A Beginner's Guide to Understanding Plastic Mold and Injection Molding

A Beginner's Guide to Understanding Plastic Mold and Injection Molding
7 min read

Plastic molding is a manufacturing process used to produce plastic products by shaping molten or pliable plastic material into a desired shape. Plastic molds are required for the shaping process, which involves injecting the plastic into the mold cavity where it cools and solidifies into the final product. This process is also known as plastic injection molding.

Plastic molding is popular due to its ability to produce inexpensive, high-volume plastic parts with great accuracy, repeatability, and low labor costs. It is used to make a variety of plastic products we use every day, from plastic bottles and containers to automotive dashboards, electronic device casings, and medical equipment.

Understanding the basics of plastic molds and the injection molding process is important for those interested in product design, manufacturing, or sourcing custom plastic parts. This beginner’s guide will cover:

What Is a Plastic Mold?

A chair mould, also known as a mold tool, mold or die, is a custom-machined metal block that forms the core component used in molding plastic parts. The mold has a hollow cavity cut into the desired part shape. During injection, molten plastic fills this cavity and forms the plastic part.

Molds are precision-machined usually from steel or aluminum. The choice of material depends on production run length and part complexity. Steel molds are stronger and suitable for long production runs of over 10,000 units. Aluminum molds are cheaper but can only handle short production runs.

The two main components of a mold are:

  • Cavity – the negative space that forms the shape of the desired plastic part.
  • Core – the protruding part of the mold that goes into the cavity.

Types of Plastic Molds

There are several types of plastic molds categorized by the molding process used:

Injection Molds

The most common mold type used in injection molding machines to produce volume plastic parts like PET preform mould, containers, automotive bumper mould, etc. They consist of two halves that separate from each other to eject finished parts.

Blow Molds

Used to shape hot plastic into hollow shapes like bottles and containers via air blowing. They consist of two to five split segments that separate for part ejection.

Compression Molds

Simpler molds with a male and female component used for molding parts with simple shapes by compressing plastic powder or preforms. Used for products like caps, gaskets, bushes, etc.

Rotational Molds

Used for molding hollow plastic parts like tanks, kayaks, and toys by rotating the mold while heating/cooling the plastic. The mold is made of two parts that separate to release the finished part.

Thermoform Molds

Single-piece shallow molds used for thermoforming thin-gauge plastic sheet into shapes via heat and vacuum/pressure. No mold separation required for part ejection.

Casting Molds

Made of flexible materials like silicone rubber to mold liquid resins and liquid plastics into shapes. Used for small production or prototype runs.

Foam Molds

Open-topped molds for pouring reactive foams to produce cushioning and insulation products.

The Plastic Injection Molding Process Step-By-Step

Plastic injection molding is a cyclical manufacturing process where raw plastic is melted and injected into a mold cavity to form a part. It consists of the following steps:

1. Clamping

The two halves of the injection mold are securely clamped together by a hydraulic or electric clamping unit. Tons of pressure are applied to keep the mold securely closed during injection.

2. Injection

Molten plastic is injected into the mold cavity through the sprue, runners and gates using an injection unit. Pressurized hydraulic fluid pushes the plastic forward.

3. Cooling

Cooling channels circulate water to cool and solidify the molten plastic in the mold cavity into its final shape. This may take 10 seconds to 2 minutes depending on part thickness.

4. Ejection

Pins eject the finished plastic part out of the opened mold. The mold closes again and repeats the injection molding cycle.

5. Trimming

Excess plastic is trimmed from the sprues and runners to separate the part from the mold framework. The scrap plastic is recycled.

Benefits of Plastic Injection Molding

Plastic injection molding offers several benefits:

  • High Production Volumes – capable of producing thousands of identical parts efficiently
  • Part Accuracy – parts are highly consistent and accurate within micrometers
  • Material Flexibility – can use various thermoplastic materials like ABS, nylon, polyethylene etc
  • Complex Geometries – ability to produce complex and detailed shapes not possible with other processes
  • Assembly Consolidation – multiple components can be integrated into one injection molded part
  • Low Labor – automated process requires minimal workforce monitoring
  • Short Lead Times – injection molded parts can be produced quickly with short production runs once the mold is ready

Plastic Injection Molding Applications

Plastic injection molding produces parts and products across many industries including:

  • Packaging - bottles, containers, caps
  • Automotive – interior/exterior components, dashboards, bumpers, grills
  • Consumer/Electronics – cases, enclosures, gears, camera components
  • Medical – IV components, syringe parts, oxygen masks
  • Toys/Sporting Goods – action figures, protective gear, helmets
  • Housewares – kitchenware, furniture, storage containers

Nearly all plastic products we interact with daily are manufactured using the injection molding process. It enables efficient, affordable, mass production of identical plastic parts.

How to Design Parts Suitable for Injection Molding

When designing parts for injection molding, consider:

  • Draft angles – include 1-2° draft on vertical surfaces so parts can eject cleanly from the mold
  • Uniform wall thickness – maintain even thickness to prevent defects like warping and shrinkage
  • Radii and fillets – add generous internal radii to avoid stress concentrations
  • Ribs and gussets – use thick reinforcing ribs to improve strength
  • Textures and finishes – simpler is better to allow demolding; avoid undercuts
  • Parting lines – position so they don’t affect aesthetics or performance
  • Ejector pins – place to avoid surface defects and ensure easy ejection

Properly designing parts for injection molding ensures they can be manufactured efficiently, within tolerance, without defects. Consulting with experienced injection molders early in the design process is recommended.


Plastic injection molding uses steel molds with precision-machined cavities to shape molten plastic material into fully formed parts and products. It is a versatile, cost-effective manufacturing process capable of producing plastic components with high accuracy, repeatability, and complexity in large volumes.

Understanding the different types of plastic molds, the step-by-step injection molding process, its benefits and applications, and design considerations will provide critical insight for designers, engineers, manufacturers, and anyone interested in how mass-produced plastic parts are made.

Bin Zheng 7
Joined: 3 months ago
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