Testing for new Plastic Injection Mold

Shanghai Olimy--Plastic Injection Molding
Their length is 646.25MM,plastic raw material ABS.We spent 56 days to finish their tooling.These days we are testing them.After that we will make necessary improvements with Dimension Control report.

Nozzle Drool

At the nozzle of the barrel, molten material can flow continuously, especially during part ejection. This can be noticed inside between two open mold platens. Once in contact with the cooler mold surface, this continuously flowing material cools and solidifies. As the mold closes on this cooled material, this solidified material can be caught in the closed mold and cause mold damage. This process of continuously flowing material out of nozzle is referred to as drool. Drooling can be found not only at the nozzle but can also occur at the sprue bushing, hot tip, or at the gate. Here are some causes and solutions below for Nozzle Drool.

The presence of moisture causes a reduction in the molecular weight of the material and promotes drooling as well. Solutions: Drying the material according to recommended times and temperatures is needed to prevent drooling in this case.

The excessive melt temperature of the material is another cause of drool. Barrel temperatures set too high will increase the melt temperature of the material. Solutions: Lowering the barrel temperatures starting with the middle zones, will lower the material viscosity and reduce drool. Lowering the setting of the nozzle temperature will also reduce drool.

Location of the nozzle heater too close to the nozzle orifice can cause drool. Solutions: Relocating the nozzle heater farther back on the nozzle will reduce the temperature at the tip and orifice of the screw. Nozzle shutoff valves have been used as another option to prevent material from drooling between mold halves.

High back pressures can create drooling due to excessive working of the material, increasing the melt temperature. Solutions: Lowering back pressures can eliminate or reduce drool. The use of decompression or suck back forces the melt back into the nozzle and can be used to prevent nozzle drool.
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Brittleness of plastic injection parts

Plasticinjection part brittleness is caused most often by a loss of molecular weight in the polymer material. A reduction in molecular weight results in a reduction of the mechanical properties of the material, such as tensile strength, elongation, and impact strength. Here are some causes and solutions below for Brittleness of plastic injection parts.

Brittleness from material degradation can be caused if materials are not dried sufficiently. This applies specifically to hygroscopic materials. In this case, a chemical reaction called hydrolysis takes place when materials are melted with moisture present. When hydrolysis occurs, a loss in molecular weight results in a reduction in physical properties.

Excessive use of regrind can cause part brittleness since regrind has already been exposed to heat and as a result suffers a loss in molecular weight.

Melt temperatures may be higher than recommended process temperatures. In this case, melt temperatures at the middle of the recommended melt temperature range should be used.

The molecular weight reduction is a result of material sitting in the barrel for long periods of time, referred to as residence time. The BSR should fall between 30 and 65%. If this falls well below 30%, this will indicate a residence time problem that can lead to brittleness.

High screw speeds and back pressure can overwork the material, resulting in an increase in melt temperature.

Solutions: 1.The barrel-to-shot ratio (BSR) is measurement, in terms of percentage, used in determining residence time.  2. A reduction in nozzle temperature will reduce melt temperature avoiding brittleness form occurring. 3. Reducing screw speed and lowering back pressure can prevent resin melt temperature. 4.Increasing wall thickness, rib designs, avoidance of sharp corners, and addition of radii can reduce the chance of brittle parts.
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Flashing

Flashing is defined as excess material that has exceeded the boundary of a mold beyond the mold cavity. Flash can be found in a number of areas. The parting line is one of the more frequent locations where flash is found. When a mold does not clamp fully, this provides a space for material to flow. Vent areas are another location of flash, especially if vents are cut too deep and wide. If not fully closed, movable mold sections such as slides can move back, due to the pressure of the melt front permitting material to flash into the space resulting from the relocation of the slide. Here are some causes and solutions below for flashing.

The cause of flash on a part is inadequate drying of the material, especially hygroscopic material that can absorb moisture. The addition of water to many hygroscopic materials, such as nylon and polycarbonate, can reduce the viscosity of the material by breaking down the molecular weight of the polymer. Moisture in the form of bubbles creates an easy-flowing polymer. Solutions: Drying the material according to recommendations needs to be followed to avoid this phenomenon.

Flow properties of the material, such as viscosity and melt flow rate, affect the amount of flashing found. Low-viscosity, high-MFR material will have a tendency to flow easier, raising the potential of flashing. Solutions: High-viscosity, low-MFR materials can resist flashing since they display stiffer flow properties. Decreasing melt temperature will also prevent flashing since the viscosity of the material increases resisting flow.

