The picture below is a pair of black plastic sunglasses.

 

This article takes a deep look at the core process of plastic glasses, the injection molding process.

Injection molding (IM) is a manufacturing process in which a polymer is heated to a highly molten state and forced into a mold under high pressure. The molten plastic then cools and solidifies in the desired part shape cavity. Finally, the molded part is removed from the mold.

The core requirement of an injection molding machine is to melt plastic pellets, push them into the mold cavity, and open the mold cavity to release the part after cooling. Therefore, an injection molding machine consists of a plastic injection unit, a mold clamping unit, and a mold.

The plastic molding machine was adapted and developed from the metal die-casting machine pioneered by John Hyatt, who patented the plastic injection molding machine in 1872.

The image below shows a typical injection molding machine showing all the basic components, namely the injection unit, clamping unit and mold.

 

Injection Unit - The injection unit consists of a material feed hopper, a barrel with an extrusion screw, and a hydraulic cylinder to push the injection unit toward the mold cavity entrance. The screw mixes the material, then heats and pushes the molten plastic into the mold. The screw and barrel geometry are optimized to help build pressure to the right level and melt the material.

Clamping Unit - The injection molding machine clamping unit clamps the two halves of the mold together during the injection cycle to resist the injection force. It also helps the mold open and close correctly in sync with the injection cycle, keeping the two halves aligned.

Mold - A mold is a segmented piece of metal with a cavity that is the same shape as the desired part. Manufacturers typically make molds from tool steel, aluminum, and beryllium copper. A mold can have one cavity, multiple identical parts, or multiple parts, which allows manufacturers to mold multiple parts together in one injection cycle.

The first step in the injection molding process is mold making. Once the mold is designed and built, trials begin. First, manufacturers conduct trials to set the process parameters. Typically, the first batch of samples, called T1 samples, is for customer approval.

Manufacturers load a feed hopper with dry polymer pellets, color pigments and other polymer fillers and additives.

The machine then feeds the material mixture into the barrel, where a reciprocating screw simultaneously heats, mixes and moves the material toward the mold.

When the material reaches the correct temperature in the barrel, the injection molding machine is ready to start the injection molding cycle.

A typical injection molding cycle begins.

Once the injection molding cycle begins, the machine closes the mold and the clamping unit clamps the mold halves together.

When the material reaches the correct temperature and the mold is ready, a hydraulic cylinder (Ram) moves forward to inject molten plastic into the mold cavity through the mold runners. This is called injection.

The molten plastic cools as it contacts the cooler mold surfaces. The Ram maintains pressure to inject more molten plastic into the cavity to compensate for the material shrinkage during cooling.

After the material solidifies in the mold cavity, the injection unit retracts from the clamping unit, which opens the mold.

When the mold tooling opens, built-in ejector pins eject the molded part.

The finished part goes through several post-processing steps. These procedures include removal of excess material and structural supports, finishing of surfaces, and, if required, heat treatment, painting, and electroplating.

Injection molding is one of the efficient manufacturing processes for producing plastic parts. Therefore, it is used in various consumer goods, industrial, automotive, manufacturing, and food processing industries. Others include injection molded glasses, also known as plastic glasses.

1. Injection molding can produce complex shapes with very tight tolerances.

2. The weight of injection molded parts ranges from 50 grams to about 25 kilograms.

3. It is easy to scale up production, and production costs fall as the number of parts increases, and to produce repeated parts.

4. Low labor costs, because injection molding is a semi-automated manufacturing process, with only one operator managing the machine.

5. A wide range of material options, although the most common injection molding materials account for 90% of injection molding. Compared with other manufacturing processes such as CNC machining, IM can produce near-net-shape parts with little scrap, thus reducing waste.

6. Design freedom with function, material and color options. Materials can also be modified to suit the application and operating environment by adding additives such as UV additives, fibers and glass beads.

7. IM is the most widely used molding process for thermoplastics. Compared with CNC machining and 3D printing, IM manufacturing is very efficient and fast.

What are the disadvantages of injection molding.

1. The initial cost is too high due to the cost of the mold. Therefore, injection molding is only economical for large-volume parts.

2. IM requires several prototypes and iterations to correct the part, adding time and cost; therefore the initial lead time is long.

3. Not suitable for large parts because the mold and machine limit the size. This can be overcome by making multiple parts to create large components. Multiple parts can be cut into the same mold with multiple cavities.

