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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.

Injection Molding Process

Process of Injection Molding

The process of injection molding process is best explained as heating up a type of plastic and under a forced type or pressure is poured into a predesigned mold, after the hot liquid is poured into the mold the mold is clamped shut to prevent any air from getting in. After the initial work is done, the mold hardens and takes the shape of the mold.

Then the next step in the process is when the resin (hard plastic) which is now in the shape of a small pellets are poured into the feed hopper, the hopper is a large open bottomed contained and what it does is that is filters the pellets into the screw.

As the screws turn the resin pellets are moved into the screw and then they go through a very intense pressure. Then friction is created and when that happens heat is generated to melt the pellets. There are heaters on both sides of the screws and there is temperature control during the melting process.

The oil gets pumped from the tank to the injection molded parts that run along the tie bar equipment, then that’s when the liquid plastic gets injected into the mold. Then the water-cooling technique is applied in assist in cooling off the mold. The process is complete when the mold is pulled from the pre-designed mold.

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.

How Injection Molding Works

How Injection Molding Works

In this post I’m going to explain how injection molding works. Creating polymers is an amazing process. Then there is the question of forming the plastic or rubber into useful objects….another fantastic discipline. One of the most common methods of forming rubber or plastic resins is a process called injection molding. Injection molding is made possible by large machines called injection molding machines.

Material, either rubber or plastic resin is fed to the machine. This can be in the form of a hopper for plastic pellets or an auger for thicker, heavier compounds. Colorants are usually fed to the machine directly after the hopper. The resins enter the injection barrel by gravity though the feed throat. Upon introduction into the barrel, the resin is heated to the appropriate temperature to make it melt.

This now viscous material is injected into the mold by a reciprocating screw or a ram injector. A reciprocating screw provides the advantage of being able to inject a smaller percentage of the total shot1. The ram injector on the other hand, must typically inject at least 20% of the total shot. A screw injector can inject as little as 5% of the total shot. Many factors also come into play such as the type of mold, how the material is injected, etc., effect the shot.

The Plastic mold is a cavity in the machine that receives the material and shapes it accordingly. In order to make the injected material solidify, the mold is cooled constantly to a temperature which makes the solidification possible. The mold plates are forced together, usually by hydraulic force. The clamping force is defined as the injection pressure multiplied by the total cavity projected area. Molds are typically over-designed with regard to the pressures they must endure depending on the material to be cast. In addition, each injectionable material has a calculated shrinkage value associated with that has to be accounted for as well.

Some Typical Complications

Burned or Scorched 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.

Warpage of Parts: Uneven surface temperature of the molds. Non-uniform wall thickness of mold design.

Surface Imperfections: 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.

Incomplete Cavity Filling: 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.