Holding pressure can cause flashing, especially when packing pressures are too high, forcing more material into cavity. Solutions: Reducing hold pressure will reduce the chance of flashing. Reducing the amount of screw feed or reducing the cushion will reduce the risk of flashing, due to less material being forced into the cavity.

When a mold is designed with not enough support to the mold surface, flexing of the mold surface may occur, which will flash the tool. Solutions: Support inside the mold to resist the high pressures applied is important in preventing flash. Sufficient support of the mold surface and cavity needs to be designed to prevent flexing of the surface.

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Undercuts Design in plastic injection parts

An undercuts is an indentation or projection on a plastic injection part usually means that the core moving direction is not parallel to the draw direction of mold, so that the molded plastic injection part makes ejection from the sample two- part mold almost impossible. Because of the movement way they are oriented, some feature is introduced to injection mold design. The cores or protrusions, which used to form these indentation, referred to as undercuts, are typically used in articles such as enclosures to provide a snap fit to assemble plastic injection parts.

Internal undercuts can be molded by using two separate core pins, as shown in Fig 1. This is a very practical method to use, but flash will sometimes occur where the two core pins meet.

Fig 1 An internal undercut can be molded by using two separate core pins

    Fig 2 shows different types of undercuts, which are frequently necessary in a molded plastic part design. It can be classified as an undercut on one side wall, internal undercut, external undercut, and circular undercut of a part.

There is a very fine distinction between undesirable and impossible undercuts. Fig 2a is an internal undercuts which can be formed using movable cores that are low efficiency and expensive, and that should be avoided. A more efficiency method to molding an internal undercut is redesign the products as shown in Fig 2b.

External circular undercuts are loaded in the outside contours of the part as shown in Fig 2c. A more efficiency method to molding an external undercut is to use two retractable ejector wedges. The other less expensive method is forming by grooving the part in a lathe with a properly designed cutting tool. Side wall undercuts as shown in Fig 2d must be molded by retractable forming core or wedge which is quite expensive.

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Fig 2 Different types of undercuts in molded plastic injection parts

Nominal Wall Thickness

The nominal wall thickness effects not only on the strength of plastic injectionparts, but also on the characteristics such as performance, surface aesthetics, appearance, moldability, and economics, so that the nominal wall is the ground floor for plastic injection part design. The best plastic injection part thickness is often a trade-off between strength versus parts weight, durability versus parts cost. For injection molding, it is important to keep the thickness of a part as constant, or nearly as constant. Constant wall thickness can assures that uniform mold shrinkage will prevent part warpage problem, and to obtain accuracy dimension of parts.

Fig 1 illustrates both poor and optimum object nominal wall thickness design. The first is to make a constant or nearly constant wall thickness. The second is that if there is a transition in wall thickness, the key consideration is to make wall transitions less drastic and abrupt. The corner should be design in a radius.

Fig 1 Wall thickness design

A rule of thumb is to avoid designing with nominal wall thickness above 4.0mm for most thermoplastics products. When the thickness is higher than 4.0mm that will cause excessively long cycle times for that the long cooling times is needed to remove heat from a thick wall, and will risks the increase of voids in a plastic injection part, affects the part performance negatively. If wall thickness higher than 4.0mm is required, it is best way to use other molding process technologies, such as structural foam or gas-assist injection molding.

Fig 2 illustrates the corner design of plastic injection parts. This modification is benefit to improve cooling of the part, and to reducing cycle times without sacrificing part structural integrity. Thick, heavy solid section such as are found in knobs and handles can be designed into two individual moldings and keeping the nominal wall as uniform as possible avoids voids in plastic injection parts. The special products such as handle usually are designed with texture in exterior for aesthetics.

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Fig 2 Corner of plastic parts

Delamination in plastic injection parts

Delamination is a condition that exists when the surface skin of a molded part can be physically separated from the part which can lead to a reduction in mechanical strength . Here are some causes and solutions below for Delamination of plasticinjection parts.

One of the main cause of delamination is incompatibility of mixed materials due to material contamination. For example, nylon is contaminated or mixed with polyethylene, delamination will take place, due to the difference in the rheological properties of the two materials. Solutions: Recommended drying temperatures and times should be used based on material supplier recommendations.

