Polymers Used in Plastic Moulding

The term “polymer” describes classes of molecules with large numbers of repeating structural units connected through covalent bonds. The major identifying feature distinguishing polymers from other molecules is the repetition of many similar, identical, or complementary subunits.

Most manufacturers decide upon polyethylene as the polymer of choice for the plastic moulding process because of its availability, ease of use, and suitable properties. According to recent reports 80% – 90% of all polymers used in the plastic moulding industry are polyethylene compounds (HDPE, LPDE and LLPDE). Although PVC, nylons, and polypropylene compounds are also used.
Nylons
Polyethylene
Polypropylene
PVC

A paper written by J.D. Ratzlaff of Chevron Phillips Chemical Company LP in 2004 entitled “Polyethylene: Process Sensitivity in Rotational Moulding” presents the results of a study of the impact sensitivity of polyethylene to processing conditions and discusses methods to maintain high impact standards.

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What is Gas-assisted injection molding

Gas-Assisted Injection Molding is a process where an inert gas is introduced at pressure, into the polymer melt stream at the end of the polymer injection phase. The gas injection displaces the molten polymer core ahead of the gas, into the as yet unfilled sections of the mold, and compensates for the effects of volumetric shrinkage, thus completing the filling and packing phases of the cycle and producing a hollow part.

Traditionally, injection molded components have been designed with a relatively constant wall thickness throughout the component. This design guideline helps to avoid major flaws or defects such as sink marks and warpage. However, apart from the simplest of parts, it is impossible to design a component where all sections are of identical thickness. These variations in wall thickness result in different sections of the part packing differently, which in turn means that there will be differentials in shrinkage throughout the molding and that subsequently distortion and sinkage can often occur in these situations.

By coring out the melt center, gas injection molding enables the packing force (which compensates for differential shrinkage) to be transmitted directly to those areas of the molding which require attention. This dramatically reduces differentials in shrinkage and thus the sinkage. In addition, the internal stresses are kept to a minimum, considerably reducing any distortion that may otherwise have taken place.

Maximum clamp pressures are normally required during the packing phase of a molding cycle. This is due to the force which has to be exerted at the polymer gate in order to pack melt into the extremities of the mold cavity in an effort to compensate for the volumetric shrinkage of the solidifying melt. In comparison to compact injection molding, gas injection molding typically has considerably shorter distance over which the solidifying melt is required to be packed because of the gas core. This means that proportionally lower packing pressures are required to achieve the same results and in turn, lower machine clamp forces are required.

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What is Overmolding

Overmolding is an injection molding process where two materials are molded together. Types of overmolding include two shot sequential overmolding, multi-shot injection molding or insert overmolding.

Multi-shot injection molding injects multiple materials into the cavity during the same molding cycle. Insert overmolding uses a pre-molded insert placed into the mold before injecting the second material. Two shot sequential overmolding is where the molding machine injects the first material into a closed cavity, and then moves the mold or cores to create a second cavity, using the first component as an insert for the second shot using a different material.

Materials are usually chosen specifically to bond together, using the heat from the injection of the second material to form that bond. This avoids the use of adhesives or assembly of the completed part. It can result in a robust multi-material part with a high quality finish.

When designing an overmolded part, wall thicknesses of both the insert and the overmolded component should be as uniform as possible to ensure an even and robust bond. Avoid ribs and sharp corners to reduce flow problems.

Overmolded parts take longer to cool than single shot injection molded part, and cooling systems are less effective. The insert acts as an insulator and heat is less efficiently extracted from the part. However, optimising the cooling system can help reduce the cycle time.

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Procedures for control of injection pressure

1.4.3.2 procedures for control of injection pressure
The control of the injection pressure is usually divided into an injection pressure, the second injection pressure (holding pressure) or more than three times the injection pressure control. Switching time pressure if it is appropriate to prevent mold pressure is too high, to prevent overflow or lack of material are all very important. Mold packing stage the specific volume depends on the gate closed when the melt pressure and temperature. If the holding pressure time to switch to products from the cooling phase of the same pressure and temperature, then the specific volume of products will not change. Molding at a constant temperature to determine product size is the most important parameters of the packing pressure, affecting the product dimensional tolerances of the most important variable is the holding pressure and temperature. For example: after the end of the filling, packing pressure decreased immediately, when the surface to form a certain thickness, packing pressure to rise again, thus forming a low clamping force of large thick-walled products, elimination of collapse pits and flash.

