Custom Injection Molding Process injection molding

Custom Injection Molding Process

Plastic injection Molding

Plastic Injection Molding process is the principal method for manufacturing plastic parts. Plastic is known to be a very resourceful and cost-effective material that is used in many applications. Although the tooling can be expensive, the cost per part ends up being very low.

Injection molding process involves taking plastic in the form of pellets or granules and heating these bits and pieces until a liquefied state is obtained. This transition is known as plasticizing. If the melt temperature is too low the pressures will tend to rise and cause flashing or short shots in the part. If the temperature is too high you can encounter mechanical failure of the part due to material degradation. In the design process you must keep this in mind because if your part is not able to accept the melt without suffering other problems you will have wasted time and money. 

Once the material is liquefied it is forced into a mold where it is allowed to "chill". If all goes well you will have the part that you were trying to build.  During this cooling the phases more pellets are loaded from the hopper into the barrel which heats these pellets to the melt state. The mold is then opened and the part is ejected. The mold is closed and the cycle begins again.

There are two types of injectors. The first is a simple ram that travels forward to inject the plastic and back to receive more pellets. This unit relies only on the heat of the barrel to melt the plastic. The second type of ram is a combination of the first with the addition of  threads along the ram that serve two purposes. The first purpose is to draw plastic pellets into the barrel as the ram is reversed. The second is to aid in the melt process by adding mechanical fraction to the heat equation.

Problem Areas

Surface is not smooth: Melt temperature may be too high causing resin decomposition and gas evolution (bubbles). Excessive moisture in the resin. Low pressure causing incomplete filling of mold.

Burned Parts: Melt temperature may be too high. Polymer may be becoming trapped and degrading in the injection nozzle. Cycle time may be too long allowing the resin to overheat.

Short Shot: Injection stroke may be too small for mold (ie. not enough resin is being injected). Injection speed may be too slow causing freezing before mold is filled.

Part disfigured: Uneven surface temperature of the molds. Non-uniform wall thickness of mold design.

Design

Part design should include draft features to facilitate removal from the mold. This can be evaluated in many of today's 3D CAD programs before starting the mold. Draft angles as low as one half a degree can be used in some situations. Typical draft angles should be about 1 too 2 degrees.  Depending on the tolerance limit requirements most parts can be manufactured using Injection Molding. The more complex the part the more complex the mold and the higher the cost. 

Wall Thickness

It is possible to build parts with very thin walls, as well as thick walls. When the two come together is when you run into problems.  Plastic flows much differently in thick regions than thinner regions. Keeping the thickness uniform will help in the predictability of the part as well as lessen the cost of the mold.  With uniform wall thicknesses you will lessen the chance of warpage due to uneven shrinkage. Remember that the thicker the wall the more mass and the more mass the more heat there is to dissipate.

Corners and radiuses

A good rule to follow is to keep to a size that will maintain strength but not so large that it will create problems. Also maintaining a uniform wall thickness at corners.  If you have pins you should not have a larger diameter than the thickness of the wall it is extruded from. Cross walls should follow the same rule. The minimum radii should not be less than 1/4 minimum wall thickness.  Design for radii to be 1/2 to 3/4 of the  nominal wall thickness.  When significant stress is present, design in larger radius as a larger radius distributes stress uniformly.

Ribs

Ribs should be 1/2 to 2/3 of the nominal wall thickness and less than 3 times thickness in height.  Taper of 1 deg. is typical.  Note: excess thickness promotes shrinkage.  Excess rib height combined with taper will produce thin sections requiring extra fill time at the mold .

Bosses

Weld lines 

If you are able to use a flow simulation program on your part before making the mold you will be able to determine the location of your weld lines relatively easily. The location of these weld lines are determined by the melt front. When the plastic flows in opposite directions and eventually meets again, this is where the weld lines develop. Weld lines are produced at the mating of the flow fronts of the plastic during molding.  The weld line area is more susceptible to cracks and stress failure.

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