Material selection comes down to one sentence: first check whether wall thickness and shrinkage rate can be matched, then check whether the surface process suits that material, and only last discuss flame retardancy, glass fiber, and welding. Get the order backwards and everything downstream is rework.
I once did a wall-mounted energy storage power supply housing. The client specified flame-retardant PC/ABS. The mold shop's samples came back badly shrunk — the snap positions were off by 0.6mm and wouldn't assemble. It took a while to find that it wasn't a mold problem: we had used a 0.5% shrinkage grade on a 2.5mm-wall main housing without giving the snaps their own shrinkage compensation. This article walks through the pitfalls of ABS, PC, flame-retardant grades, and glass-filled grades in the order I actually hit them.
What You'll Find Here
- Why shrinkage rate and wall thickness are the first gate in material selection, and how to calculate it correctly
- The hidden material requirements of no-paint, electroplating, and two-shot molding — and which materials simply can't be used
- The three areas with the most pitfalls: flame-retardant grades, glass-filled grades, and ultrasonic welding
1. Shrinkage Rate and Wall Thickness: The First Gate in Material Selection
Many junior designers start by asking "what strength does this product need" and then hunt the material handbook for the highest tensile strength. That approach works in the lab and gets you scolded by the mold shop in production. The real first gate is: can this material's shrinkage rate be matched with your wall thickness?
Common numbers: ABS shrinks 0.4%-0.7%, PC 0.5%-0.7%, PC/ABS alloy about 0.4%-0.6%, glass-filled grades drop to 0.2%-0.4% but with strong directionality. These numbers look small — but on a 200mm housing, 0.5% equals 1mm. If your snap clearance is only 0.3mm, shrinkage will cause interference.
Core answer: Before selecting a material, work backwards in the order "wall thickness determines shrinkage behavior, shrinkage rate determines dimensional compensation." Keep the main wall between 1.5mm and 3.0mm — ABS recommended 2.0-2.5mm, PC 2.0-3.0mm. Above 4mm, consider coring out or adding ribs. Don't just read the handbook for shrinkage: confirm the actual measured shrinkage of the specific grade with the mold shop, then leave clearance margin in the 0.3%-0.5% range.
On wall thickness, my rule: the thicker the wall, the harder shrinkage is to control and the higher the internal void risk. ABS starts showing sink marks above 3.5mm; PC can get internal bubbles above 4mm. We once made a handheld vacuum battery compartment housing with a 5mm local wall to fit the battery pack — the sample came out with an obvious surface depression that even paint couldn't hide, and we had to modify the mold with an insert to core it out. Since then I've set an internal rule: no plastic part exceeds 3mm main wall without coring.
Another pitfall is abrupt wall thickness changes. Jumping from 2mm to 4mm will always shrink at the junction. The correct approach is a gradual transition with a slope of at least 1:3, or split into separate parts. We made a beauty device where the top and bottom housings were originally one piece with a thick ring in the middle; after shrinkage the roundness was off by 0.4mm and assembly showed a step. Splitting it into two parts with uniform walls each solved it.
Ribs follow the same logic. Rib thickness is typically 0.5-0.7x the main wall — 0.6 for ABS, 0.5 for PC. Too thick and the root sinks; too thin and it's short-filled. Rib height shouldn't exceed 3x the main wall; beyond that, add support structure. These numbers aren't dogma, but they're the starting point — deviate far and you should ask yourself why.
2. Hidden Material Requirements of No-Paint, Electroplating, and Two-Shot Molding
Many designers only think about surface processes after the structure is finished. By then it's often too late, because some materials are inherently incompatible with certain processes.
No-paint. The essence of no-paint is mixing colorant or pearl powder directly into the plastic, relying on mold texture and flow marks for the effect. Its biggest hidden requirement: good flowability and no obvious weld lines. ABS is the friendliest material — good flow, even pigment dispersion. PC is poorly suited: high melt viscosity makes weld lines very visible where flow fronts meet, and pearl effects especially show "black lines." We made a no-paint pearl air purifier panel with PC first — weld lines were plainly visible. Switching to high-flow ABS with pearl masterbatch passed on the first try.
Another no-paint pitfall is gate location. The weld line forms opposite the gate; if it lands on the front-facing surface, the no-paint effect is ruined. So the gating plan for no-paint parts must be decided at the structure review stage, not adjusted after the mold is cut.
Electroplating. The material requirements are nearly rigid: it must be plating-grade ABS with butadiene content of 18%-23%. Ordinary ABS has poor plating adhesion and peels off. PC can't be plated alone, and PC/ABS with too high a PC ratio also fails — generally PC content must stay under 30%. Another hidden requirement is internal stress: plated parts must be annealed after molding, or the plating solution corrodes along stress lines and produces pitting.
A pitfall I hit: an electroplated decorative ring with a thin-wall snap. It went straight to plating without annealing and came back covered in pitting — over 300 pieces scrapped. We added an annealing step at 80°C for 2 hours and the problem was solved. That cost must be budgeted into the project.
Core answer: For no-paint, prefer high-flow ABS and avoid PC and long flow paths. For plating, use plating-grade ABS with PC content under 30%, and anneal after molding. Two-shot molding requires compatible materials with close shrinkage; common pairs are ABS+TPE and PC+TPU — rigid plastic as the body, soft plastic for sealing or grip. The interface needs mechanical interlock, not just chemical bonding.
