Plastic Structural Design 1—Materials

Time:2024-03-28 18:52:09 / Popularity: / Source:

For series of  Plastic Structure Design, Please refer to Series 1, 2, 3, 4, 5, 6, 7.
Plastic structure design series stipulates basic design principles, basic design knowledge and basic mold-related knowledge of plastic structures, which can guide mechanical engineers to quickly and accurately complete structural design of plastic products. This article mainly talks about selection of plastic materials and introduction of key points of material mold design; details are as follows:
Plastic Structural Design 

(1) Introduction to plastic material selection

Main component of plastic is synthetic resin. Term resin was originally named after lipids secreted by animals and plants, such as rosin, shellac, etc. Currently, resin refers to a polymer that has not been mixed with various additives. Resin accounts for approximately 40% to 100% of the total weight of plastic. Basic properties of plastics are mainly determined by nature of resin, but additives also play an important role. Some plastics are basically composed of synthetic resins with no or less additives, such as plexiglass, polystyrene, etc. So-called plastic is actually a kind of synthetic resin. Its shape is similar to pine resin in natural resin, but it is called plastic because it is synthesized through power of chemistry.
Various plastics are shown in table below:
Scientific name Abbreviation Commonly known as Recycling logo Usage
Polyethylene PE     Low temperature items
Polypropylene PP 100% glue, plastic 05 Microwave lunch box, can be used at around 100℃
High Density Polyethylene HDPE Hard soft glue 02 Cleaning supplies, bath products
Low Density Polyethylene LDPE   04 Cling film, plastic film, etc.
Linear Low Density Polyethylene LLDPE      
Polyvinyl Chloride PVC Vinyl 03 Rarely used in food packaging
General Purpose Polystyrene GPPS Hard glue    
Expansible Polystyrene EPS Styrofoam    
High Impact Polystyrene HIPS Impact resistant hard glue    
Styrene-Acrylonitrile Copolymers AS, SAN Transparent strong glue    
Acrylonitrile-Butadiene-Styrene Copolymers ABS Super unbreakable glue    
Polymethyl Methacrylate PMMA Acrylic Plexiglass    
Ethylene-Vinyl AcetateCopolymers EVA Rubber glue    
Polyethylene Terephthalate PET Polyester 01 Mineral water bottles, carbonated drink bottles
Polybutylene Terephthalate PBT      
Polyamide(Nylon 6.66) PA Nylon    
Polycarbonates PC Bulletproof glue 07 Kettles, cups, bottles
Polyacetal POM Saigang, Duugang    
Polyphenyleneoxide PPO Noryl    
Polyphenylenesulfide PPS Polyphenylene sulfide    
Polyurethanes PU Polyurethane    
Polystyrene P.S. Polystyrene 06 Bowl instant noodle box, fast food box
Polytetrafluoroethylene PTFE Teflon, Teflon    

(2) Classification of commonly used plastic materials and related performance tables

