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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