How to Run a Gate Seal Study for Injection Molding

An injection molded cover has a shallow depression opposite a screw boss. Adding a second of hold time makes the part heavier and reduces the mark. Adding another second makes little difference. A gate seal study uses this change in part weight to find how long the cavity responds to holding pressure. It also helps separate a hold time problem from a thick section that needs a different gate, cooling layout or part geometry.

For a gate that seals by freezing, holding pressure can feed more material into the cavity while the gate and flow path remain open. After the gate seals, a longer hold cannot feed the cavity through it. The weight plateau estimates that transition under the conditions tested. BASF troubleshooting guide for engineering thermoplastics

Set up the trial so hold time is the variable

Start after the tool and molding process have stabilized. Record the resin grade, drying conditions if applicable, mold temperature, fill settings, transfer point and holding pressure. Confirm that the press maintains a consistent cushion during the trial. A changing fill or inconsistent shot can move part weight enough to obscure the result.

Agree how the parts will be weighed. Separate the cold runner consistently if the study calls for part-only weight. For a multicavity mold, identify and weigh each cavity separately. A total shot weight can look steady while one cavity is still gaining weight or shows more variation than the others.

Keep the time from fill to ejection comparable where the controller permits it. Some presses count hold and cooling on separate timers, so a change in hold time can also change the part's time in the mold. Record the actual cycle and inspect samples at a consistent time after molding.

Increase hold time and look for a repeatable plateau

1. Start with a short hold time within the approved trial range. Use the same holding pressure across the study.

2. Increase hold time in planned increments. At each setting, allow the process to settle and collect several consecutive parts from each cavity.

3. Record individual weights and note sink, flash, sticking or incomplete features. Keep the ejected runner out of the part weights if that was the agreed method.

4. Compare the average and spread at each setting. A plateau is more convincing when the difference between adjacent settings is small relative to the repeatability of the process and the resolution of the scale.

The following numbers illustrate a trend for one cavity. They are not production settings.

If repeated readings at 3.0 to 5.0 seconds overlap, the result suggests that useful weight gain has largely stopped in that region. Use finer steps around the transition if you need a better estimate. Then verify a candidate production setting with the expected variation in resin, mold temperature and cycle conditions. The holding time selected for production should also meet the part's dimensional and appearance requirements.

Decide what to change if the sink remains

Compare the defect location with the wall section and the path from the gate. A thick boss junction can cool more slowly than the adjacent wall. A thin region between the gate and that junction can freeze early and restrict the flow of additional material. A whole part weight measurement does not reveal how much pressure reached a particular feature. Autodesk Moldflow guidance on sink marks and voids

If the weight rises and the sink improves during the hold time study, verify the longer setting before changing steel. If the weight plateaus and the mark remains, review the holding pressure while the gate is open, the local wall thickness, gate location and cooling. Check near-gate flash, ejection and dimensional changes if you increase pressure. Where internal voids matter, define a separate check such as a cut section or a suitable imaging method. Surface appearance alone cannot establish that the core is sound.

For example, a cover may still show a faint depression opposite a deep boss after the weight levels off. If the near-gate edge starts flashing during a pressure trial, compare a cored boss or a revised feed path with the original design. Use the same appearance criteria and check screw engagement and assembly fit before approving the revision.

What to record for mold approval

• Mold revision, resin grade and cavity identification.

• Fill and transfer settings, holding pressure, tested hold times and actual cycle timing.

• Individual sample weights, scale resolution and whether runner material was excluded.

• Sink location and the lighting or measurement used to judge it.

• Results for critical dimensions and assembly at the selected settings.

For a new mold or a corrective trial, share the CAD, resin requirements, gate layout if available and photos that show where the mark sits. Include previous trial weights and settings if you have them. Request an injection mold review from Aluprototype

Frequently asked questions

Can I use the same hold time for every cavity?

A shared recipe can work when it meets the requirements across all cavities. Compare the weight curves and finished parts by cavity so a slow response or uneven filling is visible before approval.

Does this study apply to a valve gate?

A valve gate can stop flow mechanically when the pin closes. Record the valve timing and sequence. Interpret a weight plateau against that timing, along with any thermal restriction in the flow path.