Custom Polypropylene (PP) Injection Molding
Polypropylene is the workhorse of injection moulding: light, chemically resistant and cheap enough to use in huge volumes, yet capable of living hinges that flex millions of times. We mould homopolymer, copolymer, glass filled and talc filled PP with the process control that high shrinkage demands, from our China workshop.
Free Consultation24h
Rapid Quote
100 pcs
Molding MOQ
1.0 to 2.5 percent
Shrinkage Control
1.5 to 3.0 mm
Wall Thickness
Materials
| Grade | Properties | Typical Applications |
|---|---|---|
| Homopolymer PP | Stiff, hard and chemically resistant with good fatigue strength at room temperature, but losing impact resistance below about 0 degree C. The lowest cost grade and the one for caps, closures and rigid containers. | Caps and closures, rigid containers, laboratory ware, living hinge bases where impact is not the dominant load |
| Block Copolymer PP | Ethylene modified for markedly better impact strength and low temperature toughness, with a small trade in stiffness and clarity. The standard choice for parts that must survive dropping or cold environments. | Living hinges, battery cases, automotive bumpers and trims, crates, tool bodies |
| Random Copolymer PP | Lower ethylene content giving improved transparency and a softer feel with good low temperature properties, used where appearance matters more than maximum stiffness. | Transparent food containers, laboratory and diagnostic disposables, cosmetic packaging, clear overlays |
| Glass Filled PP (10 to 30 percent) | Glass fibre raises stiffness and heat deflection temperature by tens of degrees and cuts mould shrinkage roughly in half, at the cost of surface abrasion and lower weld line strength. Shrinkage becomes anisotropic and must be modelled. | Structural brackets, appliance bases, fan housings, under bonnet parts, load bearing frames |
| Talc Filled PP | Mineral filled for dimensional stability, reduced creep and higher heat deflection at low cost, with a matte surface and lower coefficient of thermal expansion. The appliance industry default for large housings. | Washing machine and dishwasher panels, heater housings, liners, large appliance bodies |
| UV Stabilised and Impact Modified PP | Additive packages for outdoor weathering and very low temperature impact, used where the part lives outside or in a freezer. Colour and stabiliser selection depends on the service life required. | Outdoor furniture, garden equipment, cold chain containers, playground and water sport parts |
Process Parameters
| Parameter | Standard Range |
|---|---|
| Drying Requirement | Not required for virgin PP; surface moisture on regrind dried at 80 degree C for 1 to 2 hours if needed |
| Melt Temperature Window | 200 to 280 degree C depending on grade and filler, higher for glass filled |
| Mould Temperature | 20 to 60 degree C, higher for filled grades and large parts |
| Injection Pressure | 500 to 1,200 bar depending on flow length and wall |
| Recommended Wall Thickness | 1.5 to 3.0 mm uniform; living hinge 0.2 to 0.5 mm |
| Mould Shrinkage | 1.0 to 2.5 percent unfilled, 0.5 to 1.2 percent glass filled |
| Clamp Force Requirement | 2 to 4 tonnes per square centimetre of projected area |
| Cycle Time | 15 to 50 seconds per part depending on wall and size |
| Part Tolerance | plus or minus 0.05 mm standard, tighter by agreement |
| Maximum Moulded Part Size | up to 900 x 600 x 500 mm |
Surface Finishes
Moulded Colour and Texture
Colour compounded in the resin with a VDI 3400 textured cavity, the cheapest and most durable finish and the default for internal and industrial parts. PP takes colour and texture well and the finish does not scratch off.
SPI Polish (A-1 to A-3)
Polished cavity for gloss surfaces on copolymer and random copolymer grades where clarity or shine is wanted. Requires a steel cavity and good venting for defect free gloss.
In-mould Label (IML)
A pre-printed film placed in the mould before injection becomes part of the surface, giving scratch resistant decoration and labelling in one shot. Common on food containers and branded packaging.
Pad Printing and Hot Stamping
Logos, part numbers and markings applied after moulding; pad printing for detail, hot stamp for metallic effects on cosmetic grades.
Vapour and Chrome Effect
Metallisation over a lacquer base for a chrome look without electroplating, used on premium cosmetic and automotive trim parts.
Anti-scratch and Soft Touch Coating
A clear coat improving mar resistance and grip on handled parts such as appliance controls and tool grips.
