Engineering Insights: Resolving Machine Nozzle Issues in Injection Molding
Proper machine nozzle setup is a critical factor in injection molding. “A chain is as strong as its weakest link, and an injection molding machine is as effective as its nozzle,” said Bill Fierens, Lead Senior Technical Development Engineer at M. Holland.
The machine nozzle facilitates controlled delivery of molten plastic into the mold cavity, ensuring reliable filling and associated pressure loss. When machine nozzles aren’t set up correctly, part quality can be negatively affected.
Identify Machine Nozzle Issues
Most molding machines do not offer a monitoring solution to indicate when the machine nozzle is the source of an issue, but Bill shares common clues.
“In cold runner molding, you can visually assess the cold sprue. Typically, if the nozzle is set at the correct temperature, a uniform cone-shaped vestige appears at the end of the sprue,” Bill said. Discoloration can indicate that the temperature potentially exceeded what’s tolerable for the polymer in-use.
According to Bill, you may hear a thump when the machine nozzle is causing a problem. More advanced machines may record the thump as a spike in the hydraulic pressure curve. The spike happens when pressure busts through a cold slug in the nozzle — a sign that your nozzle setup may not be optimal.
Best Practices for Machine Nozzle Selection
The average individual ordering equipment may not know all the nuances that can go into nozzle selection. Bill recommends involving the processing and maintenance teams in purchasing machine nozzles to ensure all requirements are met.
Once you have the proper team involved, ask these questions to select a machine nozzle:
- What material is being molded?
Understanding a polymer’s thermal behavior in relation to the nozzle attributes will minimize material hang-up and subsequent material degradation. Degraded material can negatively impact the color of the final part, causing streakiness and compromising part quality. To minimize material degradation, Bill recommends matching the nozzle orifice and internal geometry of the nozzle body to the material you plan to work with. A properly angled nozzle assembly will help provide a wiping effect that decreases the likelihood of degradation in temperature sensitive materials.
“You should also account for any material properties that might impact the type of nozzle you choose,” Bill said. “A corrosive or abrasive material will require appropriate metallurgy to avoid pitting or premature erosion. For example, when molding rigid PVC, a corrosion resistant full taper internal design might be advantageous. Choosing the right material for your nozzle can also minimize thermal transfer to the sprue.” - Is the machine nozzle the right length?
Bill’s number one tip for machine nozzles is “the shorter, the better.” The longer a nozzle is, the harder it is to control. Consider whether you can shorten the nozzle if the mold and machine will allow.
When a given mold design isn’t ideal, you may need to use a longer nozzle. When a longer nozzle is necessary, strive to cover as much of the nozzle body as possible with a heater band. According to Bill, the heater band will minimize temperature swings and helps to ensure overall uniformity. - Is the machine nozzle sized appropriately?
The nozzle sits between the barrel adaptor and the mold. According to Bill, proper attributes include temperature uniformity and residence time.
“A uniform plastic temperature and exposure time is a must. If not configured correctly, the result could come in the form of degradation, leakage, and/or part defects,” Bill said. “One of the more significant issues lies in orifice matching. A gradual orifice reduction should occur, and if hang-ups or mismatch exists, problems result. As an example, the nozzle tip orifice should be 1/32 inch to 1/16 inch less than the sprue rear opening. Nozzle tip radius will need to be identical, as leakage can occur if mismatched.”
Record Nozzle Attributes
After you have taken the time to determine which nozzle to use with a machine and mold, Bill recommends recording several attributes in your master setup sheet. The metal type, design and body length of the nozzle’s tip orifice, along with heater length will be critical factors to consider when repurchasing.
A simple solution is to use a photo editing application on your cell phone or computer to mark up a picture of your setup with the appropriate measurements that can be inserted into your master setup sheet. Bill also recommends recording normal operating conditions in your master setup sheet, such as melt temperature, internal nozzle temperature, steel temperature and even a thermal map of the mold if you have the capability. These details will simplify the setup process and help with troubleshooting any issues that arise during molding.
Standardize Barrel Adapter Diameters
If you are purchasing machines or setting up a new shop, you have a fantastic opportunity to standardize barrel adapter diameters across your owned machines, according to Bill. Shops with standardized barrel adapter diameters can buy one type of nozzle with confidence that orifices will align correctly. Standardizing barrel adapter diameters across molding machines streamlines machine setup and nozzle purchasing procedures.
Evaluate Specialized Nozzles
In some cases, you could use specialized nozzles to minimize temperature swings. According to Bill, internally heated nozzles can help with temperature control, temperature-resistant nozzle insulators minimize sprue heat loss or sprue sticking, and reverse taper nozzle tips can aide with sprue sticking woes for nylon processing. Bill recommends having multiple tools in your toolbox to use when challenges arise.
Engineering Insights: Practical Plastics Solutions
M. Holland’s highly skilled team of technical development engineers can help you find success in injection molding. Industry pros like Bill are sharing their expertise in our Engineering Insights series. Don’t miss future tips. Subscribe to receive the latest news and insights from M. Holland.
Disclaimer: The information provided in this post is intended solely for general guidance. Outcomes vary based on individual circumstances, and M. Holland Company, LLC does not ensure a specific result. Clients shall use their own independent skills and expertise when testing any application of technical support. M. Holland is not responsible and will not be liable for any discrepancies between expected and actual outcomes. M. HOLLAND DISCLAIMS ALL WARRANTIES, WHETHER EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION ANY WARRANTIES OF MERCHANTABILITY OR FITNESS OF A PRODUCT FOR A PARTICULAR PURPOSE.
Bill has 31 years of plastics experience in technical service and engineering. In his position with M. Holland, Bill strategically applies his extensive knowledge of plastics manufacturing processes to various client scenarios. He reviews workload requirements to recommend thermoplastic resin materials that meet customers’ component, functional and logistic requirements.