Injection molding of plastic is a technology of unique significance in the world of the and products. Injected molded parts are used in homeware, electronics, medical products, automotive and furniture products just to mention a few.

During the injection molding process small plastic pellets are melted and injected with high pressure to a temperature controlled mold. Once the plastic cools it solidifies and stays in the shape of the mold’s cavity (similarly to ice cubes). Once the cooling process is done the part is ejected from the mold and proceeds to post processing or assembly.

Advantages of injected molded plastic

The design and production of injected molded products has many advantages that make it such a popular method. In this section we will review a few of these advantages:

High accuracy: Injection molding delivers consistent and high precision parts with tight tolerances across production runs, which are a key factor in the quality of products and is essential for assembly with other parts (electronics, other plastic parts or parts from other materials).

Speed: After finishing the mold manufacturing and mold set up processes, injection molding allows for high-speed production that can produce thousands of parts a day, making it an ideal choice for products which require mass production.

Cost effectiveness: Injection molding is one of the most suitable production methods for mass produced goods because once the mold is created, the per-unit cost of injection molded parts decreases significantly.

Lightweight: Plastics is a relatively light weight material compared to other materials such as metal or glass. This attribute combined with the ability to produce plastic parts with thin wall thickness makes it an ideal material for products which require light weight materials.

Material versatility: Injection molding supports a wide range of thermoplastic and thermosetting polymers including Polypropylene, Nylon, ABS, Silicons and more. This wide range of material options allow to match the right material for the product’s requirements. Additionally it is possible to combine different polymers to reach a combination of characteristics such as strength, flexibility, transparency, heat resistance and more.

Minimal post processing: Plastic molded parts usually require very little to no post processing after the part is ejected from the mold. This is an important aspect that affects both the speed of production and the price of the final product which does not have to bear additional post production costs.

CMF versatility: Plastic can be injected in a huge variety of colors, opacities and surface finishes.

Morphological freedom: Injection molding allows for vast design flexibility in the creation of custom shapes and intricate geometries, in comparison to other materials and manufacturing methods such as wood or metal (excluding die-casts). Due to the fact that molten plastic (just like water) does not have a shape of its own, it takes upon itself the shape of the mold it is casted to. This is nearly impossible to do industrially in any other materials.

Design consideration when designing parts for injection molding

While there are many advantages to designing plastic products, the design of plastic parts also requires a lot of knowledge, experience, attention to details and a good collaboration between the design, engineering and manufacturing. We can easily say that even after 15 years of designing plastic products we still learn something new in every new product we design. In this section we will explain the main design considerations that need to be taken into account during the design and development of plastic products in order to reach a high quality product:

Material choice: Different polymer types have different behavior during the manufacturing process in regards to the wall thickness they need to flow in the mold correctly, their shrinkage percentage during the cooling process, knit lines and jettings and much more. For these reasons it is important to choose the material type early in the design and development process so the product can be designed in a way that is suitable for the polymer requirements from the get go.

Uniform wall thickness: Injected molded plastic parts require consistent wall thickness. The reason for this is that consistent wall thickness ensures even material flow inside the mold and helps to minimize variations in cooling rates during the cooling process, which reduces the risk of defects such as warping, sink marks, and voids.

Ribs: Due to the fact that injected molded parts require a uniform wall thickness, most plastic parts are what we call “shells”. This “shellnes” is needed for high quality parts (as explained in the previous paragraph) but it also creates weak parts that need strengthening. Strengthening of plastic shells is usually done with what is called a “rib”. Ribs are thin walls that support the plastic parts usually from the inside or bottom of the product which is not visible to the user. Rib placement and thickness is crucial to avoid warping and sinking in the material.

Draft angles: Draft angles are essential in injection molding to facilitate the ejection of molded plastic parts from the mold. The specific draft angle requirements may vary depending on factors such as part geometry and surface finish, but typically range from a minimum of 0.5 to 1 degree. However, in cases where draft angles are undesirable for aesthetic or functional reasons, an alternative solution is to incorporate “sliders” into the mold design.

Sliders, also known as lifters, are mechanical components used in injection molding molds to facilitate the ejection of parts with undercuts or features that cannot be released with conventional straight-pull motion. Unlike the main mold halves (core and cavity), which open and close along a straight line, sliders move in a perpendicular or angular direction to the mold’s main axis. When the mold opens, the sliders move laterally or angularly to release the undercut features of the molded part, allowing for smooth ejection without causing damage or deformation.

Placement of manufacturing marks: Injected molded parts have several manufacturing marks that are left during the manufacturing process. Amongst these marks are gate marks (the point where the plastic material enters the mold cavity), parting lines (the meeting point between the mold parts) and ejector pin marks (ejector pins are used to eject the molded part from the mold cavity after it has solidified). These marks must be taken into careful consideration when designing the part so the marks are placed in a way that does not hurt the product functionally or aesthetically.

Conclusion

Injected molded plastic products have many advantages especially for products which are mass produced. However, the industrial design and manufacturing of plastic products which are of high quality requires a lot of knowledge, experience and attention to details.

Studio Kuchik & Amitai has vast experience in designing plastic products in the fields of furniture, electronics, medical, defense and more. If you have a project which requires design and development of plastic products give us a call and we will be happy to assist you!