PLASTICS 101 – Introduction to Polymers

  • introduction to polymers
  • basic polymer properties
  • product testing
  • polymers comparison chart
  • molding methods
  • container & closure decoration
molecule

This tutorial section was created to give packaging designers and buyers a brief overview of plastic packaging, its properties, advantages, disadvantages, and techniques of manufacture.

While this is by no means an exhaustive study of plastics packaging, it will help orient the newcomer to the possibilities of this medium.

Simply select a section from the buttons above and begin your exploration of packaging with polymers.

 

Introduction to Types of Polymers

moleculeThis section provides a brief overview of the basic properties (i.e., advantages, disadvantages and compatibility issues) of plastic resins utilized in cosmetic and personal care packaging. It is a general overview of plastic resins commonly used in packaging. However, it should not be used as a substitute for compatibility testing. All of the following materials are available in FDA grades.

High Density Polyethylene (HDPE)

recycle arrowsMost commonly used for extrusion-blown bottles. In its natural state, HDPE is translucent and flexible. The addition of color will make it opaque. Adding extra weight will yield a more rigid package. Polyethylene is economical, impact-resistant, and provides a good moisture barrier. HDPE is compatible with a wide range of products that contain caustics and acids.

Polypropylene (PP)

recycle arrowsTypically used for injection-molded closures and jars, injection, extrusion or stretch-blow molding methods are employed to produce polypropylene bottles. In its natural form, polypropylene is semi-clear to clarified, and provides excellent contact clarity. It also produces a high-gloss finish when produced in colors. Polypropylene provides a rigid package and its greatest strength is high-temperature application (i.e., hot filling, autoclavability). It offers the ability to be steam sterilized. Polypropylene has excellent chemical resistance. Subfreezing temperatures dramatically reduce this resin’s impact resistance. Oriented polypropylene provides improved impact resistance and clarity at low temperatures.

Polystyrene (PS)

recycle arrowsThis plastic is commonly used for injection-molded jars due to its clarity and stiffness at an economical price. This resin lends itself to most decoration methods without the need of flame treating. Polystyrene has a good oxygen barrier rating and a fair moisture barrier. Polystyrene is used to package dry goods such as spices and vitamins. It is also used for petroleum jellies and creams. Polystyrene demonstrates a fair resistance to chemicals and acids. Its oil and grease resistance is rated as good. This polymer has poor impact resistance.

Styrene-Acrylonitril (SAN)

recycle arrowsThis styrene derivative offers the clarity and rigidity of polystyrene in an engineering-grade plastic. SAN has superior barrier properties, as well as improved chemical resistance when compared to polystyrene. SAN is a fairly costly resin.

Acrylic Butadiene-Styrene (ABS)

recycle arrowsThis is a styrene derivative that offers greater impact resistance than polystyrene. ABS is only available in solid colors.

Polyvinyl Chloride (PVC)

recycle arrowsTypically used for blow-molded bottles. PVC is extremely resistant to chemicals and oils, has a very low oxygen transmission, and provides an excellent barrier to gases. PVC is a clear (marked yellowing over time) drop-impact resistant, semi-rigid material.

PVC is vulnerable to solvents and public relation difficulties. Many environmental scientists and activists are of the opinion that chlorine expelled in the manufacture of this resin is a key factor in ozone depletion and global warming. None of Pearce Plastics’ products use this resin.

Polyethylene Terephthalate (PET)

recycle arrowsThis plastic is durable, with an excellent gloss, clarity and sparkle, and is most commonly used for blow-molded bottles. Oriented PET (OPET) offers a very good chemical resistance, especially to alcohol and essential oil. PET is vulnerable to ketones and acetone attack. The orienting process improves gas and moisture barriers. PET is not recommended for high-temperature applications (160 degree Fahrenheit is its maximum temperature). However, heat-set PET will accept high-temperature applications to 195 degrees F. PET is typically used for food and beverage products, such as carbonated beverages and juice, and cosmetic items that are high in alcohol, like hair styling products.

Glycol Polyethylene Terephthalate (PETG)

recycle arrowsGenerally used for shampoos, soaps and detergents, PETG demonstrates a good impact strength and gas barrier. The chemical resistance of PETG is fair, and product testing is highly recommended.

Introduction

We hope the information in this tutorial will help you streamline this procedure, but this step in the design process should always be comprehensive. All polymers have compatibility strengths and weaknesses in regards to how they react to ketones, acetones, solvents and oils. All packaging designs have their own characteristics as to how they address the filling procedures, gaseous build-up, and changes in air pressure due to altitude during shipping. Insuring that a container is free from negative reactions due to your product’s formulation and that it withstands the demands of the marketplace is critical to your project’s success.

