Troubleshooting Tips For Plastic Injection Molding

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When injection molding issues rear their head, the methods and accuracy of your troubleshooting are often directly related to the speed and effectiveness with which you’re able to remedy a problem (or problems — remember that symptoms of an issue will often have more than one underlying cause). Not all troubleshooting approaches are created the same, even when applied via the standard “checklist” method. Below, we’ll first take a look at some troubleshooting techniques and ideas that may be missing from your approach, and then we’ll examine some common injection molding issues and their causes.

Advanced Troubleshooting Techniques

The concepts outlined below can help apply a more scientific process to your injection molding troubleshooting, which can reduce remedy time and result in greater accuracy and fewer issues in the future.

  • Examine the entire part and process. Prior to moving forward with troubleshooting for one particular problem, such as sink marks, be sure to fully examine the part, internally and externally, to determine whether any other injection molding defects are occurring. The presence of additional problems — when considered as evidence of a process issue — can help you eliminate certain underlying causes and narrow the culprit more quickly.
  • Review and create documentation. When good documentation practices are in place throughout a process, it can serve to eliminate many questions about an issue before they even arise. In the ideal scenario, documentation can prevent issues before they occur. Any adjustments made to a process to remedy a molding issue should be incrementally documented to provide as much information as possible for the future (whether for the next machine operator, or for yourself). Conversely, documentation should always be consulted when starting a new process.
  • Include machine outputs as well as inputs. To provide a bit more detail for the important documentation point above — in addition to documenting machine inputs during troubleshooting, outputs should be documented. This helps you (and anyone else consulting the documentation) to have a better sense of the actual results of the standard process inputs (with respect to the expected results), and how they affect the quality of end products. Outputs that especially merit documentation are second-stage packing time, screw recovery time, cooling time and finished part weight.
  • Consider process relationships. During troubleshooting, when you’re isolating processes to identify potential sources of problems, be sure to consider latent or interactive effects that adjusting (or eliminating) one process may have on another. Whether during the troubleshooting process or when enacting the remedy, a good rule of thumb to remember is the simple adage that “nothing occurs in a vacuum” and that changing one aspect of the process is likely to affect 10 (or many more) other aspects. It is in this area that fine-tuning and documenting become especially important.

Troubleshooting Common Injection Molding Issues

Now that you have some additional troubleshooting methods to add to your toolbox, let’s take a look at common injection molding issues, and how you can identify and remedy them. For the most part, the remedies below focus on process — those that can most easily be identified and remedied by reviewing and adjusting machine settings.

Short molding: Short molding simply means that a mold cavity did not get filled all the way prior to cooling and ejection. This usually occurs due to one of two issues: (1) not enough material was present for injection into the mold, or (2) the material solidified (either completely or at a gate) too quickly to allow the cavity to fill completely.

To remedy short molding, look into:

  • A larger capacity machine
  • Faster injection speed
  • Higher mold and/or material temperature (to slow down cooling)
  • Higher injection pressure

Sink and void issues: Sinks and voids are areas where the molding material cooled and shrunk at a disparate rate to its surrounding material. A sink occurs externally (and results in a gap or depression in the part surface), while a void occurs internally (and results in hollow areas or gaps within the piece). For sink and void issues, the issue may be helped by:

  • Increasing injection pressure
  • Increasing cooling time
  • Increasing hold time
  • For design issues, introducing ribs instead of thick sections may also help for more uniform cooling

Delamination: Delamination means that the plastic resin did not bond or “set” properly, and surface areas of the piece can be peeled in layers. Delamination presents a major threat to the quality and integrity of the part, and pieces with this error must, almost always, be discarded. Typically, delamination is caused by contaminants in the material (or the mold), though it may also be caused by excess moisture in the material or mold.

Solutions to delamination can include:

  • Purging and cleaning mold and tool
  • Drying the material
  • Raising the material or mold temperature

Flashing: Flashing occurs when excess material is present along the mold parting lines of a part. Flash is not typically seen as a cause for part rejection, but excess material must be trimmed, creating extra production time and costs. In addition, excess material can be re-used (with thermoplastic materials), but only to a certain degree, meaning that flash will, over time, increase material costs. Flashing typically occurs due to misaligned mold halves, or due to the mold opening under pressure. To remedy flashing, try:

  • Re-aligning, flattening and/or cleaning mold faces
  • Increasing clamping pressure
  • Increasing fill time (thus, decreasing the fill rate)

Weak weld line connections: Weak weld lines mean that a material did not bond properly at a point of contact within the mold (for instance, where the mold halves meet, or where two material flows meet when injected from different areas of the mold). Poor weld lines can also occur as a result of contaminants in the mat. Potential remedies for weak weld lines are:

  • Decreasing mold lubricant
  • Further homogenizing any pigments used in the material through greater dispersion
  • Purging and cleaning mold and tool
  • Increasing material and/or mold temperature
  • Increasing injection speed
  • Aligning weld lines with vents

At RevPart, we work to ensure that our parts are as defect-free as possible — and we aim to identify and resolve any issues that occur as quickly and efficiently as we can. For more information about our molding services, contact us today.

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