Mould trial and sample approval: how to get a moulded part signed off

A moulded part gets signed off cleanly when you and the buyer agree three things before the mould goes on the machine. The first is what the trial run will prove. The second is how and when each check is made. The third is whose signature closes it. After that, keep a signed set of approved parts on both sides, with the settings they were moulded at. Many arguments after a trial are not about the part. They are about what "OK" meant on the day, and nobody can find the piece it was said about.

A trial run is not the approval, unless you both say so

The first shots from a new tool tell the toolroom whether it fills, vents and ejects. They do not show whether the part holds its size on your production cycle, in your resin lot, on the machine it will run on. A development engineer sees a clean part at the press, fits it on their assembly and says "go ahead". A month later their quality team rejects the first production lot.

So name the run before it happens. Tell the buyer in writing which trial this is: a tool check, a process trial at production settings, or the approval run. Only the last one should carry a signature.

Agree these before the mould goes on the machine

Send this list to the buyer's development engineer and copy their quality engineer. Ask for a reply on each line.

• Drawing number and revision, and which dimensions are critical. For a multi-cavity tool, say whether every cavity is measured or a sample from each.
• When parts are measured. Moulded parts keep settling after they leave the tool. The buyer's quality plan decides how long they rest first; get it in writing.
• The visible face. Mark it on the drawing. Then agree what is acceptable there: gate mark, flow lines, sink, parting line flash.
• Colour, matched against whose master, under which light, and who keeps that master.
• Resin grade and supplier named on the approval, with a line saying a substitute needs fresh agreement.
• Fit and function tests: snap fit on the mating part, screw boss torque, a drop, a trial assembly on the buyer's line.
• Approval lot size, and who pays for it. Our post on handling a sample request covers the charges.
• Signatories by name: dimensions, cosmetics, and colour if the buyer's own customer must see it.
• A "yes with comments", and how it is recorded. A deviation agreed on the phone is not a deviation.

Words like "minor flow lines acceptable" settle nothing. Cosmetic standards are agreed with limit samples: one part that is just acceptable and one that is just not, both signed.

Who should be at the trial

The person who says "looks fine" at your press is often not the person who signs the approval. Purchase placed the order. The development engineer cares about fit. A supplier quality engineer, or whoever runs the buyer's incoming inspection, decides whether your production lot is accepted.

Invite the quality engineer to the trial. If neither of them can attend, call the run a process trial, not the approval, and say so in your covering note. If they cannot travel but will join, run a video call from the machine and show the cavity numbers, the gate area and the parts beside the master. On your side, the setter who will run production should be there, not only the toolroom.

Lock the settings and seal the samples

An approved part is approved at a set of settings. Write them on a process sheet signed at the trial. Include the machine number, barrel and mould temperatures, injection and hold pressures, hold and cooling times, cycle time, resin lot and masterbatch ratio. If you later move the job to another machine or change the resin source, the buyer hears it from you first.

Then seal the samples. Take a set from each cavity, tag them, put them in a bag and label it. Both sides sign the label. The buyer keeps one bag; you keep the other, away from the production area.

APPROVED SAMPLE: RETAIN, DO NOT DISPATCH
Part, drawing no., revision:
Mould no.:              Cavity:
Resin grade and lot:    Masterbatch and lot:
Trial date:             Machine no.:
Process sheet ref.:
Signed for buyer:       Signed for moulder:
Set 1 of 2. Set 2 held by the buyer.

Photograph the bag, the label and each cavity's part on a plain sheet. File the photographs with the process sheet.

Imagine: a moulder in Waluj and a fuse box cover

Imagine a moulder in Waluj MIDC, Chhatrapati Sambhajinagar, making a four-cavity fuse box cover for an auto-electrical maker in Chakan. The details are illustrative.

At the trial, the buyer's development engineer watched, clipped two covers onto a box and said they were fine. The shop measured parts warm off the press, and the report showed every dimension in tolerance. The first production lot went to Chakan and came back. Covers from cavity 3 were tight on the box once they had cooled overnight. The grey also looked darker than the engineer remembered. Nobody had signed a sample, and the trial pieces were in a drawer somewhere in the engineer's office.

The owner asked for a second trial with the supplier quality engineer present. Parts rested overnight before measurement, as the buyer's plan required, and every cavity was checked. Colour was compared with the buyer's master plaque under the inspection lamp. Cavity 3 needed a small tool correction. Two bags of signed covers, one cover per cavity in each, carried the process sheet number. When the shade question came up again, it took ten minutes. Both sides opened their bags.

When a lighter sign-off is enough

Not every part needs the full routine. A colour change in an existing tool needs a colour approval and a signed sample. A cap or a standard container sold to a dealer needs a signed sample and little else. A buyer three states away with a simple part may accept a video trial and sealed samples by courier.

Some buyers work under their own approval regimes: automotive, electrical safety, medical and food-contact parts among them. Their requirements differ by sector and by customer, and they change. Get their document list in writing and ask their quality team before assuming anything. For the report and covering note that go with a first lot, our post on first-article approval sets out a format that also suits moulded parts.

One more rule. Do not start production on a verbal "go ahead". If the buyer needs parts before the paperwork is done, ask for an email saying production may start before formal approval, and on what terms.

Before the next tool goes on the press

Take the next tool due for trial and send the buyer the checklist before it reaches the press. Print the bag labels the day before. Not at the machine. The plastic manufacturing section covers the rest of the route from enquiry to repeat order. If you want approval terms written around your own parts, as part of customer onboarding, tell us what you need.