Hot runner vs cold runner injection molding is a trade between tooling capex and scrap/cycle opex. Wrong runner choice shows up as stringing, long color-change downtime, gate vestige rejects on cosmetic faces, or piece prices that hide runner regrind you cannot use — especially on engineering grades where regrind caps apply.
Procurement compares mold quotes where one supplier included a hot manifold and another did not — a $10,000 spread that is meaningless without scrap math. Operations then lives with the choice for years: either trimming sprues on every cycle or replacing hot tips on maintenance weekends.
This guide explains when each runner type wins, how gate technology interacts with cosmetics, maintenance ownership, and what to specify in RFQ so quotes compare the same process — not different hidden cost structures.
Cold runner — when it wins
- Lower initial mold cost — simpler manifold, no heaters or controllers
- Frequent material or color changes during NPI
- Lower annual volumes where runner scrap is acceptable financially
- Commodity parts where sprue trim marks are not customer-facing
- Resins where regrind is allowed, tested, and does not degrade CTQs
- Simple open-and-shut tools where hot half access is difficult to service
Cold runner scrap is visible — you weigh it. That makes TCO modeling easier if you honest about scrap rate and regrind policy.
Hot runner — when it wins
- High volume — runner scrap cost exceeds hot manifold investment over tool life
- Cosmetic parts — controlled gate vestige, no sprue trim on Class A surfaces
- Engineering resins where regrind is restricted or degrades properties
- Fast cycle — no runner cooling, handling, or regrind logistics
- Multi-cavity layouts where cold runner balance is difficult or runner too long
- Automated assembly lines that cannot tolerate sprue tabs
Gate technology within hot runners
| Gate style | Cosmetic control | Cost / complexity |
|---|---|---|
| Thermal hot tip | Good for hidden gates | Lower than valve |
| Valve gate | Best vestige control on Class A | Higher capex, sequencing logic |
| Hot edge | Thin parts, uniform fill | Design-sensitive |
| Insulated runner (semi-hot) | Hybrid economics | Less common, maintenance nuance |
Valve gates cost more than thermal gates but improve vestige control on visible surfaces — specify in RFQ when cosmetic gate marks are CTQs. Ask for drop count, gate type per drop, and brand of manifold if maintenance matters to you.
Decision table
| Scenario | Lean toward |
|---|---|
| < 20k parts/year, simple PP/PE commodity | Cold runner |
| Class A bezel or lid, 100k+/year | Hot runner, likely valve gates |
| Glass-filled nylon, cosmetic gate zone | Hot runner with wear-resistant tips |
| NPI with material grade still TBD | Cold until resin frozen |
| Color changes every production run | Cold or hot with documented purge plan |
| Medical / low regrind allowance | Hot runner to minimize scrap |
| Large cold runner > 30% of part weight | Recalculate hot runner ROI |
Scrap math procurement should run
Estimate runner weight per shot × shots per year × resin cost. Add regrind handling labor if applicable. Compare to hot manifold premium amortized over expected tool life.
Example framework: 18g runner × 200,000 shots/year × $3.50/kg resin ≈ $12,600/year in material alone — before counting grind, storage, and quality risk of regrind ratio. If hot half adds $9,000 upfront, payback can be under one year. Your geometry and volume will differ — run the calculation.
Color change and downtime
Cold runners purge through sprue and runner volume — wasteful but straightforward. Hot runners require controlled purge procedures; carbon buildup in manifolds causes streaks for days if purged wrong.
If you change color weekly, model downtime cost in OEE. Sometimes cold runner wins on OEE even when hot runner wins on scrap at steady state.
Maintenance and risk
Hot runners add heater, thermocouple, and tip wear failure modes — require a supplier with documented hot-half service and spare tip policy. Cold runners are mechanically simpler but hide scrap cost in piece-price quotes that omit regrind policy or assume 100% regrind acceptance.
Contract items for hot runners:
- Spare tip inventory on site or lead time guarantee
- Heater failure response time
- Annual hot half inspection interval
- Who pays tip replacement after shot count threshold
Interaction with mold design and DFM
Runner choice changes gate location options, wall thickness requirements near gates, and fill balance in multi-cavity tools. A part that works with edge gate cold runner may fail cosmetic review with hot tip on the same face.
Include runner preference in DFM review before cavitation lock — retrofit from cold to hot often requires new plates and weeks of delay.
How Deuchi specifies runners
Runner choice appears in mold quotes with drop count, gate type, and maintenance expectations. We align with DFM so gate location options are evaluated before steel, not argued at T0.
Runner decisions feed tooling cost breakdown and production contract manufacturing quotes — same assumptions end to end.
Regional and supplier capability notes
Hot runner service depth varies by molder. A tool built with Brand A manifold at mold shop may land at production molder who prefers Brand B controllers — budget interface check at mold design.
Some low-cost quotes use generic hot halves with long spare tip lead times. Ask spare tip price and lead time before award; a cheap manifold with eight-week tips is not cheap in production.
Runner design and part geometry
Thin-wall parts with long flow paths may need hot gates to avoid high injection pressure and cosmetic gate blush. Thick sections may tolerate cold tab gates hidden on functional faces. Weld line location from runner layout is a DFM output — request weld line prediction or flow comment on Class A zones.
Energy and material waste accounting
Cold runner scrap consumes resin and regrind energy; hot runners consume electricity for heaters continuously during production. Sustainability reporting increasingly asks for scrap rate — runner choice affects both metrics. Document baseline scrap at launch and track after runner or process changes.
Validation testing by runner type
Cold runner programs should validate regrind ratio impact on CTQs — tensile, impact, cosmetic streaks. Hot runner programs should validate gate vestige dimensions across temperature window and after simulated heater cycle failures. Quality plans should list these tests explicitly in PPAP where applicable.
Cosmetic correlation across cavities
Class A programs should define whether all cavities must match color and gloss within delta-E spec, or whether cavity-to-cavity cosmetic variation is acceptable within limits. Texture depth can vary slightly between cavities if polish or EDM timing differed — set visual limit samples at FAI.
Industrial design should sign cavity sample board at production release, not only single-cavity T1 from mold build shop — build and production cavitation may differ.
FAQ
Can we convert cold to hot later?
Often requires significant mold rework — decide at design if volume justifies hot half; retrofit rarely matches greenfield economics.
Does hot runner affect DFM?
Yes — gate location options, balance analysis, and wall thickness near gates change; include in DFM before cavitation lock.
Who maintains the hot runner?
Production supplier typically owns preventive maintenance during the contract; define heater failure response time and spare tip inventory in the tooling agreement.
Are hot runners always faster cycle?
Usually yes on thick cold runners, but not universally — thin parts with short cold sprue may differ. Require cycle time estimate at quoted cavitation.
Next step: Ask for runner recommendation with resin, volume, and cosmetic class.