How to Control Gate Vestige and Cosmetic Quality on Class A Parts

A glossy black Class A trim panel left the press with a 0.4 mm raised vestige at every submarine gate. The buyer pulled 4,800 parts at receiving under an AQL 0.65 major plan, and the rework line burned six days. The steel was not wrong. The gate freeze had been timed late, so each shot pushed a melt plug 0.2 mm proud of the surface before it solidified. This guide treats the vestige as a measurable feature you set, not a defect you inherit.

Scope runs from witness anatomy and gate choice through freeze timing, mold-side correction, trimming and final sign-off, all judged on the face the customer sees first.

The Snapshot

• The gate type sets the cosmetic ceiling. An edge gate leaves a 2 to 4 mm tang on the parting line, a pin-point a 0.3 to 1 mm boss witness, a valve gate a flush mark near 0.05 mm. No process tuning removes a tang.
• Freeze timing is worth 0.1 to 0.5 mm. A vestige standing tall means the gate froze late: drop the gate zone 10 to 20 °C or shorten hold by 0.5 to 1 s. A sink at the gate means the opposite.
• Rework is a real cost line. Gate trimming and cosmetic inspection run 0.2 to 0.8 dollars per part. Moving the gate to a hidden face costs 200 to 1,500 dollars of tooling change and pays back under 50,000 parts.
• Acceptance must exist before the first cut. Zone the part A, B or C, fix the lighting angle at 15 to 45 degrees for gloss surfaces, cut limit samples per gate and hold AQL 0.65 major on Class A faces.

Anatomy of the Witness: What a Gate Vestige Really Is

A gate vestige is everything the gate leaves behind after ejection. Three elements make it up: the frozen plug that sat in the gate land, the shear line where the part tore or was cut away, and sometimes a pull crater around both. On a well-tuned submarine or valve gate the total mark measures 0.1 to 0.5 mm raised or sunk. On a direct sprue the separation pull leaves a crater 1 to 3 mm across.

The vestige grows during the first hundred milliseconds after the cavity fills. Melt still stands in the gate, and packing pressure keeps feeding it into the part. The moment the gate cross-section solidifies, that feed stops and the plug shape freezes. Time the freeze late and the plug stays fat and proud of the surface. Time it early and the part shrinks away from the gate and leaves a dimple.

The plug is also a copy of the steel. Whatever geometry the gate land carries, its polish and any burr at its edge, prints onto the part every shot at 40 to 80 MPa of packing pressure. That is why two tools cutting identical parts can disagree cosmetically for the life of the mold. A 0.005 mm burr on the gate edge repeats on every single shot, and no process window removes it.

Read a gate problem in three places, always in this order: gate land geometry in the steel, freeze timing in the process, and shear or trim in post-operations. Most shops start at the press because it is free, and finish at the mold shop anyway. The same 0.3 mm mark means something different on a Class A face and on a rib underside, which is exactly why the acceptance definition comes next.

Define Cosmetic Acceptance Before the Steel Is Cut

A vestige is only a defect against a written limit. Before any steel is cut, pin the surface reference: an ISO 100-A comparator plate for gloss and flatness, and the SPI or SPE texture class for molded grain. Record both on the drawing, because "smooth and shiny" is not a tolerance and a texture sheet number is.

Then fix the viewing conditions, because the same mark disappears or appears with the light. Glossy Class A faces are inspected at a 15 to 45 degree lighting angle, the band where reflections rake the surface and expose flow and gate disturbances that frontal light hides. Set the viewing distance and the lamp type in the same document, or two inspectors will disagree on identical parts.

Zone the part. A cosmetic map drawn on the parting drawing marks each face Class A, B or C: A is the always-visible face, B is glimpsed in use, C is hidden. The gate should land in B or C wherever the part allows. If a required gate position lands in zone A, write the maximum witness into the map: a height, a diameter and a trim allowance per gate, each checked against a physical limit sample before production approval.

Buyers routinely skip this and argue about it later. A receiving plan of AQL 0.65 major on Class A faces, with no limit samples behind it, turns every gate witness into a negotiation at the dock. The gate type itself decides how much of the argument you will ever have, and the gate families are compared in the injection mold gate type guide. Layout rules that keep gates off visible faces follow the same placement logic as the filling balance rules in gate design practice.

Gate-by-Gate Cosmetic Map: Witness Size and Trim Load

Each gate family leaves a signature you can measure before the tool is built. The numbers below are what a vestige actually costs on a Class A face, and they explain why gate selection is a cosmetic decision disguised as a flow decision.

An edge gate on the parting line leaves a tang 2 to 4 mm wide that must be trimmed flush to 0.05 mm by hand or fixture. A tunnel gate buries the mark below the parting on a 1.5 to 2 mm land, so the witness sits on the C face and reads as a small crescent. The submarine gate shears clean at 0.2 to 0.5 mm of relief because the part tears at the land under its own ejection load. A pin-point gate leaves a 0.3 to 1 mm boss witness that grows worse on thick parts, since more packing volume stands behind it.

