Preventing Jetting by Resin: Commodity, Filled and Engineering Plastics

Jetting can leave a folded stream near the gate and affect appearance or performance. Start with the observed flow pattern, exact resin grade and gate-entry geometry. Select a controlled gate or velocity trial from that evidence, then verify filling and finished-part acceptance rather than assuming one setting works for every resin.

How Can Gate Design Prevent Jetting in Different Plastic Types?

A gate that directs the initial stream against a nearby cavity surface can help establish a stable flow front. Review gate size, direction, local thickness and the initial velocity profile together. A fan or tab gate is a candidate design change, not a guaranteed cure; confirm its effect with short-shot observations and full-part tests.

Understanding Gate Influence

The primary goal of the gate in preventing jetting is to slow down the initial, uncontrolled rush of plastic. It needs to encourage the melt to flow like a broad front, not a narrow stream.

Gate Types and Material Considerations

Different gate types offer different solutions:

• Tab Gate: This gate directs the flow along a wall or a core pin. This allows the plastic to spread out and fill more gently. It’s great for flat parts and works well with many resins.
• Fan Gate: This type spreads the flow out into a wide, thin sheet before it enters the cavity. This significantly reduces the velocity at the entry point. It’s excellent for wide parts and materials that are sensitive to shear, like PC.
• Submarine (Tunnel) Gate: While often used for automatic de-gating, its position below the parting line can sometimes help if it directs flow against an obstruction. However, its small size can sometimes increase velocity if not designed carefully for the specific material’s viscosity.

• Edge Gate: A common gate, but its placement is key. If it directs flow into a wide open space, jetting is likely. It’s better if it flows against a wall or core. Do not rank jetting risk simply by commodity versus engineering resin. Viscosity depends on the commercial grade, temperature and shear rate; gate entry must be checked for each application. The gate land length should also be short to minimize pressure drop, especially for shear-sensitive materials.	Gate Type	Suitability for Jetting Prevention	Common Materials	Notes
  Tab Gate	High	ABS, PS, PP, PE	Directs flow against a surface.
  Fan Gate	Very High	PC, Acrylic, ABS	Spreads flow, reduces velocity.
  Pin Gate	Low (if direct entry)	PP, PE (for small parts)	Prone to jetting if not aimed at an obstruction.
  Film Gate	High	Large, flat parts	Similar to fan gate, but for longer edges.

  Thinking about how the plastic will behave as soon as it passes the gate is key. We want it to roll into the cavity, not squirt.

  Do Commodity Plastics Like Polypropylene Require Special Anti-Jetting Strategies?

  Even easy-flowing commodity plastics can suffer from jetting. Are you overlooking simple fixes? Small adjustments can make a big difference for materials like Polypropylene.
  Commodity plastics like Polypropylene (PP) benefit from optimized gate location to impinge flow, and slightly reduced injection speeds. Their lower viscosity means jetting is often due to excessively fast fill or poor gate aim.
  
  Commodity plastics, such as Polypropylene (PP), Polyethylene (PE), and Polystyrene (PS), are generally known for their good flow characteristics. You might think jetting wouldn’t be a big issue with them. However, their lower viscosity can sometimes make them more prone to jetting if the conditions are wrong. Because they flow so easily, a high injection speed or a poorly aimed gate can send them racing into the cavity uncontrolled.

  Key Factors for Commodity Plastics

  When dealing with jetting in these materials, here’s what I focus on:

  1. Gate Location and Type: As discussed before, the gate should not direct material into an open, unrestricted area of the cavity. For PP, a tab gate or a fan gate that makes the material flow against a mold wall or a core pin is usually very effective. Even a well-placed edge gate can work if it forces the melt to spread.
  2. Injection Speed Profile: While a fast fill is often desired for cycle time, the initial injection speed is critical. For commodity plastics, I often start with a slower initial speed as the melt enters the gate and begins to fill the cavity. Once a dispersed flow front is established in the cavity, the speed can be increased. This “profiled” injection speed helps the flow front to advance more uniformly.
  3. Melt Temperature: Running commodity plastics too hot can further lower their viscosity, making jetting worse. Sticking to the recommended melt temperature range is important. Sometimes, going to the lower end of the range can help increase viscosity just enough to mitigate jetting, without causing other issues like short shots.
     

     Example with Polypropylene

     Illustrative PP trial sequence, not a customer case: compare the initial flow from the existing gate with a proposed gate-entry direction, then trial a slower entry stage followed by the speed needed to complete filling. Establish the transition from labeled short shots and record appearance, fill completeness and cycle impact before accepting the change.

     What Makes Engineering Resins Prone to Jetting and How Can We Address It?

     Engineering resins present unique jetting challenges. Their properties demand precise control. Are your advanced materials suffering from this common flow defect?
     Engineering resins (e.g., Nylon, PC, PBT) often have higher viscosity or are shear-sensitive. Jetting can be prevented by optimized gate design (fan/tab gates), higher, stable melt/mold temperatures, and controlled, profiled injection speeds.
     
     Engineering resins like Nylon (PA), Polycarbonate (PC), ABS, and PBT offer superior mechanical and thermal properties. However, these same properties can make them more challenging to process, and jetting is a common issue. Their higher melt viscosity, sensitivity to shear, or specific temperature requirements need careful management.

