What is a Plastic Mold?
A plastic mold is a cavity made up of aluminum or stainless steel used to design and manufacture various plastic parts for multiple purposes. A mold can be single or multi-parted, depending on the application.
A mold is a metal structure in which a product takes its life form for a brief period of time before being ejected from the machine. Various products can be shaped using a mold. A good amount of supervision goes into designing and manufacturing a mold as it is an integral part of the entire plastic processing.
A mold is assembled or constructed using many components like pins, bushings, plates, ejector systems. A mold can vary in size and weight depending on what it is being used for. The weight of a mold varies by 75 kgs. To 500 kgs.
A mold has several assembled parts, but the main two halves are injecting half of the mold and the ejector half of the mold. Let’s have a detailed discussion about them.
The Injection Half of The Mold –
This half contains several components. Let’s have a brief look at all of them.
The Sprue Bushing –
The A half of the mold mostly contains Sprue bushing. The term “Sprue” is borrowed from the die-casting industry. Sprue bushing is a component responsible for molten plastic entering the mold and traveling to the cavity image.
The material is directed through the runner system, past the gate, to the cavity image. Sprue bushing is the mediator between the mold and the injection machine.
The sprue busing is made up of high-end steel and is highly polished to avoid any sticking. The dimensions of sprue busing are usually dependent on the viscosity of the molding material.
The A and B plates –
The molded parts in the mold are formed by A and B plates. To take advantage of the ejector system ( The B part of the mold ), we can mold the B plate material whenever possible. However, sometimes the plastic products require some cavity image to be placed in the A plate.
Mainly there are four kinds of mold used in different plastic molding processes. 1. Molds of Injection molding, 2. Molds of Blow molding, 3. Molds of Compression Molding and Mold of Extrusion Molding.
Runners and Gates –
When the molten plastic leaves the injection machine and enters the mold through the sprue bushing. It must be allowed to enter the cavity image to structure the finished part, which can be accomplished using a runner system and gates.
The gate is placed at the end of the runner system and is considered an opening of the mold. It is useful for filling the cavity quickly. The design of the component allows it to do it flawlessly.
The thickness of the gate determines the overall cycle of the injection molding process. It should be kept as thin as possible for smooth flow and no disturbance.
The Cavity Image –
The cavity image is responsible for the shape of the final product. The image needs to be machined with the plates using common machining techniques like EDNC grinding, milling, turning, and polishing.
By far, EDNC ( electrical discharge numerical control ) is the most used technique. EDNC, as the name suggests, applies electrical current to polish metal from the cavity plates.
The Ejector half of the mold –
The parts found in this half of the mold include support pillars, support plates, ejector pins, return pins, ejector housing, etc.
The Ejector Housing –
The ejector housing is used to attain maximum strength and minimum deflection during molding. It looks like a U shaped metal box, forges in a solid one-piece unit.
To cut costs and maximize profit, some companies tend to replace the ejector housing with bolted plates, which is a very unsafe practice as it will create cracks at the side of the box. The distortion also creates additional pressure on ejector pins.
Support Plate –
The U shape structure of ejector housing leaves many unguarded spaces under the mold’s B plate. The open area can cause bending under injection pressure. The pressure can result in a lot of flash n the parting line and a weakened B plate.
The support plate helps to create a much better resistance to distortion. Using a support plate is always advisable rather than using a thicker B plate.
Support Pillars –
Support pillars have the same purpose a support pillars. They are used to reduce the B plate stress by providing additional assistance to support pillars. They are shaped like round, made u of steel, and located between the mold’s ejector housing and the support plate.
Ejector Pins, Return Pins, and Sprue Pullers –
Ejector pins are made up of steel, and as the name suggests, they are used to push the finished article out of the mold at the end of the injection molding cycle. Ejector pins are also useful to the eject surface style runner system.
The ejector pin always causes an access amount of material stuck with the final product, which can be removed through manual labor.
Talking about return pins, they have used to pull back the ejector system to its place when the mold closes just before beginning the next molding cycle. These pins never contact the surface of molded parts but contact the A part’s steel surface as the mold closes.
Talking about Sprue puller pins, they are an extension of ejector pins. Their purpose is to lend assistance to sprue bushing in releasing the sprue when the mold first opens after one injection molding cycle is complete.
The sprue puller pins usually go through with some undercut in the end.
There are several types of mold used for various applications. All the major plastic manufacturing techniques utilize molds.
Let’s take a detailed look at all the types of molds –
1. Mold of Injection molding –
Every injection molding machine is composed of A” Plate and B” Plate, the same principle will be applied to the mold, and it will be called “A” half and “B” half. The A half is often called the dead half as it never moves, while the B half is referred to as the Life half as it is constantly moving.
Every part is made in mold cavities. The line here, two halves are joint, is called the parting line.
To ensure that the mold has no defects, the sprue puller provides friction against the injection’s impact. In the end, the ejector pins are used to push out the finished products, and the return pin is used to pull the ejector pin back.
2. Mold of Rotational Molding –
Rotational Molding mainly utilizes heated hollow mold, which is pored with the shot weight of the material. The mold will start rotating steadily, spread the softened material, and stick them to the mold walls.
The constant rotation is the key here, avoiding wilting during the heating phase and distorting during the cooling phase.
3. Mold of Extrusion Molding –
Extrusion molding molds working are quite different from other methods. It is a custom plastic mold designed based on the company’s demand, and it is also called steel die.
The molten material enters in powder form, then the motor steadily rotates the material into the heated area.
At last, the martial is melted, squeezed, and pushed out the steel die.
Interesting Read – What is Warpage? | Causes of Warpage | Warpage Variations
Common Troubleshooting For Plastic Mold Design –
|Fault||Common Cause||Possible Cures|
|Burn Marks||Lack of mold venting||
Improving the mold venting
|Weld and flow marks, short molding sink and voids, poor surface finish||Cavity not filling properly||
Increasing injection rate and Injection pressure, increasing cylinder and mold temprature, increasing the size of gate sprue and runner, increase feed, improve mold venting.
|Decomposition, silvering, and flash marks||Overheating of material||
Reducing injection pressure
|Jetting||The linear velocity of melt too high||
Reducing velocity or breaking up the stream of melt
|Warping, mechanical sticking flash||Excess pressure of the cavity||
Reducing injection pressure
|Crazing, lamination||Frozen-in stresses||
Ensuring rapid filling at the lowest possible temperature
|Distortion||Ejecting is too hot||
Eject over a larger area, increase the cooling time.
Final Thoughts –
Thus, it was my effort to share my knowledge on plastic molds. The knowledge of having accurate molds can really make an astronomical difference in the quality and speed of your production run. I’ve tried to keep the topic as brief a possible.
Feel free to comment your thoughts and reviews in the comment box.