Wear in Injection Molds: To Float or Not to Float

One of the most difficult and challenging mold design issues is how to reduce or eliminate wear in injection molds. After all, the more wear that occurs, the higher the mold repair costs and/or the shorter the life of the mold. It’s a given that both the mold designer and the mold builder are committed to designing and building molds with the least wear, but it often seems to be an elusive target, especially with mold designs that have various actions taking place during the molding cycle.

The Wear Challenge

Mold designers and builders try to use the typical methods of minimizing wear—such as using differential metals; using low-wear coatings; specifying high metal hardness thus equating high hardness with low wear; designing shutoffs with high angles thus avoiding locking shutoff angles; and, many more concepts that their experience leads them to believe works best in specific applications. However, one design principle that is not well understood by mold designers and mold builders is wear caused by mold components that are under some load or stress during the molding cycle.

Mold designers and builders see the mold as it is built and especially during the mold assembly phase. Every attempt is made by them to assemble the mold components such that all inserts are fitted so the mold can be assembled by hand. In the better mold building shops, the use of hammers, pry bars and clamps is either strictly forbidden or the use is severely limited. Most molds then are assembled by hand with many of the mold components fitted with close tolerances but, by design, are not under a stress. On the bench then, the mold is in a Static condition.

The molder then sees the mold in a Dynamic condition. Components may be moved out of position due to a variety of molding conditions—such as being clamped up under tonnage; cams moving back and forth with the opening and closing of the mold; thermodynamic changes in the mold due to differential heating and cooling; and, stresses caused by injection pressures trying to move mold components out of position. Therefore, to the perceptive molder, a mold is alive or a dynamic device, not a static device during the molding process.

For years there has been a small group of molders and mold builders that recognize that molds are dynamic devices. They have recognized that this dynamic condition often has caused mold components to be forced out of a designed position. As forces are exerted on mold components, they move out of their desired position and may be forced against their mating components such that very high loads or stresses between these components occur. One of the more serious results of these stresses is high rates of mold wear. It is almost inevitable that increasing loads will increase the wear between these mold components.

Mold Alignment Approach
Molders who recognized that excessive wear was being caused by unanticipated stresses on the mold components began to use a variety of designs to address the problem. One of the most common was to utilize increasingly expensive mold alignment features—such as high cost straight line interlocks. The reasoning was that adding complex alignment features would not allow components to move regardless of the forces being exerted in the mold. Thus, instead of attacking the problem of movement, they continued to try to stop movement.

Wear Minimization Solution

The best solution to the problem of movement causing stress was very simple. If component A is forced against component B thus putting both components under stress, remove the stress. Removing the stress can be accomplished by a revolutionary principle and that is to incorporate designs that allow components to move in the mold.

Specifically, if component A moves due to stresses, allow component B to move with component A. If both move together, stress is reduced or eliminated—depending on the design concepts. Result: when stresses are eliminated, mold wear is substantially reduced.

Float Concept
This concept of allowing components to move or float flies in the face of the mold builder teachings. It is a concept that is foreign to their experience and teachings. The industry generally agrees that the one type of mold that has the most potential for wear is unscrewing molds (i.e., molds designed to make a part with an internal thread). Today, molders that have adopted this concept and incorporate this into their designs are building very successful molds and are seeing substantial savings in repair costs.

Molders that are having their molds built with copper alloys to take advantage of increased mold cooling discovered that copper alloys are easily damaged. When copper alloys are formulated to resist wear, thermal conductivity can be significantly reduced. Molders then were forced to choose between fast cycle times due to increased mold cooling or better wear resistance. Molders that are using copper alloys successfully in molds have learned that float mould components may be the most successful mold design concept to minimize wear. Floating mold components therefore allow the molder to have minimal wear and fast cycle times.

Key to Success

The key issues in floating mold components are to determine which components will be floated and how to actually design them to float. Molders that have successfully determined how to achieve this are seeing faster cycle times and lower mold repair costs.

