Designing and Building Your Plastic Injection Mold 

The engineer who designs your part should have in-depth knowledge of the injection molding process or be able to collaborate with an experienced mold engineer. Incorporating Design for Manufacturing (DFM) early in your project ensures critical details of the injection molding process are considered in your part design. 

Oftentimes, part design is either complete or in a very advanced state before an OEM starts sourcing its mold. In this case, your mold maker will want to review your part design to confirm it can be manufactured feasibly. Some questions they will likely explore include the following: 

Undercuts: 

• Can any undercuts be edited out to remove unnecessary complexity in the mold design, while still fulfilling the part’s intended purpose? 
• Is it realistic for the mold design to accommodate any requisite undercuts while staying within the budgetary requirements of the project? 

Resin Selection: 

• Is your part design compatible with the selected resin(s) as it relates to injection pressure and mold flow? 
• What challenges or delays could occur related to the part’s cooling, finish, etc. that might be resolved by minor changes to the part design and/or choice of resin? 

Tolerances: 

• What are your part’s tolerances and how might they affect tooling (i.e., changing the material selection for your mold, necessitating additional quality checks, or creating the need for a more complex mold)?
• How might the design of the part/mold take the part’s performance requirements into account in order to balance out unnecessary increases in lead time or cost?  

From the standpoint of mold construction, DFM is a great place to start, but manufacturability isn’t the only important consideration at this point in the process. In fact, Medical Design and Outsourcing stresses the need for a more comprehensive approach they call DFx, which “requires consideration of both the product and the business objectives, including reliability, regulatory requirements, cost, and supply chain.” It’s one thing to be sure your part can be manufactured, but you’ll also want to make sure your design is optimized for assembly and kitting, your part is likely to pass any necessary certifications or quality requirements, the resin(s) you’ve selected satisfy your cost requirements and are available in the quantities needed to meet your production schedule, and so forth. 

As you might imagine, mold design is very closely tied to part design and can’t begin until the part design is (mostly) finalized. Beyond accommodating the basic geometry of your part, the tool designer must consider a number of important factors related to the manufacturing process: 

• Production Needs: Not only will production needs affect the selection of metal for your mold, they’ll also fundamentally impact your mold design. As an example, a high planned production volume will benefit from adding more cavities per mold, increasing the mold’s efficiency by producing additional parts per cycle. At smaller volumes, the additional costs associated with a multi-cavity mold likely outweigh the savings from the mold’s increased efficiency. 
• Gates and Runners: How and where resin flows into your mold has a profound impact on the success of your mold. Selecting the appropriate type and placement of gates can avoid issues with the part’s function and will ensure the final product is aesthetically pleasing, as well. 
  Parting Line: Because an injection mold is generally created by two or more pieces that are pushed together, there’s often a visible line in the final part where these pieces meet. An experienced mold designer can strategically place any parting lines to minimize functional or aesthetic challenges. 
• Coolant Lines: Having an appropriate number of well-placed coolant lines allows for efficient cooling and prevents unanticipated shrinkage or warpage. 
• Shrink Rate: Shrinkage in the 0.4 to 2% range can be expected on all plastic parts produced via injection molding and will vary depending on the materials selected, gate and coolant line locations, and other conditions related to the manufacturing process. The mold design must take shrinkage into account for a successful part. 
• Ejection: Of course, your mold design will need to account for the ejection of your cooled part(s). There are various ejection systems, each with different implications in terms of your cycle time and the quality of the final product. Learn more about some considerations for identifying the right ejection method. 
• Secondary Operations: If your part requires more than one color or material, it may make sense to build a mold that can support two- or multi-shot molding (wherein two or more materials are injected into the mold simultaneously) and/or overmolding (where additional, often softer, materials are injected onto the cooled underlayer, or substrate part). 

While minimizing costs is an important consideration in the creation of any mold, some additional cost may be necessary to support your part’s intended purpose/longevity, as well as the production needs (volume, lifespan, etc.). Having a firm understanding of these factors upfront will guide the mold design process. Take it from Plastics Today: “No amount of savings is worthwhile if it compromises the ability to maintain the condition of the mold, has the potential to affect part quality, or complicate start-up and daily production. Relatively small tooling savings will be dwarfed by the cost of scrap, drain of resources, lost time, and repair activities, so carefully consider how cost savings can impact overall efficiency.”  

To produce your mold, the tool shop will employ a highly precise, multi-step process entirely guided by computers. Each phase of the process further refines and validates the mold. While the order of some of the steps below can vary, below is a high-level overview of the mold machining process: 

• Initial Milling: Machining the mold base to the proper size and shape 
• Rough CNC Machining: Basic shaping of the mold cavity/cavities 
• Heat Treatment: applying heat to harden and strengthen the metal  
• Precision CNC Machining: More refined machining for molds that need more detailed features 
• Electrical Discharge Machining (EDM): Applying electrical discharge to the mold to smooth and finely shape it 
• Polishing/Finishing: Putting the final touches on the mold to guarantee the parts it produces have the desired finish 
• Measurement: Ensuring the mold dimensions exactly match the CAD design through the use of an Automatically Programmed Measurement (APM) system 
• Fitting & Assembly: Putting the mold together and adjusting for a tight fit between the mold parts and within the injection press 
• Mold Trial: Molding initial parts to validate the final product’s accuracy vs. the approved design/tolerances (these are the toolmaker’s samples and are created prior to the official first article for internal validation) 

To allow for further visibility and control during production, an RFID (radio frequency identification) chip is attached to your mold. During manufacturing, these sensors feed real-time data to the factory’s centralized computer, providing a live look into production progress, enhancing inventory management, proactively identifying potential issues, and avoiding unplanned downtime. 

Now that the tooling is complete, your mold maker will provide first articles for you to inspect before beginning production on your part. By this time, enough simulation and testing has been done that your mold should consistently produce quality parts, but full-scale production won’t begin until you’ve signed off on the output. 

At this point, it’s important to document materials and processes to support quality control and allow future flexibility in the process. Ideally, your manufacturer will leverage a cloud-based network of machinery that allows automated setup and production based on your defined specifications. In instances where your mold may need to be moved to a new press and potentially set up manually, having the proper documentation is critical to maintain continuity in your manufacturing.

With so much to think about related to mold design and construction, you need a partner that sees the injection molding process holistically. NMG has years of design and manufacturing expertise, supplemented by the knowledge of reliable and trusted partners worldwide, to facilitate successful injection molding.