4. Part design limitations (such as undercuts) increase costs. Design changes and interactions are very expensive and time-consuming. Therefore, processes such as 3D printing and vacuum casting are used to prototype parts before injection molding.

5. Thermosets and elastomers can only be injection molded by modifying the process and equipment. Manufacturers need to clean the sprues and runners on the parts ejected from the mold. Sometimes clever three-plate tools can remove sprues and runners, but the cost is very high.

Injection molding is the most widely used molding process for thermoplastics. An estimated 390.7 million metric tons of plastic parts were produced worldwide in 2021.

Because thermoplastics and thermosets react differently to temperature, some thermosets, such as silicones and elastomers, require modified equipment and operating parameters when molding in order for these materials to crosslink. A typical example of an improved injection molding manufacturing process is the liquid silicone rubber molding process.

Acrylonitrile Butadiene Styrene (ABS) – 27% of world production

Polycarbonate (PC)

Polycarbonate + Acrylonitrile Butadiene Styrene (PC/ABS)

High Impact Polystyrene (HIPS) – 8% of global production

Polyphthalamide (PPA)

Polymethyl Methacrylate Acrylic (PMMA)

High Density Polyethylene (HDPE) (15% of world PE production)

Low Density Polyethylene (LDPE) (15% of world PE production)

Polypropylene (PP) – 38% of world production

Polyoxymethylene (POM)

Both plastic injection molding and 3D printing are manufacturing processes for making plastic parts, but they are produced differently. Therefore, the mechanical properties and surface finish of the final product will be different.

3D printing, also known as additive manufacturing, creates 3D objects by adding material layer by layer. As mentioned in the previous section, injection molding creates parts by injecting molten plastic into a mold cavity. Let's look at the differences between the two.

1. The initial mold cost is the biggest disadvantage of the injection molding process compared to 3D printing.

2.3D printing is an excellent choice for prototyping injection molded parts to make fine adjustments through rapid iterations.

3. Although 3D printing product cycles are slower, the initial setup time is fast because most 3D printing technologies can print directly from 3D CAD models.

Injection molding is cost-effective for high-volume production.

4.3D printing is faster to set up, allows frequent design changes, and is more suitable for complex designs.

5. Use 3D printing to make small batch prototypes and pre-production test units. This is a good choice for the early product design stages when rapid design changes need to be tested.

Understanding how to design injection molding and getting involved in the design process early is critical to the injection molding process because it allows you to avoid common injection molding defects.

1. Shrinkage. A common problem with injection molding is shrinkage. Since polymers have a higher coefficient of thermal expansion than metals, shrinkage can occur during the cooling cycle of the molding process.

2. Short shot. A short shot is when the injected molten plastic solidifies inside the mold before filling the mold cavity. This is caused by insufficient pressure, low temperature, and low injection volume.

3. Flash. Flash is when the molten plastic squeezes through the gap between the two mold halves. It usually occurs near the ejector pins.

4. Dents. Dents occur when the inner surface shrinks and pulls the solidified outer surface inward, creating a depression in the surface.

5. Voids. Voids are very similar to dimples, but instead of pulling the outer surface, the inner surface pulls away from the solidified outer surface, creating an internal void.

6. Weld lines. When plastic flows around a specific feature and meets from opposite directions, a visible line is formed, called a weld line. These weld lines affect the function and aesthetics of the product.

7. Burn marks and discoloration. Burn marks are stains on the surface of the injection molded part, usually rust-colored.

8. Flow marks. Flow marks are streaks, patterns, or lines on the molded part caused by the physical path and cooling curve of the molten plastic as it flows into the injection mold cavity.

9. Delamination. Thin surface layers form on the component due to contaminants, a condition called surface delamination. These layers are similar to coatings and often flake off.

10. Warpage. Warpage occurs when shrinkage is uneven.

The premise of a good injection molded eyewear is that the injection mold is reasonably designed and well made, so that the surface of the injection molded eyewear product is smooth, even mirror-like, and the parting line and burr are not obvious. The use of injection molding to manufacture eyewear products is generally used for single models with a large number of models, otherwise the production cost will be relatively high. Injection molded products can achieve more complex structures and shapes, and the products are full of personality. In order to give injection molded glasses products a colorful appearance and fashionable features, surface treatment and spraying are required after injection molding. The injection molding process is the key process for making plastic glasses. To make high-quality injection frame glasses, you must first master the injection molding process.