A simple explanation of the principle of blow molding is a balloon. much more amenable to the blow molding process and opened up the way to further. the injection moulding, blow moulding (pictured), film extrusion, pipe extrusion and rotational moulding markets. Qenos produces a full range of Alkatane HDPE . In the continuous process, second parison is extruded through the die whilst the first one in being inflated with air. Note: Stretch extrusion blow moulding is also.
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Blow Molding. STRETCH-BLOW MOLDING. 2. Page 3. Blow Molding. INJECTION-BLOW MOLDING Blow Molding. NECK RING BLOW MOLDING PROCESS. Blow molding is a manufacturing process that is used to create hollow plastic REF: raudone.info . Molding process in which air pressure is used to inflate soft plastic into a mold cavity (2) injection mold is opened and parison is transferred to a blow mold.
Vertical strength 2. Wall thickness uniformity 3. Highlight deflection 4. Push-up strength 5. Label considerations 6. Rigidity 7. Shape 8. Hot-fill capacity If the bottle is subjected to vertical loadings, horizontal corrugations or bellows on the part should be avoided. Higher value means easy flow of the melt. Articles blown from these grades exhibit good stiffness.
Extrusion Blow Moulding process 2. Extrusion Blow Moulds 3. Blow Mould Construction 4. Now, this technology is continuing to develop to improve such production methods, using tweaks in design and moving towards pure automation. It promotes a higher level of productivity. This technology has always been moving forward with the purpose of becoming universally used with plastic production. It has allowed for a significant increase in production capabilities, allowing manufacturers to produce greater quantities in just a short period of time.
Using machines that allow for 3D moldings, it has allowed for a production cycle that is much faster. It offers the benefits of automation. Blow molding was taken to greater heights when the Placo X-Y machine from Japan was developed, giving rise to 3D blow molding. Basically, this is an automated production method that allows for minimal flash excess polymer around the material, seamless part incorporation and increased speed of production, thanks to the precise receptacle it creates.
List of Disadvantages of Blow Moulding 1. It is highly dependent on petroleum. Like the gas industry, the blow molding sector highly relies on millions of gallons of petroleum to be able to produce plastic product. As oil is an essential factor in thermoplastics that is becoming automated and more streamlined, the technology has become an ongoing threat to the diminishing oil supply in the world.
It creates a huge impact on the environment. As this technology depends greatly on petroleum and is an integral part in producing polymer, it carries a huge risk on the destruction of the environment. Aside from the fact that it contributes to the diminishing oil resources, it creates plastic that is not biodegrade. In simpler terms we are comparing the difference in weight between resins when suspended in water.
If the sample material floats in water it may be PE or PP. These materials have a specific gravity less than water. Water is always referred to as 1. PE and PP are in the 0. If the sample material sinks in water it may be any of the materials that has specific gravity greater than water.
For example: Specific gravity of PC is 1. For a list of specific gravities see Table 2. Table 2. The flow behaviour will vary with the molecular weight of the material. The test is measured in the number of grams of resin forced through a 2. If the MI of a resin is high, the melt flow resistance during the process is low. If a material is downloadd in large quantities it is important to test them according to their criteria to ensure quality.
If material is displaying poor hang strength or unusually low motor current, it signals the need for a MI check.
With many plastics, moisture must be kept below 0. This number is 37 Practical Guide to Blow Moulding determined by the level of how much water is absorbed in 24 hours at room temperature, if greater than 0. Resins that absorb moisture are called hygroscopic.
HDPE Blow Moulding Project Report
A popular method for drying materials is the hot air desiccant process. Air is passed over and through a bed of moisture absorbing desiccant. This removes the water in the air and lowers the Dew Point. This makes the air very dry and causes it to pull moisture from the resin. A hopper drier can be mounted on the machine to keep material dry after it has left the primary unit.
These units utilise hot air only. In general, hardness refers to scratch and abrasion resistance. Hardness is often associated with other properties to characterise materials.
Material can be described as hard and tough, hard and brittle, or hard and strong. The test for hardness is usually made by the indentation of a pin into the plastic surface.
Refer to Table 2. The tensile strength of plastic materials range from Tensile properties see Table 2. This is defined as the total energy that is needed to break the sample.
The tougher the material the more difficult it is to break. High tensile materials with good elongation such as PC and ABS are used in applications where impact is important. High tensile materials such as PS that have low elongation are too brittle for some applications unless they are rubber modified.
Creep resistance is an important property for products that carry heavy loads which must be stacked. Temperature resistance is an important factor in creep. High impact strength b. Low temperature toughness c. Excellent resistance to chemicals d. Good electrical insulating properties e. Poor ultraviolet resistance Drying required: No Melt temperature range: Pellet type: Virgin has a spherical shape.
