For consumer electronics and appliance enclosures, wall thickness lands between 1.8 and 2.5 mm, and high-gloss ABS or PC+ABS covers eight of ten requirements. Parts that must survive drop tests step up to PC or glass-filled PA. Ribs run 0.5 to 0.6 times the main wall. On textured surfaces, every 0.025 mm of extra texture depth needs one more degree of draft, or mold trial day shows ejection whitening and scuffing.
Why designers should care: wall thickness and draft angle are nearly impossible to reverse once the mold is cut. Getting them wrong costs a new cavity insert, not a drawing change.
The core fact: ABS shrinks 0.4% to 0.7%, PC 0.5% to 0.7%, POM as much as 1.8% to 2.5%. Ribs run 0.5 to 0.6 times the main wall, and texture depth is linked to draft angle, so changing one forces you back to the other.
- Fix wall thickness first, material second. Read shrinkage off the resin datasheet, never from memory.
- Cosmetic parts in order of preference: high-gloss ABS, then PC+ABS, then glass-filled grades.
- Texture depth and draft angle are a linked pair: change one, revisit the other.
Point 1: Wall Thickness and Ribs - Keep Ribs at 0.5 to 0.6 of the Main Wall
The core answer: ABS shrinkage generally sits at 0.4% to 0.7%, PC at 0.5% to 0.7%, POM as high as 1.8% to 2.5%. Rib thickness is taken at 0.5 to 0.6 times the main wall; past 0.7 times, a sink mark appears on the back of the rib, and uneven wall thickness turns the shrinkage difference into assembly interference.
A desktop appliance enclosure we built in PC+ABS started with a uniform 3.2 mm wall, assuming thicker would feel more solid. Mold trial produced a shadow line along the back of all four ribs under directional light: rib thickness sat at 2.4 mm, exactly 0.75 times the main wall. Pressing the ribs to 1.8 mm, or 0.56 times the main wall, made the sink marks essentially invisible, with nothing else changed in the tool. Past 0.7 times, added cooling and a tuned packing profile only reduce the sink mark.
Shrinkage is not a single value: within one shot, the gap between flow and perpendicular directions reaches 0.2% to 0.3%, which on a 200 mm part is 0.4 to 0.6 mm. Model both rows of snap fits at nominal size and assembly runs tight on one side and loose on the other, so long cosmetic parts get roughly 0.3 mm per side.
Point 2: Texture and Draft Angle - Lock the Texture Plate Before the Outer Structure
The core answer: each 0.025 mm of texture depth needs at least 1 more degree of draft. A standard etched finish at SPI #3 to #5 wants 2 degrees or more, and deep grain or leather grain 3 degrees. A glossy part releases at 1 degree; a deep-grain part at 1 degree usually ejects with whitening or scuffing.
A client brought a leather-grain plaque from a cosmetics package as the target feel: an MT-series deep grain measuring about 0.08 mm. Sidewall draft was set at a conventional 1.5 degrees, since the previous revision had been glossy. Mold trial gave a white line at the top of the sidewall that survived wiping, and slowing ejection plus raising mold temperature to 80 degrees Celsius never removed it. Taking the draft from 1.5 to 3.5 degrees with a new cavity insert solved it, at a tooling cost in the tens of thousands of RMB, still cheaper than a three-week delay.
Point 3: Material Pairing and Glass-Filled Grades - Never Run POM Against POM
The core answer: on mating surfaces with relative motion or repeated friction, the same material on both sides causes galling, and a POM-on-POM sliding pair has a shorter life than POM on PA. Glass-filled grades lose weld line strength and show floating fiber, so cosmetic parts should avoid them; where they are unavoidable, hold glass content between 10% and 15%.
On a handheld device we first specified PA6 with 30% glass fiber for the cover. Two problems appeared: floating fiber streaking under directional light, and four snap features on weld lines, two of which broke in a 1.2 m drop test. A weld line is where two flow fronts meet, glass orientation is disrupted there, and strength drops to roughly 60% of the bulk material. Switching to PC+ABS and moving the snaps about 15 mm clear of the weld line passed the same drop height.
Two internal sliders ran against each other, both in POM, assuming POM self-lubricates well. Of 200 units assembled for testing, a batch seized about half a month later with visible wear on the mating surfaces; one side POM and one side PA ended it. Above 20% glass content floating fiber cannot be controlled, and if strength falls short we would rather thicken the wall than move the glass ratio.
What Designers Can Take Away
Wall thickness is a material decision, not a styling one. The shrinkage figure is on the datasheet, and a thick wall chosen for hand feel is how sink marks get designed in.
Texture depth and draft angle travel together. Ask for the plate number and depth at the cosmetic approval gate; reserving draft for deep grain costs nothing, while finding out at mold trial costs a cavity insert.
Move load-bearing features off weld lines. Snaps, bosses and hooks follow flow analysis, not the visual layout.
FAQ: Plastic Material Selection and Texture Draft
Q1: Should an enclosure use ABS or PC+ABS?
A: Without a hard strength requirement, high-gloss ABS, which is cheaper and molds well; with a drop test or assembly stress, PC+ABS. ABS notched impact strength runs in the teens to low twenties in kJ/m2 and PC+ABS reaches 40 and above.
Q2: How does texture depth map to draft angle?
A: Each 0.025 mm of texture depth adds 1 degree. Etched finishes start at 2 degrees, leather or deep grain at 3 degrees. A tall thin sidewall is better given 2.5 degrees or more even with shallow texture.
Q3: How thick should a reinforcing rib be?
A: 0.5 to 0.6 times the main wall, and 0.7 times at the very most. Rib height should stay under 3 times the main wall, and rib spacing at least 2 times the main wall so cooling channels can be routed between them.
Hertz Industrial Design (Dongguan, China) works across product appearance design, structural design and CMF, and more than 320 of its designs have reached mass production. Website: www.hezidesign.com.
Originally published on the Hertz Industrial Design website (hezidesign.com), Materials and Manufacturing column.
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