How to Prepare a Product for Injection Molding

Ready to quote and ready to cut steel are different states. A quote can be a lesson about actions and risk. Steel is a commitment to shrink, texture and a parting line. This article is the pack you should be able to hand over, and the questions you should refuse to leave blank. It is not a substitute for the moulder's own review of the steel. Figures are hypothetical teaching maths, not BrahmWorks prices, yields, timelines or test results. How tooled parts fit a wider release is covered in How to Take a Hardware Prototype to Production.

Name the resin family, and say what is still open. "ABS" with no grade, no colour plan and no exposure is a placeholder the shrink will later contradict. Food contact, flame, outdoor ultraviolet or a disinfectant throw out families. A family change after the tool is cut can scrap the shrink allowance. Write the claim now.

Name the split. One part or two, which half is the cavity, where a witness is allowed, which faces are show faces, which faces seal. Mark every feature that is not in the draw: a side hole, a hook, an undercut clip. Each one is a slide, a lifter, a shut-off, or a deleted feature. Leaving the choice implicit is how the quote grows a line you did not budget and the tool grows a witness you did not draw.

Name the neighbours: board, gasket, lens, cable. Those interfaces need datums, not a hope that a prototype fit reappears after shrink. Write the engagement, the stop, and what a missing or rotated part does.

You need not freeze cavity count, press size or a commercial texture code on day one. You must know which of those the quote is allowed to assume. Three moulders assuming three cavitations and three textures have not given you three prices. They have given you three products.

Separate tool class from the geometry review. A softer tool can be right while the shape is still moving and the quantity is small. It is the wrong teacher if you needed the real resin, a fine texture, or a cycle you will use in the piece price. Write what this tool may prove. Learning fits in the real resin is a legitimate purpose. Calling it the production tool is a different one, and it needs a model you will live with.

Do not smuggle prototype cheats into the pack. Filed snaps, a glued light pipe, a screw in a hole that was drilled after printing, a gasket chosen because it was in a drawer. List them as not in the tool. A moulder who is handed a tidy STEP file cannot see the knife.

Send the pack when the split and the undercuts are honest, even if a few ribs can still move. Read the quote for actions, assumed resin, cavity count, and exclusions such as texture or inserts. A low price that excludes the slide you need is not low.

Cut steel when the functional interfaces are on a drawing, the cosmetic map matches the parting line, and you know which feedback you have rejected. Cutting in order to "see parts" before those exist is how the first moulding becomes another prototype, at tool prices. You will see parts. They will teach you what the files already contained.

Family tools and two-shot need the same honesty, earlier. Parts in one tool share a resin, a colour and a volume balance. Two-shot shares a press and a bond. A clear lens in a custom-colour body may make two-shot a complication rather than a saving. Decide before a specialist tool is the only option left.

The teaching product is a small outdoor sensor, two housings and a clear window. It is not a BrahmWorks product. The team prepared a quote pack that called for a two-shot front: body colour plus a clear window, no gasket. The window also had to come off for a board connector during service. Two-shot does not come off.

A teaching preparation, not a mould trial: one body moulding, one clear window as its own simple tool, a gasket bead or a separate elastomer specified against the outdoor claim, and screws in bosses that a service technician can reuse. The body stays in the draw, with the connector exit on the parting line. Show faces are the front only, so the parting witness sits at the edge. The quote asks for one cavity first, because the outdoor resin is not yet confirmed and a second cavity would copy a mistake. The pack lists the printed prototype's glued window as a cheat, so nobody cuts a shut-off that assumes glue.

The two-shot version could still be right for a product that is sealed for life and high enough in volume. Preparation did not "reject two-shot". It refused to let a service requirement and a process assumption contradict each other in silence.

Hypothetical teaching maths. Not a tool quote and not a BrahmWorks result.

Assume a one-cavity tool at INR 3,50,000 and a two-cavity tool at INR 5,40,000. The extra steel is 5,40,000 − 3,50,000 = INR 1,90,000. Assume the piece price falls by INR 12 with the second cavity, because the press cycle is shared, and assume that INR 12 is an illustration, not a moulding study. Units to recover the extra steel on piece price alone = 1,90,000 / 12 ≈ 15,800.

If you expect far fewer than that before the geometry changes, the second cavity is a bet that the first cavity was already right. If the resin is unconfirmed, the bet is larger, because both cavities are wrong together. The sum ignores cycle risk, maintenance and a demand spike. It only asks whether cavitation is a decision or a hope. Quote both if you wish. Cut the second when the first cavity's resin and fits have a disposition.

• Resin family is named. Claims that constrain it are written. Open items are marked open.
• Part split, cavity side and allowed witness lines are drawn.
• Every undercut is a slide, a lifter, a shut-off, or gone.
• Show faces, seal faces, gate faces and ejector faces do not silently share a surface.
• Board, gasket, lens and cable interfaces have datums and a failed-assembly condition.
• Prototype cheats are listed, and none of them are in the STEP file by accident.
• Tool class states what the tool is allowed to prove.
• Cavity count is a decision, with the quantity it assumes.
• Two-shot or a family tool is justified by resin, colour, service and volume, not by part count alone.
• You can reject a moulder comment without losing the requirement it would have broken.

Usually not, unless the geometry and the resin are already stable and the quantity will use the extra cavities before you want to change the part. A second cavity copies the current design, including the mistakes. Spend the first tool on learning the resin, the cosmetics and the fits. Add steel when those have a written disposition. The break-even sum above is a way to have that conversation, not a rule.

You need the family and the constraints before production steel. A trial tool in the right family can be the way you choose the grade, if that purpose is written and the tool is not then relabelled as production. Cutting hardened production steel "in ABS" and sorting the grade afterwards is how shrink, colour and strength become a surprise you have already paid to repeat.

They will choose if you do not. Their choice will be mouldable. It may not preserve your fits, your seal or your show face. Arrive with a proposed draw and the faces that must stay clean. Let the toolmaker correct it with a reason. Do not arrive with a vertical-walled print and a request to "add DFM".

A prototype tool is allowed to answer a named question in something close to the real process: fill, warp, a texture, a snap in the real resin. A production tool is allowed to repeat a design you have accepted, at a cavity count you mean to run. The steel hardness is not the difference. The difference is whether you are still allowed to change the part when the answer arrives. If you are not, you bought a production tool, whatever the purchase order says.

Bring the model, the cheat list, the cosmetic map and the sentence that says what this tool must prove. BrahmWorks can review that pack before a quote is treated as permission to cut.

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