     Why Engineering Resins Can Be Tricky

     Several factors contribute to jetting in these materials:

• Higher Viscosity: Many engineering resins are more viscous than commodity plastics. This means they require higher pressure to flow. If the initial flow isn’t controlled, the high pressure can easily lead to jetting once the resistance at the gate is overcome.
• Shear Sensitivity: Some resins, like PC, can degrade if subjected to excessive shear rates, which can occur at the gate if it’s too small or injection speed is too high. While not directly jetting, degraded material can flow erratically. Conversely, some materials are shear-thinning, meaning their viscosity decreases with increased shear rate – this can exacerbate jetting if speed is too high.
• Temperature Sensitivity: Engineering resins often have a narrower processing window for melt and mold temperatures. Too low, and viscosity increases, potentially leading to short shots or requiring excessive pressure that causes jetting. Too high, and material degradation can occur. Consistent temperature control is vital. For instance, Nylon is hygroscopic; moisture content can drastically alter its flow behavior and proneness to jetting. Proper drying is non-negotiable.
  

  Strategies for Engineering Resins

  To combat jetting with these materials, I always recommend:

  1. Generous Gate Design: Fan gates or tab gates are often preferred. They help to spread the flow and reduce the linear velocity of the melt entering the cavity. The gate land should be kept short to minimize pressure loss and shear heating.
  2. Optimized Temperatures: Ensure both melt and mold temperatures are within the material supplier’s recommended range, and importantly, are stable. For materials like PC, a higher mold temperature can improve flow and reduce the tendency to jet by keeping the melt more fluid as it enters.
  3. Profiled Injection Speed: A slow initial injection speed as the melt passes the gate and begins to fill the cavity is critical. Once a stable flow front is established, the speed can be increased. This is even more important than with commodity plastics.
  4. Adequate Venting: Trapped air can resist melt flow and contribute to erratic filling patterns, including jetting. Ensure vents are properly sized and located.
     I worked on a project with glass-filled Nylon where jetting was a persistent issue. The glass fibers tended to orient with the flow, and if jetting occurred, it created very visible and weak flow lines. The solution involved a combination of a larger fan gate, slightly increased melt temperature, and a carefully profiled (slow-fast-slow) injection. It took some fine-tuning, but the results were excellent.

     Are Processing Parameter Adjustments Key to Solving Jetting Across Various Materials?

     After reviewing the gate and flow path, test the initial velocity profile within the grade supplier’s processing limits. Record actual melt and mold conditions and distinguish shear, moisture, gas and cold-slug effects. Evaluate each change against the same filling and part-acceptance criteria.

     Critical Processing Parameters for Jetting

     Let’s break down the most impactful parameters:

  5. Injection Speed: This is often the number one culprit and the first thing to adjust.
     • Initial Speed: As the melt first enters the cavity, test a controlled lower velocity where a free jet is observed; excessive slowing can cause hesitation or freeze-off. This allows the plastic to create a stable flow front that can then advance more uniformly. Modern machines allow for profiled injection speeds, so you can have a very slow start, then ramp up.
     • Overall Speed: If the overall fill time is too short (i.e., average speed is too high), this can also contribute, especially with lower viscosity materials.
  6. Melt Temperature:
     • Too High: Can lower viscosity too much, making the melt too fluid and prone to jetting, especially with commodity plastics. It can also lead to material degradation.
     • Too Low: Increases viscosity, requiring higher pressure. This might lead to hesitation and then a sudden burst of material, causing jetting. It can also cause short shots or poor surface finish. Finding the sweet spot in the material’s recommended range is key.
  7. Mold Temperature:
     • Too Low: Can cause the melt to cool too quickly upon contact with the mold walls. This “freezing off” effect can impede smooth flow and contribute to jetting as the subsequent melt tries to force its way through.
     • Too High: While generally good for flow, extremely high mold temps (beyond recommendations) can prolong cycle times or affect part dimensions. However, for some engineering resins, a higher mold temperature is beneficial to promote flow and reduce stresses.
       

       A Balanced Approach

       It’s rarely just one parameter. Often, it’s a combination. For example, if you lower injection speed, you might need to slightly increase melt or mold temperature to ensure complete filling without increasing pressure too much.
       Here’s a general troubleshooting sequence I use for jetting related to process:

  8. Reduce initial injection speed significantly.
  9. If still jetting, try slightly increasing melt temperature (within spec).
  10. Try slightly increasing mold temperature (within spec).
  11. Check melt preparation, recovery consistency and cushion repeatability separately; back pressure is not a direct substitute for gate-entry velocity control.
  12. Check actual nozzle and barrel-zone temperatures and investigate cold slugs or unstable heating. Correlate any abnormal condition with the defect pattern before changing the process; this diagnostic step does not establish a verified customer result.

Use a Resin-Grade Trial Card

Record the manufacturer and exact grade, filler content, drying/moisture requirements, melt/mold temperature limits, gate-entry geometry and cavity-specific observations. Separate a surface stream from splay before changing temperature. A grade with better flow does not compensate for a gate firing into an open cavity.

For each candidate resin, compare the same short-shot sequence and the finished part’s weld-line strength, dimensions and appearance. Check filler orientation and wear as well as cosmetic improvement. The Autodesk jetting remedies concern flow entry; grade-specific processing instructions must come from the resin supplier.

For a new resin or a material substitution, send the grade data and part requirements before approving a gate modification. The simulation-scope guide explains what a model can and cannot establish.

Conclusion

Effectively preventing jetting requires understanding material behavior, optimizing gate design, and carefully adjusting process parameters like injection speed and temperature for smooth melt flow.