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Everything You Need To Know About Injection Molding

What is Injection Molding?
Injection Molding is a producing process for manufacturing products in large quantity. It is commonly used in large-scale production processes where the same components are created thousands or even millions of times in a row.

Why Use Injection Molding?
The main advantage of injection molding is the ability to concentrate on expanding production. Once the initial cost is paid, the price per unit during the injection molding process is very low. As more parts are produced, prices will drop dramatically. Other advantages include:
• Compared to traditional manufacturing processes (such as CNC machining), injection molding produces a low scrap rate that reduces the majority of the original plastic block or sheet. However, this may be negative for additive manufacturing processes for 3D printing with even lower scrap rates. Note: Waste plastics in injection molding typically come from four areas: runners, runners, gate locations, and any overflow material that leaks from the chamber itself (called “flash”).

The gate is simply a passage that directs molten plastic from the nozzle of the injection molding machine to the entry point of the entire injection mold. It is separate from the mold itself. A runner is a channel system that meets a gate, typically within a mold or as part of a mold, directing molten plastic into a component cavity within the mold. There are two main categories of runners (hot and cold). Finally, the gate is part of the channel after the runner has entered the chamber directly. After the injection mold cycle (usually only a few seconds), the entire molten plastic will cool, leaving solid plastic in the runners, runners, gates, part chamber itself, and possibly a little overflow on the edge of the part ( If the seal is not 100% correct).
Thermoset materials, such as epoxy resins that cure upon exposure to air, are cured materials that burn after curing if attempted to melt. In contrast, a thermoplastic material is a plastic material that can be melted, cooled and solidified, and then melted again without burning. With thermoplastic materials, materials can be recycled. Sometimes this happens on the factory floor. They grind the gate/runner and any scrap parts. They then added this material back to the raw materials that entered the injection molding machine. This material is called "re-grinding." Often, the quality control department will limit the amount of recycling that can be allowed to be reintroduced into the press. (Some performance characteristics of plastics are reduced with molding again and again). Or, if they have a lot, the factory can sell this regrind to other factories that can use it. Typically, the regrind material is used for low quality components that do not require high performance properties.
Injection molding is very repeatable. In other words, the second part of your production will be identical to the first part. This is a good feature when trying to produce brand consistency and component reliability in high volume production.

What Is The Downside To Injection Molding?
Due to design, testing and tool requirements, upfront costs are often very high. If you want to mass produce parts, you need to make sure you design correctly for the first time. This is much more complicated than you think. Getting the right design includes:

Designing the part itself and prototyping it into specifications

  • Initial prototyping is usually done on a 3D printer and usually uses a different material (eg ABS plastic) instead of the final partDesigning an injection mold tool for an initial production round.
  • Typically, 300-1000 injection molding prototypes are produced in production materials that require the development of injection molds.Refining any and all details in the injection mold tool prior to mass-production in an injection mold manufacturing plant.

Potential negative effects of injection molding include:
The two main drawbacks of injection molding are the high tool cost and the long lead times required. The mold itself is almost just a project and is just one stage of the entire injection molding process. Before producing an injection molded part, you must first design and make a prototype of the part (possibly by CNC or 3D printing), and then you must design and make a mold that can produce a copy of the part. Finally, you can usually inject parts after extensive testing in both phases. It is conceivable that all the iterations required to make the tool correct before mass production takes time and money. It is difficult to prototype your injection molding tools. But it does happen, especially for parts that will be made in multi-cavity tools. Let’s suppose we want to inject a new shampoo cap. The cap may have threads to attach it to the bottle, live hinges, snaps, and possibly some overmolding. The company can choose to manufacture a single-cavity tool for the part to ensure that all features are molded as needed. Upon approval, they will create a new tool, such as forming 16 tools at a time. They first use a single-chamber tool, so if there are any problems, they don't have to pay and wait for each cavity to be fixed 16 times.

  • Since the tool is usually made of steel (very hard material) or aluminum, it is difficult to make changes. If you want to add plastic to the part, you can make the tool cavity larger by cutting steel or aluminum. But if you want to take away the plastic, you need to reduce the size of the tool cavity by adding aluminum or metal. This is very difficult, and in many cases it may mean that the tool (or part of the tool) needs to be completely discarded and restarted. In other cases, you may be able to weld metal into an undesired cavity.
  • Injection molding requires a uniform wall thickness. If you are cutting the cross section of the upper Panasonic mold, you will find that the entire wall thickness is about 2-3 mm. Keeping the walls too thick is important to prevent inconsistencies in the cooling process from causing defects such as dents. 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 you use and the longer the cycle time, the higher the cost per part. Conversely, if the wall thickness exceeds about 1 mm, it may cause problems (causing gaps or short shots) when filling the mold. Designers can compensate for this potential by using materials with higher melt flow indices, such as nylon, which are typically suitable for walls as thin as 0.5 mm. Different manufacturing techniques such as CNC do not require uniform wall thickness at all.
  • In general, large parts cannot be produced as a single piece by injection molding. This is due to the size limitations of the injection molding machine and the mold itself. For example, large injection molded parts consider Target's shopping cart. Although there are machines for molding very large parts (for example, a 1000 ton press, which is roughly equivalent to the size of a train car), it is very expensive to use. For this reason, objects that are more capable than typical injection molding machines are usually multi-piece. CNC machines have similar limitations in terms of product size, while 3D printing has more restrictions. The CNC is limited to the stroke and size of the bed in the milling machine, while large 3D printed parts usually require multiple pieces of print and then glue together.
  • Large undercuts require an experienced design to avoid and often increase project costs.

What Are Some of The Considerations For Injection Molding?
Before you produce parts by injection molding, please consider the following points:

  • Financial Considerations
    1) Entry costs: Preparing products for injection molding requires a large initial investment. Make sure you understand this key point in advance.
    2) Production Quantity
    (1) Determine the number of parts that are injection molded into the most cost-effective manufacturing method.
    (2) Determine the number of parts you want to earn from your investment (considering the cost of design, testing, production, assembly, marketing and distribution, and the expected price point of sales). Built on a conservative edge.
  • Design Considerations
    1) Part Design: You want to design the part from day one with injection molding in mind. Simplifying geometry and minimizing the number of parts early on will pay dividends down the road.
    2) Tool Design: Make sure to design the mold tool to prevent defects during production. For a list of 10 common injection molding defects and how to fix or prevent them read here. Consider gate locations and run simulations using moldflow software like Solidworks Plastics.
  • Production Considerations
    1) Cycle Time: Minimize cycle time. Using machines with hot runner technology will help with well thought out tools. Small changes can make a big difference, and you can save a lot of money when you're producing millions of parts by shortening your cycle time by a few seconds.
    2) Assembly: Design your parts to minimize assembly. The main reason for injection molding in Southeast Asia is the cost of assembling simple parts during the injection molding process. If you can design an assembly in a process, you will save a lot of labor costs.

Injection molding is a great technology for large-scale finished product production. It is also useful for the final prototype for consumer and/or product testing. However, in this later stage of production, 3D printing is more affordable and flexible for the product in the early stages of design.

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