Can these be injection molded? PTFE, PVC, Rubber, Silicone, Polypropylene, Polylactic Acid, and Polyethylene Terephthalate

Introduction

Injection molding is a manufacturing process in which a molten material is injected into a mold cavity under high pressure and allowed to cool and solidify into a desired shape. This report aims to comprehensively analyze the feasibility and specific considerations of injection molding for seven common industrial materials: polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), rubber, silicone, polypropylene (PP), polylactic acid (PLA), and polyethylene terephthalate (PET). The suitability of injection molding depends largely on the unique physical and chemical properties of the material, which determine the required processing conditions and achievable part characteristics.

Overview:

PTFE Injection Molding

PTFE is a high-performance polymer known for its excellent chemical resistance, low friction, and thermal stability. Its unique molecular structure gives it a high melting point of approximately 327°C (621°F). However, even above its melting point, PTFE does not flow as easily as other thermoplastics, but becomes a rubbery elastomer and is very shear-sensitive in its amorphous state, prone to melt fracture. PTFE also has an extremely high melt viscosity, and is able to maintain its original shape in the molten state, similar to a gel that does not flow. In addition, PTFE has a non-stick surface.

Due to its high melt viscosity and non-flowability, conventional injection molding methods are not suitable for PTFE. PTFE behaves very differently in the molten state than typical thermoplastics, which decrease in viscosity as temperature increases, making them easy to inject. In contrast, PTFE's high viscosity and gel-like state mean that pressure alone is not enough to make it flow into complex mold cavities in conventional equipment. PTFE also has a high thermal expansion rate and poor thermal conductivity, which can cause 2-5% shrinkage and part warping if not properly controlled during the molding process. In addition, PTFE requires very high injection pressures (over 10,000 psi) and is prone to damage during demolding due to its high surface energy, requiring careful handling and specialized mold design. PTFE parts also often require additional processing, such as annealing or machining, and the high reactivity of PTFE with mold materials can result in a shortened mold life, requiring frequent maintenance or replacement of specialized equipment.

Despite these challenges, PTFE can still be molded using some specialized techniques. Press molding is currently the most widely used PTFE molding process. The method involves uniformly filling PTFE powder into a mold and then compressing it at a pressure of 10 to 100 MPa at room temperature. The compressed material is then sintered at a temperature of 360°C to 380°C (680°F to 716°F) to bond the particles together. Depending on different needs, press molding can be divided into ordinary press molding, automatic press molding, and isostatic pressing. **Push molding (paste extrusion)** is another method, in which a 20-30 mesh screened resin is mixed with an organic additive into a paste, pre-pressed into a billet, and then extruded in a push press, and finally dried and sintered. Screw extrusion uses a special extruder design in which the screw mainly plays a conveying and pushing role, sintering and cooling the PTFE powder through the die head. Isostatic pressing is to fill the PTFE powder between the mold and the elastic mold, and then press the powder from all directions by fluid pressure to make it combined, which is suitable for products with complex shapes. It is worth noting that KingStar Mold claims that PTFE injection molding can be performed, but they emphasize that this requires specialized equipment and technology, such as using fine powder or granular PTFE, and may involve compression molding or plunger extrusion before injection to ensure that the material flows and forms complex shapes. This shows that although there are inherent difficulties in directly processing PTFE using traditional injection molding processes, a certain degree of "injection molding" may be achieved through improved methods such as injection preforming or specially formulated PTFE materials.

PTFE molded parts are widely used in applications that require excellent chemical resistance, low friction, and high thermal stability, such as seals, gaskets, and electrical insulation. Due to its excellent chemical resistance, PTFE is also widely used in the chemical industry. Its high temperature stability makes it indispensable in parts that require durability under extreme conditions in the aerospace and automotive sectors. PTFE's low friction makes it ideal for parts that require smooth movement and minimal wear, such as bearings, seals, and gaskets. Due to its biocompatibility, PTFE is also suitable for medical applications.