30 holes egg tray mould use for paper pulp machinery

Egg tray moulds features:

1. Our mould is durable and easy for maintenance and replacement.
2. The price of our mold is reasonable comparable to its high quality.
3. We make lineation before drilling holes to ensure all holes distributed evenly. The holes size and spacing of holes of our suction molds are better for high efficiency. We use stainless steel mesh and use pre-forming technology when making them, this make the molds looks better and increase the efficiency, and the products produced are with better looking.

Our pulp molds have been mounted successfully on the international pulp molding machine, such as reciprocating pulp molding machine, rotary pulp molding machine. And we made many molds for pulp molding machines made by some famous company in the world.

In the process of producing plastic chemical barrel moulds, due to the influence of raw materials, molds, equipment and processes, the surface of the plastic chemical barrel moulds will appear to be blooming. At this time, we need to conduct on-site inspections. The plastic chemical barrel mould manufacturers introduce the process as follows:

Whether the material is wet: check whether there is obvious water and moisture phenomenon in the data. You can visually observe that there is no water and moisture phenomenon in the outer package. Open the material bag and touch the data to see if there is moisture. Difficult to dry.
2. Feeding link: Check the feeding link to ensure that the barrel mould and feeding process are clean to prevent foreign materials from mixing in and check the data particles for obvious bees.
3. Checking the drying equipment: Check the drying equipment to see if the heating temperature of the drying equipment is normal (there is a thermometer on the bottom of the barrel mould), whether the air blows in and out smoothly, and you can feel the wind out of the air outlet with your hand.
4. Is there a foaming phenomenon in the shot block? Exit the shot block to see if there is foaming in the block. There should be no foaming in the normal production materials. If there is foaming, there is gas. Continue to dry or lower the material. Tube temperature.
5. Check the nozzle heating ring and barrel mould: Check whether the nozzle heating ring and barrel mould are heated normally. The nozzle can be tested with a strip on it. The strip is normal if it melts. The temperature of the barrel mould can be viewed on the temperature screen, and the deviation from the set value cannot exceed 20 degrees.
6. Injection process: Check the key conditions that affect the silver pattern in the injection process. The loosening position should be as small as possible, the back pressure can be appropriately increased, and the injection speed can be increased and decreased in both directions.
7. Mold problem inspection: The inspection mold has no effects of oil leakage, air leakage, and water leakage.

Why Does Mold Grow on Plastic?

Many people believe that mold cannot grow on plastic or polymer materials. Mold generally can't break down plastic easily.

However, plastic contains many additives, such as plasticizers, cellulose, lubricants, stabilizers, and colorants to help provide desired features, and these additives ARE very easy for trash can mould to break down.

Once mold becomes established by breaking down the easily digested ingredients, the acids they produce as by-products of growth break down the resin into a more useable food source.

To learn more, read up on the ASTM G 21 test method that describes this process in greater detail.

What Does Mold Need to Grow on Plastic?

Mold is a broad term that covers a variety of species. Able to survive in the strangest of places, mold only has a few requirements to survive and thrive in any environment.

In addition to additives already embedded in the plastic itself, as mentioned above, food (dust, dirt, organic materials), moisture, and suitable temperature all provide enhanced conditions for mold growth to take hold.

While plastic might not be the preferred surface for sustaining mold growth, it can easily meet all the prerequisites if exposed to food, humidity, dirt, and dust.

These conditions vary widely between species. In fact, even the International Space Station (ISS) astronauts spend hours every week battling mold growth.

However, their fights have not gone unnoticed. Through their research, we have gained an indispensable look into the harsh environments of mold and its spores. The ISS scientists replicated the X-ray and UV exposure found around the space station and found spores surviving up to 100 gray worth of X-rays.

To put that in perspective, 5 gray is all that is needed to kill a person, while ? gray is enough to induce radiation sickness.

So, is mold the next Terminator T-1000? Well, not for us resourceful humans!

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