Resin transfer moulding (RTM)

Resin Transfer Molding (RTM) is a low pressure, closed molding process which offers high-quality dimensional and surface finish composite moldings using liquid thermoset polymers reinforced with various forms of fiber reinforcements. Typically polymers of Epoxy, Vinyl Ester, Methyl Methacrylate, Polyester or Phenolic are used with typically fiberglass reinforcement. Other reinforcements, are offered for more demanding applications such as Arimid, Carbon and Synthetic fibers either individually or in combination with each other.
A process using a two-sided mold set that forms both surfaces of the panel. The lower side is a rigid mold. The upper side can be a rigid or flexible mold. Flexible molds can be made from composite materials, silicone or extruded polymer films such as nylon. The two sides fit together to produce a mold cavity. The distinguishing feature of resin transfer molding is that the reinforcement materials are placed into this cavity and the mold set is closed prior to the introduction of matrix material. Resin transfer molding includes numerous varieties which differ in the mechanics of how the resin is introduced to the reinforcement in the mold cavity. These variations include everything from vacuum infusion (see also resin infusion) to vacuum assisted resin transfer molding. This process can be performed at either ambient or elevated temperature.

The matrix selection of polymer and reinforcement, dictates both molding material cost, as well as molding mechanical and surface finish performance. Along with the polymer and reinforcement the addition of mineral fillers may be added to enhance fire retardantcy, flex modulus and surface finish.

Reinforcements are presented in their dry form to the mold in either binder-bound chopped mat, random-continuous strand mat or woven cloth format. The fiber has been either "preformed" to the exact shape of the molding tool in a previous operation or is hand-tailored during the loading process in the molding tool. After the fiber is installed into the mold, a premixed catalyst and resin is injected into the closed mold cavity encapsulating the fiber within. The primary surface of the molding may be gel-coated, a process of spraying the mold surface before installing the fiber. If a gel coat is not required, the exterior finish would be the same from the front to back of the molded part. RTM having the inherent advantage of low-pressure injection usually does not exceed 100 psi of resin injection pressure during the mold-fill process. the most common application of RTM composites is molded with a cross-section thickness of 4mm with 30% fiber content by weight of the total laminate.

Resin Transfer Molding (RTM) is a low pressure, closed molding process, where a mixed resin and catalyst are injected into a closed mold containing a fiber pack or preform. After the resin has cured, the mold can be opened and the finished component removed. A wide range of resin systems can be used including: polyester, vinylester, epoxy, phenolic and methyl methacylates, combined with pigments and fillers including aluminum trihydrates and calcium carbonates if required. The fiber pack can be glass, carbon, arimid, or a combination of these.

EDO has placed considerable emphasis on the development of in-house resin transfer molding for the fabrication of high performance structural composites. Resin transfer molding produces parts to net shape with good surface finish, low void content and required levels of fiber volume. Raw material costs for the resin transfer molding process are significantly lower than those associated with pre-impregnated materials. Being a "closed mold" process capable of automation, resin transfer molding does not pose the health and environmental risks associated with traditional methods of resin impregnation.

Resin transfer molding is a logical choice for the impregnation of dry fiber preforms. Braided preforms can be held in inventory indefinitely without concern for resin out-times and cure schedules. As separate processes, braiding and RTM schedules can be designed for optimum production rates. The RTM production area can be configured to operate a variety of molds simultaneously to accommodate the diversity of preforms being produced by the braiding operation.

The Benefits of using RTM

· Closed Molding process is cleaner and healthier which attracts higher skilled employees.

· Closed Molding operator turn-over is dramatically reduced through improved working conditions.

· Closed Molding area has organized process flow to maximize throughput efficiency.

· Moldings can be manufactured to close dimensional tolerances.

· Components will have good surface finish on both sides.

· Selective reinforcement and accurate fiber management is achievable.

· Ability to build-in fiber volume fraction loadings up to 65%.

· Uniformity of thickness and fiber loading, resulting in uniform shrinkage.

· Inserts may be incorporated into moldings.

· Tooling costs comparatively low compared to other manufacturing processes.

· Uses only low pressure injection.

· Low volatile emission during processing.

· Ability to produce near net shape moldings, reducing material wastage.

· Process can be automated, resulting in higher production rates with less scrap.

· Ability to mold complex structural and hollow shapes.

· Low resultant voidage in molded components.

· Ability to achieve from 0.1mm to 90mm laminate thickness.

