QUICK ANSWER
Choose the gate and runner system together. The feed path must fill the part, support packing where needed, place flow-front meetings acceptably and allow practical separation or finishing. Simulation should compare physically feasible options, while sampling must verify the actual resin, machine and tool. No gate type is best for every part.
Engineering reference, not a material specification or machine-setting recipe. Validate decisions against the released drawing, resin-grade data and equipment instructions. Equipment work must follow the applicable safety procedures.
1. Translate the part requirements into a gate brief
Start by marking appearance zones, load paths, sealing features, minimum sections and surfaces that cannot accept a gate vestige. Add the exact resin grade, reinforcement, color or optical requirements and the expected production range. A location that is convenient for trimming may be poor for a long thin flow path; a location with a short flow path may leave an unacceptable mark on a user-facing surface.
Record what must be true after molding: for example, a clip must survive the specified loading, a cover must meet flatness in its defined assembly state, or a sealing surface must remain uninterrupted. These are the criteria against which gate alternatives should be judged. A picture of an apparently complete part is not evidence that its weld lines, residual stress or dimensions are acceptable.
2. Compare gate concepts rather than isolated openings
A cold runner solidifies with the molded shot; a hot runner keeps material in a heated feed system. Each approach changes tooling, operation and maintenance requirements. Evaluate runner waste, permitted use of regrind, color changes, thermal exposure, controls, service access and expected output. Do not calculate savings from part weight alone or assume every resin can tolerate the same residence history.
Use the table as a discussion prompt, not a catalog of guaranteed capabilities. Gate separation, vestige size and accessibility depend on the selected construction. A valve-gated arrangement adds sequencing and shutoff decisions; a tunnel gate introduces its own separation geometry. The final detail must be designed for the material and checked against the tooling supplier's demonstrated process.
| Concept | Potential reason to consider it | Question that can reject it |
|---|---|---|
| Edge or tab gate with a cold runner | Accessible feed and trimming location | Can the allowed edge tolerate the vestige and finishing operation? |
| Automatically separating gate | Reduce a manual separation task | Will the actual resin and gate geometry separate consistently without damage? |
| Hot-tip or valve-gated feed | Feed-system and production objectives justify heated tooling | Are thermal control, service, resin stability and surface requirements compatible? |
| Multiple gates | A single feasible location does not meet the fill strategy | Where will the fronts meet, and how will balance or sequencing be verified? |
3. Track flow fronts, air and packing access
Flow splits around a hole and then rejoins, or fronts from separate gates meet. Review those meeting regions against mechanical and appearance requirements. A simulated line is a location prediction, not a measured strength rating. Material, orientation, venting and the conditions at the meeting region influence the result. Move the line away from a critical load path where practical, and validate the remaining requirement through an appropriate test.
Review filling and packing together. A thin section can become a restriction between the gate and a thicker region that still needs pressure transmission. Also identify where displaced air can escape. Gate relocation can move an air trap or weld line rather than remove the underlying issue. Compare alternatives with the same assumptions so that an apparent improvement is not simply the result of changing the material model or process inputs.
4. Use simple quantities to expose assumptions
For an illustrative cavity volume of 20 cubic centimeters, a one-second fill corresponds to an average cavity delivery rate of 20 cubic centimeters per second; a two-second fill corresponds to 10. This is volume divided by time, not an injection-speed recommendation. It excludes runner filling, melt compressibility and the machine's conversion from screw motion to delivered flow.
The comparison makes one review question visible: when fill time changes, what happens at the gate and at the final flow-front meeting? Obtain grade-specific limits and evaluate pressure, shear-related indicators and temperature history using suitable tools. Do not convert the illustrative rate directly into a machine setting or assume that a faster fill necessarily improves a weld line. The part's required performance remains the acceptance criterion.
5. Close the loop with a structured trial
Before release, document the selected feed concept, accessible alternatives and the reason for rejection of each alternative. Include gate dimensions and steel-safe adjustment provisions where justified, runner layout, vent strategy and expected separation method. A gate-location solver can rank candidate regions, but it does not know every physical or cosmetic constraint of the product. Engineering review is still required.
At trial, identify samples by cavity and condition. Compare filling progression, gate vestige, mass stability, appearance and the relevant dimensional or mechanical tests. Review packing studies and process repeatability before correcting steel for a result that is not stable. Record every approved change so that the final production tool, process record and part drawing describe the same gate arrangement.
PUT IT INTO PRACTICE
Engineering review checklist
Use these prompts for a project review. This page does not record or certify completion.
- No-gate zones and permitted vestige are defined.
- The exact resin grade and thermal constraints are known.
- Physically feasible gate options are compared consistently.
- Flow-front meetings and vent paths are reviewed.
- Packing access to critical regions is considered.
- Cavity-specific trial and functional tests are planned.
Questions engineers ask
Does adding a gate always make filling easier?
It can shorten a flow path, but also introduces additional flow-front meetings and balancing or sequencing requirements. Compare the complete result instead of counting gates.
Is a hot runner automatically the lowest-cost choice?
No. Compare tool investment, usable output, resin handling, changeovers, maintenance and the intended production program. The answer is project-specific.
Sources and application limits
This is independently written educational guidance. The linked manufacturer and engineering references support specific mechanisms or further study; they do not endorse PFMOLD or replace project approval. Supplier recommendations apply to their stated grades, tooling and equipment.
- Protolabs: How to Solve Gating Problems in Moulding ↗
Primary manufacturer discussion of gate tradeoffs and vestiges; service-specific capability statements are not PFMOLD specifications.
- Autodesk Moldflow: Selecting Polymer Injection Locations ↗
Primary simulation documentation on gate placement, flow-front meetings and venting.
- Autodesk Moldflow: Find an Injection Location ↗
Explains that a gate-location result does not itself establish physical practicality or cosmetic acceptance.