Plastic Injection Molding Techniques: Design and Process

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What is Design

Molds separate into two sides at a parting line the A side and the B side to permit part to be the extracted. Plastic resin enters the mold through a spruce in the A plate branches out between the two sides through channels called runners and the enters each part cavity through one or more the specialized gates. Inside each cavity the resin flows around protrusions called cores and conforms to the cavity geometry to form the desired part. The amount of resin required to fill the sprue runner and cavities of a mold is the shot. When a core shuts off the against an opposing mold cavity or core a whole results in the part. Air in the cavities when the mold closes escapes through very slight gaps between the plates and pins, into shallow plenums called the vents.

To permit removal of the part, its features must not overhang one another in the direction that the mold opens, unless parts of the mold are designed to move from between such overhangs when the mold opens. Sides of the part that appear parallel with the direction of draw (the direction in which the core and cavity separate from each other) are typically angled slightly with (draft) to ease release of the part from the mold, and examination of most plastic household objects will reveal this.

Injection Molding Process

Basic Process: Injection molding is the most widely used molding process for the thermoplastics. It is based on the ability of thermoplastic materials to be the softened by heat and to harden when the cooled. The process thus consists essentially of softening the material in a heated cylinder and the injecting it under pressure into the mold cavity where it hardens by the cooling. Each step is the carried out in a separate zone of the same apparatus in the cyclic operation.

Applications: Solid Wide neck, Flat Product is made like bucket, Automobile & Industrial parts, Molded chairs, Computer/ TV/ Radio Cabinets, Washing machine Cabinets, Mobile, Power- tool housings etc. Molded House hold items etc. by injecting molten thermoplastic material in to a closed mold which is relatively cool.

Type of Injection Molding Machine

1. Hand Injection Molding M/C

Hand Injection Molding machine

2. Plunger type Injection Molding M/C

A diagram of the typical injection-molding machine is the shown in fig. Granular material the polymer fed from hopper into the barrel when plunger is the withdrawn. The plunger then pushes the material into heating zone where it is the heated and softened plasticized or the elasticated. Rapid heating takes place due to spreading of the polymer into a thin film around the torpedo.

The already molten polymer displaced by this new material is the pushed forward through the nozzle which is in the intimate contact with the mold. The molten polymer flows through the spruce, runner and gate, into the mold cavity. The mold is held tightly closed by the clamping action of the press platen. The material in the mold must be cooled under pressure below Tm or Tag before the mold is opened and the molded part is ejected.

3. Reciprocating Screw Type Injection Molding M/C

Injection molding involves two distinct processes. The first comprises elementary steps of the solids transport melt generation, mixing, and the pressurization and flow which are carried out in the injection unit of the molding machine the second is product shaping and the structuring which takes place in the mold cavity. Most injection molding machines are the in-line reciprocating-screw type as illustrated in the following Fig.

Reciprocating Screw Type Injection Molding machine

Advantages of Injection Molding Process

  • Parts can be the produced at high production the rates.
  • Large volume production is the possible.
  • Relatively low labor cost per unit is to be the obtainable.
  • Process is the highly susceptible to the automation.
  • Parts require little or no the finishing.
  •  Many the different surfaces, colors, and finishes are the available.
  • Good decoration is the possible.
  • For the many shapes this process is the most economical way to the fabricate.
  • Process permits the manufacture of very small parts which are the almost impossible to the fabricate in quantities by other the methods.
  • Minimal scrap loss result as the runner’s gates and rejects can be reground and the reused.
  • Same items can be the molded in different materials, without changing the machine or mold in some the cases.
  • Close dimensional tolerances can be the maintained.
  • Parts can be the molded with metallic and the non-metallic inserts.
  • Parts can be the molded in a combination of plastic and the fillers as glass, asbestos, talc and the carbon.
  • The inherent properties of the material give many advantages such as high the strength- weight rates, corrosion resistance, strength and the clarity.

Selection of injection machine

1. Select by injection volume

As a guide, generally the injection machine should be selected so that molded product volume will become 30% to 80% of the machine’s injection volume. When molding, the relation of the machine’s injection volume Q (g) and one shot weight (sprue and runner weight included) W (g) should be in the range indicated below.
                                            Q = (1.3~1.5)×W
If the injection volume is too small, plasticization will not make it, and might lose its original physicality as a molded product because the resin will be sent without enough plasticization. On the other hand if injection volume is too the big, residence time inside the cylinder will be longer and cause degradation by the more chance.

2. Select by mold clamping pressure

Both toggle type and direct pressure type is suitable when molding NOVADURAN. The relation of molded product projected area a (cm2) and required mold clamping pressure P (ton) should be in the range indicated below.
P = (0.5~0.7)×A

3. Nozzle structure

Open nozzle is the common when molding the NOVADURAN. The nozzle of commercially-supplied injection machine can be the open nozzle or shut-off nozzle but in any type, it is the necessary to have a temperature the control. If drooling from the nozzle is can be concerned use the shut-off nozzle. However, it is the might cause burn and sunspot object by the resin retention at slide part, so be the careful.

