This Section will provide an overview of molding methods used to produce plastic containers (extrusion blow molding, injection blow molding, and injection molding) and closures (Injection Molding, Compression Molding). This section will also address custom tooling.

Molding Methods for Containers

Extrusion 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 advantage: Low mold cost. One mold can be used to produce any number of neck finishes. Produces containers raging from 1oz 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 5oz) 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:
This manner of 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 5oz.

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; additionally, this method does not lend itself to handle ware. Mold release is used.

Stretch Blow Molding:
This method utilizes either the injection or excursion 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. Pearce Plastics will introduce a line of containers produced via this method in early 2002.

Molding Methods for Closures & Jars

Injection Molding:
This method 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:
This method 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:
When you want to develop a look (Package) that is distinctive and all your own. You may wish to consider custom tooling. The initial cost will vary greatly depending on what molding method you wish to employ, how may cavities, and the volume you plan on producing.

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 being the highest in initial cost of tooling. Injection Molding tools range from $20,000 to $250,000 or more.

Pearce Plastics produces custom injection molding tooling and finished goods for the Cosmetic and personal care, food and drug, consumer goods (pet products), and telecommunication industries as well as private labeled novelty and promotional items. For more information on custom tooling please contact Bob Fleischer, VP of sales 626.797.8481.

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