Design for Manufacturing (DFM)
- Design for the high shrinkage of PP: at 1.0 to 2.5 percent it moves more than most plastics, so size the tool from the conditioned part and add ribs rather than thickening walls to hit stiffness
- Keep wall thickness uniform between 1.5 and 3.0 mm; PP sink marks are large and uneven walls warp, so thick local sections are the usual cause of rejected parts
- For living hinges use block copolymer with a thin uniform section of 0.2 to 0.5 mm and a generous radius, then fold the hinge a few times during finishing to orient the molecules and extend life
- Apply 1 to 2 degree draft, more on textured surfaces, because PP has low stiffness and drags on ejection if under drafted
- Place gates away from cosmetic and hinge areas; PP flow lines and gate vestige show on gloss surfaces, and a hinge gate interrupts molecular orientation
- Consider shrinkage anisotropy on filled grades and model warp rather than expecting it away, because glass and talc shrink differently along and across flow
- Avoid sharp corners with a minimum 0.5 mm radius to reduce stress concentration and sink at the root of features
- Plan for post-mould shrinkage when tolerancing; PP continues to settle for up to 24 to 48 hours, so critical dimensions are checked after conditioning
- Use talc or glass filled grades where heat deflection matters, since unfilled PP softens early and sags near hot components
- Upload STEP, IGES or DXF and receive a free material and DFM review within 24 hours covering wall, gate, draft and shrinkage risk
Tooling & Capacity
| Type | Lead Time | Suitable For |
|---|---|---|
| Aluminium Soft Tooling | 2 to 3 weeks | 25 to 1,000 pieces, pilot runs, living hinge trials and market testing where surface finish can be matte |
| Production Steel Tooling | 4 to 6 weeks | Volumes above roughly 10,000 pieces, gloss cavities, filled grades and long run cosmetic parts |
| Multi-Cavity Tooling | 5 to 7 weeks | High volume caps, closures and containers where cycle time and balanced fill dominate |
| Stack and Hot Runner Tooling | 5 to 8 weeks | Very high volume thin wall packaging where cycle time and material savings justify the tool investment |
Quality Assurance
- Resin grade verified on incoming material against the supplier certificate, with lot number recorded rather than assumed
- Melt temperature checked at the nozzle, because PP degrades if overheated and a machine setting is not the true material temperature
- First article inspection with dimensional report before the balance of the run is released
- Wall thickness and shot weight checked per cavity to catch short shots and flash early
- Shrinkage and warp verified on conditioned samples, since PP dimensions settle after moulding
- Visual inspection for sink, warpage, splay and burn on every setup change
- Living hinge flex tested on samples where the part has a cycle requirement
- Colour measured and recorded against the reference with delta E reported per lot
- Resin lot traceability retained per production run, with the supplier certificate filed against it
- Batch dimensional report, material certificate and Certificate of Origin shipped with every lot; third-party inspection by SGS, Bureau Veritas or TUV arranged on request
Applications
- Caps, closures and dispensing pumps for bottles and jars
- Living hinge enclosures, crates and fold-flat containers
- Automotive interior trim, bumpers and battery cases
- Appliance housings, panels, liners and heater bodies
- Laboratory and diagnostic equipment disposables and housings
- Food contact containers and packaging, grades selected per applicable regulation
- Industrial bins, trays, pallets and stacked storage
- Garden, outdoor and playground equipment
- Consumer goods, toys and cosmetic packaging
Why Choose FHY Precision
All four PP families moulded in house: homopolymer, copolymer, glass filled and talc filled, so the grade is matched to the load instead of to what the tool already exists for
Process control tuned to the high shrinkage of PP: tool sized from conditioned parts, ribs over thickness, and warp modelled on filled grades
Living hinge know-how: thin uniform copolymer sections folded to orient molecules, giving hinges that survive millions of cycles rather than dozens
Free material and DFM review within 24 hours, including grade selection where the best choice is not obvious
Pilot quantities from 25 pieces in aluminium tooling before committing to production steel
Moulded colour, IML and printing quoted with the moulding so the finished part is one purchase order
Resin lot traceability retained, with third-party inspection by SGS, Bureau Veritas or TUV available on request
Tooling and moulding in-house in China, no trading layer between quote and shipment
FAQ
1. What is the difference between homopolymer and copolymer polypropylene?
Homopolymer PP is stiffer, harder and cheaper, with good chemical resistance but poor impact below freezing. Block copolymer adds ethylene for much better low temperature toughness and drop resistance, at a small cost in stiffness and clarity. Random copolymer adds transparency and a softer feel for cosmetic and food containers. We pick the grade from the impact and appearance requirement rather than from price per kilogram.
2. Can you mould living hinges in polypropylene?
Yes, and PP is the best material for it. The hinge is a thin uniform section, typically 0.2 to 0.5 mm, in block copolymer, with a generous radius and a gate that does not interrupt molecular orientation. Folding the hinge a few times during finishing aligns the molecules and extends life to millions of cycles. We trial hinge geometry in aluminium tooling before production so the flex life is proven, not assumed.