Holding pressure and the speed is usually highest when the plastic cavity filling pressure and speed of 50% to 65%, which is holding pressure lower than the injection pressure of about 0.6 ~ 0.8MPa. As the packing pressure lower than the injection pressure, holding pressure in the considerable time, the pump load is low, solid pump life is extended, while oil pump motor power consumption is also reduced.

Three pressure injection filling both the smooth parts, and weld line does not appear, depression, flash and warpage. For thin-walled parts, small pieces of long, long process of molding large parts or even the cavity mold configuration is not balanced and not too close parts of the molding as a whole.

1.4.3.3 the amount injected into the mold cavity filling process control of plastic
Regulate the use of certain measures in advance so near the end of the injection stroke, the screw end is still a small amount of residual melt (buffer capacity), according to the mold filling was further applied injection pressure (injection pressure two or three times) , add little melt. This can prevent depression or adjustment products product shrinkage.

1.4.3.4 screw back pressure and speed of the process control
High back pressure can be intense shear melt at low speeds, will also get a long plastic tube in the machine plasticizing time. Therefore the current use of more back pressure and speed of parallel programming control. For example: measurement of the whole trip in the screw first high speed, low back pressure, and then switch to a lower speed, high back pressure, and then switch to high back pressure, low speed, and finally in the low back-pressure, low speed for plastics , so that the front screw melt most of the pressure be released, reducing the rotational inertia of the screw, thereby improving the accuracy of measurement of the screw. Often result in excessive back pressure increases the degree of stain color; pre-plastic bodies in increased mechanical wear screw and barrel; pre-plastic periodic extension of decline in production efficiency; nozzle prone to drooling, regeneration feeding amount increased; even with self-locking nozzle, if the back pressure higher than the design of the spring closure pressure may also cause fatigue failure. So, back pressure must be properly adjusted.
As technology advances, the small computer into the injection molding making machine control system, using a computer to control the injection molding process has become possible. Japan Steel Works N-PACS (micro-computer control system) can do four feedback control (holding pressure adjustment, molded adjustment, automatic measurement adjustment, resin temperature adjustment) and four process control (injection speed process control, pressure inspection, screw speed process control, process control back pressure).

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Automatic insert molding systems design choices Note

Automatic insert molding systems design choices Note

1, the metal insert molding mold shrinkage prone to heterogeneity, should be done in advance important parts of the shape, dimensional accuracy of the limit test.

2, injection of metal embedded in the process of easy deformation and displacement, should take full account of the structure and easy to maintain the metal mold insert mold the shape of the design. The inserts can not change the shape of the product, prior testing is indispensable.

3 the arrangement of metal inserts separate occasions using conveyor, the metal inserts and the inserts and vibration between the ball contacts the surface of inserts will slightly damage, affect product quality. In advance to confirm the scope of permissible limits of its quality.

4, metal inserts shall be determined in advance because the stamping process caused by serrated warpage, material thickness difference, difference in diameter, the thickness of deposited gold due to poor processing. On the basis of the automated installation options for the design and mold construction design.

5, mold gate location methods, molding cycle, mold construction restricting the predictability of the matter, as far as possible to resolve or there is a corresponding improvement measures.

6, should recognize the need for preheating metal inserts or dried. Purpose is to ensure product quality and shape stability.

7, mold set up various testing devices in the mold to heat, force, vibration under the influence of environmental conditions to ensure stability by forming action, shall determine whether to use.

8, in order to avoid the metal insert, molded fine film deposited on the mold cavity, if necessary, can be assembled to air blowing device.

9, due to the high price of equipment investment system, to fully consider the possibility of using the former to ensure the running of the production equipment. Special occasions by machine, it is necessary to ensure that no form of updated products in the past few years the premise of continuous production.

10, adopt a common plane when the occasion required to confirm many varieties of small batch insert the number of combinations of production. If the whole mass production can not be guaranteed when the daily, each a product of the fixed assets of the recovery is difficult. In this case, to be replaced after a certain portion of the devices within the species adapted to update the requirements.

11, injection molding machine, injection molds, automation equipment and effective combinations and how to function in a short time, is to determine the automatic insert molding systems critical.

12, decided to insert molding rate, the cost of production and molding conditions of metal goods, precision inserts, insert shapes, molds are beneficial to insert molding and molding products and technologies and other factors shape decisions clever.

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Plastics Processing insert molding processes

Insert into the mold forming means of the different materials prepared after the inserts into the resin, the melt solidified bonding material and inserts, made of molded construction method of integration products.

Substrate injection molding refers to the local board on metal injection molding parts embedded in the construction method. Its characteristics are as follows:

1, ease of molding of resin, metal bending rigidity, strength and heat resistance of each combination of added complexity and sophistication can be made of solid metal to plastic integrated products.

