So you need custom molded plastic parts.
Maybe it’s for a marine engine application. Maybe it’s plastic injection molded parts for medium or heavy trucks. Or maybe it’s molded plastic parts for high temperature applications. Whatever your custom part needs, it’s important to make sure your plastic parts are designed for manufacturability.
What does that mean? And how does plastic injection molding work in the first place? Let’s take a look at the world of plastic injection molding, and the importance of different design features that can reduce cost when it comes creating the perfect plastic part.
An Overview of Plastic Injection Molding
Plastic injection molding offers a panoply of manufacturing advantages, including efficiency, consistency, and the ability to accommodate complex parts.
How does it work? In the injection molding process, heated thermoplastic material is forced by a large screw into a closed mold. First, pellets of material are fed through a hopper into a heated chamber. Next, a reciprocating screw drives heated plastic into the front of the chamber. The material is then forced at a high pressure into the mold. Finally, the plastic in the cavity cools, the mold is opened and the finished part is removed.
With a clear understanding of how plastic injection molding works, now let’s talk about the importance of design.
Why Design For Manufacturability?
Design for manufacturability is engineering products with the goal of enhanced productivity and lower costs.
When it comes to plastic parts, design for manufacturability ensures consistency, prevents susceptibility to defects, streamlines efficiency and saves material. Above all, it is a method to control costs. In our industry, it’s widely accepted that there are essential features in designing the perfect plastic part, including wall thickness, rib design, boss design, gate placements, weld lines, corner transitions, and properly placed vents.
Each of these design features needs be taken into consideration to design the perfect plastic part.
Plastic Part Design and Wall Thickness
Choosing the optimal wall thickness is the first step in creating the perfect plastic part. Wall thickness is related to various criteria, including performance and cost.
Creating a standardized wall thickness throughout the part can prevent a myriad of problems that can take place during the rest of the manufacturing process. For example, since melted plastic travels to where there is the least amount of resistance, a wall with uniform thickness will prevent melted plastic from flowing to unwanted areas. Furthermore, a wall with standardized thickness will cool at the same rate, whereas a wall with varying thickness cools unevenly — making it susceptible to voids and sinking issues. Ultimately, the best wall thickness balances strength and weight to optimize quality, longevity and total cost.
Rib Design and Injection Molded Plastic Parts
When it comes to supporting the walls — as well as the entire stability of a plastic part — ribs are another important element. This is the second essential feature in designing for manufacturability. Often compared to flying buttresses in architecture, ribs provide support without increasing wall thickness. Importantly, the effective design of ribs can minimize material expenditures. Ultimately materials dictate the rib thickness, but in general, ribs should not exceed 60% of wall thickness.
Boss Design for Custom Plastic Parts
Bosses are another fundamental design element. These cylindrical protrusions on plastic parts are typically used for mounting or as reinforcement. Think of the top of a lego. In general, bosses correspond to the thickness of ribs. When it comes to plastic part assemblies, bosses are utilized through the use of screws or pins. Like ribs, bosses are also subject to sink defects in cooling when they are too thick.
Now let’s talk about gate placement. Gates help direct plastic flow to fill your mold correctly. Since gate placement is related to plastic flow filling and packing properly — which affects the overall quality of your plastic parts — it is another essential design feature. Your plastic part will fill optimally if the gates flow from thick to thin walls, which also reduces the risk of flow marks. All of these design precautions ensure your perfect plastic part works from a performance and aesthetic perspective.
Weld Lines on Plastic Injection Molded Parts
Weld lines, another important design consideration, are created when two flows of material meet. These are frequently cited as one of the most significant problems in injection molding today, due to the aesthetic and structural issues they create. This is where strategic design is so important since improper selection of weld line place can result in performance issues.
Corner Transitions Design Features for Plastic Parts
In addition to the aforementioned design considerations, corner transitions are an additional important design feature. Since melted plastic travels to areas of least resistance, dramatic changes such as acute corners, can prevent the plastic melt from flowing to their intended areas. This can result in cavities not filling correctly, and cause part defects. A good rule for corner transitions is that the radius should be uniform on the inside and outside of the wall so as to create a consistent thickness. That will allow the melted plastic to flow to the target areas.
Proper Ventilation for Your Plastic Parts
Vents are the final design feature we should talk about. What’s so great about vents? For one, vents create areas for gas in the cavity to be released. In addition, properly placed vents can reduce the risk for weld lines.
Now that we’ve discussed important elements, including wall thickness, rib design, boss design, gate placements, weld lines, corner transitions, and properly placed vents, you understand the important design features required to make the perfect plastic part.
But that’s not all it takes. We believe in manufacturing partnerships, too. In our design process, we partner with your engineering department to create product designs in an effort to use the least amount of material, enhance process reliability and perform well in the application. Bottom line, provide true value.
Whether it’s rapid prototyping for product design, plastic part design for injection molding, or quick turnaround time plastic parts, our experienced team of professionals ensures you’re equipped with trusted parts from one company, from design to delivery.