3. When should I use glass filled or talc filled PP?
Use glass filled PP when stiffness and heat deflection govern the design: it roughly doubles rigidity and raises the softening point by tens of degrees, at the cost of surface abrasion and weaker weld lines. Use talc filled PP for large appliance housings where dimensional stability and low cost matter more than peak stiffness. Both cut shrinkage to about half of unfilled PP, which changes how the tool is sized.
4. Does polypropylene need drying before moulding?
Virgin PP is non-hygroscopic and normally needs no drying, which is one reason it runs cheaply. The exception is regrind that has picked up surface moisture, which we dry at about 80 degree C for one to two hours, and any filled grade stored in humid conditions. We check moisture on incoming material rather than assuming it, because surface water is what causes splay on gloss parts.
5. How do you control the high shrinkage of PP?
Two ways. First the tool is sized from the conditioned, settled part rather than from the drawing nominal, because PP moves 1.0 to 2.5 percent as it cools. Second we design for stiffness with ribs and gussets instead of thick walls, since thick sections sink and warp. On filled grades the shrinkage is anisotropic, so we model warp from the flow pattern rather than hoping it stays flat.
6. What wall thickness should I design for?
1.5 to 3.0 mm uniform for most parts. PP shows sink marks heavily, so uneven walls are the leading cause of rejected housings. Living hinges are the deliberate exception at 0.2 to 0.5 mm. Where extra stiffness is needed, add ribs at no more than 60 percent of wall thickness rather than thickening the wall, which would increase cycle time and sink.
7. How chemical resistant is polypropylene?
Very. PP resists most acids, alkalis and solvents at room temperature, which is why it dominates containers, laboratory ware and battery cases. It is not resistant to strong oxidising agents and some chlorinated and aromatic solvents at elevated temperature. We review the service fluid at DFM stage and select the grade and any coating accordingly, because chemical attack is a field failure mode worth designing out.
8. Can polypropylene be used for food contact parts?
Yes, with the correct grade. We select food contact approved PP and supply the resin maker documentation for the grade used, and we keep resin lot traceability per run. To be precise, the food contact approval belongs to the resin grade rather than to our factory, so what we provide is the correct grade and its supporting paperwork, plus the process controls that keep contamination out.
9. What is your minimum order quantity for PP parts?
Standard moulding MOQ is 100 pieces. For pilot runs and market tests we machine aluminium soft tooling and produce from 25 pieces, which is the fastest and cheapest way to get functional PP parts in hand. High volume caps and containers run in multi-cavity and hot runner tools where the economics shift toward cycle time.
10. How long does PP tooling take?
Aluminium soft tooling takes 2 to 3 weeks and suits pilot and low volume work. Production steel tooling takes 4 to 6 weeks. Multi-cavity and hot runner tools for very high volume packaging take 5 to 8 weeks because the runner balance and cooling layout decide the cycle time. Trial moulding and first article approval add about a week before release.
11. What surface finishes can you apply to PP?
Moulded colour with VDI 3400 texture is the default and most durable, followed by SPI polish for gloss on copolymer and random copolymer. In-mould labelling gives scratch resistant decoration in one shot for packaging, and pad printing or hot stamping add logos afterwards. A clear anti-scratch or soft touch coat improves grip and mar resistance on handled parts.
12. What tolerances can I hold on PP parts?
Plus or minus 0.05 mm is standard on critical features, tighter by agreement on short runs. The practical limit is shrinkage, not the machine: PP settles for 24 to 48 hours after moulding, so we measure critical dimensions on conditioned parts and we mark which dimensions are shrinkage controlled on your drawing during DFM review.
13. Can you mould PP automotive parts?
Yes. Block copolymer and filled PP are widely used for bumpers, trims, battery cases and under bonnet components where heat deflection permits. We select the grade for the temperature and impact load, model warp on filled parts, and keep lot traceability. Where a part sits near a hot component we move to talc or glass filled PP rather than expecting unfilled material to hold shape.
14. Do you use recycled or reground PP?
We can mould regrind where the specification allows it, and we dry and verify it before use. For cosmetic and food contact parts we mould virgin grade and keep the regrind stream separate so there is no cross contamination. The resin certificate and lot record tell you exactly what went into the run.
15. How does PP compare with HDPE or ABS for my part?
HDPE is more flexible and tougher at low temperature but harder to glue and print and it creeps more under load. ABS is stiffer and more cosmetic but less chemical resistant and more expensive. PP sits between: better chemical resistance than ABS, easier processing than PC, and good enough stiffness for most housings. We compare all three against your load, chemical and cost targets at review.
16. Do you arrange third-party inspection for PP parts?
Yes. We arrange SGS, Bureau Veritas or TUV inspection at any stage you specify, including incoming resin verification, dimensional checks and coating verification. For food contact and automotive programs this is often what your end customer needs, so we build the inspection point into the order rather than treating it as an afterthought.