Plastics are classified as follows:
Plastics can be divided into three types according to their uses: general plastics, engineering plastics, and special plastics; according to physical and chemical classification, they can be divided into two types: thermosetting plastics and thermoplastic plastics; according to molding methods, they can be divided into molding, lamination, injection, and blow molding , extruded, cast plastics and reaction injection plastics and other types;
Properties of commonly used plastics are shown in table below
Name Advantage Shortcoming Key points of mold design Application
1. High impact strength and good creep resistance.
2. Good heat resistance, low embrittlement temperature (-130℃), and can resist influence of sunlight, rain and temperature changes.
3. Good chemical properties and high transparency.
4. Good dielectric properties.
5. Good dimensional stability.
1. Poor solvent resistance.
2. There is stress cracking.
3. It is easily hydrolyzed if immersed in boiling water for a long time.
4. Poor fatigue strength.
1. Due to poor fluidity of PC, size of runner system and gate should be larger, side gates, sector gates, and ear-protecting gates are preferred.
2. Due to high melt viscosity, cavity material is required to be relatively wear-resistant.
3. Solidification speed of melt is relatively fast, and unbalanced flow has a significant impact on filling process. In order to prevent stagnation, cavity should be filled in a good order.
4. PC is more sensitive to notches, so wall thickness of product is required to be uniform, existence of sharp corners and gaps should be avoided as much as possible. Arc transitions should be used at corners, and arc radius should not be less than 1.5mm.
5. In order to prevent poor exhaust during molding process, it is necessary to open an exhaust slot with a depth of less than 0.04mm.
Used in instrument covers with a wide temperature range, parts in aircraft, automobiles, and electronics industries, textile reels, carburetors, timer components, safety helmets, impact-resistant aviation glass, etc. Also commonly used in daily necessities.
Acrylonitrile-butadiene-styrene (ABS) 1. It has good mechanical properties and thermal properties, high hardness, and surface can be easily plated with metal.
2. Resistant to fatigue and stress cracking, and high impact strength.
3. Resistant to chemical corrosion such as acid and alkali.
4. Lower price.
5. Easy to process, shape and modify.
1. Poor weather resistance.
2. Heat resistance is not ideal
3. Not resistant to organic solvents.
In order to prevent defects such as poor exhaust, burns, and weld marks during mold filling process, it is required to open an exhaust groove with a depth of less than 0.04mm. Machine covers, covers, instrument casings, hand drill casings, fan impellers, radio, telephone and television casings, some electrical parts, automobile parts, machinery and conventional weapons parts.
Plastic alloy (ABS+PC) 1.Excellent UV resistance
2.Good impact strength
3. Excellent molding and processing performance
4. High temperature resistance (80~120℃)
5. Flame retardancy.
  In order to prevent poor exhaust during molding process, it is necessary to open an exhaust slot with a depth of less than 0.04mm.
Used in automotive interior and exterior parts, computers and interface equipment, communication equipment, and home appliances.
Low pressure high density polyethylene (HDPE) 1. Excellent electrical insulation.
2. It has good wear resistance, water impermeability and chemical resistance.
3. It has good low temperature resistance and is still flexible at -70℃.
1. Poor resistance to sudden cooling and sudden heating.
2. Mechanical strength is not high and thermal deformation temperature is low.
1. Polyethylene molecules are oriented and prone to warping and distortion. When designing mold, attention should be paid to determination of gate position and selection of shrinkage rate.
2. Polyethylene is soft and smooth in texture and easy to demould. For products with shallow grooves on side wall, forced demoulding can be used.
3. Due to good fluidity of polyethylene, depth of exhaust groove should be controlled below 0.03mm.
It is mainly used to make hollow products such as blow molded bottles, and secondly used for injection molding to make turnover boxes, cocks, small load gears, bearings, electrical component brackets, etc.
High pressure low density polyethylene(LDPE) 1. Excellent electrical insulation properties and chemical resistance.
2. Its softness, elongation, impact resistance and light transmittance are better than HDPE.
1. Mechanical strength is slightly poor,
2. Poor heat resistance,
3. Not resistant to light and heat aging.
Same as HDPE. Extruded packaging films, sheets, packaging containers, wire and cable coverings, soft injection molding and extruded parts.
Polypropylene (PP) 1. Fluidity at melting temperature.
2. Heat resistance is very good.
3. High yield strength and high bending fatigue life.
4.Low density
1. Molding shrinkage rate is large.
2. Poor rigidity.
3. Poor weather resistance.
1. Molding shrinkage rate is large. When selecting gate position, melt should fill cavity in a more balanced flow order to ensure that shrinkage of product is consistent in all directions.
2. For products with hinges, attention should be paid to selection of gate location, and flow direction of melt is required to be perpendicular to axis of hinge.
3. Due to good fluidity of PP, depth of exhaust groove should not exceed 0.03mm.
Tableware, kettles, etc. and medical equipment that require high temperature sterilization.
Polyoxymethylene (POM) 1. Tensile strength is higher than that of ordinary nylon, and it is resistant to fatigue and creep.
2. Good dimensional stability.
3. Less water absorbent than nylon.
4. Good dielectric properties.
5. Can be used normally at 120℃.
6. Small friction coefficient.
7. It has excellent elasticity and acts like a spring.
1. No self-extinguishing property.
2. Molding shrinkage rate is large.
3. Fast solidification speed.
1. In molten state, solidification speed is fast, crystallinity is high, and volume shrinks greatly. In order to meet normal filling and pressure holding, gate size is required to be larger and flow balance is better.
2. It has good rigidity but insufficient toughness. Arc-shaped gate is not suitable for POM to prevent gate from breaking and preventing normal demoulding.
3. To prevent POM from decomposing and corroding cavity, cavity material should be corrosion-resistant.
4. POM melt has good fluidity. In order to prevent defects such as poor exhaust, welding marks, burns and discoloration, mold is required to have a good exhaust groove with a depth of no more than 0.02mm and a width of about 3mm.
Various gears, bearings, bushings, cages, automobiles, agricultural machinery, plumbing parts, etc.