Contamination can occur from moisture in material that has not been dried sufficiently. Bubbles or blisters are formed and trapped on the part surface in a fully packed cavity, and the skin formed by the trapped moisture can be separated. Solutions: Better mixing and better melt uniformity as well as increasing melt temperature is needed to prevent delamination in this case.

Nonuniform melt temperatures are yet another cause of delamination. Colder material will flow much slower than the hotter material, causing the cold skin to separate from the hotter forming skin. Solutions: Better screw design will also help in providing improved melt uniformity.

High packing pressures can cause delamination at the gate. When a object shrinks away from the gate or sprue bushing, high packing pressure can force excess material into the gap between the object and mold cavity. A thin layer of material is formed on the surface of the part. Upon workpiece ejection, this thin layer can separate and delaminate. Solutions: Reducing pack pressure will prevent this buildup of excess material from forming.

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Injection molds

Injection mold is principally used for the production of thermoplastic parts, although some progress has been made in developing a method for injection mold some thermosetting materials. The principle of injection mold is quite similar to that of die-casting. Plastic power is loaded into the feed hopper and a certain amount feeds into the heating chamber when the plunger draws back. This plastic power under heat and pressure in the heating chamber becomes a fluid. Heating temperature rang from 265 to 500° F. After the mold is closed, the plunger moves forward, forcing some of the fluid plastic into the mold cavity under pressures ranging from 12000 to 30000 psi. Since the mold is cooled by circulating cold water, the plastic hardens and the part may be ejected when the plunger draws back and the mold opens.

Injection-molding machines can be arranged for manual operation, automatic single-cycle operation, and full automatic operation. Typical machines produce molded parts weighing up to 22 ounces at the rate of four shots per minute, and it is possible on some machines to obtain a rate of six shots per minute. The molds used are similar to the dies of a die-casting machine with the exception that the surfaces are chromium-plated. The advantage of injection mold are:

(1) A high molding speed adapted for mass production is possible.

(2)There is a wide choice of thermoplastic materials providing a variety of useful properties.

(3)It is possible to mold threads, undercuts, side holes, and large thin sections.

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Injection Mould Parts

The parts of injection mould are divided into the following categories based on their different functions.

Forming part: The forming parts consist of the punch (also know as the core ), the cavity plate ( also known as the cavity ), inlay and insert. When the whole mould is closed, a cavity is established. In the mould, the cavity is produced by fixed half of a mould and punch.

Feed system: The feed system refers to the passage-way through which the molten plastic passes from the nozzle of the injection machine into mold cavity. It consists of four parts: sprue, runner, gate and cold-slug well.

Ejecting mechanism: The ejecting mechanism means the equipment push-off plastic part from mould after parting. In the mould, the ejecting mechanism consists together of ejector plate and ejector retainer plate, and sprue puller pin, and ejector guide pillar, and ejector guide bush, and ejector pin, and return pin.

Temperature adjustable system: In order to meet the demand on temperature of the mould in the injection technology, the temperature of the mould must be controlled, so a temperature adjustment system is normally installed for cooling or heating the mould. The cooling system is generally realized through the cooling channel on the mold. The heating system consists of the heating elements inside or around the mould.

Exhaust and overflowing system: In order to exhaust gases inside the cavity in the process of injection and forming, an exhaust and overflowing system mush be set up. Gases can be exhausted by the parting line, by air vent purposefully made on the parting line, or through the fit clearance between the ejector pin or core and the mould plate.

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Injection mould

Plasticinjection mould is used for forming thermoset plastic products or thermoplasticity  plastic products.

Granulous or powdery plastic is fed by the hopper of an injection machine into a heated charging barrel, where it is heated and molten and is then driven and injected by the force plunger or thread institution into a closed mould through the front-end jet of the charging barrel. The molten plastic is then solidified under pressure through cooling ( for thermoset plastic) or heating ( for thermoplasticity plastic ), and it will take the shape the injection mould cavity produces. When the closed mould is opened, a plastic product is obtained. When open the mould and take out plastic products, a moulding period is completed. Such moulding technology is called injection moulding. Here are two injection moulds, single parting line injection mould (Fig 1) and double parting line injection mould (Fig 2).