Main Disadvantages:

• Matched tooling is expensive, and heavy in order to withstand pressures.
• Generally limited to smaller components.
• Unimpregnated areas can occur resulting in very expensive scrap parts.
  

Autoclave Composite Moulding

Autoclave composite moulding is a similar to pressure-bag and vacuum-bag moulding but uses a high pressure chamber instead. This advanced composite process produces denser, void free mouldings because higher heat and pressure are used during curing. It is widely used in the aerospace industry to fabricate high strength/weight ratio parts from pre-impregnated high strength fibres for aircraft, spacecraft and missiles. Autoclaves are essentially heated pressure vessels usually equipped with vacuum systems into which the bagged lay-up on the mould is taken for the cure cycle. Curing pressures are generally in the range of 50 to 100 psi and cure cycles normally involve many hours. The method accommodates higher temperature matrix resins such as epoxies, having higher properties than conventional resins. Autoclave size limits part size.

A process using a two-sided mold set that forms both surfaces of the panel. On the lower side is a rigid mold and on the upper side is a flexible membrane made from silicone or an extruded polymer film such as nylon. Reinforcement materials can be placed manually or robotically. They include continuous fiber forms fashioned into textile constructions. Most often, they are pre-impregnated with the resin in the form of prepreg fabrics or unidirectional tapes. In some instances, a resin film is placed upon the lower mold and dry reinforcement is placed above. The upper mold is installed and vacuum is applied to the mold cavity. The assembly is placed into an autoclave pressure vessel. This process is generally performed at both elevated pressure and elevated temperature. The use of elevated pressure facilitates a high fiber volume fraction and low void content for maximum structural efficiency.

An alternative to the conventional autoclave process is a patented technique developed by the Australian CSIRO called ‘Quickstep’. Quickstep uses a unique, fluid filled, balanced pressure, floating mould technique, combined with vibration through the fluids to produce advanced fibre and glass reinforced composite components. The quickstep process produces composite structures having superior properties in a much faster time than conventional autoclave processes.

Pressure bag moulding

A hand lay-up process in which the lay-up is cured under pressure generated by applying air or steam pressure up to 50 psi between a tailored bag placed over the lay-up and a pressure plate placed over the top of the mold.
This process is related to vacuum bag moulding in exactly the same way as it sounds. A solid female mould is used along with a flexible male mould. The reinforcement is place inside the female mould with just enough resin to allow the fabric to stick in place. A measured amount of resin is then liberally brushed indiscriminately into the mould and the mould is then clamped to a machine that contains the male flexible mould. The flexible male membrane is then inflated with heated compressed air or possibly steam. The female mould can also be heated. Excess resin is forced out along with trapped air. This process is extensively used in the production of composite helmets due to the lower cost of unskilled labor. Cycle times for a helmet bag moulding machine vary from 20 to 45 minutes, but the finished shells require no further curing if the moulds are heated.
Pressure bag molding is similar to the vacuum bag molding method except that air pressure, usually 30 to 50 psi, is applied to a rubber bag, or sheet that covers the laid up composite to force out entrapped air and excess resin. Pressurized steam may be used instead, to accelerate the cure. Cores and inserts can be used with the process, and undercuts are practical, but only female and split molds can be used to make items such as tanks, containers, and wind turbine blades.

Stretch Blow Moulding

Stretch blow molding

Stretch blow molding produces a part with biaxial molecular alignment. In the process a preform, or parison, elongated mechanically in the mold and than expanded radially in a blowing process. A desirable resulting molecular orientation yields a material with increased strength. This means that products that are strength-based designs can be produced using less material than if they were to be produced using simpler blow molding techniques.

A goal in stretch blow molding is a designed work material developed by producing desirable molecular orientation. In order to produce and retain desired structure and specified properties the stretching and blowing processes need to be carried out at temperature lower than in other blow molding processes and the allowable temperature range will be smaller and so more difficult to control. A temperature conditioning station in-line is required, or a re-heating operation needed for preforms allowed to cool before use or for purchased preforms. This increased the difficulty of process design and operation and material specification since polymer properties depend on temperature history, e.g., on temperature, time at temperature and number of temperature cycles.

Important polymer properties to be considered:

• Tensile strenth and yield above Tg
• Effect of orientation on gas permeability through the polymer

In the Stretch Blow Molding (SBM) process, the plastic is first molded into a "preform" using the Injection Molded Process. These preforms are produced with the necks of the bottles, including threads (the "finish") on one end. These preforms are packaged, and fed later (after cooling) into an EBM blow molding machine. In the SBM process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high pressure air into bottles using metal blow molds. Usually the preform is stretched with a core rod as part of the process. The stretching of some polymers, such as PET (Polyethylene terephthalate) results in strain hardening of the resin, allowing the bottles to resist deforming under the pressures formed by carbonated beverages, which typically approach 60 psi.