Die Swell: Blow up rate: Very good when at proper processing temperatures. Material in natural state is milky white. Burns easily with the smell of candle wax. It can be cut easily with a knife or scratched with a fingernail.
Residence Time: In the absence of air, it remains stable for up to four hours. Temperature consideration: Try to hold melt temperature at the lower end of the range. Temperatures above this range may result in high gloss streaking. Mould cooling: Requires temperatures of 7.
Cycle Times: Often restricted by the shrinkage of the part. A hard, tough material b. Good impact resistance c. Good electrical insulation properties d. Versatile additive acceptance Drying required: ABS is hygroscopic, that is, it will absorb moisture from the atmosphere. A dryer is recommended. Maximum drying time is twelve hours.
Melt temperature range: A recommended temperature profile is given in Table 2. Regrind may need to be dried if not used right away. Virgin pellets are cylindrical in shape. Die swell: Good compared to most engineered resins.
Dependent upon melt temperature, parison weight distribution and additive package. Material identification: Material will sink in water. Begins to melt at approximately the temperature of PE. Has a sweet styrene smell when dried. Residence time: At high temperatures fumes can be a problem. It remains stable for only a short period of time. The higher the temperature the better the texture reproduction and parting line weld strength.
Part design may require that mould halves be maintained at different temperatures. Excellent resistance to heat b. Hard, tough material c. Good impact resistance Drying required: PC is hygroscopic and will readily pull moisture from the atmosphere.
Maximum drying times are eighteen hours. Melt temp range: Regrind has a negative effect on hang strength of a parison after extrusion. Regrind material must be dried before use. Virgin material has a cylindrical shape.
Will require larger head tooling than other resins. Typically 1: Quick set up rate of this material also plays an important role. Hang strength: Poor stiffness at melt temperature. Parison support is critical. Top pinch bar may be required: Pellets will sink in water. Material will degrade quickly if flow is stopped. Keep material moving. Degradation will occur at some point even if melt flow has not been stopped. This can be seen by the appearance of yellow and brown streaking.
Purging with ABS may increase degradation. Temperature considerations: High mould temperatures produce texture reproduction and improve dimensional stability of the part. Pinch edge weld is also improved. Start up: Temperatures can then be lowered when a proper parison is produced. Run machine completely dry. Purge with a high molecular weight material. As temperatures are brought down, the cooling PC will pull contamination off the interior walls of the extruder and head. Head tear down may be necessary after long runs with PC.
Cycle time: Because of the quick set up properties of PC cycle times are faster than many other materials. Good impact strength b. Good chemical resistance c. High abrasion resistance d. High melt strength Drying required: In PP that is loaded with white pigment, yellow streaking may appear when re-grind is used. Virgin pellets are spherical in shape.
Blow up ratio: A maximum of 3: Mould design may decrease this ratio. Very good at proper melt temperatures. Virgin material is milky in colour. Pellets will float in water. Pellets have hard, dry feel. They have a candle like smell when burned. Similar to PE but will degrade more readily. If melt flow must be stopped be sure to fill and purge the head every twenty minutes. Mould texture will reproduce at these temperatures. Cycle times: Like most materials with high shrink rates, parts made of PP are often cycle restricted.
Use the lowest mould temperatures possible that will still provide the desired surface finish. Polyphenylene Oxide Characteristics of polyphenylene oxide that make it good for blow moulding are: Good flame retardancy b. Good impact resistance d. Retains mechanical properties in high heat environments Drying required: Re-grind will need to be dried if not used within an hour. A recommended temperature profile is shown in Table 2. Pellets often come pre-coloured. May require larger head tooling than most resins.
Diverging tooling will be required in most cases. Marginally better than PC. Staying on the low end of the melt range will provide best stiffness.
Parison support is required in most cases. Material has a strong styrene smell when processed. Smoking may be a problem when melt temperatures are too high.
Material will begin to degrade in hours if melt flow is stopped. Purge each half-hour to avoid degradation. Higher mould temperatures provide better reproducibility and weld strength. This material sets up quickly and provides for fast cycle times. Wall thicknesses above 2. The colour can be provided in a pre-coloured base plastic or by adding solid or liquid colour concentrates at the machine just prior to plasticisation.
Colour concentrates are high pigment content dispersions of colorants in carrier resins. The concentrate supplier matches the desired colour with a blend of colorants. Additives such as antioxidants, stabilisers and anti-blocking agents are often co-blended at this point.
