How to Use Low Volume Injection Molding Properly?

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Posted on:  Jan 3nd, 2019, | By kenzi, WayKen Project Manager

Low Volume Injection Molding is a new way to look at the established conventional Injection Molding. For a long time, it has been associated with mass production with part count ranging from 500,000 to several million parts. However, new Computer Numerical Control Technologies, touch probes, and other innovations have enabled us to create the injection mold tooling for low volume part batches.

Injection Molding Tool Design

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First of all, let’s look at how conventional Injection Molding is designed to understand what we managed to change in the process to move it from mass production into that low volume manufacturing area. The process and the Tools used for conventional injection molding and low volume injection Molding are actually quite similar.

The molding tool consists of three main elements: the injection mold that makes molten plastic take the desired form; the clamping system that is used to hold the mold halves together and prevents them from shifting and creating a step defect; The plastic feed system. The feed system consequently consists of the feed screw that is surrounded by heaters and the bunker with plastic pellets inside.

Injection Molding Process

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The process is done in the following way. The pellets fall from the bunker onto the screw and are simultaneously melted end fed into the cavity of the mold. The cavity is filled with liquid plastic until the required pressure is reached. Afterward, when the part gets cool enough, the clamping system unclamps the mold and the part is ejected. Some unnecessary elements are mechanically removed and the process can be set up again. If the workflow is set, manufacturing each part takes only a few minutes.

Low Volume Injection Molding VS Conventional Process: Differences

So, now you know how the conventional injection molding process works but what’s the difference compared to the rapid variation? Well, the most expensive part of the molding system is the mold, which contains the product cavity. It is usually made to last. Special hardened steels are used, they are heat-treated and processed to get outstanding hardness, precision, and surface finish of the part. Don’t forget, that the cavity is hard to work with since bigger tools won’t fit everywhere and smaller tools break easily. The mold halves need to be precisely matched to each other or the part will have a shift along its middle.

The cost of a heat-treated hardened steel mold is colossal and the time to manufacture it is significant. The result of Injection molding is a near-perfect part in terms of quality, but it only becomes profitable when the cost of the mold is spread over the vast volume of products it can produce.

So, that’s where rapid IM comes in as it strikes directly into conventional IM weak spot. Rapid IM, you see, is aimed at lowering the cost of the mold. It is usually manufactured from softer metals (aluminum alloys for instance) and has many more joints. The result is that the mold becomes considerably cheaper but serves less so it’s perfect for low volume, low-cost injection molding.

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The advantages of Rapid Injection Molding

Even Rapid Injection molds are quite hard to manufacture. Surely, harder than silicone casting forms or 3-d printing. However, RIM holds a number of advantages not available to the other rapid prototyping processes.

Accurate features

Injection molds are exposed to very high pressure. This leads to the plastic inside the molds being stacked harder on the mold in comparison to other low volume manufacturing techniques. This makes it possible to produce parts with very fine features or thin walls. Due to the intensive pressure, the plastic fills the cavity better and no air bubbles are left.

Increased efficiency

Injection molding itself is carried out significantly faster than other plastic manufacturing processes. Once the molds are done, less than an hour is required for each part to be molded. And that is considering higher quality compared to other processes. Besides, despite the mold not being hard enough for millions of injections, it can still withstand tens of thousands.

Improved strength

You have the option of using fillers ( special small particles to harden the plastic) on injection molds in the plastic injection molding procedure. The fillers help reduce the density of the plastic when it is being molded and also help in adding more strength to the part once it has been molded. In sections where parts need to be strong and sturdy, the plastic injection has a number of advantages that other molding processes do not offer. Injection molding machines enable the production of the same part from multiple materials without making any significant changes in the system.

Great Flexibility

Rapid Injection Molding is known for increased flexibility. The cavity of the mold is usually a separate part so it is much easier to change the design of the cavity if changes are necessary. This makes it a good option for prototyping at later stages when a sample batch of the product is manufactured for real-life tests. In addition to that, it is much easier to make adjustments for aluminum molds rather than for heat-treated steel.

Low Volume IM Materials

LVIM copies conventional molding in regard to the material. Aluminum alloys have a higher melting point and thermal stability than any industrial plastic used in modern industry. The most widespread IM materials are Polypropylene (PP), Acrylonitrile Butadiene Styrene (ABS), Polyamide (Nylon), High-Density Polyethylene (HDPE), and Polycarbonate (PC). In addition to that, the flexibility of the IM process allows one mold to be used for multiple plastic materials without any change in the design. Only the molding parameters have to be changed.

Design for Low Volume Injection Molding

Design for low-volume plastic parts production is similar to the design for conventional processes. The main point is to keep wall thickness uniform so as to prevent cavities but at the same time to prevent the part thickness from being too large. Another important issue is to keep in mind the elements for part ejection. Vertical walls should be 85-87 degrees instead of 90  and each sharp angle should instead have fillets. Other notable issues are undercuts and shutoffs. Those are basically elements that are overhanging over the vertical wall or vertical pockets. Those elements must be prevented at all costs. It is better to make holes to prevent undercuts rather than leave those elements intact.

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Cost Analysis

As was already mentioned, the main initial cost of the injection process lies in the cost of the mold and the variable cost lies in the material. The second point is very hard to minimize since you can’t really decrease the cost of the material without sacrificing part quality and material waste with IM processes is minimal. However, the initial cost can be varied to achieve the desired injection molding cost.

The ways to control the initial costs comprise of variable quality and sturdiness of the mold. If we choose a softer aluminum alloy for the cavity, the machinability grows and lead time gets much shorter with it, however, the number of injections it can withstand lowers though. In addition, the mold can be made into an assembly that consists of the cavity and the block to be installed into the molding tool. That way, you can manufacture mold parts simultaneously and decrease the lead time. However, the part quality is worse because of additional joints’ lower precision. Basically, you will have to choose between three connected parameters: lead time, mold sturdiness, and part quality.

About Us Can Do

In  WayKen, you can enjoy the service of low volume injection molding to help you to create specific material parts within only 2-5 weeks. Besides, if you take advantage of high-speed machining, EDM, and a wide selection of materials, you will get your complex and intricate parts faster and at a reduced cost. Therefore, rapid injection molding is really economic and efficient.

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There is also a situation the client will ask us to give some constructive suggestions that are better for production when quoting so that they can change some design to make the mold easier to go and parts easier to remove from the mold. For example, for the part thickness if it is too thick, we will suggest client make it thinner, or when parts come out from the mold, the part with too thick thickness will become shrinkage, but some clients will think that will be OK, then we will need a limit sample of this shrinkage for future mass production the quality check.

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