LAB REPORT

B.S. MECHANICAL ENGINEERING

MECHANICAL ENGINEERING LAB # 1

Title: “Injection Molding”

Group #: 4

Submitted by: Muhammad Ahsan MirMuhammad Ali keyaniMuhammad Ali NaveedMuhammad Hassan RazzaqMuhammad Asif Khan

Submitted to: Mr. Naeem ul Haq Tariq

Abstract:

In this experiment we understood the working of an NC Injection molding machine. We learned how to operate an NC Injection molding machine and we made a final product of Low Poly-ethene with the help of injection molding.

Introduction

Injection molding is a process in which a polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity, where it solidifies. The molded part, called a molding, is then removed from the cavity. The process produces discrete components that are almost always net shape. The production cycle time is typically in the range of 10 to 30 sec, although cycles of 1 min or longer are not uncommon for large parts. Also, the mold may contain more than one cavity, so that multiple moldings are produced each cycle.

Complex and intricate shapes are possible with injection molding. The challenge in these cases is to fabricate a mold whose cavity is the same geometry as the part and that also allows for part removal. Part size can range from about 50 g (2oz) up to about 25 kg (more than 50 lb), the upper limit represented by components such as refrigerator doors and automobile bumpers. The mold determines the part shape and size and is the special tooling in injection molding. For large, complex parts, the mold can cost hundreds of thousands of dollars. For small parts, the mold can be built to contain multiple cavities, also making the mold expensive. Thus, injection molding is economical only for large production quantities. Injection molding is the most widely used molding process for thermoplastics

Equipment:

Figure 1

PROCEDURE

Before the injection molding process is started, we should know the material which is to be used in the injection molding. After this, we should set the machine setting according to manufacturing part. Note that the temperature settings for actual machine parameters. The % of the machine’s total pressure and speed are settings that determine the limits the machine will operate at during the cycle.

Warm the injection molding machine. Once the injection molding machine has reached the proper temperatures for molding, processing can begin. The machine operating settings are then need to be determined. The first parameter that needs to be determined is the shot size. Start by making parts with a small shot size and increase the shot size until the mold is filled completely (and packed). Once the shot size has been determined, other operational settings need to be determined that will produce high quality parts with a minimal cycle time. Now we will have the chance to adjust the temperatures, holding times, and limiting pressures and speeds.

Injection Molding CycleA typical sequence of Injection moulding cycle is as follows :

Starting with an empty cylinder, raw material from the feed hopper falls onto the rear flights of the screw which conveys material to the front of the cylinder. During its passage along the cylinder it is plasticized to a fluid state with the help of external heaters on the barrel. Some material may escape through the nozzle but the back pressure is generally sufficient to push the screw back in the cylinder and to provide a reservoir of fluid plastic in the front of the cylinder for injection. The mould closes and the cylinder moves forward on its carriage until the nozzle is in contact with the entrance of the mould. The screw is moved forward by the hydraulic cylinder and the injection takes place. After a short interval (the holding time), the screw rotates, creating some pressure in the barrel, which forces it back against low pressure in the hydraulic cylinder, until the limit switch operates, stopping the rotation. This plasticizes material ready for the next shot .The mould opens, the article is ejected and the mould closes again ready for the next cycle.Repeat the process as discussed above for next cycle.

Discussion

Following are some of the common defects found in injection molded parts;

Shrinkage:During cooling the molten polymer may shrink and results in the decrease of

desired dimensions. This can be decreased by adding fillers.

Short shots : As in casting, a short shot is a molding that has solidified before completely filling the cavity. The defect can be corrected by increasing temperature and/or pressure. The defect may also result from use of a machine with insufficient shot capacity, in which case a larger machine is needed.

Flashing: Flashing occurs when the polymer melt is squeezed into the parting surface

between mold plates; it can also occur around ejection pins. The defect is usually caused by (1) vents and clearances in the mold that are too large; (2) injection pressure too high compared with clamping force; (3) melt temperature too high; or 4) excessive shot size.

Sink marks and voids: These are defects usually related to thick molded sections. A sink mark occurs

when the outer surface on the molding solidifies, but contraction of the internal material causes the skin to be depressed below its intended profile. Avoid is caused by the same basic phenomenon; however, the surface material retains its form and the shrinkage manifests itself as an internal void because of high tensile stresses on the still-molten polymer. These defects can be addressed by increasing the packing pressure after injection. A better solution is to design the part to have uniform section thicknesses and use thinner sections.

Weld lines: Weld lines occur when polymer melt flows around a core or other convex detail in

the mold cavity and meets from opposite directions; the boundary thus formed is called a weld line, and it may have mechanical properties that are inferior to those in the rest of the part. Higher melt temperatures, higher injection pressures, alternative gating locations on the part, and better venting are ways of dealing with this defect.

USESInjection moulding is used for the formation of intricate plastic parts with excellent dimensional accuracy. A large number of items associated with our daily life are produced by way of injection moulding. Typical product categories include housewares, toys, automotive parts, furniture, rigid packaging items, appliances and medical disposable syringes.