assorted plastic bottles

Plastic molding is the process of manufacturing plastic parts by pouring liquid plastic into a mold. During the molding process, a liquid or powder such as polypropylene or polyethylene has to be placed into a hollow mold to ensure that the polymer takes the desired shape. There are various methods of plastic molding, such as blow molding, insert molding, extrusion molding, and compression molding. Each technique involves different factors, such as varying pressure, heat, temperature, cooling, and melting point. Here are the most popular plastic molding techniques considered in the manufacturing process.

Rotational Molding

Also known as roto-molding, rotational molding is done when producing large hollow products such as storage tanks, containers, car parts, recycling bins, and playground slides. This manufacturing process doesn’t involve pressure but uses high temperatures and rotational movements to coat the inside of the mold creating the desired plastic product.

The first step in rotational molding involves filling a metal mold with polymer powder or a liquid resin. Next, the mold is rotated constantly in an oven until the products placed inside coat the mold. This constant rotation of the mold creates a centrifugal force that forms even walled products. Finally, the mold cools down, and the final product is removed.

Rotational molding is cost-effective and environmentally friendly as it wastes few materials, and the excess can be reused. Other features of rotational molding include:

  • It doesn’t have any limitations when it comes to sizes.
  • The products produced are uniform in whole thickness with straight and thick corners.
  • It can support other production intricacies such as fixtures, ribs, and inserts.

Blow Molding

This plastic molding technique is used to manufacture hollow, thin-walled products. Its main objective is to make plastic molds with a uniform wall thickness where the shape is considered. The blow molding process shares the same principle as glass bowling. The process starts with pouring molten material through a parison. Parisons are tube-shaped plastic structures with holes that pass compressed air through them. They are created when a molten material is hard-pressed over an extruder.

The blow molding machine then heats the plastic material until it liquefies, and the air is injected into it to create a balloon-like shape. Next, the plastic is blown into the mold and expands to press against the mold walls to take shape. After the balloon fills the mold, it is cooled, hardened, and ejected.

Features of blow molding include:

  • The fast process takes an average of less than two minutes.
  • Processing costs and machinery are meager, making it a great choice for small and medium businesses.
  • The process is versatile and can produce plastic products such as water bottles, cases, fuel tanks, and containers.

Extrusion Molding

Extrusion molding produces long and straight plastic parts such as straws, PVC pipes, and hoses. This molding technique works by extruding melted plastic materials directly into a hole known as the die. Unlike other molding techniques, extrusion product shapes are determined by the shape of the die. Typical die shapes include solid rounds, L-shapes, rectangles, and T-shapes.

Extrusion molding uses tensile strength and compressive stress to shape and deform the materials. Features of extrusion molding include:

  • Production of flexible products that have consistent cross-sections
  • Cost-effective. The waste materials can be reused multiple times without trading off the quality of the final product. In sedition, the machinery can run for twenty hours without incurring additional costs for maintenance.
  • High-volume production while maintaining speed and quality.
  • Freedom to do post-extrusion alterations if the products are still warm

Injection Molding

This is a versatile plastic molding technique that can be sued to create a variety of parts that range in shape and size depending on the pressure exerted and tonnage. While injection molding principles are simple, the actual process can be complex. Injection molding creates molted fluid by heating the solid granules above the melting point. The liquid resin produced contains low resistance flow and is considered the best option for large-scale production.

The injection molding process starts with injecting melted plastic into a steel mold. To shape the material to desired size using the aesthetics of the mold. Once the material in the mold cools, it is injected into a heating chamber and finally ejection.

Features of injection molding are:

  • It requires low operation and maintenance costs.
  • The process is versatile, efficient, and quick, even for large-scale production.
  • Accurate. This technique provides exceptional accuracy even when a thousand parts are produced.
  • Injection molds are complex and heavy, which may result in more expenses, but it can be very economical if you produce in bulk.
  • Flexibility. The products produced are created with increased flexibility allowing the engineers and designers the freedom to innovate constantly.

Compression Molding

This technique is done by placing a heated plastic material into a pre-heated mold. The plastic is then compressed into the required shape. The process uses excess heat and then the hydraulic presses to heat and contact molten plastic to every part of the mold. The heating is to ensure that the plastic retains its maximum strength. Finally, the process is completed by cooling, trimming, and removing the plastic.

Compression molding is best used when replacing metal parts with plastic parts. Additionally, the technique is usually used in the automotive industry since the final products are durable and robust.


Thermoforming molding consists of when a thermoplastic sheet is heated over a specific temperature to make it soft and flexible. The heated sheet is then formed into a particular shape and size to create the desired final product. Thermoplastics range in colors, materials, thickness, and finishes. The thermoforming technique uses very low pressure making it an inexpensive manufacturing process.

Thin-gauge thermoforming can be used to manufacture trays, disposable cups, clamshells, lids, blisters, and other products commonly used in food and general retail industries. The thick-gauge thermoforming products include utility vehicle beds, doors, refrigerator liners, and dash panels.


Every plastic molding technique is unique and offers different advantages and disadvantages. Understanding the various plastic molding techniques can help you select the best option that suits your specific needs and one that will meet customer expectations and produce quality products.

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