(3) Main injection molding process parameters of common plastics

As shown in table below
Commonly used plastics Process parameters
Barrel temperature/drying process Melt temperature/℃ Mold temperature/℃ Injection pressure Injection speed Gate system
PVC Usually no drying is required. 185~205 20~50
Can be as large as 150MPa To avoid material degradation, a relatively high injection speed is generally used. All conventional gates can be used. If you are processing smaller parts, it is better to use a pinpoint gate or a submersible gate; for thicker parts, it is better to use a fan gate. Minimum diameter of pinpoint gate or submersible gate should be 1mm; thickness of fan gate should not be less than 1mm.
PC PC material is hygroscopic and drying before processing is important. Recommended drying conditions are 100~200℃, 3~4h. Humidity before processing must be less than 0.02%. 260~340 70~120 Use as high an injection pressure as possible. Use low speed injection for smaller gates and high speed injection for other types of gates. Small products can use needle gates, and gate depth should be 70% of the thickest part. Other gates include ring and rectangular gates.
The larger gate, the better to reduce defects caused by excessive shearing of plastic.
HDPE Feeding area 30~50℃ (50℃)
Zone 1 160~250℃ (200℃)
Zone 2 200~300℃ (210℃)
Zone 3 220~300℃ (230℃)
Zone 4 220~300℃ (240℃)
Zone 5 220~300℃ (240℃)
Nozzle 220~300℃ (240℃)
220~280 20~60 It has good flow properties and avoids using excessively high injection pressure of 80-140MPa; except for some thin-walled packaging containers, which can reach 180MPa. Thin-walled packaging containers require high injection speeds, and medium injection speeds are often more suitable for other types of plastic products. Point gate; heated hot runner, thermally insulated hot runner, inner pouring sleeve; cross-sectional area is relatively small, which is sufficient for thin-section products during machine downtime, and there is no need to use other materials for special cleaning work.
PP Feeding area 30~50℃ (50℃)
Zone 1 160~250℃ (200℃)
Zone 2 200~300℃ (220℃)
Zone 3 220~300℃ (240℃)
Zone 4 220~300℃ (240℃)
Zone 5 220~300℃ (240℃)
Nozzle 220~300℃ (240℃)
220~280 20~70 It has good flow properties and avoids using excessively high injection pressure of 80 to 140MPa; except for some thin-walled packaging containers, which can reach 180MPa. Thin-walled packaging containers require high injection speeds (with accumulators), and medium injection speeds are often more suitable for other types of plastic products. Point gate or multi-point gate; heated hot runner, thermal insulation hot runner, inner gate; gate position should be at the thickest point of product, otherwise large shrinkage is likely to occur.
PS Feeding area 30~50℃ (50℃)
Zone 1 160~250℃ (200℃)
Zone 2 200~300℃ (210℃)
Zone 3 220~300℃ (230℃)
Zone 4 220~300℃ (230℃)
Zone 5 220~300℃ (230℃)
180~280 40~50 20~60MPa It is recommended to use high injection speed. All conventional gate types can be used.
PET Drying before processing is necessary because PET is highly hygroscopic. Recommended drying conditions are 120 to 165℃ and 4 hours of drying. Required humidity should be less than 0.02%. For non-filled type, it is 265~280℃; for glass-filled type, it is 275~290℃. 80~120 30~130MPa Higher injection speeds can be used without causing embrittlement. All conventional types of gates can be used. Gate size should be 50% to 100% of thickness of plastic part.
POM If material is stored in a dry environment, drying is usually not required. Homopolymer materials are 190~230℃; copolymer materials are190~210℃ 80~105 70~120MPa Medium or high injection speed Any type of gate can be used. If a tunnel gate is used, it is better to use shorter type. For homopolymer materials, it is recommended to use a hot nozzle runner. For copolymer materials, either an internal hot runner or an external hot runner can be used.
Polyamide 6 or nylon 6 (PA6) Special attention should be paid to drying before processing. If material is packaged for use in waterproof material, container should remain sealed. If humidity is greater than 0.02%, it is recommended to dry it in hot air above 80℃ for 16 hours. If material has been exposed to air for more than 8 hours, it is recommended to perform vacuum drying at 105℃ for more than 8 hours. For non-enhanced varieties, it is 230 ~ 280℃, for enhanced varieties, it is 250 ~ 280℃ 80~90 75~125MPa High speed (slightly lower for reinforced materials) Gate hole diameter should not be less than 0.5t (t is thickness of plastic part). If using a hot runner, gate size should be smaller than if using a conventional runner because hot runner can help prevent premature solidification of material. If a submersible gate is used, minimum gate diameter should be 0.75mm.
PMMA Drying conditions are 90℃, 2~4h 240~270 35~70   Medium injection speed  

For read more about this series, please read: Plastic Structural Design - Mold (Introduction).

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