8 types of molding for plastic products

March 30, 2022

By siedery

This article introduces the common plastic products in daily life and their molding types. Give beginners and people just in the injection molding industry an intuitive understanding.

You can click here to learn about the classification and properties of common plastics.

  1. Injection Molding
  2. Rotational Molding
  3. Extrusion Blow Molding
  4. Injection Blow Molding
  5. Reaction Injection Molding (RIM for short)
  6. Vacuum compound mold
  7. Hot press molding
  8. Compression molding

Plastic products are commonplace in everyday life, and there are many different ways to produce both everyday consumer goods and industrial supplies. Below is an introduction to the seven most common molding methods for plastic supplies, which will hopefully help you choose the right molding method for your project.

1. Injection Molding

About 80% of plastic products in daily life are injection molded. Injection molding is produced by using an injection molding machine with an aluminum or steel mold, which consists of a core and a cavity. The injection molding machine heats the raw resin until it melts, and uses pressure to inject the molten plastic material into the cavities of the mold, after which the core and cavity are separated and the product is ejected from the mold.

The advantages of injection molding are high volume, speed and low cost with good surface finish. However, the structural complexity of injection molds depends on the geometry of the product, the more complex the structure the higher the cost of the mold; in addition, the engineering design of the mold must be perfect in order to prevent defects and optimize product quality and processing speed.

2. Rotational Molding

The mold used in rotational molding is also composed of core and cavity, but the working principle is different. In rotational molding, the plastic material is first added to the mold, then the mold is rotated and heated along two vertical axes, so that the plastic material inside the mold is gradually and evenly coated, melted and adhered to the mold cavity under the action of gravity and heat. Afterwards, the mold is removed and the product is removed after cooling and shaping and is used for the next round of production.

Rotational molding is used to produce large, hollow or curved products, such as outdoor kayaks or water tanks. Because the product is formed under a high-temperature rotational process and the product is seamless, it has high strength and a relatively simple and inexpensive mold. The disadvantage is that the mold needs to be repaired after a few thousand uses, and the quality of the product is average, so it is not suitable for precision products.

3. Extrusion Blow Molding

Extrusion blow molding is commonly used to make thin-walled and inexpensive containers, such as disposable water cups or bottles. Production is extremely fast and the tooling is simple, but it is not suitable for complex or high-precision products.

After the mold is closed, compressed air is introduced into the mold to swell the mold until it is all adhered to the cavity walls. The mold walls are water-cooled and the plastic parts are quickly cured and then eject to obtain the finished product.

4. Injection Blow Molding

Injection blow molding is a process that uses air pressure to inject molten resin into the mold cavity. This process is more controllable and repeatable and is typically used to produce clear plastic beverage bottles with excellent surface finishes, but is not suitable for thicker wall thicknesses.

Polyester and polyester ether ketone are both common raw materials for plastic bottle manufacturing because both resin pellets are transparent and stable, and they are rated as safe for consumer products and easy to recycle.

5. Reaction Injection Molding (RIM for short)

Reaction injection molding is often used in the automotive industry because it produces a light weight but hard surface product that can be painted to make body parts, instrument panels or other components. This molding method can only process thermoset plastic materials. The thermoset plastic material undergoes a chemical reaction in the mold, which extends like a foam and fills the entire mold cavity.

The rapid tooling cost for this molding method is relatively low, but the mass production tooling cost is relatively high. The main cost of using this molding method is still the raw material, and after the product is produced, additional steps such as painting must be used to treat the surface effect, which also increases the cost of the product.

6. Vacuum compound mold

Vacuum compound mold is a good choice if you have limited investment in tooling or materials and only need a small number of high quality, fast model products.

A prototype or any solid model (usually a 3D printed prototype) is placed inside a sealed box, which is subsequently filled with urethane or silicone. After the prototype is removed, a mold cavity is formed and the resin material is poured into the cavity to produce the restored part. Air is removed from the mold by vacuum pressure to avoid air bubbles in the mold. The surface effect of the products made by vacuum re-molding is outstanding, the details are perfect, and the resin material that can be poured can imitate different engineering grades of plastic products. The only disadvantage is that the molds are not durable, and they have to be downgraded after about 20 uses.

7. Hot press molding

Hot press molding is a type of vacuum forming where plastic sheets or plates are placed on a die-casting mold and heated to soften the material, allowing the plastic material to stretch over the surface of the mold while using vacuum pressure to form it.

This molding method uses relatively simple molds and equipment and is often used to produce thin-walled, hollow plastic samples. For industrial use, it is commonly used to produce plastic cups, lids, boxes, and open-and-close packaging, as well as thicker sheets for automotive body parts. Only thermoplastic materials can be used for hot press molding.

8. Compression molding

Compression molding is a process in which the material is preheated and placed in the die cavity of a die casting mold, and the plastic is cured by pressure and heat after the mold is closed. This molding method is suitable for the production of plastic buttons, washers, O-rings, or other soft, easily molded, thin-walled products.

This molding method is relatively inexpensive and has less material waste, but the stability of the product is difficult to control and more attention must be paid in the mold design stage.

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