Product Compatibility
"Pearce Plastics, Inc. assumes no responsibility for product compatibility. Customers are advised to perform compatibility, fill and decoration testing of Pearce Plastics products with any process, chemicals, bottles, seals, decoration, or any other materials that constitute a complete package. Tests should be complete and favorable before an order is placed for any packaging components. Pearce Plastics, Inc. will be happy to provide product samples for testing."

We start with our disclaimer because this part of the design process is so important. Any reputable vendor will hesitate to fill your order without testing. Product compatibility is the responsibility of the Packaging Designer, Research Department or Project Director/Manager. As a vendor partner or supplier we are not privy to your trade secrets (nor should we be) and would only be able to foresee general compatibility issues.

Leak/Seal Testing

First your bottle is filled with water to 3/4 of capacity and the closure is torqued to the filler's or packaging manufactures' specifications. Next, external pressure is applied creating about 10 to 15 PSI of internal pressure. Then the package is inspected for leaks. Pearce Plastics offers this test at no charge to customers considering our closures.

Stability Testing

These testing procedures are employed to insure the package you have chosen and your products are free of unforeseen compatibility issues. First the container should be filled with your product. You should reproduce the filling process as close as possible, (i.e. the exact formulation, temperature, the torque of the closure,) and you should also determine if any modifications are needed for your filling line to accommodate the container.

Accelerated Shelf-Life Testing

Packages are held at an elevated temperature for several weeks and then inspected for any change in appearance or weight. Accelerated shelf life testing effectively reproduces what will happen if a container sits on the shelf for one year. This test is one of the more important elements of Stability Testing.

Vacuum/Pressure Testing

This test is used to determine if there will be any changes in appearance or weight during shipping. Changes in altitude may create gaseous build-up that might result in a container blowing out or a closure losing its seal. If you ship via air or over the Rocky Mountains this test should be performed.

Pearce Plastics offers this test upon request. If it is determined that gaseous build-up is an inherent problem due to product formulation, our Venting Cap Technology can be employed to reduce or eliminate packaging failure. For more product information on our patented venting technology please contact our customer service department or call us at 626.797.8481 for more information. More elaborate testing methods may also be needed depending upon your product's formulation and requirements. You may need to test over a wide range of temperatures and humidity conditions.

 

As compatibility is an important consideration when working with plastic packaging, this chart should help streamline your compatibility testing and help you pick a resin that is best-suited for your product.

Comparison of Polymers
Resin PET LDPE HDPE PP PS PETG

Density

1.35

.91 to .925

.94 to .965

.89 to .91

1 to 1.1

1.27

Clarity

Clear

Hazy

Hazy

Hazy

Clear

Clear

Moisture Barrier

Fair to Good

Good

Good to Excellent

Good to Excellent

Poor to Fair

Fair

Oxygen Barrier

Good

Poor

Poor

Poor

Good

Good

Acid Resistance

Fair to Good

Fair to Very Good

Fair to Very Good

Fair to Good

Good

Fair

Grease & Oil Resistance

Good

Good

Good

Good

Fair

Good

Distortion Temperature

125 to 160F

160 to 220F

175 to 180F

230 to 260F

200 to 220F

140 to 160F

Rigidity

High

Low

Moderate

High

High

High

Impact Resistance

Good

Excellent

Good to Excellent

Fair to Good

Fair to Good

Good

Sunlight Resistance

Good

Fair

Fair

Fair

Fair to Poor

Good

Max. Hot Fill (F)

120

150

190

200

150

140

Cool Tolerance (C)

-40

-100

-100

0

-

-40

INTRODUCTION TO MOLDING METHODS

This Section provided an overview of molding methods used to produce plastic containers, closures and jars. It will also address custom tooling.

  • molding methods for containers
  • molding methods for closures and jars
  • extrusion blow molding
  • injection blow molding
  • stretch blow molding

In extusion blow molding, a parison, or hollow tube is formed when an extruder forces material between a die and a mandrel. The mold cavity consists of two halves that close around the parison and pinch off one end. Compressed air is then injected and the parison is expanded to conform to the shape of the cavity. Once cooled the container is removed and the flash (excess plastic) is trimmed.

This molding method has some strong advantages:
• Low mold cost. One mold can be used to produce any number of neck finishes. Produces containers ranging from 1 oz. to 55-gallon drums.
• Unrestricted container shape. Bottles can be multi-layered.
• No mold release is needed.

This molding methods disadvantages include:
• Containers have flash that must be removed.
• Requires highly skilled operators.
• The cost to produce small containers (under 5 oz.) is high when compared to injection blow molding.
• The weight, shape, neck finish, and the over all quality of the container will vary.