2, in particular the use of insulating resin and a combination of conductive metal, moldings made of electrical products to meet the basic function.

3, multiple insert the pre-forming composition, makes the product portfolio after the unit works whenever necessary.

4, insert product entirely limited to the metal, there are cloth, paper, wire, plastic, glass, wood, line rings of class, and other electrical components.

5, the rigid moldings, rubber seals on the flexural plate moldings through injection molding substrate made of integrated products, eliminating the need for sealing rings of the complex arrangement of work, so after the combination of process automation easier.

6, because the material is molten metal inserts in the joints, and pressed into shape Comparatively, metal inserts can be designed more narrow gap, forming composite products more reliable. .

7, select the appropriate resin and molding conditions, that is easy to Bian-shaped broken for products (such as glass, coil, electrical parts, etc.) can also be sealed by resin fixed.

8, select the appropriate mold structure, insert materials can also be completely enclosed in resin.

9, vertical injection molding machine with mechanical hand, insert the entire product series combination of devices, insert molding can achieve a large-scale automated production.

10, insert molding, after a deal to core hole can also be made into a hollow cavity Bands products.

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What Is the Plastic Manufacturing Process

The plastic manufacturing process refers to the series of steps required to turn raw oil and natural gas into plastic products, such as cups, hoses, pipes, and straws. It is a very complicated process that first begins with transforming oil and natural gas into hydrocarbon monomers that are then linked together into long chains known as polymer plastics. Based on the type of monomers linked together, the process can produce a variety of different types of polymer plastics, such as polyethylene terephthalate (PET), high density polyethylene (HDPE), and polyvinyl chloride (PVC).

Once these plastic polymers have been created, the next step in the plastic manufacturing process is to transform these simple polymers into complex consumer products. The four sub-processes that may be used to do this are injection molding, extrusion molding, blow molding, and rotational molding. Each plastic manufacturing process produces a different type of plastic product.

Injection molding is a plastic manufacturing process wherein plastic polymers that have been compressed into plastic pellets are fed into an injection molding machine. These machines mold the pellets into the correct shape and add additives to give them the correct color and texture. This process produces plastic products such as butter tubs, yogurt containers, closures, and fittings.

Extrusion modeling is practically the same as injection modeling, in that it molds plastic pellets into shapes. The difference is that this process also uses a shaped die to give the plastic mold a tube like shape. As such, this plastic manufacturing process is used to create pipes, tubes, and wire insulation.

Blow molding is a more complicated plastic manufacturing process that is used to create hollow plastic products with an opening or neck. The two types of blow molding are injection blow molding and extrusion blow molding. Both processes involving turning plastic pellets into molds and then injecting air into the molds to make them hollow. The difference is that injection blow molding can only produce small containers, whereas extrusion blow molding isn’t limited by size.

The fourth plastic manufacturing process, rotational molding, is the most complicated and expensive. Like the other processes, it turns plastic pellets into a mold. The difference is that the mold is then rotated to create complex hollow shapes. Where rotational molding differs from blow molding is that the hollow products it produces are entirely closed, and they do not have an opening. Such products include plastic toys and furniture.

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What Is Rapid Injection Molding

Rapid injection molding refers to the process of making injection molds for the prototype, bridging, and short production run parts in less time and a reduced cost than full production molds. The injection molds typically made for prolonged production runs are very expensive and time consuming. In situations where production of injection molded parts requires fast-tracking and initial cost control, inexpensive and quick rapid injection molding techniques can produce molds better suited to low volume production runs.

Injection molding is a production method that delivers low unit costs during production but can be cost intensive and time consuming in its roll-out phase. Molds made for extended production may take several months to produce. This is not always feasible where small numbers of parts are required as prototypes for function and fit testing or marketing tests. In these cases, molds identical to the full production examples regarding design and dimension can be made, although they lack the longevity of the final molds.

Parts manufacturing subject to small final production volumes also benefits from the use of rapid injection molding techniques as the per-unit cost of the final product can be kept within viable limits. Bridging is another area of production that benefits from rapid injection molding; this allows manufacturers to start production while the final molds are being made, thereby “bridging” the gap between prototype and final product. Considering that rapid injection molds may be produced in approximately two to five weeks, these production needs can be met without incurring large peripheral expenses.

Rapid injection molding equipment is usually produced from aluminum using techniques such as rubber plaster molding, rapid prototyping, and CNC machining which allows for very quick production times. Although these molds generally lack the life span or qualities of a final production set, they can produce thousands of parts, are suitable for most materials and colors, and may even be textured. Modern machining techniques also means that rapid injection molding can produce ever increasing levels of complexity and size differences which further expands the list of possible benefits the process offers.

