OEM's Guide to Design for Manufacturing for Injection ...

What is DFM?

Design for manufacturability (DFM) refers to the practice of designing products to achieve optimal manufacturing results. By employing DFM, original equipment manufacturers (OEMs) and contract manufacturers (CMs) can pinpoint existing or potential issues and rectify them during the design stage, which constitutes the most cost-effective moment to address these challenges. DFM can impact several project decisions, including but not limited to:

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• Types and forms of raw materials utilized
• Dimensional tolerances
• Wall thickness and draft angle
• Secondary processes, such as finishing
• The overall method of producing the product

What is DFM in plastic injection molding?

While DFM principles can be applied across varied manufacturing sectors, they hold particular importance in plastic injection molding, addressing unique challenges. Defects in plastic injection molding can stem from issues in part design, mold design, temperature of materials, injection pressure, cooling duration, and other facets of the production process. Most of these defects can be anticipated and mitigated through effective DFM practices.

Who is involved in DFM?

OEMs collaborating with a CM for DFM can rely on the engineers at the CM to guide this process, conducting analysis, identifying issues, and proposing solutions. The engineers involved in DFM for plastic injection molding should have substantial expertise in custom plastic injection molding and injection mold tool manufacturing.

Throughout any given project, both engineers and product designers from the OEM and CM will participate in the DFM process. Engineers and product designers at the OEM are tasked with supplying the CM with all relevant plans, materials, and specifications related to the component, along with thorough information about the product and its intended applications. Effective communication among all parties involved greatly enhances the chances of a successful outcome during the DFM process and often throughout production.

What does the DFM process entail in plastic injection molding?

DFM can be applied at various stages throughout the product lifecycle, including the initial concept and design phases. Regardless of when it occurs, DFM in plastic injection molding incorporates several key phases and activities:

Evaluation of Plans and Identification of Issues

The OEM provides the CM with all existing plans, documentation, and relevant information regarding the project. This encompasses comprehensive details about not just the component, but also the product as a whole and its applications. The OEM communicates any concerns regarding manufacturability or challenges they foresee with the manufacturing process. The engineers at the CM take these concerns into account while reviewing all plans and documentation to pinpoint potential issues affecting manufacturability.

DFM Simulation

CM engineers utilize specialized mold flow simulation software, like Sigmasoft, to model the injection molding process. This software produces a 3D simulation of the flow, thermal behavior, and warpage during injection molding, encompassing the entire mold design. Throughout the simulation, they analyze the project against the checklist for DFM in plastic injection molding, identifying problems that might arise unless design modifications are applied.

Presentation of Findings and Suggestions

Once the simulation is completed, CM engineers compile the results and present them to the OEM, along with the most viable solutions to any identified issues. A crucial deliverable from the CM to the OEM is a comprehensive document detailing the DFM process results and recommendations, supplemented with screenshots from the simulation. This document encompasses:

• Simulation conditions, including:
  • Material
  • Shot Size
  • Material Temperature
  • Mold Temperature
  • Fill Time
  • Pack Time
  • Total Mold Closed Time
  • Gate Size
  • Nozzle Size
  • Gate Freeze Time
  • Pack Pressure
• Press setup details, covering:
• Material datasheet with specifications from the material supplier (e.g., ExxonMobil). The DFM process should include guidance from the CM on selecting resin and other raw materials to meet the requirements of the project’s plastic components.)
• A section addressing each parameter tested in the simulation with accompanying results. A complete list of parameters can be found in the checklist for DFM in plastic injection molding below.
• Multiple sections comparing results from various alterations in the simulation (e.g., varying packing pressures and their effect on sink marks, hot spots, and voids).
• A summary of concerns arising from the simulation.
• A summary of recommendations and solutions provided by the contract manufacturer to finalize the DFM process.

Prototyping, Testing, and Finalization

Frequently, the DFM process extends to utilizing 3D printing (additive manufacturing) for prototyping and creating parts for the OEM to test. Necessary parts of the DFM simulations may be reiterated alongside prototyping and testing until the product reaches readiness for manufacturing.