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    Fig 1 Single parting line injection mould

    1-Ejector pin    2-Ejector retainer plate 3-Ejector guide bush 4-Ejector guide pillar

    5-Ejector plate 6-Sprue puller pin 7-Return pin  8-Support pin

    9-Guide pillar   10-Guide bush  11-Cavity plate of fixed half  12-Feed system

    13-Plastic part  14-Core 

    Fig 2 Double parting line injection mould

    a) Mould closing state      b) Mould opening state

    1-Mould base leg  2-Support plate  3-Punch-retainer plate  4-Stripper plate

    5-Guide pillar  6-Stop pin  7-Spring  8-Limit plate  9-Sprue bush

    10-Clamping plate of the fixed half  11-Middle plate  12-Ejection pin for guide pillar

    13-Ejection pin  14-Ejection retainer plate  15-Ejector plate

Gloss for plastic injection parts

Gloss is defined as the amount of light that is reflected off a surface. The rating used for gloss is based on how much light is reflected at different angles and the degree of light that is scattered. Parts with high gloss reflect the majority of light with very low scatter. Low gloss is low reflection at differing angles with large scatter. The correct amount of gloss is determined by the requirements of the end-use application. Here are some causes and solutions below for gloss of plastic injection parts.

Cold melt temperatures provide low gloss on untextured surfaces. Solutions: If higher gloss is needed, processing at higher temperatures is needed. In some cases, too hot a melt temperature can also cause low gloss. In this case, additives that are mixed in with the base resin, such as plasticizers or some flame retardants, can bloom on the surface. Reducing the melt temperature may help in this situation.

Low packing pressure is another cause of low gloss, since the plastic material is not fully packed onto the cavity surface and does not replicate the surface. Solutions: Increasing pack pressure will allow the melt to pick up all the details of the mold cavity.

Injection speeds that are too slow will reduce the gloss on the surface since these also may cause the melt not to replicate the mold surface. Solutions: Increasing injection speed will raise the melt temperature and pack out the part.

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Thread Design

Thread are used in plastic for the purpose of providing a secure anchorage or locking device for a mating plastic injected part. There are two types of threads are used in plastic injected part design: internal and external thread. External or internal screw threads can be molded. An internal thread is on the inside of the plastic injected parts which molded-in by a threaded core. An external thread is on the outside of a plastic injected part which molded by threaded pipe or threaded hold or by two half-molds.

Threads in plastic injected parts are obtained by four methods: molded-in; trapped; threaded metal inserts and thread inserts are pressed or cemented into plastic injected parts after injection molding.

Threads formed by thread plug or tube should be taken to eject the plastic injected parts from the mold. Usually, mechanical unscrewing or collapsible cores can be used in injection molding. There are different types of threads used in the plastic industry. It includes the V sharp, square, acme, buttress, round profile thread, unified screw thread. Fig 1 shows profiles of different types of threads which usually used in plastic injected parts.

Fig 1a shows a developed unified screw thread which is used frequently. The feature is that the tip or crest of the thread is flat, the root of the thread has a radius and does not leave a sharp edge. It can be quick and easy assembly of the plastic injected parts which are desired, and for all work where conditions do not use of fine-pitch threads. 

A square thread (Fig 1b) is used where the highest strength is desired such as in pipe fittings. The thread unscrewing is not as easy as unified screw thread during molding.

The acme thread (Fig 1c) is widely used in applications requiring strength because this type of thread is much easier to mold or unscrew than the square thread.

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Fig 1 Profiles of different types of threads

Designing with Weld Lines

Knit lines, known also as weld lines, are formed when two melt fronts converge and joined together, forming a thin, fine line in a plastic injected part. In a plastic injected part, weld lines can also be formed as a result of flow interruptions such thin sections, holes, slots, and other types of cored-out areas. Weld lines affect the strength and aesthetics of plastic injected plastic.

If there are holes, ribs, bosses, and opposite gates in plastic injected parts(Fig 1), the plastic flow in a mold is split by an obstruction such as a pin, insert, corner, or slot, weld lines will usually result when the flow fronts meet, as in Fig 1 a and Fig 1 b. Weld lines are particularly noticeable in transparent and translucent plastic materials.

Weld lines not only distort the aesthetics of plastic injected parts, but also affect the strength of plastic injected part for the point of potential failure. This problem occurs mostly to thermoset  and thermoplastic materials used in injection. Weld lines can be prevented by designing the mold so as to permit the material to move with maximum freedom.

An overflow located at the converging melt fronts is a technique. That permits the plastic material at the weld line to flow into a pocket, allowing the two melt fronts to bond together with higher injection pressure, as Fig 1 c. However, after molding, this overflow may need to be removed manually.