Polyvinyl chloride (PVC) injection molding

Polyvinyl chloride (PVC) is a versatile thermoplastic that can produce a variety of parts through the injection molding process. PVC is non-hygroscopic and has good chemical resistance. It can be divided into hard PVC and soft PVC, and soft PVC is made more flexible by adding plasticizers. PVC is usually supplied in granular or powder form and needs to be melted before processing. The injection molding process involves injecting molten PVC into a mold cavity under high pressure and then cooling and solidifying it into the desired shape. Typical melt temperatures range from 160-190°C and should not exceed 200°C. Mold temperatures are usually maintained at 20-70°C. Injection pressure should be above 90MPa, and holding pressure is usually between 60-80MPa. To avoid surface defects, moderate injection speeds are usually used. PVC has a relatively low shrinkage of 0.2% to 0.6%, but uneven shrinkage during cooling can cause warping. To ensure smooth demolding of the part, a draft angle of 0.5% to 1% is recommended in PVC part design.

PVC injection molding has several advantages, including high cost-effectiveness. Compared with other specialty plastics and polymer blends, PVC is a common injection molding material with a lower price. It has good chemical resistance to many acids, bases, salts, fats and alcohols, and is a good electrical insulator. PVC is also flame retardant and water-resistant, and is durable, easy to color and recycle. However, PVC also has some disadvantages. It has poor thermal stability, starts to degrade above 60°C, and decomposes into harmful byproducts when overheated, such as hydrochloric acid (HCl), which is extremely corrosive. PVC also has a relatively low heat distortion temperature, deforms under load above 82°C, and loses strength at higher temperatures. In addition, PVC may wear when exposed to oxidizing acids.

PVC injection molding is widely used in various fields, such as for the production of pipes, fittings, and housings. Other common applications include adapters, RV parts, computer housings and components, and doors, windows, and machine housings in the construction field (rigid PVC). Soft PVC is mainly used to make medical catheters, car interiors, and garden hoses. In the automotive industry, PVC injection molding is used to make parts such as dashboards, interior panels, and sealing strips. Many household items, such as containers and furniture parts (excluding drinking glasses and washbasins that come into direct contact with the human body), can also be made using PVC injection molding. PVC is also widely used in the electronics, medical, and industrial fields. Other applications include toys, hoses, decorative displays, and labels.

Rubber Injection Molding

Rubber injection molding is a process where uncured rubber is injected into a metal mold cavity and then vulcanized (cured) under heat and pressure to form a usable product. This method is applicable to both natural and synthetic rubber. The general rubber injection molding process involves feeding uncured rubber into the injection molding machine, heating it to liquefy it to a gel state, then injecting it into the mold cavity through runners and gates, vulcanizing it under high pressure and temperature to crosslink the polymer chains, and finally cooling and ejecting it from the mold.

Injection molding has several significant advantages over traditional rubber molding methods such as compression molding and transfer molding. It is able to produce products with higher precision and tighter tolerances and allows the design of more complex and delicate geometries. The production cycle of injection molding is generally shorter, and in many cases, pre-molding is not required, which reduces material waste and flash. In addition, injection molding can accommodate a wider range of rubber hardness (Shore hardness) and can better achieve material flow and mold filling. The process also has the potential for automation, which reduces labor costs and can achieve better surface finish. Due to its speed and precision, injection molding is well suited for mass production of rubber parts and the ability to produce overmolded parts (bonding of rubber to metal).

