Plastic Injection Molding

Abstract:

This paper is concerned with the injection moulding process, in which hot molten plastic is injected under high pressure into a thin cold mould. Assuming that the velocity and temperature profiles across the mould maintain their shape, a simple steady state model to describe the behaviour of a Newtonian fluid during the filling stage is developed. Various phenomena of the process are examined, including the formation of a layer of solid plastic along the walls of the mould, and the relationship between the flux of liquid plastic through the mould and the average pressure gradient along the mould. In any given situation, it is shown that there is a range of pressures and injection temperatures which will give satisfactory results.

Injection molding has been a challenging process for many manufacturers and researchers to produce products meeting requirements at the lowest cost. Faced with

global competition in injection molding industry, using the trialand- error approach to determine the process parameters for injection molding is no longer good enough. Factors that affect the quality of a molded part can be classified into four categories: part design, mold design, machine performance and processing conditions. The part and mold design are assumed as established and fixed. During production, quality characteristics may deviate due to drifting or shifting of processing conditions caused by machine wear, environmental change or operator fatigue. Determining optimal process parameter settings critically influences productivity, quality, and cost of production in the plastic injection molding (PIM) industry. Previously, production engineers used either trial-and-error method or Taguchi’s parameter design method to determine optimal process parameter settings for PIM. However, these methods are unsuitable in present PIM because of the increasing complexity of product design and the requirement of multi-response quality

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characteristics. This article aims to review the recent research in designing and determining process parameters of injection molding. A number of research works based on various approaches have been performed in the domain of the parameter setting for injection molding. These approaches, including mathematical models, Taguchi method, Artificial Neural Networks (ANN),Fuzzy logic, Case Based Reasoning (CBR), Genetic Algorithms (GA), Finite Element Method(FEM),Non Linear Modeling, Response Surface Methodology, Linear Regression Analysis ,Grey Rational Analysis and Principle Component Analysis (PCA) are described in this article. The strength and the weakness of individual approaches are discussed. It is then followed by conclusions and discussions of the potential research in determining process parameters for injection molding.

Introduction:

This project takes a look at a half factorial experiment on plastic injection molding and how independent variables affect the dependent variables. These independent variables include Barrel Temperature, Nozzle Temperature, Dwell Time, Injection Pressure, and Flow Rate. This independent variable will be tested to see what affects they have on the final part weight (dependent variable). The range of the half factorial and 3 replication will help give a broad range of data to further understand what affects have the most impact on the final outcome.

Theory:

The first plastic injection molding machine was introduced in 1872 by John Wesley Hyatt and his brother Isaiah. When World War II hit in the 1940's the industry boomed and mass production plastic injection came about. Today's industry supplies a wide variety of injection plastics ranging from bottle caps to automotive dash boards and hair combs. Plastic injection can be very cost efficient to run with high production rates, high tolerance repeatability, low scrap and little finishing needed. Most injected polymers are either thermoplastics, thermosets or some elastomers. They range in a wide variety of alloys and blends from epoxy, phenolic, nylon, polyethylene, and polystyrene.

The plastic injection machine consists of a heating and injecting unit to make the plastic injection. A heated barrel with a screw type feeder or ram feeds the material into the machine to be heated before injection. By means of pressure and the ram the material is fed into the heater to melt the plastic pellets. With more pressure and force of the ram the plastic flows into the nozzle as a liquid and into the mold where it fills the mold and cools until it is hardened. The mold is place near the nozzle of the plastic injection machine and clamped into place with a large force to keep the mold firmly in place during injection. This helps to reduce flash from the mold. After cooling the part can be ejected from the mold and the sprue can be moved as well.

Many factors play into the final outcome of the injected plastic part. Temperature of the barrel and nozzle play a vital role in how viscous the plastic is when it is being

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injected. Pressure and flow rate aid in the injection rate and flow, too high and jetting can occur from turbulence in the flow. Flow marks can also occur is the speed is to slow for injecting. Mold design is critical to make sure that the mold fills with material and that it can be ejected from the mold. Draft angles and fillets or chamfers can help reduce voids in molds and help with ejection from the mold.

Experimental Procedure:

Variables: By varying the independent variables stated down below will conclude the interactions between them and show if there is significant cause of that variable. Also interactions of combinations of variables will be taken into account as well.

Independent Variables: Barrel Temperature, Nozzle temperature, Dwell Time, Injection Pressure, Flow Rate

Dependent Variable: Part Weight

Equipment Used:

Safety Goggles Plastic Injection Machine Air Compressor Injections Die Molds Polyethylene Gloves Cutting Snips Weight Scale

Procedure:

Preliminary:

Turn on the plastic injection machine and let it warm up to proper operating temperature.

Put clean Die Mold into alignment bolts.

make sure the table clamp holds the mold securely. Clamp should click into place insuring proper mold clamping. (between 7 and 10 Tons) if necessary use allen wrench to adjust clamp pressure.

Check to following: o Barrel Temperature

o Nozzle Temperature

o Tooling Temperature

o Clamp Pressure

o Injection Pressure

o Flow Rate

Injection pressure control is controlled by a T screw at air line.

Flow rate is controlled in the back by turning the adjustment knob.

Double check all settings before continuing.

Injection Procedure:

Place Die Mold into alignment bolts. Clear nozzle drool with aluminum

scraper. Lower table guard. Push Clamp Valve to raise bottom

platen (check to hear click of the clamp)

Use aluminum scrapper to cylinder hopper.