A recommendation is then made as to the amount of concentrate required to blend with the base resin to obtain the desired colour. This is referred to as the let down ratio.
The reason for this is the pigments need to be mixed in the screw, the less time it spends in the screw, the less mixing takes place. The carrier resin is preferably the same as the let down resin for good compatibility.
The melt flow of the carrier resin should be high enough that it will mix readily and uniformly throughout the let down resin. The important quality check for colour concentrates is the colour match. The colour chip must match when compared to the moulded part and colour references. It is important that the type of light under which a match is checked be specified.
Ultraviolet, fluorescent and sunlight are common choices. Colours may match under one source of light but not another. This is known as metamerism. Colour concentrates are heat sensitive and may reduce available residence times.
Generally, reds are the most heat stable while yellows and oranges work in the moderate range. Darker colours work in the low heat tolerance range. They may require re-drying prior to mixing in an extruder to ensure that good parts are obtained. It is an economic must that this flash be recovered as regrind. In order for this regrind to be used for finished product it must be kept clean. Foreign material can harm surface appearance and degrade the properties of the part or resin.
Foreign material may also hang up in the head and cause additional foreign material build-up. All material should be covered and all material handling equipment grinders, boxes, loader should be clean. The amount re-grind used in a given product is determined by several factors. Reprocessing material tends to reduce these values.
Parison length consistency will also be a problem. Most resin suppliers recommend that a re-grind have no more that three heat histories.
Parts, which need to withstand impact or repeated stress, should be produced with precisely controlled levels of re-grind. Quantities of flash can be reduced by keeping the mould as close to the head as possible and by using the proper size head tool for the job. This will help prevent premature melting of the plastic.
This relates to the fact that many colours are heat sensitive and will change shades after being reprocessed.
Regrind material will be conveyed differently to virgin pellets, because of the irregular shape of the reground pellets. PP materials have a tendency to bridge due to pellet shape. Other materials are not blow moulded in volumes that are finding their way into the recycling stream. HDPE in a variety of grades, especially for blow moulding is the largest volume plastic available. The items to be considered most in handling the recycled material is that chips or flakes will flow differently to pellets.
This will cause a problem in both the initial feed area due to more chance of bridging at the extruder inlet. The presence of fines in the flake increases this 45 Practical Guide to Blow Moulding situation. Determination of the amount of fines is often a very important specification to have in order to evaluate the potential for this type of problem. The problem of optimising the concentration level of the recycled HDPE to the virgin HDPE in a given application centres on the long-term property requirements, as well as, the short term properties.
Exposure of test bars of the material to accelerated or natural UV radiation is necessary to establish the long-term toughness of the material and compare it to the initial value for the HDPE. This will ensure the selected material ratio will meet the short-term and long-term requirements for the product. The same procedure would be duplicated for any recycled plastic material. Blending virgin and recycled material together requires a determination of which blend will meet the specifications that are normally applied to virgin materials.
The feed streams for HDPE and other plastics will give different characteristics. Staying with HDPE for example, milk jugs are at one end of the spectrum with fractional melt indices.
This is good for blow moulding, bad for injection moulding where high flow MI of is necessary. Detergent bottles of mixed colours, where melt indices of fractional to 5 or so are common - once again, this is good for extrusion and blow moulding, less attractive for injection moulding. Films trash bags, pallet wrap, and so on are also poor for injection moulding but good for extrusion or blown film applications.
The point being, testing must be done when a given stream of recycled material is being considered. This will ensure that the proper recycled material gets into the proper production part or product. References 1. General Reading L. Kerzner, Project Management: Rosenau, Jr. It helps provide an end product with the essential physical properties and the desired appearance. But frequently, minor adjustments or improvements, which would not justify its being returned to the mould maker, can be made with equipment and knowledge available in the blow moulding shop.
The blowing mould may have a number of parts, counting its various inserts, but it usually consists of two halves. When closed, these halves will form one or more cavities which will enclose one or more parisons for blowing. The two mould halves are usually alike. There are usually no male and female sections. Pinch-off edges are generally provided at both ends of the mould halves. A blowing pin may have the additional function of shaping and finishing the neck inside.
Both mould halves must have built-in channels for the cooling water. Sets of guide pins and bushings or side plates in both mould halves ensure perfect cavity alignment and mould closing. Accurate guiding devices in both mould halves reduce setup time. Figure 3. On some blowing presses, mould closing is carried out in two steps, first at high speed, with lower pressure to say, 6.