There are a variety of natural and synthetic rubbers suitable for injection molding. Natural rubber has high tensile strength as well as good friction and wear properties. However, due to its high viscosity and sensitivity to temperature, injection molding of natural rubber requires specific techniques. There are many different types of synthetic rubbers, each with unique properties suitable for different applications. Nitrile rubber (NBR) has excellent resistance to oils, solvents, water, and abrasion. Ethylene-propylene-diene monomer rubber (EPDM) has enhanced resistance to light, ozone, and heat, making it ideal for outdoor applications. Neoprene is widely used and has fire, weather, temperature, and wear resistance. Silicone rubber has excellent heat resistance, high and low temperature flexibility, and biocompatibility (which will be discussed in detail in the silicone section). Fluorosilicone rubber has excellent resistance to fuels, chemicals, and oils. Thermoplastic elastomers (TPEs) combine the properties of plastics and rubbers, flow easily when heated, and can be recycled, including TPR, TPU, and TPV. Hydrogenated nitrile rubber (HNBR) has high resistance to petroleum-based oils and is widely used in the automotive field. Butyl rubber has low gas and moisture permeability and is suitable for vacuum and high-pressure gas systems. Styrene-butadiene rubber (SBR) is a common synthetic rubber with good wear resistance. Isoprene rubber is the best choice if color is important. Fluororubber (Viton/FKM) has excellent heat and chemical resistance and is suitable for extreme environments.

Rubber injection molding is widely used in various industries, such as for the manufacture of seals, gaskets, O-rings, rubber plugs, and pipes. In the automotive industry, it is used to produce transmissions, engine parts, valves, extrusions, as well as instrument panels, interior panels, and seals. The defense industry uses rubber injection molding to manufacture weapon parts, shock and noise reduction parts, and seals. In mass transportation, it is used for brakes, steering systems, tubing, wire insulation, and engine parts. Rubber injection molding is also used to make household appliances, electrical components, building components (such as shock absorbers and sealing gaskets), medical devices, and rubber handles on kitchen utensils and tools. In food processing and manufacturing, natural rubber is often used to produce shock absorbers on production lines. Due to its wear resistance, natural rubber is also commonly used in the railway and defense industries and is nuclear certified. Its wear resistance also makes it suitable for speed bumps in the transportation industry.

Silicone injection molding

Silicone injection molding is mainly divided into two types: liquid silicone rubber (LSR) injection molding and high consistency rubber (HCR, also known as solid silicone rubber) injection molding. LSR is a low viscosity platinum-cured silicone rubber that requires a cooled barrel and heated mold. It is a two-component system where the A and B components are mixed before injection. HCR has a higher viscosity, is usually peroxide cured, requires a heated barrel and mold, and has a longer cure time. HCR is supplied as a pre-mixed compound or as a base component that needs to be mixed.

The LSR injection molding process involves metering two liquid components (base silicone and catalyst) together (pigment is often added) and feeding them into a cooled injection barrel. The mixture is injected into a heated mold (usually 150-200°C or 275-390°F) where rapid vulcanization occurs. LSR production cycle times are very short, typically 30 seconds to 2 minutes. The process is usually automated, produces minimal flash ("flashless" technology), and often uses automatic demolding systems. In contrast, the HCR injection molding process involves feeding solid silicone rubber (in blocks, strips, or a mixture) into a heated injection barrel. This is then injected into a heated mold (150-200°C or 302-392°F) for vulcanization. HCR has longer cure cycles than LSR, often requires manual loading and demolding, and is more prone to flash, requiring trimming. LSR injection molding has many advantages, including high precision, ability to manufacture complex designs, suitability for high-volume production, consistent quality, fast production cycles, low material waste, biocompatibility, good heat and chemical resistance, and self-adhesive grades are available. Its disadvantages are higher initial tooling and specialized equipment costs, and the need for expertise. HCR injection molding has advantages in certain applications that require durability and toughness, has lower equipment costs than LSR injection molding tooling, can be mixed with additives to meet unique specifications, and is suitable for large molded products. However, HCR has a higher viscosity and is more difficult to handle, often requiring labor-intensive transfer molding and compression molding methods for small batch production, has a slower cure cycle than LSR, wastes material, results in higher labor costs, often requires post-curing to remove peroxide byproducts, and requires manual operation and additional tooling equipment. LSR is commonly used in products that require high precision and quality, such as medical devices (seals, diaphragms, connectors, baby nipples, catheters, valves), automotive parts (seals, gaskets, electrical connectors), consumer products (kitchenware, electronics), industrial parts (seals, gaskets, O-rings), wearables (health monitoring, drug delivery), and overmolding onto other plastic parts. HCR is commonly used for compression molding and extrusion tubing. Medical device manufacturers use HCR to make implantable shunts, pacemaker lead sheaths, pump diaphragms, and catheters.

