What is plastic injection molding?

Plastic injection molding is a manufacturing process for the mass production of parts. It is most commonly used for mass production of the same parts, usually on a formal scale of thousands or even millions of times.

The main advantage of plastic injection molding is concentrating and scaling up production. Once the machines and molds are ready, the price per production unit is very low. As more parts are produced, prices will fall sharply.

The advantages of injection molding

Injection molding products is a low reject rate compared to traditional manufacturing processes such as machining and can reduce a large proportion of the original blank or sheet. It may be slightly higher for additive manufacturing processes such as 3D printing, with a relatively low reject rate. Waste plastics in injection molding production usually come from the sprue, gate location, and any spillage leaking from the part cavity.

The runner is simply the channel through which molten plastic is poured from the nozzle of the injection molding machine to the gate of the entire injection mold. It is separate from the mold itself. The sprue is a system of channels connected to the gate that directs the molten plastic into the cavity within the mold. After the injection mold is filled (typically only a few seconds), the entire molten plastic will cool, leaving solid plastic in the runner, gate, cavity itself, and possibly a little overflow (if poorly sealed) at the edge of the part.

Thermoplastic materials are materials that can be recycled. Crush gate/runner nozzle and any waste parts. These recycled materials are then added in proportion to the new raw material. (Some of the properties of plastics degrade as they are molded from time to time). Recycled materials are used for low-quality components that do not require high-performance characteristics.

Plastic injection molding is a highly repeatable process. The second part of the production will be the same as the first part. It allows good control of product consistency and component reliability in mass production.

The disadvantages of injection molding

The two main disadvantages of injection molding are high tool cost and long lead time. The mold itself is a project and only a stage of the whole injection molding process. Before an injection molded part can be produced, a prototype of the part must first be designed and made (possibly by CNC or 3D printing), and then a mold must be designed and made that can produce a copy of the part. After completing the above two stages and making extensive test adjustments, the part can be injected. The process of adjusting a mold before mass production takes time and money, especially for multi-cavity moulded parts.

For example, To injection molded a new shampoo bottle cap. That hat might have threads to attach to the bottle, hinges, fasteners, or some cladding. A single cavity mold for the component can be made first to ensure that all features are made as required. After testing, a new mold can be made, forming up to 16 LIDS at a time. They first tested the mold with a single cavity, so they didn’t have to modify each cavity 16 times to find any problems.

Because tools are usually made of steel (very hard materials) or aluminum, it is difficult to make changes. If you want to add plastic to a part, you can make the mold cavity bigger by cutting steel or aluminum. But if you want to reduce plastic, you need to reduce the size of the tool chamber by adding aluminum or metal. It is very difficult, and in many cases, may mean completely scrapping the mold (or part of the mold) and starting from scratch.

Injection molding requires uniform wall thickness. Generally, the entire wall thickness is about 2-3 mm. Maintaining uniform wall thickness is important to prevent defects such as dents due to inconsistencies in the cooling process. A good rule of thumb is to keep the wall thickness less than or equal to 4 mm. The thicker the wall, the more materials used, and the longer the molding cycle, the higher the cost of each component. Conversely, if the wall thickness exceeds about 1mm, problems may be encountered when filling the mold (resulting in missing material or a short shot). Designers can compensate for this by using materials with a higher melt flow index, such as nylon.

In general, plastic injection molding is not recommended for large parts to be manufactured as a single piece. This is due to the size limitations of the injection molding machine and the mold itself. While injection molding machines exist for making very large parts (for example, a 1,000-ton injection molding machine, about the size of a railroad car), they are very expensive. For this reason, parts larger than common injection molding machine products are usually multi-piece.

Large molds require an experienced design to avoid increasing project costs.

Some considerations for injection molding:

Cost factor

Upfront costs: Preparing an injection molded product for production requires a significant initial investment. It’s best to know this key point in advance.

Production Quantity

Injection molding is the most cost-effective manufacturing method to determine the number of parts to produce. Estimate parts defect rates and yield rates (considering design, testing, production, assembly, marketing, and distribution costs and the expected sales price point).

Design Considerations

Part design: Simplifying product geometry and minimizing the number of parts will pay dividends.

Tool design: take the defects that may occur during production into account in the mold design process in advance. Run the simulation analysis using Solidworks Plastics and other mold flow software, considering gate location.

Production Factor

Cycle time: Reduce cycle time as much as possible. Use proven tools. Small changes can make a big difference, and shaving a few seconds off your cycle time from the start can save a lot of money when you’re producing millions of parts.

Assembly: Design your parts to minimize assembly. The main reason injection molding is done in Southeast Asia is the cost advantage of assembling simple parts during injection molding. If the assembly process can be optimized, a lot of labor costs will be saved.

Aria is a plastic injection molding manufacturer engaged in mold, injection, spray, pad printing, thermal transfer printing, and a variety of plastic products with rich production experience. With advanced equipment, professional technology, perfect quality, and perfect management system, the pursuit of “high quality, no mistake, on-time delivery” to the greatest extent to meet customer needs, wholeheartedly to provide customers with quality products and efficient service. They developed an online quotation platform in 2021, user can get an instant injection molding quote online by Aria’s website.