The second step is slower with higher pressure to protect the mould from tools or anything else that might have fallen between the halves which, however, should never happen in a well-kept shop and for operator safety. Moulds are not necessarily positioned vertically, that is, in line with the parison. They may occasionally be tilted Figure 3.
This will result in a non-uniform distribution of resin which may be helpful, for example instance when such irregular pieces as a pitcher with a handle are being blown. It may also result in some saving in parison length.
11 Advantages and Disadvantages of Blow Moulding
Table 3. The predominant raw materials used for blow moulds are machined from aluminium billet, cast aluminium alloys, zinc alloys such as Kirksite, and occasionally, bronze. Beryllium-copper because of expense and difficulty to machine is usually reserved for pinch inserts or cores where fast heat transfer is needed.
All these alloys are excellent materials for blow moulds Table 3. Furthermore, aluminium moulds wear easily. On the other hand, they are easiest to machine.
Aluminium and beryllium-copper cast moulds may be slightly porous, and occasionally, blow moulders have experienced some permeability of such moulds to the viscous resin. This may affect the appearance of the blown part.
The remedy is coating the inside of the mould halves with a sealer such as radiator sealer. This will not affect the heat transfer between the resin blown against the mould and the mould walls. Steel moulds are heavier, more expensive and more difficult to machine than those made of nonferrous alloys. Higher weight will mean more setup time in the moulding shop.
Moreover, the heat conductivity of steel is inferior to that of the three non-ferrous mould materials. This results in a slower cooling rate and a correspondingly longer cooling cycle and consequently, a lower production rate for steel moulds.
This is a high-strength aluminium alloy which, is fully heat-treated and stress relieved. This alloy has outstanding thermal conductivity along with high strength and surface hardness and as such is suitable for polishing and texturing.
All gauge tolerances are on the plus side of nominal. This alloy is suited for use in production injection moulds, blow moulds, structural foam moulds, reaction injection moulding RIM moulds, and aluminium die sets. It is weldable and highly machinable. These are general purpose, high-strength aluminium alloys suitable for blow moulds and structural foam moulds. This high-strength alloy is heat-treated and stress relieved. It is ideally suited for large blow moulds, injection moulds, and other high-pressure mould applications.
It is also suitable for use in aluminium die sets. It exhibits very high strength and surface hardness with excellent thermal conductivity. This is a high-strength aluminium alloy suitable for blow moulds, structural foam moulds and injection moulds, which currently use alloy It has higher strength and surface hardness than even when thicker than 8. There is no copper in the alloy giving it greater resistance to corrosion.
It can be heat-treated and stress relieved. This alloy is not quench 50 Practical Guide to Blow Moulding sensitive like the alloy is and, therefore holds its high strength even at a thickness greater than 8.
This alloy combines the high corrosion resistance, thermal conductivity and weldability of with the high Brinell hardness of It machines with the same feeds and speeds, creates similar chip sizes and has a surface finish that is found in or alloys.
The material is heat-treated, aged to peak strength, and stress relieved. It is suitable for many or applications. These alloys which are heat-treated and stress relieved in the case of T and T are generally suited for low-pressure applications including large blow moulds, prototype injection moulds, compression moulds, vacuum form moulds, RIM moulds, and structural foam moulds.
These alloys have excellent thermal conductivity but exhibit relatively low strength and surface hardness.
Therefore, these alloys are not recommended for applications requiring high surface hardness. This extruded aluminium plate is heat-treated and stress relieved and is suitable for use in vacuum-form moulds and investment-cast moulds. It is also suitable for use as back plates on blow moulds and structural foam moulds. This direct, chill-cast alloy exhibits very low internal stress and therefore machines relatively stress free. Its high degree of surface finish, typically 0. This alloy is suitable for low-strength mould applications such as vacuum form moulds.
This cast alloy gets its unique properties from a proprietary thermal treatment.
Blow Molding – Problems and Solutions
This product is thermally stable with 0. It exhibits uniformly consistent machinability and polishability throughout the thickness of the plate. This alloy is suitable for use in large blow moulds, prototype injection moulds, structural foam moulds, and investment cast moulds.
It is weldable and is virtually residual stress free. These high-strength beryllium-copper alloys are suitable for mould applications requiring high thermal conductivity, high surface hardness, and high strength.
The alloys are supplied as 40 RC and 30 RC and are already heat-treated. They offer good corrosion resistance, excellent polishability, good wear resistance and excellent weldability.
They can be used for entire moulds or inserts in steel and aluminium moulds. This very high thermal conductivity beryllium-copper alloy is suitable for mould applications requiring more rapid removal of heat than can be attained in steel or aluminium moulds. It offers excellent corrosion resistance, good polishability, excellent weldability, and resistance to high temperatures.