The main applications are bottles, jars and other containers. The Injection blow molding process produces bottles of superior visual and dimensional quality compared to extrusion blow molding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash. A sign of injection blow molding is the seam where the two halves of the mold meet.

This picture shows what happens inside the blow mold. The preform is first stretched mechanically with a stretch rod. As the rod travels down low-pressure air of 5 to 25 bar (70 to 350 psi) is introduced blowing a 'bubble'. Once the stretch rod is fully extended, high-pressure air of up to 40 bar (580 psi) blows the expanded bubble into the shape of the blow mold.

Injection Blow Moulding

Injection blow molding

Injection blow molding is a two stage process since the parison is produced in a separate operation. In the first process molten plastic is injected into a heated preform mold around a hollow mandrel blow tube or core rod. This is similar to insert injection molding. The workpiece for the second, blow molding, process is the preform-mandrel assembly. The preformed parison is placed in a larger mold cavity for blow molding. Between the preform production and blow blow molding processes a heated preform may be held in a temperature conditioning stage or a cooled preform re-heated. After blow molding the part is stripped from the core rod at an ejection station.

Raw Materials

• Polyethylene (Low Density) LDPE, LLDPE
• Polypropylene PP
• Polyethylene - Terephthalate PET
• Polyvinyl chloride PVC
• Polyethylene (High Density) HDPE

These factors are critical to this process:

• Shear & temperature dependent viscosity
• Temperature-dependent tensile strength on the pin
• Tensile elongation during inflation
• Crystallization kinetics on the core pin
• Crystallization kinetics during blowing and cooling

Examples of Application

• Bottles
• Jars
• Roll-on containers

Injection blow molding

The process of Injection Blow Molding (IBM) is used for the production of hollow glass and plasticbottles. The process is divided into three steps: injection, blowing and ejection. objects in large quantities. In the IBM process, the polymer is injection molded onto a core pin; then the core pin is rotated to a blow molding station to be inflated and cooled. This is the least-used of the three blow molding processes, and is typically used to make small medical and single serve

The injection blow molding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mold. The preform mold forms the external shape and is clamped around a mandrel (the core rod) which forms the internal shape of the preform. The preform consists of a fully formed bottle/jar neck with a thick tube of polymer attached, which will form the body.

The preform mold opens and the core rod is rotated and clamped into the hollow, chilled blow mold. The core rod opens and allows compressed air into the preform, which inflates it to the finished article shape.

After a cooling period the blow mold opens and the core rod is rotated to the ejection position. The finished article is stripped off the core rod and leak-tested prior to packing. The preform and blow mold can have many cavities, typically three to sixteen depending on the article size and the required output. There are three sets of core rods, which allow concurrent preform injection, blow molding and ejection.

Another application of injection blow molding is in the production of soft elastic gelatin capsule for pharmaceutical applications. Two strips of gelatin are pressed together in a rotary die which cuts out the desired shape of capsule while the fill liquid is injected. Afterwards, they are cooled and dried to yield a firm, strong capsule.

Extrusion Blow moulding

Extrusion Blow Molding

In extrusion blow molding the parison is formed by forcing molten plastic through an annular orifice in a die that is part of the die head assembly. The orifice is formed by the space between the mandrel and the die. Extrusion may be directly from an extruder, or for large parts for which more material is needed than the extruder can continuously provide an accululator is used. The parison is extruded and drops to between the mold halves and when the mold closes the parison is sealed. Air injected into the parison inflates it to the shape of the mold cavity. After cooling and solidification the mold is opened and the part removed.

Raw Materials
This process usually use commodity materials such as:

• Polypropylene PP
• Polyethylene PE
• Polyethylene - Terephthalate PET
• Polyvinyl chloride PVC

Important factors one should consider for extrusion blow molding include the following:

• Polymer viscosity at high & low shear rates
• Melt strength (important for uniform wall thickness, no holes)
• Strain recovery (MW & Distribution)
• Crystallization rate (slow rate desired)
• Thermal properties (thermal diffusitivity, thermal conductivity, specific heat, etc.)