Two-shot molding's core requirement is material compatibility and similar shrinkage. ABS+TPE is the classic pair with strong bond. PC+TPU also works, but don't pick too low a TPU hardness — too soft and it won't grip. Keep the shrinkage difference within 0.2%; beyond that the overmold layer warps. Structurally, always cut dovetail grooves or through-holes in the rigid part so the soft material locks mechanically — pure adhesion is unreliable in mass production.
3. The Three Areas with the Most Pitfalls: Flame Retardants, Glass Fiber, Ultrasonic Welding
I'm calling these out separately because each one can delay a project by more than a month.
Flame-retardant grades. The pitfall is "the flame rating passes but mechanical properties drop." Adding flame retardants makes the material brittle — impact strength can fall by more than 30%. V0-rated ABS may have only half the notched impact strength of ordinary ABS. So snap designs for flame-retardant parts must be loosened; don't use ordinary ABS engagement values. Another pitfall is mold corrosion, especially with halogen-based retardants — the mold needs hard chrome plating. We made a charger housing requiring V0, initially in halogen-based flame-retardant PC/ABS. The mold showed corrosion pitting before 50,000 shots. Switching to phosphorus-based retardant restored mold life but raised material cost — plan that trade-off early.
Glass-filled grades. The pitfalls are anisotropy and appearance. PA or PBT with 30% glass shrinks 0.3% in the flow direction and 0.8% across it — more than double. Apply an average 0.5% and long parts will warp. The correct approach is separate shrinkage by flow direction, or confirm with mold flow analysis. On appearance, glass-filled materials show floating fibers and can't achieve a high gloss — only texture or paint can hide it. We once made a power tool body specified in glass-filled PA but requiring a matte fine texture; floating fibers were obvious in samples, and we finally switched to a lower glass content grade with texturing.
Glass fiber also has a hidden effect on ultrasonic welding: it weakens energy transmission, so weld strength is notably lower than unfilled material. If a glass-filled part must be ultrasonically welded, widen the weld line, raise the amplitude, and test the parameters.
Ultrasonic welding. Most pitfalls are in material and weld line design. On materials: amorphous plastics (ABS, PC, PMMA) weld well; crystalline plastics (PP, PE, POM) are difficult because they solidify too fast after melting to fuse. To ultrasonically weld PP you must use a dedicated weldable grade or change the joining method. On weld lines: the common triangular line is 60° with 0.3-0.5mm height, but one parameter set doesn't fit all — ABS and PC need separate tuning. Most important: the weld face must have a locating feature; relying on the weld line alone for alignment means it will shift.
Core answer: Flame-retardant grades need margin for reduced mechanical performance — snap engagement 20% lower than ordinary material, and consider hard chrome plating for molds. Glass-filled grades must have direction-specific shrinkage, and appearance surfaces can only hide floating fibers with texture. For ultrasonic welding, prefer amorphous plastics; crystalline plastics need dedicated weldable grades. Weld lines at 60° and 0.3-0.5mm height, with an independent locating structure.
What Designers Can Take Away
Material selection isn't a multiple-choice exam from a handbook — it's calculating four variables together: wall thickness, shrinkage, surface process, and joining method. My requirement for the team: before selecting, draw a table and fill in the corresponding data for each variable across candidate materials. If it can't be filled in, don't select it. In that table, get measured shrinkage from the mold shop, confirm process compatibility with the supplier, and confirm the parameter window with the process engineer. Three phone calls can settle what would otherwise be rework after the mold is cut. Eighty percent of the pitfalls we hit over ten years could have been caught by that table.
FAQ: Common Questions on Material Selection
Q: How do I choose between ABS and PC?
A: Look at transparency and strength requirements. ABS is cheap and easy to mold but opaque; PC is transparent with high strength but poor flow and high internal stress. Choose PC for transparency or high impact — but keep walls under 4mm and anneal. Choose ABS for no-paint, plating, or complex structures. To balance strength and moldability, use PC/ABS alloy with PC content between 30% and 70%.
Q: Is flame-retardant material always more brittle than ordinary material?
A: Usually yes — flame retardants reduce impact strength, but systems differ widely. Halogen-based retardants are efficient but hurt mechanical properties and corrode molds; phosphorus-based are gentler but cost more. Ask your supplier for measured impact strength data at the flame-retardant grade, and design snap engagement at 80% of ordinary material.
Q: How do I deal with floating fibers on visible glass-filled parts?
A: Floating fibers can't be fully eliminated — only hidden with texture, paint, or a lower glass content grade. High-gloss surfaces should basically never use glass-filled material. If you must, choose a 10%-15% glass grade, use fine texturing (VDI 27-30), and add a sandblasting pre-treatment if needed. Higher mold temperature helps somewhat, but the effect is limited.
Q: Can PP be ultrasonically welded?
A: Ordinary PP is very difficult — you need a dedicated weldable grade or a design change. PP is crystalline and solidifies too fast after melting, leaving an extremely narrow welding window. You can choose PP with specific additives for ultrasonic weldability, or switch to snaps, screws, or heat staking. If welding is mandatory, widen the weld line to over 1.0mm and determine amplitude and time through extensive trials.
Q: Do electroplated parts always need annealing?
A: For high-gloss plating or structurally complex parts, yes — otherwise pitting and blistering risk is high. Annealing is typically 70-80°C for 2-4 hours depending on part thickness. Annealing relieves molding internal stress and prevents plating solution from corroding along stress lines. Thin, simple parts can skip it, but the savings are far less than the scrap cost.
This article is adapted from the Hezi Industrial Design official website (hezidesign.com), "Structural Design Field Notes" column.
Top comments (0)