Injection Molding – How to Manufacture Successfully in China

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. The process 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 techniquesThe equipment usedCommon defects and how to fix them   

How it works

The 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 manufacturing 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.  

Techniques

There are two main techniques for this process– 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 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. 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.