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Plastic Injection Molding Explained

This is how plastic injection molding works

Vaughn Cook RockWell Window Wells
Jeff Crawshaw
January 19, 2021

In the first part of our three-part series on plastic injection molding, you read about the history of this multifaceted process of manufacturing large volumes of common-use plastic products. To recap, plastic injection molding is a technique by which hot molten plastic is injected into a customized metal mold. The plastic is then cooled, causing it to solidify and take the shape of the mold. Plastic injection molding is used to make a wide variety of everyday plastic products such as packaging, fasteners, tools, car components, toys, and medical equipment.

In this second piece, we take a look at how plastic injection molding works. Read on to know about:

  • The different plastic injection molding techniques
  • The equipment used
  • Common defects and how to fix them   

How plastic injection molding works

The plastic injection molding process is similar to the die casting process. The entire cycle can be completed within seconds or a few minutes. And it only takes five steps:

1. Clamping: Depending on the plastic, the two halves of the mold are sprayed with a chemical release agent to help with ejection later. They are then clamped shut. The mold halves come attached to the injection molding machine, with one fixed in place and the other allowed to slide back and forth to aid in ejection.           

2. Injection: Molten plastic is injected into the mold cavity at high speed and pressure. The amount of material injected is called the shot.           

3. Cooling: Water cooling systems in the tool allow the plastic to cool and solidify quickly, which is desirable as cooling can take up to 60% of the cycle time.

4. Ejection: Once the plastic has cooled, the mold is opened, and the molding ejected. The release agent applied earlier facilitates its smooth removal. This ends the production cycle, and the process can then be repeated.

5. Post-production treatment: The molded part might still require some finishing. For example, excess plastic from material inlet gates will still be attached to the molding and will need to be removed. In the case of thermoplastics, this scrap can be reground and reused. Some moldings require the creation of threads (small grooves in the molded part that allow it to be fastened to another plastic or non-plastic part, as in a bottle cap or drainpipe nut). Some parts might require the addition of inserts, which are usually small metal parts (though these can be inserted during the molding process itself). A final surface finish, such as powder coating or the application of paint, might also be needed.  

Plastic injection molding techniques

There are two main techniques in plastic injection molding – the traditional cold runner system and the hot runner system. Each has its benefits and drawbacks, which makes them suitable for specific applications. But first, what are runners? Simply put, runners are a network of tunnels or channels that carry molten plastic into the mold cavity.

1. Cold runner system: Molten plastic enters the mold through a channel called the sprue, flows into the runners and ultimately into the mold cavity. Because the runners are not heated, the plastic in the sprue and runners cools and gets attached to the molded part. After ejection, this excess plastic must be removed from the part.

  • Advantages: Lower cost per part, lower tooling and maintenance costs, accommodates a wider variety of plastics as heat sensitivity of the raw material is not a factor, more flexibility in terms of design changes, faster color change.
  • Disadvantages: Slower production cycle, lower production volume, higher waste rate, requires removal of excess plastic, limitations in part design and size.
  • Suitable for: Electronic casings, keyboards.                  

2. Hot runner system: In contrast, a hot runner system uses a heated distribution network that keeps the plastic in the runners in a molten state. This prevents the plastic in the runners from attaching itself to the molded part.

  • Advantages: Faster production cycle, lower waste rate, requires less pressure, less post-production work (because removal of excess plastic, regrinding and recycling are not required), capable of producing higher volumes and larger parts.
  • Disadvantages: High tooling, maintenance and inspection expenses, higher cost per part, heat sensitive plastic resins might experience slight color changes.
  • Suitable for: Bottle caps, mobile phone cases, beauty products packaging.                    