Setting a porous metal insert at the place of melt fronts converge and join is another technique. This can remove trapped gases at the converging melt fronts and providing a higher weld line strength, as Fig 1 d. This is recommended mostly for textured surfaces since on smooth surfaces a slight gloss difference can be found on the plastic injected parts.

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Fig 1 Forming and Solution of weld line

Plastic Moulds and Plastic Forming Technology

The mould used to make plastic products is called plastic mould for short. In order to get plastic products with certain sizes, shapes and properties, thermosetting or thermoplastic plastic is heated to a certain temperature, put into a mold cavity, and taken out after it cools. This is called plastic forming technology.

Plastic forming technology are classified mainly into compression forming, transfer forming, injection forming in accordance with method of moulding. Moreover there are hollow how molding forming, foaming forming, pouring forming, etc.

In accordance with plastic forming technology, plastic mould are classified into compression mould, transfer mould, injection mould and hollow blow molding mould, foaming mould, pouring mould, etc.

There are many kinds of plastic moulds, but they only can be decided into two main types, forming parts and structure parts. Now, it explain basic structure of mould with a  example of typical structure.

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Fig 1 Basic structure of Fixed injection mould

a) Mould closing state    b) Mould opening state

1-Sprue puller pin  2-Ejector pin  3-Headed guide pillar  4-Core  5-Cavity plate

6-Cooling channel  7-Locating ring  8-Sprue bush  9-Clamping plate of the fixed half

10-Fixed half of a mould  11-Moving half of a mould  12-Support plate

13-Spacer (mold leg)  14-Ejector retaining plate  15-Ejector plate

Inserts Design in plastic injected parts

Inserts generally serve an important functional purpose, but should be used sparingly because of the costs involved. They may be used in plastic injected parts to carry much higher mechanical stresses than plastic themselves, to transmit electric current, to take wear and tear, to decorate the plastic injected parts, and to aid in assembly work. Most of inserts may be made of copper, brass, aluminum and its alloy, or steel, ceramics and including plastics.

The great majority of inserts used in plastic injected parts are made by either automatic screw machines or metal stamping machines. Fig 1 shows variety of metal inserts used in molded plastic products, usually a medium or coarse diamond knurl. (a) Projecting rivet. The end of the insert to be imbedded in the plastic is rounded or chamfered. A chamfered or rounded end is desirable so that the plastics will flow easily around the insert and manufactory easily. The sharp corner on inserts which embedded may cause crack of plastic injected parts.

(b)Male insert as bolts and female inserts as nuts should be provided with shoulders to help prevent the plastic compound from flowing into the threads. Avoid using an insert that has not been provided with a shoulder which the flash may flow on the thread. A single sealing shoulder is better. A double sealing shoulder is the best, but it is more expensive.

(c)Blind female insert with internal threads. (d) Open ends female insert with internal threads. (e)Blind hole female insert with internal threads and counter bore.

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                                (a)                                                                    (b)

 
                       (c)                                                 (d)                                                  (e)

Fig 1 Various designed inserts

Voids in plastic injected part

Voids are formed inside the wall of a part and can adversely affect the structural performance of the plastic injected part. Except in transparent parts, voids can be invisible to the naked eye. Voids are formed as a result of shrinkage of the molten core after the wall has solidified on the cooler mold surface. The shrinkage of the molten core cause the layers of material to pull away, forming an opening in the part or void. Here are some causes and solutions below for voids of plastic injected parts.

Higher melt temperature slow down the cooling rate of the polymer in the plasticinjection mold, causing higher shrinkage in the hotter core, leading to voids. Solutions: Lower melt temperatures can help reduce the formation of voids.

Injecting the material at high speeds raising melt temperature due to share heating can also cause overheating. Solutions: Low packing pressures will allow voids to form since there is not enough resistance applied on the core to prevent shrinkage.

A gate that freezes-off too soon will prevent material from filling out the cavity completely, causing the molten core to shrink. Solutions: Measuring gate seal time can indicate whether the cavity is full before the gate closes.

Small runner diameters will cool faster and hinder cavity filling, creating the potential for voids to form in the part. Solutions: Larger-diameter runners, such as full round, trapezoidal, and modified trapezoidal are recommended.

A cold mold will only enhance the formation of the skin surface long before the core cools sufficiently. As a result, the core forms more slowly and shrinks more. Solutions: Increasing plastic injection mold temperature slows down the cooling rate, and the formation of the skin prevents voids. Improvements can be made in mold cooling, such as the addition of bubblers, cooling pins, thermal pins, or steel inserts to drive out more heat from the part.

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