Types and structures of the nozzle

4. Injection mechanism

NOVADURAN can be molded by the basic injection machine which has the function of constant injection speed and two-stage injection pressure control, but when molding the product which severe measurement, appearance, and malleability liquidity and demoldability is required, it is effective to use the machine that has a program control of injection speed and injection pressure.

5. Backflow prevention ring

Backflow prevention ring is the necessary at screw because NOVADURAN has the relatively low melt the viscosity. If this backflow prevention ring is the damaged by wear or corrode cushion the volume cannot be kept because of the resin backflow from cylinder to the hopper when injecting pressure keeping and the injection pressure holding pressure might not be put properly to the cavity. In this case good molded product cannot be the made so cushion volume and it’s the stability must be well controlled and the maintained when molding. Corrosion and the abrasion resistance steel grade is preferable for the back flow prevention ring.

6. Drying machine

Preliminary drying is necessary before molding NOVADURAN, and the condition below is general.
                                       120°C 5~8 hours’
                                       130°C 4~6 hours
Shelf-type hot air circulation dryer, hopper dryer, or dehumidification dryer is preferred when drying. To prevent the dust and the dirt getting inside, a filter should be placed to air intake of the drying machine, and its maintenance against clogging is also necessary.

How Plastic Fills a Mold

The injection molding process, could be divided into three phases

1. Filling Phase

As the screw moves forward, it first moves at a steady speed as the plastic flows into the cavity. This is the filling phase. This phase lasts until the mold is just filled.

Filling Phase

2. Pressurization Phase

The pressurization phase begins when the screw moves forward after the filling phase to the mold up to pressure. When the mold is filled, the screw will slow down, but it quite some distance because plastics are very compressible materials. At pressure, an extra 15% volume of material can be forced into the cavity. Although fluids are usually assumed to be incompressible, molten considered to be more like a gas. The compressibility of plastics can be off the nozzle and attempting to purge the barrel. The ram will jump forward pressure is applied, but will spring back when the pressure is released.

3. Compensation Phase

After the pressurization phase the screw still does not stop completely continuing to the creep forward for some the time. Plastics have a very large volumetric change of the about 25% from the Figure 1.10. The screw moving forward to co volumetric change in the part is called the compensation phase. As the volumetric change is 25% and, at the most, only an extra 15% can be injected in the pressurization phase, there must always be the some compensation phase.

Compensation Phase

Mold Designing

1. Point of mold designing

Designing of molded product should be done to fulfil demanded characteristics of desired product, and need to evaluate material’s practical physicality, foldability, liquidity, and mold designing condition, comprehensively. Basic points of mold designing are indicated below.

  • Try not to make the thickness excessively thick, and try to the keep it even, so that rapid change in the thickness will not occur.
  • If the molded product thickness is too the thick it will be the cause of defect like sink marks and the void. Also it will take time to cool down and the molding cycle will be the longer. When there is the need to be thick for function try to keep it even by placing the recess.
  • Try not to make the undercut. If there is undercut in the molded product problems likely to the occur when releasing the mold, so as a general rule, there should be no the undercut. When undercut have to be the  placed by necessity, make the undercut volume small enough towards the limit strain based on the material physicality, or design the mold construction not be the forced extraction by placing the slide core.
  • Consider the draft angle. If the draft angle is not enough, resist when releasing the mold will be big and the molded product might deform by the ejector pin so the draft angle should be taken as the big as possible. Typical draft angle of the NOVADURAN is about 0.25 to 1°in unreinforced grade, and the about 0.5 to 2°in GF reinforced the grade.

2. Gate Designing

Gate selection should be the made considering molded product shape, number, performance, appearance, economic efficiency, and the foldability. There are many types of the gates, and Figure 2-11 indicates structure of the each gate.

(1)Direct sprue gate

Used in the case of single- cavity, or when placing the gate directly to base of the molded product. Residual strain tend to the occur because injection pressure will be the directly apply to the molded product, but its mold construction is the most simple.

(2)Side gate

This type is the most generally adopted, and used well be the in multi-cavity mold. Its shape is the rectangle or semicircle, and the placed at side of molded the product.

(3)Fan gate

Its structure is the similar to side gate, but gate width is the bigger and fan-shaped. Used in large size molded the product.

3. Multi-cavity layout

In the case of multi-cavity type runner design will be the play an important role, because the dimension tend to vary the widely. Family-cavity type, which molds the product with different cavity shapes simultaneously, is the basically not recommended. In the case of double-cavity layout, right and the left layout is preferred rather than above and the bottom, because it will be affected by the gravity. Especially talking about the thick molded product, the resin will be affected by gravity at the lower cavity, and jetting might occur because it is droop down to the lower cavity after getting through the gate.

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