Double check settings. Push injection control valve to inject

plastic. Wait specified time by experiment.

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Pull the injection control valve to retract ram. (check hopper to make sure ram is fully retracted)

Pull clamp valve out to lower bottom platen.

Raise table guard. Remove Die Mold. Open mold, grasp plastic part and

remove part with twist method. Remove sprue with push pull method

from Die Mold. Check to make sure Die Mold is

clean or excess plastic and debris. Snip part of excess plastic

consistently. Weigh part.

Shut Down Instruction

Check the Clamp Valve is the out position.

Lower table guard. Turn off main power switch on side

of temperature control. Remove both 110V power plugs. Turn off air supply. Bleed air lines out.

Discussion:

Plastic injection molding consists of two main components: injecting and clamping. Starting with the clamping end of the process, a mold must be made for the plastic injector to push hot liquid plastic into. The mold is usually made of a machine-able metal, usually aluminum or steel. The mold consists of a closed cavity for the material to flow into. A sprue is a hollow tube leading up from the mold for the material to flow into and into the mold. A well is located at the bottom of the sprue to build up material and push it into the mold. Another important part of the mold cavity is the vents; these help push ambient air in the mold so the mold can be completely filled. The injection part of the process includes a hopper for raw plastic material to be fed into. This is usually fed into the barrel by a screw pushing the material towards the nozzle. The barrel heats up the material till it is soft and turned into a liquid. The nozzle pushes the material through down into the mold by

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pressure. The barrel and nozzle temperatures can be controlled separately. These temperatures control the viscosity of the material. The pressure of the injector controls the rate of flow of the injection into the die cavity.

Polyethylene is a synthetic fiber used in making various products like beverage and containers. It is a good barrier against gas, moisture, alcohol, and solvents. The chemical composition for polyethylene is (C10H8O4) and is a thermoplastic. Has a crystalline density of 1.455 g/cm3, a tensile strength of 55-75 MPa, a Young’s Modulus (E) of 2800-3100 MPa, and a melting temperature of 260oC. It is strong and impact resistant. It has a semi-crystalline structure and is transparent or opaque depending on the structure and particle size. Polyethylene is commonly referred to as polyester for packaging materials. Before the material can be processed it must be dried to bring down its moisture content as far as possible. Polyethylene can be formed through plastic molding injection or blow molding. After a product has reached its end of life cycle the polyethylene can be recycled back into the system and used again once it has been process and chipped back up to be used again in molding.

Polypropylene (PP) is a thermoplastic polymer with whose mer chemical formula is C3H6. PP’s density ranges from 0.855 g/cm3 to 0.946 g/cm3. Its melting point ranges from 130 to 171 degrees Celsius, however in most application it is assumed to be 160 degrees Celsius. PP is commonly used to produce both fiber and molding. Therefore, the two

main manufacturing processes are extrusion, and injection molding, respectively for fiber and plastic molds. PP is commonly used for Tupperware due to its higher melting point, which allows it to heat foods without melting. It is also highly resistive to fatigue and stress, allowing it to be used in high wear application. Its main weakness is degradation due to UV radiation.

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Conclusion

In conclusion this project was useful in exploring and understanding how different control factors have different effects on a desired characteristic for a part. Through the use of design expert it was shown how some control variables have a strong effect on a specified characteristic, while others have little to no effect. Through design expert it was found that the variables affecting the final weight of the plastic mold injected part the most, were pressure and flow weight. Each of these factors had a larger impact than the nozzle temperature, barrel temperature and dwell time combined. Even though this experiment was a half factorial, a lot of useful data was collected about the

different factors and how they interact with each other.

Finally we conclude that plastic injection molding is very helpful in making all necessary basic goods like tumblers, body of the brush etc which attract people with snazzy colours as well as shining and their smoothness.

REFERENCES

1.Mathivanan D. ; Parthasarathy, N. S. (2009) .Prediction of sink depths using nonlinear modeling of injection molding variables,International Journal of Advanced

Manufacturing Technology, vol. 43 pp.654–663.2. Protyusha Das Neogi (2009) , Comparing the Predictive Ability of T-Method and Linear Regression Method , Proceedings of the Industrial Engineering

Research Conference.3. Raviwongse ,Rawin; Allada., Venkat; (1997) Artificial Neural Network Based Model for Computation of Injection Mould Complexity International Journal of Advanced Manufacturing Technology , 13, pp. 577-5864. "History of Plastic Injection Moulding & Rotational Moulding." Plastic Moulding: Process, Polymers & History. N.p., n.d. Web. 20 Apr. 2010. <http://www.plasticmoulding.ca/history.htm>.

5. "Injection molding - Wikipedia, the free encyclopedia." Main Page - Wikipedia, the free encyclopedia. N.p., n.d. Web. 20 Apr. 2010. <http://en.wikipedia.org/wiki/Injection_molding>.

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6. "Plastic Injection Molding Company ." CNC Machine Shop | Custom Waterjet, Plasma, Laser Cutting | eMachineShop.com . N.p., n.d. Web. 20 Apr. 2010. <http://www.emachineshop.com/machine-shop/Plastic-Injection-Molding/page80.html>.

7. "Plastic Injection Molding | Thermoplastics, Thermosets, Machines, Resins and More." Plastic Injection Molding | Thermoplastics, Thermosets, Machines, Resins and More. N.p., n.d. Web. 20 Apr. 2010. <http://www.injection-molding-resource.org/>.

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