Advantages of Extrusion Blow Molding:

• Low initial mold tooling costs.
• Flexibility of tooling. Molds can accommodate interchangeable neck finishes and body sections.
• Flexibility in production: Neck inner diameters (I.D.) can be easily controlled to varying requirements. Bottle weights are adjustable.
• Container sizes can range from less than 1 oz. to 55 gallons and up. (Custom Bottle's equipment is most efficient producing containers up to 1 liter in capacity.)
• Container shape is not restricted by blow-up ratios. Bottles can be long and flat or have handles.
• Wide selection of machine sizes: Molds can be geared to volume requirements.

Applications

• Bottles and containers
• Automotive fuel tanks
• Venting ducts
• Watering cans
• Boat fenders etc

In Extrusion Blow Molding (EBM), plastic is melted and extruded into a hollow tube (a parison). This parison is then captured by closing it into a cooled metal mold. Air is then blown into the parison, inflating it into the shape of the hollow bottle, container or part. After the plastic has cooled sufficiently, the mold is opened and the part is ejected.

EBM processes may be either continuous (constant extrusion of the parison) or intermittent. Types of EBM equipment may be categorized as follows:

Continuous Extrusion Equipment

• rotary wheel blow molding systems
• shuttle machinery

Intermittent Extrusion Machinery

• reciprocating screw machinery
• accumulator head machinery

Examples of parts made by the EBM process include dairy containers, shampoo bottles, hoses/pipes, and hollow industrial parts such as drums.

Basic polymers, such as PP, HDPE, PVC and PET are increasingly being coextruded with high barrier resins, such as EVOH or Nylon, to provide permeation resistance to water, oxygen, CO2 or other substances. In dairy applications, it is possible to extrude a black light-blocking layer in the center layer of containers, with opaque white resin used in the inner and outer layers.

Compared to injection molding, blow molding is a low pressure process, with typical blow air pressures of 25 to 150 psi. This low pressure process allows the production of economical low-force clamping stations, while parts can still be produced with surface finishes ranging from high gloss to textured. The resulting low stresses in the molded parts also help make the containers resistant to strain and environmental stress cracking.

Blow Moulding

Blow Molding Process

Process of inflating a hot, hollow, thermoplastic preform or parison inside a closed mold so its shape conforms to that of the mold cavity. A wide variety of hollow parts, including plastic bottles, can be produced from many different plastics using this process.

Main steps:

• parison is formed between mold halves
• mold closes around the parison
• sealing one end of the parison
• closing the parison around a mandrel at the other end
• parison is inflated by air blown through hollow mandrel or needle in side of parison
• cooling and solidification of the part
• mold opening and part ejection

During this process the resin raw material is melted in the machine barrel, forced over a spreader and through the die head into the mold. The mold halves are held on platens which ride on tie rods or tie bars.

Raw Materials
Most commodity grade and engineering grade resins may be blow molded,. but the most common is polyethylene, which is used for food or chemical or detergent bottles. PET or polyester is used for clear beverage bottles such as water bottles or the familiar 2-liter beverage bottles. EVA is a rubber-like material used for blow molded elastomer parts. Generally the list includes: HDPE, PET, Polypropylene, LDPE, PVC, Polycarbonate, ABS, EVOH, LLDPE, TPO, PBT,Nylon, TPE, ABS/PC Blend, Polystyrene, K-Resin®, MDPE, PUR, PETG and PPO. The "melt index", or viscosity, of the plastic must be high to keep the parison from stretching too much prior to mold closure. The resin is in the form of pellets before processing.

Tooling
Machined or cast aluminum is traditional.

Cost
Part prices are generally higher than injection molded parts, but lower than rotationally molded parts. Tooling costs are moderately expensive.

Advantages
This process lends itself to any designs involving hollow shapes. Equipment availability is good in most geographical locations. Can save tooling dollars over injection molding.

Disadvantages
Cycle times are slower than injection molding. Piece prices are higher than injection molding.

Applications
All types of bottles, toys, air ducts for automobiles, chemical & gasoline tanks, household goods.

There are three general types of blow molding: extrusion blow molding, injection blow molding, and stretch blow molding. Extrusion blow molding is usually used to make items of weight greater than 12 oz. such as containers for food, laundry, or waste. Injection blow molding is used to achieve very accurate wall thickness, high-quality neck finish, and to process polymers that cannot be extruded. Usual applications include pharmaceutical, cosmetic, single serving liquor bottles that weighs less than 12 oz. Stretch blow molding is only used for difficult to blow crystalline and crystallizable polymers such as polypropylene and polyethylene terephthalate.