DFM Guidelines for Plastic Enclosures

Close the checks below before steel is cut. They are not a datasheet and not a moulder's signature. Every figure later is hypothetical teaching maths: not a BrahmWorks price, yield, timeline or test result, and not a quotation. Draft, shrink and rib proportion come from the grade and from the moulder who will cut the tool.

The model describes a cavity, a core, and the direction in which the part leaves them. If the skin and the core do not separate along one direction, you need a slide or a lifter, or you change the shape. If one region is thick and the next is thin, the thick region stays soft longer, shrinks more, and pulls. That pull is sink, warp on a lip, or a void in a boss that appears when a screw strips.

Name the process on the review: injection moulding, the resin family, texture or none, and whether a side action is allowed. Mark show faces, seal faces, and faces that may carry a pin or a gate vestige. Cosmetic class changes the tool. It does not suspend shrink.

Choose the nominal wall with the moulder, from flow length, stiffness, resin and cycle. This article will not give you that number. What the review can demand is control of variation. A local section much thicker than the wall is a shrink mark aimed at the opposite surface. A much thinner section is a short shot, a brittle wall, or two cold flow fronts meeting.

Core thick corners from the non-show side. A radius where walls meet helps flow and removes a sharp notch. A radius large enough to leave a blob recreates the sink. Ribs add stiffness without filling the part solid. A common teaching prompt, not a measurement, is to keep a rib noticeably thinner than the wall it joins, often spoken of as around half, so it does not read through. Treat that as a question. A deep rib in a textured, high-shrink resin is not the same problem as a short rib in a filled grade. Ribs need draft, and a root radius that is not itself a lump.

A screw boss is a tube, ribbed back to a wall, not a freestanding post. Depth, internal draft and real thread engagement go on the drawing. Keep bosses off the middle of a gloss face, or name an owner for the sink. Moulded-in and heat-set inserts add a shut-off and a rule for a missing insert. Include that process or delete the insert.

Draft is the taper that lets the part leave the steel. The angle depends on depth, resin, polish and texture. Texture is a field of tiny undercuts, so it usually needs more taper than a polish. There is no universal angle here. The degree in the sum below is assumed only so the shift can be counted.

Draft ribs, boss bores and the core, not only the silhouette. Agree which half is the cavity, because draft follows that choice. Put the parting line where a witness is acceptable. Flash follows the line. Sealing across that line means sealing on flash.

The gate leaves a vestige and sets flow. Fronts that meet form a weld or knit line, usually weaker and more visible than the wall beside it. Do not leave that meeting, by silence, on a snap root, a boss or a window. Ejector pins leave marks. Put them on a hidden face, a rib or inside a boss. A pin on a seal is a leak path.

A hole or hook square to the draw is an undercut. Form it with a slide or a lifter, or redesign it as a shut-off, where cavity steel meets core steel and the opening lies in the draw. A slide is legitimate. It needs room, care and money, and it leaves a witness. Count the actions before you count the clips. A thread along the draw may need an unscrewing core, or it may be damaged on the way out. A screw into a boss is often the simpler product. Write which one you mean.

This housing is invented for the exercise. It is palm-sized, wall mounted, indoors, two mouldings, resin family not yet a grade, one cable out of the bottom, front face cosmetic. It is not a BrahmWorks product.

The model arrives from a print. The front wall is thicker than the sides, so they will not shrink together. A heavy boss sits in the centre of the front, poorly cored. Outer faces are vertical. The cable port is a round hole in the side wall, an undercut in a two-plate tool. The parting line crosses the front because that is where the print was split.

A teaching disposition, not a trial result: one nominal wall; boss cored and ribbed to a side wall; draft added only after the moulder states an angle for the finish you agreed; parting line moved so the show face sits in one half; round side hole replaced by a shut-off, and the cable exit changed to match; side snap redrawn so it flexes in a direction the tool can form. No mould trial is reported here.

Assume a side wall 18 mm deep in the draw and a draft of 1.5 degrees. The angle is not a recommendation.

Offset = 18 × tan(1.5°). Tan(1.5°) is about 0.0262, so 18 × 0.0262 = 0.47 mm.

Judged against a vertical printed wall, that draft moves the steel wall by about 0.47 mm from one end to the other. A 0.3 mm clearance taken from the print disappears. Which end grows depends on whether the nominal sits at the parting line or at the top. This is plane geometry under an assumed angle, not a moulding result.

Assume a straight-pull tool at INR 2,40,000 and one side action at an extra INR 85,000. Assume the piece price does not change, which is a thin assumption: a slide can change cycle, maintenance and scrap, and those sit outside the sum. Assume 3,000 housings.

Extra tool per housing = 85,000 / 3,000 = about INR 28.

That is not a saving until a shut-off is designed and the moulder agrees the part still fills and ejects. If the new port breaks the cable, the slide may be the cheaper system. The sum only prices an action you might avoid. It is not a market rate.

The guidelines do not choose grade, colour or chemical resistance, and they do not prove a drop, a seal or a radio. A fit is a stack plus warp, not a slogan about normal tolerances. Once a prototype already works, the next question is whether another team can build that revision: How to Take a Hardware Prototype to Production.

Each line needs an owner and a revision, or it stays open.

• Resin family is named. An open grade is an open item, not a silent "ABS".
• Nominal wall is stated. Thicker and thinner regions are listed.
• Bosses are cored, drafted, and tied to a wall. Engagement is on the drawing.
• Ribs are thinner than the local wall, with a root radius that is not a new lump.
• Draft matches the parting line, on ribs and cores as well as the skin.
• Texture sits on the same note as draft.
• Show faces, seal faces, pin faces and gate faces are marked.
• Likely weld lines are not left on a snap, a boss or a window without an owner.
• Every undercut is a slide, a lifter, a shut-off, or a deleted feature.
• Inserts, a second shot and paint are in the process, or they are out of the part.
• Critical fits are on a drawing with datums.
• Prototype cheats are listed, so they are not cut into steel.

No. Uniformity is about fill and shrink. Stiffness comes from ribs and from shape, at a wall the process can make. A solid thick section often sinks, warps and lengthens the cycle. If you truly need one, isolate it and accept the cosmetic and the cycle in writing.

They will have to if the model has none. Added after you froze the fits against a print, the taper moves a lip, a button and a standoff. The 0.47 mm shift is that sort of surprise. Put draft in the model while the other parts can still move.

No. The geometry review and the tool class are different decisions. 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 production cycle. Ask what this tool is allowed to prove before you argue about steel.

Record the conflict. A boss behind a gloss face, a zero-draft mark, or a port that demands a slide can stay, with an owner and a consequence: sink, a witness, a dearer tool, or a seal you no longer claim. A conflict with nobody's name on it comes back as steel rework.

Bring the CAD, the cosmetic map, the undercuts and the cheat list. BrahmWorks can review that pack against the process you say you are using, and name what must change before a tool is cut.

Review the next stage with BrahmWorks