Injection blow molding uses a three-stage method: First plastic is injected into a mold cavity to make a pre-form parison. (The pre-form looks like a test tube with a threaded neck.) Second the parison is transferred to the blowing stage where air is blown through a core pin to expand it against the walls of the cold mold. The third and final stage is ejection.

The advantages of injection blow molding:
• A scrap-free process that is free of flash.
• The neck finish is injected to produce a high degree of accuracy.
• Weight control is precise and material distribution is uniform.
• Cost effective for containers under 5 oz.

The disadvantages:
• High tooling cost.
• The molds lack flexibility.
• High per unit cost for containers over 16oz.
• The container must be within a 3:1 (or less) neck to diameter ratio
• Does not lend itself to handle ware. Mold release is used.

Stretch blow molding utilizes either the injection or extrusion blow molding method at its foundation. A pre-form (parison) is molded and then transferred to a blow-molding cavity. Then the pre-form is stretched biaxially during blowing to orient and align the molecules. This process improves the gas barrier, stiffness, clarity, and impact strength of the container.
Containers may be made from PET, PVC, or Polypropylene. Stretch blow molding produces a high quality, lightweight container.

The advantages of stretch blow molding:
• Improved performance, better barrier and impact properties.
• Can produce lightweight containers with improved clarity.

There are little or no disadvantages in the final product; however:
• The cost of tooling can be on the high side and the multi-staged process is quite sophisticated.
• The availability of products produced via this method is limited.

  • injection molding
  • compression molding
  • custom tooling

Injection molding is used to make straight-sided containers such as jars, closures, and vials. Thermoplastic resins are heated and injected into a cavity where pressure forces the resin to conform to the mold. Then the part is cooled and ejected from the mold. Injection molding produces high-quality containers and closures that are consistent in their critical dimensions and wall thickness.

The advantages of Injection Molding:
• Produces high quality containers,
• cost effective in smaller runs,
• consistent finish size.

The disadvantages:
• initial tooling (mold) costs are quite high.
• The container must have straight sides.
• Uses Mold Release.

Compression molding is used to produce thermoset closures for glass containers. Phenolic is used for dark colors and urea is used for light colors. Closures produced in this manner are heavier and higher in price than injection molded closures.

Custom tooling is useful when you want to develop a look (package) that is distinctive and all your own, you may want to consider custom tooling. The initial cost will vary greatly depending on what molding method you employ, how may mold cavities are created, and the volume of the packaging you plan to produce.

Extrusion blow molded bottles are the least expensive custom projects with a cost of about $1,500 to $2,500 per cavity. Injection molding for custom closures and jars is the highest in initial cost of tooling. Injection Molding tools can range from $20,000 to $250,000 or more.

Pearce Plastics produces custom injection-mold tooling and finished goods for numerous industries, including cosmetic and personal care, food and drug, consumer goods (pet products), and telecommunications. We also produce private labeled novelty and promotional items.

PACKAGING DECORATION

This section provides a general overview of the different methods and products used to apply decoration to plastic packaging.

DECORATIVE RINGS

Generally used to dress-up a closure. Decorative rings can be used to pull a color theme through the closure without the headaches associated with custom colors (high minimums, long lead times, additional set-up and color charges). Decorative rings can be metalized to add a little elegance and drama to your packages' presentation.

SLEEVE LABELING

Extremely economical and are generally used for large containers. Bottles must have a recessed panel to hold the label in place. These labels can be easily applied by using automated labeling equipment.

These labels are not permanent. Problems associated with ripped, torn, and missing labels are quite common.

HEAT-SHRINK LABELING

These labels are applied to the bottle and passed through a heat tunnel where the label is shrunk and securely attached to the bottle. This method works best when several product lines share the same container but require different labeling, or when a package has numerous contours that need to be decorated. As with the standard sleeve label, this labeling process is vulnerable to rips and tears.

PRESSURE-SENSITIVE LABELING

An economical way to achieve a multi-color label design. The low cost of tooling and high speed application process adds to the cost-effectiveness of this method. Pressure-sensitive labels are available in paper, vinyl, polypropylene, polyethylene, or Mylar. This method of decoration may be used in conjunction with other printing methods, such as offset printing, silk screening, or hot stamping.

OFFSET PRINTING

This method of decoration is most often used for wide mouth containers such as jars, tubes, and vials. Offset printing can print up to six colors at once. The initial set-up costs (plating costs) are quite high. Yet, the process is fast and economical on large runs. A lacquer overcoating is recommended to protect the product’s labeling. When compared to other decoration processes (i.e. silk screening) offset printing can appear a little flat.

THERIMAGE LABELING

Cost effective for high-volume packaging projects, these labels can printed in up to five colors. Special tooling such as turrets and cylinders are required. These labels are either rotogravure printed or decal labels. Rotogravure-printed labels yield a low-cost label but the initial plating costs are high.