The plastic injection molding industry is constantly evolving. Apart from these techniques, some relatively new processes are now growing in popularity: 

1. Overmolding: The process of adding a layer of molten plastic over an existing plastic (or metal) part is called overmolding. It is also called two-shot molding because it is a two-step process. The base part is molded first using plastic injection molding. This part is then placed in an overmolding tool, where it gets an additional plastic layer. This layer is usually made of a thin, pliable, rubber-like plastic such as silicone, though hard plastics can also be overmolded. Overmolding is increasingly being used to improve product performance and aesthetics. For example, the soft-touch exterior on a plastic toothbrush, which gives it a better grip and a more colorful look, is a product of overmolding.          

2. Insert molding: This is the process of molding plastic material around an insert/inserts (which can be made of metal, plastic, ceramic or any other suitable material). What differentiates insert molding from overmolding is that the insert is placed in the plastic injection mold cavity itself and is then completely encapsulated. The faster of the two processes, insert molding is an effective alternative to assembled parts that use adhesives and fasteners. Some common applications include wires and medical syringes.

Plastic Injection Moding- Clothes Pins
Plastic injection molding is great for production in vast quantities

Plastic injection molding equipment

1. The machine

A plastic injection molding machine is a device that brings together three systems – distribution, cooling and ejection. The distribution system transfers the molten plastic from the injection unit into the mold via the sprue, runners, and gates (entry points to the mold cavity). The cooling system assists in the hardening of the plastic. The ejection system helps remove the final product from the mold.

2. The mold

The mold is the custom tool in plastic injection molding. Molds come in various forms depending on your requirements:

  • Single cavity, multi cavity and family molds: A single cavity mold, as its name suggests, has one cavity. A multi cavity mold has more than one cavity of the same part while a family mold has multiple cavities of different parts. 
  • Hot runner and cold runner molds: Molds are also differentiated on the basis of the technique used – hot runner versus cold runner. Choosing between the two depends on the choice of plastic raw material.  
  • Two-plate and three-plate molds: The more common of the two, the two-plate mold has two plates – one moving and one fixed – divided by the parting line. (The parting line is the line of separation where the two mold halves meet). Such a mold is also called a single parting surface mold. It has a simple design, is cheaper to tool and has a shorter cycle. However, it is not the best option for a multi cavity mold. A three-plate mold or double parting surface mold, on the other hand, is split into three sections with two parting lines. Its complicated design makes it suitable for large parts that require multiple gates as well as parts that cannot have gate marks along their side walls. A three-plate mold automatically eliminates the gate removal process and is also a cheaper alternative to a hot runner mold. However, its complex design makes it more expensive to tool, it has a longer injection time, it requires a higher injection pressure, and it creates more waste.
  • Unscrewing mold: This is a unique mold with a rotating threaded core that is used to make parts with a screw function, such as bottle caps, nuts and bolts. After the plastic sets in the cavity, the core unscrews and forms the threads – which can be internal or external – on the part.               

Common plastic injection molding defects and their fixes

  • Warping/shrinkage: If the part doesn’t cool uniformly, one section of it might shrink faster and get distorted or warped. Solutions: Maintain a precisely calculated cooling rate, adjust wall thickness, increase injection pressure.  
  • Flash: A flash is a thin layer of solidified plastic on the product along the parting line, caused by molten plastic seeping out of the mold. Solutions: Lower injection pressure and/or speed, adjust clamp force to ensure the mold halves are tightly secured.
  • Bubbles: These are bulges on the surface of the molding, caused by trapped air or gas formation in the plastic. Solutions: Lower injection temperature and speed, make the gates larger to reduce trapped air; remove moisture from the raw material and lower barrel temperature to reduce gas formation.    
  • Ejector marks: Small indentations on the part formed during ejection. Solutions: Lower ejection speed, allow adequate cooling.
  • Discoloration: These are color variations commonly caused by leftover plastic in the distribution system from an earlier cycle. Solution: Thoroughly flush the distribution system between cycles.
  • Short shot: When the shot doesn’t completely fill the mold cavity, it is called a short shot. Solutions: Remove gate blockages, widen the gates, increase melt temperature. 

Next: How to buy plastic injection moldings

Sourcing Allies is a team of expert China sourcing agents that has helped western customers manufacture and source products from low-cost regions since 2006.

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