Common Core Knowledge Mold Design: A Working Guide for Engineers
Shrinkage, draft, and steel choice decide whether a mold concept holds tolerance before any steel is cut. This guide is for engineers and buyers who need to judge a design early. It covers the numbers we check first and the features we machine into the cavity.
What Actually Decides a Mold
A mold is a heat exchanger with a cavity in it. Every decision below follows from that.
Shrinkage Rates and Why the Table Is Only a Starting Point
Every thermoplastic shrinks as it cools, and the mold cavity has to be cut larger than the finished part by that amount. Published shrinkage rates give you a first number, not a final one. Amorphous resins such as ABS, PC, PMMA, and PS sit near 0.5%, while semi-crystalline resins such as PP, PE, PA, POM, and PVC typically run near 2%. Blends land in between: ABS+PC around 0.4%, PC+ABS around 0.5%.
Those figures assume a nominal wall and a well-cooled tool. Change the wall from 2 mm to 3 mm and PP can move well past 2%, because thicker sections cool slower and the crystalline phase has more time to pack. Glass-filled grades shrink less in flow direction than across it, so a single number cannot describe both axes. For anything with a tight tolerance, we ask for the exact grade and let the mold shop offset X and Y separately.
The practical rule: use the table to size the cavity, then cut steel so there is room to correct. If the first shot comes out small, welding and re-cutting is expensive. If it comes out large, you can remove metal. We bias the cavity toward the high side of the shrinkage range when the material is semi-crystalline, and toward nominal when it is amorphous and the wall is uniform.
- 1Check the wall firstSection thickness moves shrinkage more than the resin data sheet suggests.
- 2Split X and YFilled grades shrink differently along flow and across flow.
- 3Bias toward metal removalA slightly large cavity can be corrected; a small one cannot.
Draft, Parting Line, and Wall Thickness
Draft is the taper that lets the part leave the steel without dragging. A textured surface needs more. As a baseline, 1° per side works for smooth vertical walls up to about 25 mm deep. Add 1° for every 0.025 mm of texture depth, so a heavy matte finish can push you to 3° or more. Ribs and bosses need more draft than the outer shell because they grip the core.
The parting line should sit where the part's largest projected area meets the shutoff, and it should be simple. Every step in the parting line adds a shutoff surface that has to be fitted by hand. If a cosmetic face crosses the parting line, the witness line will show. Move the line to a non-visible edge or accept that it will be visible and plan the finish around it.
Wall thickness drives cycle time and warpage. Keep nominal walls uniform, and where thickness must change, step it gradually rather than in one jump. A sudden 2:1 change creates a hot spot that sinks on the thick side and pulls the part out of flat. For glass-filled parts, thick walls also raise the risk of fiber orientation mismatch between the skin and core, which shows up as warp after ejection.
- 1Draft: 1° minimumMore for texture, deep ribs, and rough finishes.
- 2Keep the parting line flatEach step adds hand fitting and a visible witness line.
- 3Step wall changesAvoid abrupt 2:1 jumps that create hot spots.
Shrinkage and Draft Baseline by Resin
Starting values for a nominal 2 mm wall. Confirm against the exact grade before cutting steel.
| Resin | Shrinkage | Draft (smooth) | Notes |
|---|---|---|---|
| ABS | 0.5% | 1° | Amorphous, easy to mold |
| PC | 0.5% | 1° | Dries before molding, high melt temp |
| PMMA | 0.5% | 1° | Optical surfaces, needs polish |
| PS | 0.5% | 1° | Brittle, low cost |
| PE | 2% | 1.5° | Semi-crystalline, warps easily |
| PP | 2% | 1.5° | Living hinge grade shrinks more |
| PA | 2% | 1.5° | Moisture sensitive, anneal after |
| POM | 2% | 1.5° | Low friction, tight tolerance possible |
| PVC | 2% | 1.5° | Corrosive, use stainless steel |
| ABS+PC | 0.4% | 1° | Blend, check grade data |
| PC+ABS | 0.5% | 1° | Blend, check grade data |
Steel Selection and Cooling Layout
Cavity and core steel is chosen by resin, run size, and finish. P20 pre-hardened steel suits low to medium runs in ABS, PS, and PP where the finish is not critical. H13 or S136 hardened steel is the better call for glass-filled nylon, PVC, and PC, because abrasive fillers and corrosive byproducts wear a soft cavity fast. For optical parts in PMMA or PC, S136 polished to a mirror finish holds the surface longer than P20.
Cooling is where cycle time is won or lost. Place channels so the distance from the channel wall to the cavity surface stays roughly constant, typically 1.5 to 2.5 times the channel diameter. Baffles and bubblers reach into tall cores that a straight drilled line cannot cool. For a part with a deep draw, a straight channel around the outside will cool the rim faster than the core, and the part will bow toward the hot side.
Hot runners make sense when the runner is large relative to the part, when you run engineering resins with a narrow processing window, or when you want no regrind. They cost more up front and add maintenance. A cold runner with a well-placed gate is often the simpler choice for small parts in ABS or PP. The gate location also sets weld line position and orientation, so decide it before the cavity is modeled.
- 1Match steel to resinAbrasive and corrosive resins need hardened or stainless cavities.
- 2Keep cooling uniformConstant wall-to-channel distance controls warpage.
- 3Decide the gate earlyGate position fixes weld lines and flow orientation.
Common Questions
How do I choose the shrinkage value for a glass-filled grade?
Use the resin supplier's data for the exact grade and glass content, and expect different values along flow and across flow. For a first pass, split the difference and cut the cavity on the large side so you can remove metal after the first shot.
If the part has tight features in both directions, plan a correction cycle rather than trying to hit nominal on the first cut.
What draft angle is enough for a textured surface?
Start at 1° per side for a smooth wall and add about 1° for every 0.025 mm of texture depth. A heavy matte or leather grain can need 3° or more, and ribs need more than the outer shell.
If the texture is applied after molding, the draft can be lower, but the texture depth still drives the drag.
When should I use a hot runner instead of a cold runner?
A hot runner pays off when the runner is a large share of the shot, when the resin has a narrow processing window, or when regrind is not acceptable. It also helps when you want balanced fill in a multi-cavity tool.
For small parts in ABS or PP, a cold runner with a well-placed gate is usually simpler and cheaper to maintain.
How does wall thickness affect warpage?
Thick sections cool slower and shrink more, so they pull the part toward the thick side. Keep nominal walls uniform and step any change gradually instead of jumping 2:1 in one place.
Glass-filled parts add a second effect: fiber orientation differs between skin and core, and thick walls make that mismatch worse.
Can you machine a mold cavity to tight tolerance?
Yes. Our 5-axis and mill-turn centers hold ±0.005 mm on cavity features, and we inspect 100% before shipment. For molded parts, the achievable tolerance also depends on resin, wall, and gate location, not only on the steel.
We quote a mold and first-article plan together so the tolerance target is agreed before cutting.
Do you sign an NDA before reviewing a mold design?
Yes. Uploads are secure and confidential, and we sign an NDA on request before any design file is reviewed.
If you want the agreement in place first, send the NDA and we will return a signed copy before the DFM review starts.
Send Us the Part and the Resin Grade
We review the geometry, the shrinkage call, and the steel plan, then quote the cavity work. Quotation and free DFM analysis within 12 hours.
12-hour quote100% inspectionNDA on requestNo MOQ
