Knowledge Base

Knowledge Base

What to Know About the Different Types of Plastic Molding Processes

Mar 13, 2020, 18:04 PM

plastic pellets used in thermoplastics manufacturingIn today’s diverse manufacturing environment thermoplastics are used to make a wide range of commercial products, from automotive parts and medical equipment to toys, kitchen utensils, and bottle caps.

Getting these products to market requires a variety of manufacturing processes involving many types of plastic. Here we review the most popular plastic molding processes, highlighting the pros and cons, along with recommended applications for each.

Injection Molding

How Does It Work?

Injection molding is the process of forcing melted plastic into an engineered steel mold cavity. The plastic fills into the cavities of the mold to form the part. The mold is cooled and the parts are ejected by pins.

Pros & Cons

PROS:

  • Cost-Efficient
    The process of injection molding is mostly automated, so labor costs to run a fully-automatic injection molding facility are low.
  • High Output
    Short cooling time between injecting polymer into a mold and releasing a component means more parts in less time than with other molding methods.
  • Low Waste
    Any unused or scrap plastic can be easily recycled to use in future parts, so injection molding creates very little waste.
  • Versatility
    Injection molding can be used for a diverse array of products from small, intricate parts to large parts. Various plastic resins and additives can be utilized to create highly complex or specialized parts with many finish options.
CONS:
  • Steep Start-up Costs
    Investing in the right equipment is paramount to achieving high-quality parts, so initial costs are high. New injection molders will require precision molding equipment, precision molds, and an automated assembly line.
  • High Cost of Tooling
    New molding facilities usually require an engineer to design and test product molds because tooling often requires multi-cavity mold options.
  • Costly Repairs
    The machinery used in injection molding is generally expensive to repair. Due to the high-volume work environment, wear and tear is inevitable. That’s why it’s so important to maximize mold performance with the right mold care & maintenance.

When Does Injection Molding Make Sense?

Injection molding is ideal for high-volume, mass-produced products that require thousands or even millions of the same exact part, such as toys, kitchen utensils, surgical equipment, bottle caps, and more.

Blow Molding

How Does It Work?

The blow molding process uses the same basic principles found in glass blowing. The process begins by heating a plastic tube and filling it with air, often called a “parison.” A mold is then clamped around the tube to trap the plastic as the air continues to fill the parison to conform to the shape of the part being produced.

Designed for the manufacture of high volume, hollow objects, blow molding creates uniformly thin-walled objects at very low cost.

Pros & Cons

PROS:

  • Lower Costs than Injection Molding
    Because blow molding uses lower pressure compared to injection molding, the machinery required to perform blow molding costs less.
  • No Need to Connect Mold Halves
    Since blow molding produces containers in one piece, there is no need to connect part halves. In fact, blow molding can be used to create shapes that cannot be achieved with injection molding.

CONS:

  • Close Tolerance
    As the mold diameter increases, the corners and thinnest areas exhibit very close tolerances.
  • Longer Cooling Times
    When compared to injection molding, blow molding requires longer cooling times, which can affect productivity.

When Does Blow Molding Make Sense?

Blow molding is most often used to mass-produce one-piece, hollow objects such as containers, bottles, drums, or tubing.

Compression Molding

How Does It Work?

As its name suggests, compression molding utilizes the power of compression to create shapes from plastic. Two halves of a heated mold are pressed together with enough pressure to force the plastic resins to take the shape of the mold. The shaped resin is then left to cool before being removed from the mold.

Pros & Cons

PROS:

  • Lower Cost of Molds
    Compression molding is more cost-efficient and far less complex than injection or blow molding. Tools for compression molding are often made from lower-grade steel or aluminum, further reducing costs.
  • Small Production Runs
    Compression molding is ideal for small production runs since it doesn’t require complex tooling to achieve good results.

CONS:

  • Slow Process
    Compression molding is a much slower process when compared with injection molding.
  • Only Works with Certain Products
    Compression molding produces products with a soft plastic or rubber-like feel, making it unsuitable for many types of plastic products that require greater rigidity.
  • Less Accuracy
    The slower process results in less dimensional consistency between products, so compression molding is not recommended for products that demand high dimensional tolerances.

When Does Compression Molding Make Sense?

Compression molding works best for making large or fairly intricate parts. The most popular applications for compression molding include soft plastic and rubber products such as bottle caps, car mats, buckets, plastic containers, knobs, handles, and mechanical parts.

Rotational Molding

How Does It Work?

Rotational molding combines high temperatures and rotational movement to coat the inside of a mold with polymer powder. As the heated mold rotates on perpendicular axes, the powder adheres to the interior surface of the mold, forming an even-walled component. The component is removed from the mold after it cools.

Pros & Cons

PROS:

  • Very Little Waste
    The process of rotational molding produces very little waste since the material is added to the process as it is needed. Any waste that is produced can often be reused in future processes.
  • Produces Strong Parts
    The layering of materials during the rotational molding process produces parts with consistent wall thicknesses, as well as strength and flexibility.

CONS:

  • Slow Process
    Rotational molding takes time, both to build up the layers of the part and to complete the cooling process.

When Does Rotational Molding Make Sense?

Rotational molding is most often used for large, hollow, one-piece containers, including drums, tanks, and kayaks It is particularly effective for producing parts that demand consistent wall thicknesses and high strength.

Extrusion Molding

How Does It Work?

While other plastic molding processes insert plastic film and sheet into a mold, extrusion molding moves melted plastic directly into a die. It is the shape of the die, not the mold itself, which determines the shape of the resulting part.

Similar to injection molding, the extrusion molding machine feeds plastic resin into a feeder with the turn of a screw. Molten plastic then moves through the die, resulting in a long, tubular shape. After the extrusion cools, it is removed.

Pros & Cons

PROS:

  • Effective for shaping hard/brittle materials
  • Perfect for creating atypical cross-sections
  • The process guarantees a smooth finish on the end product.

CONS:

  • The extrusion process is only effective with two-dimensional parts.
  • A straight, linear line must be maintained throughout the extrusion process.

When Does Extrusion Molding Make Sense?

Extrusion molding works best for hollow-formed objects such as pipes, tubing, or straws. Manufacturers can create parts in a variety of shapes, including T’s, U’s, squares, I’s, L’s, or circular sections.

Want to Improve the Quality & Efficiency of Your Molding Operation?

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