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Injection Mold Design

Plastic injection molding for producing plastic products

Plastic injection molding is the principal process for producing plastic products or parts of products. Plastic is acknowledged to be a very flexible and cost-effective material that is used in many applications. Although the tooling can be expensive, the cost, per part is very low. Intricate geometries are limited only to the abiltiy to create the injection mold. Things you use everyday, such as the case that houses your monitor, the keyboard on which you type or the mouse on your desk were produced with plastic injection molding.

Plastic injection molding involves the transformation of a plastic solid, usually in the form of granules or pellets, and heating the plastic resin to a specific temperature until it melts. The melt is then forced into a mold made up of two or more dies, where it is forced to “cool”, resulting in producing the desired shape. A specific amount of time passes, usually a few seconds, and the mold is then opened and the part is released. This cycle then repeats continuously until the desired quantity is reached.

Considerations of  Injection Mold Design

The design of the part, and therefore the mold, needs to include draft features (angled surfaces) to make possible the removal of the part from the mold. Typical draft angles are about 1 to 2 degrees for part surfaces which do not exceed five inches. Dimensional tolerance specification will dictate the final cost of the part as well as its ability to be manufactured. If there is a small section of the part which needs higher tolerances, such as the location of a critical feature used for alignment,do not specify a tight tolerance, as an alternative, plan and design for post molding processes such as machining to achieve the desired results.

Radii and Corners

It is very important that uniform wall thickness be maintained at the corners. The internal and external radius need to share the same center point. External radii = internal radii + wall thickness. The minimum radii should not be less than ¼ of the minimum wall thickness. Design for radii to be ½ to ¾ of the nominal wall thickness. When a large amount of stress is going to be present, it is very important to design in larger radius as this will distribute the stress much more evenly.

Wall Thickness

The production of thin wall items such as a clamshell for retail packaging are possible with today’s technology. Products with thick walls are also easily produced. However, parts which require uneven wall thickness present a challenge to the plastic molder manufacturer. Creating a part with a uniform wall thickness and cross section will abridge manufacturing and reduce costs. One issue to be aware of is sinking. Wherever an intersection or “tee” occurs, there will be some degree of sinking. This occurs because thicker walls cool at a slower rate and therefore create this problem.

Ribs

Ribbing should be ½ to two thirds of the nominal wall thickness and less than 3 times the thickness in height.A taper of 1° is usual. Note: as mentioned above, excess thickness can result in shrinkage.An excess in rib height combined with a taper will produce thin areas requiring extra fill time at the mold.

Weld (Part) lines

The location of weld lines needs to be considered by designer before a injection mold is created. Weld lines are formed by the joining of the flow fronts of the plastic during molding. One issue of concern is the that the weld line area is more susceptible to cracks and stress failure.

Bosses

  • Diameter = (Outside Diameter) \ (Inside Diameter) = 2 to 3
  • Thickness = 1/2 to 2/3 nominal wall thickness
  • Gusset Height = 2/3 Height
  • Height = Fastener minimum requirements
  • Taper = 1 deg. all around
  • Diameter Ratio should be minimum ratio of 2., this will reduce risk of failure.

Pressure:

Another factor in the design will be the clamping pressure required to produce the part while the plastic is being injected. Smaller cavities can result in high pressures being required to force the plastic or rubber material to fully fill the mold cavity. This will, in turn, determine the thickness of the mold material, usually steel) as well as the type of machine in which can be used.

Summary:

Many factors must be taken into account when designing a mold for the creation of plastic injection molded parts. Factors such as draft angles, wall thickness, ribbing (not the kidding kind), bosses and weld lines and clamping pressure all come into play when designing a mold that will be used in a plastic injection mold machine. Each facet is important in and of itself, but as a whole, each one affects the others. Therefore the design of a mold for plastic molding can be quite involved. When done correctly, the result will be a mold which will yield thousands, hundreds of thousands, or even millions of parts over it’s lifetime.

Used Injection Molding Machines

Used Injection Molding Machines

There are constantly new techniques and new technology in the industrial world of injection molding. New machines, new plastics, new equipment. There are some companies that keep up with other companies by buying a lot of expensive equipment the second it comes out.

But these are the companies that cost so much because they have to charge more to pay for the price of the new equipment they just bought. But then you have the companies that like to keep using used machinery because they are just as good as new and as the old saying goes if it isn’t broke don’t fix it. Not every company wants to be the fancy company with all new machines.

Some companies want to just be known as the company that did my job in the most outstanding way. They want the recognition for a job well done rather than for the company who spend a lot of money on a piece of new machinery.

Companies that keep using used machinery don’t have to increase their prices and customers like to see that and they will keep coming back to the company that does great work for a great price.

Injection Molding Questions and Answers

Here are some frequently asked questions when there are problems in the process of injection molding, here some of the most popular ones and their solutions.

What happens when the molds are brittle and delaminate? The cause may be an improper injection screw. The screw that is used for molding at a low-pressure ratio fro the amount of resin being used; causes the plastic’s molecules to form incorrectly. Basically, a weak molecular structure can cause brittle parts. When this happens de lamination can occur as well. The result will be a to take a pressure reading and record the ratio. This should be performed once every six months.

Another problem may be discoloration; the possible cause for this may be excessive dwell time or what they call residence time. When a part has been discolored or changed the problem is usually found in the residence time. The material stays in the barrel too long, the base will become discolored. What can be done is the heat sensitive materials have to be set at a certain ration. After every injection shot clean out the barrel.

Discoloration may also be caused by an improper plastic mold temp. The hotter the mold is it will keep the material melted for a longer period of time. The temp of the mold should be adjusted to the supplier’s specs on material and cycle times.

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