyc-07_svg Other. Use of this code indicates that the package in question is made with a resin other than the six listed above, or is made of more than one resin listed above, and used in a multi-layer combination. Dependent on resin or combination of resins Three and five gallon reusable water bottles, some citrus juice and catsup bottles. Bottles, plastic lumber applications.

What you will learn over time is that there are a wide array polymers designed to meet the demands of a growing industry; an industry where there is a greater demand on this product line every year.

Memorizing every polymer in the industry and the various specifications is too much. Compounding the issue is the branded names for various products. However, a general understanding of polymers and possible properties and classes provide a grounds for real in-depth conversations with manufacturers. Different chemical companies may offer similar, but slightly different polymer products. Quality plastic manufactures have the ability to move across chemical companies (suppliers of polymers) and the plethora of polymers to discern the best polymer for each application, both in application and production costs. It is part of the role they play in producing quality plastic products. They are well suited to this role with continuous interaction in various polymers, on-going conversations with chemical companies and the daily experience with these products in their molding machines.

So far, we have learned the difference between polymers in general terms. If you ask a manufacture to produce a part with a higher temperature range or more resilience you may expect them to recommend a nylon polymer. You now know this may require more attention to detail in building the mold, setting up the machine and may require heaters in the molding process, making it a little more expensive to produce, but worth the price for the resulting resiliance in the final product.

If you read an article about special engineering polymers, you may find the product interesting, but will now realize it is not typical to most applications. If you see an existing product that has properties you might like to emulate or avoid you now know to look at the code stamped on the product.

If you decide you want a product with a live hinge, a bendable joint made at the time of molding, nylon is a likely choice. Decisions in plastic manufacturing start from the understanding of the finished product’s application and environment. It is important to understand the life of the product and the stresses it must endure, safety codes, health codes and recycling issues. These are all things your manufacturer will help you determine while they assess the implications of the polymer in molding.

The manufacture can investigate a number of polymers that may fit your cost concerns, satisfy customer expectations, conform to regulatory issues and create the most economical production formula.

Where you are now armed with the basics, plastic manufactures have years of experience and are constantly in communications with plastic polymer suppliers. Plastic manufactures continually receive updates on the latest materials and conditions where products can be used. A good manufacturer is your eyes and ears to the world of polymers and you will be looking to them to assist you in defining your needs.

A company that under bids everyone else may not be the best selection. Unrealized design requirements and production requirements for a polymer can result in faulty parts both mechanically and visually. The cost for rework and redesign can mean additional expenses in raw goods, production time and inevitably delays to market. Lack of in-depth experience in molding polymers can limit their selection options and your final products success. The last thing any company wants is a recall voluntary or otherwise. Where getting to market quickly and production costs are on your mind, cost and speed should not be your first concern. Consistent quality, meeting specifications and on-time delivery should always be foremost in your thoughts.

In future articles, we will continue the conversation of polymers, there is much more to learn. The topics can be challenging, but quite interesting. Certainly, a professional, experienced plastics manufacturer is your best guide, but you do need to know more so that you understand the conversation.