It is also suitable for gates and nozzle tips. Ampco This nickel-silicon-chromium-copper alloy features superior thermal conductivity, the ability to accept high surface finishes, inherent corrosion resistance, welding compatibility with other copper alloys, and is readily machined. Telmax - Tellurium-Copper: This is a highly machinable, tellurium-copper alloy suitable for manufacturing electrodes for the electrical discharge machining EDM industry. It combines low cost, mirror finishes, high strength, high resistance to DC arcing, with no dust generation.
It is recommended for use in limited-flushing applications. This is a high carbon stainless steel that has maximum hardness with other good stainless steel properties. It has excellent corrosion resistance, polishability, and resistance to wear. These characteristics make it suitable for injection moulds. Basically they have detailed texture of design such as feathers or irregular part lines, see Figures 3. Also, fuel tanks because of their irregular shape are often cast moulds Figure 3.
CNC machines are shown in Figures 3. Since aluminium billets have a limitation on thickness, moulds would need to be fabricated, but because some dimensions are critical they have to be machined.
Examples are shown in Figure 3. Good heat transfer means faster cooling, and faster cooling means more items blown per hour, that is, less expensive production.
HDPE Blow Moulding Project Report
This is the main reason why, for blowing moulds, the previously mentioned alloys are generally preferred to, the usually, more durable steel.
Occasionally, several different alloys are used in the same mould to obtain the desired strength and special cooling conditions. Different materials with consequently different heat conductivity at various points in the mould will result in non-uniform cooling.
This, in turn, might set up areas of stress in the finished piece, which are susceptible to splitting in use. The cooling water may be tap water. If it has a high content of minerals, which may settle in the narrow, cooling channels, a closed system for circulating purified water should be used. Such low temperatures may, however cause water condensation on the outside mould walls. Some moulders, though, use non-cooled tap water. Usually, the cooling water is recirculated, that is, reused time and again for a long period.
Conduit flow is determined by Reynold's number: Sometimes, a copper tubing system is cast into the mould. However, to create the most useful flow, water channels are machined into the mould halves See Figure 3. Well-placed channels will ensure that the cooling water comes as close to the mould cavity as is feasible see Figures 3.
Drilled holes are tapped sizes for National Pipe Thread Standards for inlet and outlet threaded holes. Pictures of a mould half showing an in and out cooling channel is shown in Figure 3. Larger moulds may be equipped with several - up to three or more - independent cooling zones.
Generally, in the top or bottom areas, that is, around a bottleneck or the bottom pinch-off, or both, greater masses of resin are needed than along the other areas. Such areas as well as thicker wall sections, therefore, often require additional cooling. Otherwise, these sections would still be viscous while the thinner wall sections have solidified when the piece is ejected. This may result in a deformed piece or one with non-uniform shrinkage and resulting warpage, which the customer will reject.
That is why frequently even a simple mould has two or more cooling systems for each half. Any mould run for a year for around to 1, pieces total will pay for itself in faster cooling cycles and better parts. No cooling channels or tubing should be more than three or four metres long without going into a manifold.
High turbulence in the cooling fluid is essential for fast cycles. Every molecule of the controlled temperature fluid needs to bounce off the side walls. The nature of conduit flow laminar or turbulent is determined by the value of the Reynolds number Re Equation 3. Below the critical number, laminar flow occurs - this is referred to as streamline flow.
The greater the Reynolds number is above 2,, the more efficient the cooling regardless of how the mould temperature drops, or if the internal stresses in the formed part increase.
Increasing the flow rate from a Reynolds number of 2, to a Reynolds number of 10, increases the heat transfer coefficient by about nine times. In other words, the more turbulence, the better the cooling rate.
When copper tubing is swaged into machine channels, the tubing should have an outer diameter of Air may be circulated inside the blown part to speed up its cooling. Cooling time is strongly affected by the extrusion melt temperature of the blow moulding cycle.
See Further Reading at the end of the chapter. Flash generation imposes limits on blow moulding efficiency. Economics dictate that flash materials be recaptured in a closed-loop operation.By they had machinery, which could produce 25, bottles per day.
With this type, the screw channels are deepest under the hopper and shallowest at the screw tip. This permits the speed of the output shaft to be varied, or held constant under a varying load. It exhibits uniformly consistent machinability and polishability throughout the thickness of the plate.
Two cuts are made separating the moulding into two containers leaving the transition which is later reground. Thus, the screw uses the principle of melt pool and solid bed separation to improve output at a given rpm, reduce melt temperature, and ensure complete melting.
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