The valve gate is the cosmetic benchmark: the pin shuts melt off mechanically at the gate, so the mark holds near 0.05 mm flush with no plug to trim. Fan and film gates hide the witness inside a melt land at the parting, and the land itself is cut or scored away later. A direct sprue gives none of this protection; the pull leaves a 1 to 3 mm crater that needs a boss cover, a plug insert or an acceptance exception.

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Table note: witness sizes and per-part adders are typical shop bands for glossy and Class A parts. Confirm your own trim time and cost per part against the quoting sheet before releasing gate position to steel.

Two rules fall out of the table. First, if the face is Class A and the volume is above roughly 100,000 parts per year, a valve gate or a hidden-face submarine gate usually beats trimming 0.30 dollars per part by hand forever. Second, if the gate must sit on the visible face, the valve gate or a fan gate into a cut-away land are the only options that hold a 0.05 mm flush mark without post-operations.

Freeze Timing: The Process Side of the Vestige

Once the gate is chosen, the press owns the remaining 0.1 to 0.5 mm. The variable is when the gate land solidifies relative to the part, and packing profile decides it. A vestige standing proud of the surface means the gate froze late: melt kept flowing through an open gate while the part skin pulled away, so the plug piled up. The correction is to freeze the gate earlier. Drop the gate-zone mold temperature 10 to 20 °C, or shorten hold time by 0.5 to 1 s, and the plug height falls.

A sink directly at the gate is the same clock running the wrong way. The gate froze before the part, packing could no longer feed the shrinking zone behind the gate, and a dimple appeared. Add 1 to 2 s of hold, or raise the local mold temperature around the gate, and the dimple fills. Changing hold pressure by 5 to 10 MPa moves the same defect, but it moves part weight too, so temperature and time come first.

Gate blush sits on the boundary between process and steel. It reads as a dull patch around the gate, typically 20 to 60 gloss units below the reference plate on a 60 degree meter, and it points to a gate zone running 25 to 60 °C too cold for the resin. The melt skin freezes against that steel and the part copies the chill rather than the polish. Raise the gate island temperature in 5 to 10 °C steps while watching the gloss reading, and check the cavity-pressure trace confirms the gate stays open through pack. The flow-mark family behind this shares a diagnosis tree in the flow marks fix guide.

For semi-crystalline resins like polypropylene and acetal, gate freeze arrives fast because crystallization releases heat in a narrow window. Those materials tolerate a shorter hold on the gate side and reward a colder gate island. Amorphous resins like ABS and acrylic freeze gradually, so a 10 °C change moves the vestige height by roughly the same amount as a 1 s hold change. Log both in the process sheet so the next color change starts from a number, not a guess.

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Table note: bands assume a single-cavity-class trial on an engineering thermoplastic. Re-measure the window per resin and per gate, and record the cavity-pressure trace with the final process sheet.

Mold-Side Moves: Gate Islands, Baffles and Local Heat

When the press window cannot deliver the mark, the mold has to. The core idea is local thermal control: the gate zone should freeze on schedule independent of what the cavity does, and the tool should give it that freedom.

The first tool is material. A beryllium copper gate island, at roughly 200 W/m·K against 25 to 35 W/m·K for hardened mold steel, drains or delivers heat fast enough to hold the gate on its own setpoint. On a 0.8 to 1.5 mm gate land that thermal gap moves freeze time by full seconds on thick parts. Put the CuBe insert behind the gate and pipe its own cooling circuit to it; do not let it ride on the cavity loop. The same trick run backwards, with the island heated, kills blush on parts that pack cold.

The second tool is plumbing. A dedicated baffle or conformal line 8 to 15 mm from the gate holds the island within ±2 °C while the rest of the cavity runs its own profile. On multi-cavity tools this is the difference between one cosmetic part and sixteen: probe the temperature at each gate island with a surface thermometer during the trial and reject any spread above 5 °C before sampling. A hot-runner valve gate adds its own variable, and the valve gate hot runner selection guide covers pin seat timing against vestige height.

The third tool is the freeze face itself. A gate land left at the as-machined finish pulls air against the plug and prints a matte witness even on a B2 textured part. Polish the gate land to Ra 0.05 to 0.1 µm regardless of the cavity texture class, and break the land edge with a 0.01 to 0.02 mm hone so the shear line stays where the drawing says. Then deburr at assembly: a 0.005 mm wire at the gate entrance streaks a tail across every shot for the life of the tool, because packing pressure of 40 to 80 MPa presses the melt through it each cycle.

Gate Land Finish: Polish, Burr and Texture Interaction

Texture and gate finish interact in ways that surprise shops that specify them separately. A molded grain, SPI CTG or VDI 3400 speckle, starts 5 to 50 µm deep. If a polished gate land bleeds into a textured field without a boundary, the melt flows across the transition and the witness ring reads as a bright halo a few millimeters wide. Put the land inside a machined pocket with a crisp 0.1 to 0.2 mm step, or land the gate entirely in the textured zone and accept the matte mark.

Never sand a textured surface to hide a gate mark. Abrasive flattens grain edges unevenly, and the patched zone reads worse under a 15 to 45 degree light than the original vestige. If the texture has a witness problem at the gate, re-cut the land, re-grain the insert or move the gate. Grain matching and blending rules belong to the mold texture specification guide, and remember that an EDM-cut gate pocket carries a recast layer that prints into the plug until it is stripped or polished away.