Polypropylene (PP) Injection Molding

Polypropylene (PP) is a thermoplastic polymer made by polymerizing propylene monomers. The PP injection molding process involves melting the PP (usually between 232-260°C or 450-500°F, but can range from 220-280°C or 428-536°F) and injecting it into a mold (temperature of 20-80°C or 68-176°F, 50°C or 122°F is recommended). The low melt viscosity of PP allows it to flow smoothly into the mold. It is then cooled, solidified, and ejected.

PP has several key properties that make it suitable for injection molding, including low cost and availability, high flexural strength and impact resistance, good chemical resistance to acids and bases, low coefficient of friction (smooth surface), excellent electrical insulation, resistance to moisture absorption, good fatigue resistance, suitable for making hinges, and easy coloring. PP injection molding is cost-effective, suitable for high-volume production, versatile, food-safe (BPA-free), and recyclable. However, PP also has some disadvantages, such as susceptibility to UV degradation and oxidation, high coefficient of thermal expansion, which limits its use in high-temperature applications, poor adhesion, difficult to paint or bond to other materials (welding is required for joining), poor resistance to chlorinated solvents and aromatic hydrocarbons, flammability, brittleness below 0°C (32°F), and relatively high shrinkage (1.8-2.5%).

PP injection molding is widely used in food packaging and containers (such as yogurt and butter containers), plastic parts for the automotive industry (interior trim, glove box doors, mirror housings), hinges (ketchup lids, take-out containers), medical devices, textile materials, children's toys, electronic product packaging, panels and housings, automotive batteries, laboratory equipment (beakers, test tubes), household appliances (refrigerators, blenders, hair dryers, lawn mowers), pipes (industrial and domestic), as well as furniture, ropes, tapes, carpets, camping equipment, twine, and upholstery. Typical process conditions for PP injection molding include melt temperature 220-280°C (428-536°F), mold temperature 20-80°C (68-176°F), 50°C (122°F) recommended (higher mold temperature increases crystallinity), injection pressure up to 180 MPa, injection speed is usually fast to minimize internal stress, but slower speed is recommended to avoid surface defects at higher temperatures, cooling temperature is about 54°C (129°F) to prevent deformation during ejection, and shrinkage rate 1-3%, or 1.8-2.5% (shrinkage can be reduced by adding fillers).

The following factors should be considered in the mold design for PP injection molding: Full-circle runners and gates are recommended (cold runner diameter 4-7 mm), all types of gates can be used; pin-point gate diameters are typically 1-1.5 mm (down to 0.7 mm), and side gates are at least half the wall thickness deep and twice the wall thickness wide. Hot runner molds can be used directly. Cold wells should be designed at the branching points of the runners, and the gate location is important, ideally before the vertical core.

Polylactic acid (PLA) injection molding

Polylactic acid (PLA) is a biodegradable thermoplastic polyester derived from renewable resources such as corn starch or sugar cane. PLA can be injection molded in amorphous or crystalline forms by adjusting the molding conditions. Since PLA is hygroscopic, it needs to be carefully dried before molding (moisture causes degradation). It is recommended that the moisture content is less than 0.025%. Drying conditions are: 2-3 hours at 80°C with air at -40°C dew point or 2-3 hours at 80°C under vacuum. PLA generally has a lower melt temperature than other commonly used injection molding plastics, typically between 150-160°C (302-320°F), but the recommended range is 180-220°C (356-428°F). Mold temperature affects crystallinity: amorphous PLA requires mold temperatures below 24°C (75°F), while crystalline PLA requires mold temperatures above 82°C (180°F), preferably around 105°C (220°F). Crystalline morphology improves heat resistance. PLA generally requires longer cooling times due to its slower crystallization rate. PLA's high viscosity requires higher injection pressures. PLA’s main features include biodegradability and environmental friendliness, food safety (certain grades) (US FDA Generally Regarded as Safe (GRAS) for all food packaging applications), good mechanical and physicochemical properties, glossy and smooth surface, easy molding, and recyclability. However, PLA’s heat resistance is lower than other plastics (amorphous PLA starts to soften above 55°C), and crystallization can improve heat resistance up to a melting point of 155°C. PLA has relatively low strength and can be difficult to machine and is sometimes brittle.

