Plastic Injection Molding Process Explained

Plastic injection molding (or moulding) is one of the many processes which shape plastic into useful objects just as extruded plastic profile is made by blow molding process. This injection molding process is used for manufacturing plastic products from both thermoplastic and thermosetting plastic materials. Due to many advantages of injection molding such as high production rates, repeatable high tolerances, ability to use a wide variety of materials, low labor cost, minimal scrap losses, and almost negligible requirement to finish parts after molding makes it a very popular process for making such things as wire spools, packaging, bottle caps, automotive dashboards, pocket combs, and even entire body panels of cars along with most of the other plastic products commonly available today.

Basic Plastic Injection Molding Process
In the injection molding process, plastic is taken in the form of pellets or granules. These are then heated until a melt is obtained. The resultant plastic melt is forced into a split-die chamber or mold where it is allowed to cool into the required shape. After cooling, the mold is opened and the part is ejected. The cycle is repeated for further production of more parts.

Plastic resin is fed to the injection moulding machine through the hopper. Colorants are generally fed to the machine directly after the hopper. The resins which enter the injection barrel are heated to the appropriate melting temperature.

The resin is injected into the mold by a reciprocating screw or a ram injector. Reciprocating screw can inject a smaller percentage of the total amount of melted resin in the barrel. Ram injector should typically inject at least 20% of the total amount of melted resin while a screw injector can inject as little as 5% of this amount. Thus, screw injector is better suited for producing smaller parts. After the resin is injected, mold is cooled constantly to a temperature that allows the resin to solidify. The mold plates are held together by hydraulic or mechanical force. Typically molds are over designed depending on the resin to be used.

Injection Molding Equipment
Injection molding machines and other tools equipment are very expensive and thus this is one of the main disadvantages of plastic injection molding process. However, because, its advantages outweigh the disadvantages (other drawbacks being potentially high running costs, and the need to design moldable parts) this process is widely adopted by most of the plastic products manufacturers.

Injection molding machines consist of a material hopper, an injection ram or screw-type plunger, and a heating unit. They are also called presses as they hold the molds in which the components are shaped. Presses are rated by tonnage ( varying from less than 5 tons to 6000 tons) which represents the amount of clamping force that the machine can exert. The clamping force keeps the mold closed during the whole injection process. Clamping force is defined as the injection pressure multiplied by the total cavity projected area. The total clamp force required is determined by the projected area of the part being molded. This area is then multiplied by a clamp force within 2 to 8 tons for each square inch of the projected areas. Generally, 4 or 5 tons/square inch can be used for most of the products. More stiff plastic materials need more injection pressure to fill the mold. In such a case, more clamp tonnage is required to hold the mold closed. The required force for plastic molding can also be determined by the material used and the size of the part where larger parts need higher clamping force.

Plastic Injection Mold Design
Mold is the part of the plastic molding machine which receives the plastic and shapes it in the desired form. Their are many design considerations that have to be kept in mind while designing plastic injection molds. For example, part design should include draft features (angled surfaces) so that the removal from the mold becomes easier. Depending surface length draft angles down to half a degree are reasonable. Typical draft angles should be around 1-2 degrees for part surfaces not exceeding 5 inches. Another consideration is the dimensional tolerance specification which governs the part cost and manufacturing capability. For example, if its is a small region of the part that needs higher tolerances, say the location of a critical feature used for alignment, tight tolerance should not be specified. Instead, one must design and plan for post molding processes such as machining using "assembly intent" fixture. There are many other considerations for mold designs like wall thickness, radii and corners, ribs, bosses etc.

Complications in Plastic Injection Molding Process
There are certain typical complications that take place during the plastic injection molding process.

Burned or Scorched Parts- This complication arises when melt temperature is too high. The other reasons for burned parts during injection molding include trapping of polymer or degrading in the injection nozzle. Even the process cycle time may be too long allowing the resin to overheat.

Warped Parts- Parts can come out warped due to uneven surface temperature of the molds or non-uniform wall thickness of mold design.

Surface Imperfections- This is the result of too high melt temperature which may cause resin decomposition and gas evolution (bubbles). Other reasons of surface imperfections include excessive moisture in the resin or low pressure which doesn't allow mold to fill completely.

Incomplete Cavity Filling- During the plastic injection molding process, the injection stroke may be too small for mold because of which enough resin is not injected. Injection speed may also be slow leading to cooling or even freezing before mold is filled completely.