Trimming and Post-Operations: Ultrasonic, Blade and Solvent

The gate mark does not end when the part ejects. On Class A work the trim operation is a second cosmetic risk, and the method chosen decides whether the witness reads as a spec or as a defect.

Ultrasonic cutting at 20 to 40 kHz with a 0.1 to 0.3 mm penetration depth gives the cleanest flush cut on ABS, PP and PC. The blade melts and severs in one stroke, leaving a white-free edge that disappears under paint and texture. It needs fixture work per part family, and the cycle add is 1 to 3 seconds per part.

Blade cutting on a hardened fixture runs cheaper and faster but risks stress whitening around the cut line. Hold the cut to 0.05 to 0.1 mm below the surface and check the white mark under raking light before shipping. Solvent smoothing is a legitimate finish on acrylic only. Keep it off PC, PC blends and PBT: solvent crazing on polycarbonate shows up days later as hairline cracks around the gate, long after your customer has the parts.

Inspection Discipline: Zone Maps and Limit Samples

Verbal acceptance criteria fail on cosmetic work. Two inspectors, two shifts and two opinions. Lock the standard to artifacts before T1.

Issue a cosmetic zone map drawing that divides the part into Class A faces (visible in use at arm's length), Class B (visible only when opened or touched) and Class C (never seen). Write the witness limit per zone: a common rule is 0.1 mm max protrusion on Class A with no visible color break, 0.3 mm on Class B, anything on Class C ignored. The map travels with the PPAP.

Build a limit sample library at each gate location: photos of pass, marginal and reject taken at the agreed 15 to 45 degree light angle, plus physical parts for the trial review. Sign the set with both parties before the balance invoice. AQL 0.65 major on Class A faces is a reasonable inspection level for visible consumer parts; tighten c=0 sampling on automotive interiors.

The Hidden Cost: Gate Rework Versus Gate Relocation

Count the trim-and-inspect line before defending the gate position. Ultrasonic or blade trimming with cosmetic inspection runs 0.2 to 0.8 dollars per part depending on cycle add and reject rate. On a 500,000 part program that is 100,000 to 400,000 dollars of permanent process cost carried in the piece price.

A mold change that moves the gate to a hidden face costs 200 to 1,500 dollars in steel work: fill the old gate, re-cut a new land, polish the insert. The payback crosses under 50,000 parts. This is why the gate location review at DFM is one of the highest-leverage half hours in the program. Decide with the piece price on the table, not with the runner diagram alone.

Cosmetic Sign-Off Checklist

• Zone map issued. Class A, B and C faces drawn and signed before steel is cut.
• Gate on a hidden face where possible. Or a written piece-price justification for the visible witness.
• Gate type matches the witness target. Valve gate or submarine for flush marks; edge gate only off-parting or with trim budget.
• Freeze timing proven. Vestige height measured on 10 consecutive shots at 10 to 20 C gate-island settings recorded in the process sheet.
• Gate land polished. Ra 0.05 to 0.1 micrometer on the land regardless of part texture class; zero burr on the gate edge verified under loupe.
• Trim method fixed. Ultrasonic, blade or break-away designed in; solvent never on PC.
• Limit sample library complete. Photos at the agreed light angle plus physical parts per gate, both parties signed.
• Inspection level set. AQL 0.65 major on Class A faces at minimum.

Gate Vestige and Cosmetic Quality FAQ

How big is an acceptable gate mark? There is no universal number. A defensible rule is 0.1 mm maximum protrusion with no color break on Class A faces viewed at 15 to 45 degrees, and the number belongs on the zone map drawing before the mold is built, not in an email afterwards.

Which gate type gives the best cosmetic result? Valve gate holds the flattest witness at 0.05 mm flush because the pin shuts melt off mechanically. Submarine gates shear clean at 0.2 to 0.5 mm relief below the surface. Pin-point gates concentrate the witness into a boss and are reserved for thick parts where the boss can hide.

Can texture hide a gate mark? Partially, and at a price. Fine grain in the gate land masks small witnesses, but sanding textured surfaces to fix a mark flattens grain edges unevenly and the patched zone reads worse than the original vestige. Re-cut the land or move the gate instead.

Why does my vestige stand tall on one cavity only? Check that cavity's gate island temperature first. A 10 to 20 C colder gate island on one side freezes the land late and lets material keep flowing after the rest of the family has solidified. Balancing the water circuit usually fixes it without touching the steel.

The Bottom Line

The gate witness is the first thing your customer's customer touches, and it is decided three times: at gate-type selection, at freeze-timing setup and at the trim line. Define acceptance with a zone map and limit samples before the steel is cut, hold the gate island 10 to 20 C cooler than the cavity with a polished Ra 0.05 to 0.1 micrometer land, and move the gate to a hidden face whenever the rework cost crosses the 200 to 1,500 dollar mold change inside 50,000 parts. A mold design and DFM review at DieStrike checks the gate cosmetic map against your zone plan before cutting steel, with feedback in 24 hours.