The recommended processing conditions for PLA injection molding include a melt temperature of 180-220°C (356-428°F) and a mold temperature below 24°C (75°F) for amorphous PLA and above 82°C (180°F) to about 105°C (220°F) for crystalline PLA. PLA needs to be dried to a moisture content of less than 0.025% before molding. A back pressure of 10-30% is usually used. Cooling times are usually longer due to slow crystallization.

Mold design for PLA injection molding requires a low shear, dead-angle-free hot runner system to prevent material degradation. Good venting is important due to the high viscosity of PLA. It is recommended to start with minimal venting and gradually increase as needed. The barrel length should be at least 3-5 times the shot size, and the screw aspect ratio should be at least 20:1.

Common applications for PLA injection molding include food packaging (containers, fast food boxes), disposable tableware, nonwovens (industrial, medical, sanitary, outdoor, tent fabrics, floor mats), surgical sutures and bone nails (absorbable), disposable infusion devices, removable surgical sutures, drug sustained-release packaging materials, tubing, and sanitary products.

Polyethylene terephthalate (PET) injection molding

Polyethylene terephthalate (PET) is a thermoplastic polyester that can be processed by injection molding. PET has a high melting point, with the melting point of unreinforced PET being 265-280°C (509-536°F) and the melting point of glass fiber reinforced PET being 275-290°C (527-554°F). The temperature of the injection mold is usually 80-120°C (176-248°F). PET is very sensitive to moisture and must be thoroughly dried before production. It is recommended to dry it at 120-165°C for 4 hours to keep the humidity below 0.02%. Since PET has a short stability time after melting and a high melting temperature, an injection system with multi-stage temperature control and less self-frictional heat generation during plasticization is required. Hot runner molds are usually used for molding PET preforms. Fast injection speeds are often required to prevent premature solidification during injection.

The main properties of PET include high strength and durability, light weight, naturally clear with a high gloss surface, resistance to moisture, alcohols and solvents, good dimensional stability, impact resistance, good electrical insulation properties, recyclable (resin identification code "1"), designated as a food-safe material, and good resistance to acids and oils (especially glass fiber reinforced grades).

Process considerations for PET injection molding include the importance of thorough drying to prevent molecular weight degradation and brittle, discolored products. The melt temperature needs to be precisely controlled (270-295°C for unreinforced types and 290-315°C for glass fiber reinforced types). The mold design should use hot runners with heat shields (about 12 mm thick). Adequate venting is required in the mold (venting depth does not exceed 0.03 mm) to avoid local overheating or cracking. The gate should be opened in the thick part of the PET product to avoid excessive flow resistance and too rapid cooling. The gate direction affects the flow of the melt. Lower back pressure is recommended to reduce wear. The residence time of PET at high temperature should be minimized to prevent molecular weight degradation.

Common applications for PET injection molding include beverage containers (soft drinks, water, juice), food packaging (salad dressing, peanut butter, cooking oil), health and beauty product containers (mouthwash, shampoo, liquid hand soap), take-out food containers and prepared food trays, electronics and appliances (motor housings, electrical connectors, relays, switches, microwave oven internals), automotive parts (reflectors, headlight reflectors, structural parts), plastic parts in electronics, electrical encapsulation or insulation, electrical connectors, household appliances, and bottles and rigid bottles for cosmetic packaging.