A further benefit of rapid injection molding in the pre-production phase of development is the low cost of geometry or design modification. Should unforeseen issues arise with fit and finish or the general design of the product, the retool losses incurred can generally be kept at a minimum. This sort of mold adjustment involving a final production mold set could have catastrophic financial implications. Rapid injection molding is therefore a highly desirable production method for prototype and short run injection molded plastic products.

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What Is an Injection Molding Machine

Shaping liquid plastic resins into solid, usable objects and parts is a modern science that is commonly performed using an injection molding machine. The machines come in various sizes, with the size of the machine determined by the desired dimensions of the finished product. Invented in 1872, the injection molding process revolutionized the plastics industry. It is currently a multi-billion dollar business that produces about 32% by weight of all the world’s plastics. Its popularity is credited to the injection molding process, and the machines that make millions of consumer products and manufacturing parts and components that are affordable, yet strong and long-lasting.

The injection molding machine construction and operation are fairly simple. The machine consists of six basic parts. The major components of the machine include a hopper where the raw materials are inserted, a barrel to carry the materials to the heating unit, a heater to break down the materials into liquid, a nozzle to pump the liquid into the mold, a clamping unit to solidify the shape, and an ejector to expel the finished product.

To produce an injection molded product, liquid resin is poured into the injection molding machine hopper, followed by dyes or tints. Gravity draws the resin into the barrel, and the heating process melts the resin into a smooth liquid. An injection mechanism, usually a reciprocating screw or ram injector, forces the liquid into the mold. If small amounts of the resin are desired in the mold, the reciprocating screw is used as it can inject as little as 5% of the entire amount in the hopper. The ram injector is used when at least 20% of the total amount in the hopper needs to be forced into the mold.

The mold determines the shape for the finished product, and cools the liquid into a solid. While the cooling is in process, the plates of the injection mold are held together by either mechanical or hydraulic force. This clamping procedure determines the final shape of the finished product. Since different resins have different shrinking values, molds are designed with specific resins in mind.

Problems with injection molding machine performance are normally uncomplicated and easily solved. Burned or singed parts can be avoided by lowering the hopper temperature or reducing processing time. Warpage is normally solved by adjusting the mold’s surface temperature or adjusting the mold’s thickness. Imperfections in the product’s surface may be corrected through adjusting the mold’s temperature, moisture levels, or pressure.

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How Do I Choose the Best Injection Molding Company

Injection molding works by melting beads of colored polystyrene, plastic, or resin, and then injecting the material into a mold. The process is commonly used to manufacture items, including plastic cups and cutlery, outdoor furniture, bottle caps, and automobile parts. There are four main considerations when choosing an injection molding company: quality, reputation, price, and lead time. You may want to consider all four elements together, to ensure that you are choosing the vendor that best meets your needs.

Quality is important because a badly-formed piece can be not only unattractive, but unstable. Weakness in products like automotive parts, furniture, or containers for chemicals increases the risk of injury and liability. An injection molding company should produce pieces that are free from air bubbles and foreign materials. Consider asking for samples of previous projects along with the quotation so that you can compare quality across vendors.

An injection molding company should also have a good reputation with past customers, as well as within the business community. Asking for, and checking, a company’s previous client references is a good way to find out if it regularly meets delivery dates, produces quality products, and provides good customer service. You can also ask for a business credit report to determine the company’s credit-worthiness. A potential vendor with a habit of not paying suppliers might have deeper financial problems and may not be the right vendor for you.

Realistically, price is always a major concern. A competitive bidding process is the best way to ensure that you are paying a fair amount for your product. To get the most accurate comparison, you will need to provide identical specifications to each vendor. You should also ask who owns the molds. If you own the molds, you can switch manufacturers on your next production run without having to pay for molds again.

A product that arrives too late may not be useful, so lead time can be a critical factor when selecting a vendor. Availability of material, the existing production schedule, and testing of a custom color all contribute to the production timeline. It is best to make sure you have a written timeline that includes due dates for actions you must complete, such as approving samples. You may also want to find out how much of the lead time is comprised of waiting for raw materials, and ask if the potential vendor has a back-up supplier.

Ultimately, you will want to choose an injection molding company that will be a good partner down the line. This means selecting a company that has a great track record of production, produces quality goods, is financially stable, and offers fair pricing and on-time delivery. Spending extra time in the initial vendor selection process will save time and headaches later.

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