How to Design Plastic Products for Injection Molding?

Injection molding is a technology evident in our daily lives but is often overlooked and goes unnoticed. 95% of plastic products are all made with injection molding which means almost all of plastics surrounding us are created through this technology. This wiki article aims to enlighten us on how to design plastic products for injection molding.


Getting to Know Injection Molding

Injection molding is a manufacturing process used on a wide variety of products from tiny components up to large items. It is popularly used in the plastics manufacturing industry. The process uses a granular plastic gravity fed from a hopper which is dumped into a heated chamber through a screw-type plunger. The plastic is melted while pressed against the mold. The object is allowed to cool and is removed as a solid product. In some aspects of manufacturing, aluminum molds are also being used since they are quick to mass produce while other large manufacturing companies use steel molds of various kinds. It depends on for how long or better for how many pieces the mold has to last.

Injection molding makes it possible to produce plastic product designs in large volumes which is why it is popularly used in large-scale plastic custom manufacturing. Thousands of plastic part designs can quickly be manufactured after several hours of work in a manufacturing production line. The main disadvantage of injection molding is that the process is not suitable for very small-scale production given the setup costs and equipment needed for this type of technology. You can learn a lot more online, for instance here with the free course from BASF.


4 Stages of Injection Molding

1. Clamping

This is the initial process wherein two sides of the molds are securely held in place by a clamping device. Hydraulic power keeps the mold in place preparing it to be injected with the specific material to be used.

2. Injection

As the name suggests, plastic materials usually thermoplastic are loaded on to the injection molding apparatus subsequently melted and injected into the mold.

3. Cooling

The cooling process lets the molten plastic solidify and follow the shape of the mold. There are different cooling times for each plastic product designs depending on its wall thickness including the thermodynamic properties of the plastic substance used in the injection molding process.

4. Ejection

This is the part wherein the molded part is now taken out of the clamping device. A mold ejection agent will be sprayed on the surface of the mold cavity before the injection process is facilitated to make the ejection easier later on and to make sure the plastic part design remains intact after ejection.


Advantages of Injection Molding

1. Cost effective

The biggest advantage of injection molding has to be its low production costs as plastic product designs created this way have very low price per unit compared to other plastic production following a different type of manufacturing technology. Its ability for mass production defeats the costs of materials since everything is in a lump amount making it cheaper overall. However, be reminded of the setup’s cost and tooling as well to balance its cost effectiveness.

2. Less scraps

Most traditional plastic manufacturing processes cut out a substantial part of its plastic sheet during production. With injection molding, scraps and wastes only come from overflows in the mold and leaks from runners and sprue. Injection molding follows cuts accurately keeping wastes at an all-time low in plastics manufacturing.

3. Versatile

As a manufacturing process designed for mass production, injection molding can easily be modified to create different plastic part designs as long as the same injection molding design guidelines are being followed. After mastering how to design plastic products and pinned it down, you can always expect to produce the same results for each injection molding batch which can consistently be repeated for maximum production.


How to Design a Plastic Product Mold for Injection Molding

The biggest challenge in injection molding is the process of designing a mold. There are almost no limits to your creativity (see the MOMA section on plastics here), but you can do the designs in clever and less clever way from a manufacturing point of view. Plastic manufacturers highlight this aspect as the most crucial part of injection molding. There will be no successful plastic part designs for injection molding if there is no perfect mold from where it is formed. The biggest factors to designing a mold are part and tool design. Getting these factors right means faster production, better quality and reduced costs while having them wrong could substantially affect these production aspects in a negative manner. Here are other factors you need to consider with injection molding during the product development and prototyping.

1. Wall thickness

Thin walls are advisable in plastic manufacturing for shorter cycle times and being able to produce more plastic part designs in shorter production lead time. Thinner walls also makes the cooling process faster. Ideal wall thickness from 0.08″ (2mm) to 0.16″ (4mm), but thin wall injection molding can go as thin as 0.02″ (0.5mm).

However, wall thickness also depends on the type of plastic material being used. Here are the recommended wall thickness for various plastic materials; ABS resin is between 0.045 and 0.140 inches, Acrylic is at 0.025 –0.150, Liquid crystal polymer at 0.030 -0.120, Nylon at 0.030–0.115, Polycarbonate at 0.040 – 0.150, Polyester at 0.025 –0.125 and Polypropylene at 0.025– 0.150.

2. Ribs

Adding ribs helps increase the bending stiffness due to the increased moment of inertia. This is a suggested option instead of adding thickness to the wall. Here are recommendations with ribs in plastic production;

  • Rib’s thickness should not be more than 60% of the nominal thickness value.
  • Height should be three times lower than the wall thickness.
  • Draft angle is at 25 degrees.
  • The ribs position must be perpendicular to the axis where the bending occurs.
  • Corners of the attachment points must be rounded instead of having them pointed.

3. Bosses

Bosses are layers wherein fasteners are attached and threaded inserts are located. Recommended specifications for bosses are the following;

  • Bosses’ wall thickness should not be more than 60% of the main wall.
  • Base radius should be at least 25% thickness of the main wall.
  • Bosses should be supported by ribs connecting to walls adjacent from their position or gussets at the bottom the mold design.
  • Ribs should be used to isolate bosses in corners of the design.

Conclusion

Injection molding is not the one and only perfect plastic manufacturing method. However, it has its set of disadvantages but the benefits clearly outweighs its cons with a huge disparity. All of these information on how to design plastic products through injection molding may seem so sketchy if you are still new to plastic manufacturing. It needs time and experience before you get yourself fully acquainted to injection molding as a method on how to design plastic products. An experienced plastics manufacturing company can help you with the details.

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