Defects in plastic injection molding can often be attributed to flaws in part design, mold design, material temperature, injection pressure, and cooling duration—all of which can frequently be predicted and prevented through DFM.

Common Challenges in Plastic Injection Molding

Typical issues encountered in plastic injection molding include:

Flash

Weld lines

Sink marks

Short shots

Burn marks

Brittleness

Delamination

Jetting

Sinks

Voids

Splay

Bubbles/blisters

Warping

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Flow lines

Flash

Flash refers to a thin layer of plastic that escapes outside of the mold cavity, typically at the junction point of the mold halves, leading to excess material that needs trimming. Flash deteriorates product quality, prolongs production cycles, and may damage your injection mold, hence addressing this issue is crucial.

Weld Lines

Weld lines manifest as lines or color changes in the molded part due to the convergence of two distinct flows of molten plastic, possibly affecting aesthetics and structural integrity. While weld lines are unavoidable, there are strategies to mitigate their impact.

Sink Marks

Sink marks appear as localized depressions on a part's surface, frequently resulting from excessive thickness for the specified resin type. Understanding strategies to rectify sink marks in injection molded components is invaluable.

Short Shots

A short shot occurs when the molten plastic fails to completely fill the mold cavity, resulting in portions of the part missing material (e.g., a prong on a fork absent). The causes of short shots vary, and addressing them necessitates analyzing each individual case.

Burn Marks

Burn marks are discolorations on the surface of parts, typically dark black or red due to overheating of the melted plastic against the mold.

Brittleness

Brittleness results from insufficient strength in a part, causing it to crack or break easily and can be attributed to various causes.

Delamination

Delamination refers to visible surface layers on a product that can be peeled apart, which has multiple causes and solutions available.

Jetting

Jetting leads to deformation in the final product impacting both strength and aesthetics, occurring when some molten plastic enters the mold cavity prematurely.

Sinks

Sinks occur as surface depressions in a part when the mold is inadequately filled or if heavy thickness leads to significant localized shrinkage.

Voids

Voids are air pockets located beneath the surface of a finished part, jeopardizing its strength and structural integrity, potentially resulting in failure.

Splay

Splay is a cosmetic defect appearing due to moisture in the material, causing streaks on the surface of components.

Bubbles/blisters

Bubbles or blisters form when air cannot escape the mold cavity during material injection, adversely affecting structural integrity and aesthetics.

Warping

Warping occurs when the surfaces or walls of a part bend or twist during cooling and is attributable to various causes.

Flow lines

Flow lines are visible streaks or waves on the surface, arising from inconsistent cooling of the injected material.

Implementing design for manufacturability significantly reduces or eliminates the prevalent defects associated with plastic injection molding. Below is a case study exemplifying how DFM effectively addressed quality concerns in the injection molding process.

Case Study: Resolving Quality Challenges for a Medical Device

Facing manufacturing quality concerns, the OEM of a medical device turned to Crescent Industries, a plastic injection molding CM, for resolution. This CM implemented robust manufacturing design principles, effectively eliminating quality issues and establishing a value-added, streamlined production process.

Crescent Industries began by familiarizing themselves with the device, which facilitates the healing of broken bones using ultrasound, while consulting with the engineers from the OEM. The OEM had struggled with substrate material jetting in the device’s clear window, leading parts to fail inspections due to unclear windows. Following thorough information-gathering, the CM identified the OEM’s existing injection process as the culprit behind the quality deficiencies. The re-melting and mingling of substrate material with the clear resin during filling was causing the failures, primarily due to the co-injection of three varied materials.

To enhance the process, design modifications were necessary, but the OEM lacked up-to-date data for the device. Instead of relying solely on 2D and 3D models, Crescent Industries employed 3D printing to produce prototypes demonstrating their recommended design changes. The OEM was able to incorporate the redesigned and 3D printed parts from their assembly while testing their functionality just as if they had completed the entire manufacturing progression. This methodology expedited the redesign process and assured the OEM of receiving a medical device that fully met their specifications.

The revamped approach to manufacturing the device encompassed:

• Modification of product design
• Design and construction of new tooling
• Implementation of ultrasonic welding and product printing
• Additional value-added manufacturing interventions