Injection Molding Design Guide
The rules that make a part moldable, affordable and repeatable: draft, wall thickness, ribs and bosses, radii, tolerances by material, gates, undercuts, surface finish and flame ratings. Read the summary below, or download the full PDF with checklists, tolerance tables and a DFM review form.
Get the Injection Molding Design Guide
Draft angles, wall thickness, ribs and bosses, tolerances by material, gate types, SPI finishes and UL 94, with checklists. Free PDF.
Design for injection molding, in nine rules
Draft angles
Every surface parallel to the direction of mold opening needs draft so the part releases without dragging. Use 1 degree per side as the floor for smooth surfaces, 1.5 to 2 degrees for light textures, and 3 degrees or more for heavy textures and deep ribs. Zero-draft faces are possible with side actions or lifters, which add tool cost and cycle time; design them out when you can.
Wall thickness and uniformity
Nominal walls of 1.5 to 3.5 mm suit most engineering resins; glass-filled grades tolerate thicker, TPEs thinner. Uniformity matters more than the number: a thick section next to a thin one cools at different rates and sinks, warps or voids. Core out thick areas and transition walls gradually, no more than a 25 percent step.
Ribs and bosses
Ribs stiffen without adding wall. Keep rib thickness at 50 to 60 percent of the adjoining wall to avoid sink, height under three times the wall, and draft of 0.5 to 1 degree per side. Bosses for screws or inserts follow the same thickness rule, with a radius at the base and gussets rather than thick walls for support.
Radii and corners
Sharp inside corners concentrate stress and restrict flow. Inside radii of at least 0.5 times the wall thickness, outside radii of inside radius plus wall, keep the wall uniform through the corner and keep the part from cracking under load.
Tolerances
Injection molding holds about ±0.005 inch (±0.13 mm) on typical dimensions in a Class 102 or better tool, and ±0.002 inch (±0.05 mm) on critical features with tight process control. Shrink varies by resin, so tolerances tighten or loosen with material: unfilled nylon and PP move more than glass-filled nylon or PC. Put tight tolerances only where the function needs them; every one adds inspection and cost.
Gate location and type
The gate decides fill pattern, weld lines, cosmetic marks and part stress. Edge gates are simple and robust; sub (tunnel) gates degate automatically; hot-tip and valve gates remove runner scrap on high-volume tools. Gate into the thickest section, away from cosmetic faces, and tell us which faces are show surfaces.
Undercuts and side actions
Snap fits, side holes, threads and windows perpendicular to the draw need a slide, lifter or collapsible core. Each one is a moving component in the tool: more cost, more maintenance, longer cycle. Many undercuts can be designed out with a pass-through core or a bump-off in a flexible resin.
Surface finish
Specify finish by SPI grade: A-1 to A-3 are polished (optical, show surfaces), B and C are semi-gloss and matte, D grades are bead-blast textures. Polished surfaces need more draft than you think to avoid drag marks, and textures need more still. Mold-Tech textures are specified by pattern number and depth.
Material notes and flame ratings
UL 94 ratings (HB, V-2, V-1, V-0, 5VA) are tested at a specific thickness; a V-0 resin at 3 mm may not be V-0 at 1.5 mm. Check the yellow card for the thickness you are molding. Glass fill improves stiffness and stability and worsens surface finish and tool wear. Hygroscopic resins (nylon, PC, PET) need proper drying or they lose properties and show splay.
Tolerances and shrink by material
Typical values for guidance. The PDF has the full table with flow direction, wall thickness and fill effects.
| Material | Shrink (typical) | Standard tolerance, 1 in. dimension | Fine tolerance, 1 in. dimension |
|---|---|---|---|
| ABS | 0.4 to 0.7% | ±0.005 in. | ±0.003 in. |
| Polycarbonate | 0.5 to 0.7% | ±0.005 in. | ±0.003 in. |
| PC/ABS | 0.5 to 0.7% | ±0.005 in. | ±0.003 in. |
| Nylon 6/6 (unfilled) | 1.0 to 2.0% | ±0.007 in. | ±0.004 in. |
| Nylon, 30% glass | 0.3 to 0.6% | ±0.004 in. | ±0.002 in. |
| Polypropylene | 1.2 to 2.5% | ±0.008 in. | ±0.005 in. |
| HDPE | 1.5 to 3.0% | ±0.009 in. | ±0.005 in. |
| Acetal (POM) | 1.8 to 2.5% | ±0.006 in. | ±0.003 in. |
| TPE / TPU | 1.0 to 2.5% | ±0.010 in. | ±0.006 in. |
| PEEK (unfilled) | 1.0 to 1.5% | ±0.005 in. | ±0.003 in. |
What is in the full PDF
- Nine design rules with diagrams
- Tolerance and shrink tables for 20 resins
- Draft, rib and boss calculators
- SPI surface finish chart
- UL 94 and material selection notes
- A one-page DFM review form you can send with any RFQ
Frequently asked questions
What draft angle do I need for injection molding?
One degree per side minimum on smooth surfaces, 1.5 to 2 degrees on light textures, 3 degrees or more on heavy textures and deep ribs.
What wall thickness should a molded part have?
1.5 to 3.5 mm for most engineering resins, kept as uniform as possible. Core out thick sections rather than leaving them solid.
What tolerances can injection molding hold?
About ±0.005 inch on typical dimensions and ±0.002 inch on critical features with tight process control, depending on the resin and the tool class.
Will you review my design for free?
Yes. Every quote includes a DFM review. Upload the part on the quote page and we will send notes with the pricing.
Or skip the reading and send us the part
Upload CAD and we will do the DFM review for you, with pricing, within one business day.
Phoenix, AZ 85040
480-256-9773
b.hanke@legacy-molding.com
Mon–Fri 8:00 AM – 5:00 PM