Bottle Cap Injection Molding Machine

Working Principle of Bottle Cap Injection Molding Machine

Benefits of Bottle Cap Injection Molding Machine

High-Volume Manufacturing of Plastics

Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. The recommended minimum production volume for injection molding is 500 units. At this point economies of scale start to kick-in and the relatively high initial costs of tooling have a less prominent effect on the unit price.

Wide Range of Materials

Almost every thermoplastic material (and some thermosets and silicones) can be injection molded. This gives a very wide range of available materials with diverse physical properties to design with. Bottle caps produced with injection molding have very good physical properties. Their properties can be tailored by using additives (for example, glass fibers) or by mixing together different pellets (for instance, PC/ABS blends) to achieve the desired level of strength, stiffness, or impact resistance.

High Productivity

The typical injection molding cycle lasts 15 to 60 seconds, depending on the size of the part and the complexity of the mold. In comparison, CNC machining or 3D printing might require minutes to hours in order to produce the same geometry. Also, a single mold can accommodate multiple parts, further increasing the production capabilities of this manufacturing process. This means that hundreds (or even thousands) of identical parts can be produced every single hour.

Great Repeatability and Tolerances

The injection molding process is highly repeatable and the produced bottle caps are essentially identical. Of course, some wear occurs to the mold over time, but a typical pilot-run aluminum mold will last 5,000 to 10,000 cycles, while full-scale production molds from tool steel can stand 100,000+ cycles. Typically, injection molding will produce parts with tolerances of ± 0.500 mm (0.020’’). Tighter tolerances down to ± 0.125 mm (0.005’’) are also feasible in certain circumstances. This level of accuracy is enough for most applications and comparable to both CNC machining and 3D printing.

Main Parts of Bottle Cap Injection Molding Machine

The Machine Base Unit
Often referred to as the machine "bed", this provides a mounting frame for the clamp and injection units. The base unit must have dimensional stability, accuracy, and strength. The process of injection moulding imparts high levels of stress to the unit during use which have to be absorbed without undue deformation occurring. The presence of high cyclic stress loading during service dictates a rigid heavyweight construction of the unit. Modern machine base units are generally fabricated from heavyweight steel beams welded together using thick steel connecting side plates. Due to the relative ease of the manufacturing method employed, the machine design can be readily modified during construction to fulfill specific requirements as necessary. The base unit also acts as a housing for the hydraulic oil storage tanks. The pump and electric drive motor are often mounted on the back of the base frame or directly to the bottom of the structure.

The Injection Unit
The basic function of all machine injection units is that of:

• Melting and preparation of the polymeric resin;
• Pressurizing and feeding the molten resin to the mould under controlled conditions.
• Injection units are usually based upon two distinct designs of the system, the plunger and the reciprocating screw designs. The latter is the most commonly employed on modern moulding machines.

Solid granules of resin are drawn into the screw flights as the screw is rotated using the screw drive motor. Heat is supplied to the resin through the barrel wall and additionally by friction generated as a result of the shearing action of the screw. The screw geometry creates a reduction in flight volume towards the front of the barrel thus compressing the softening resin into a molten phase.
This process is often referred to as plasticizing. The pressurized molten resin is then metered through a ring valve or collar, into the cavity between the screw tip and the nozzle. The build-up of resin in front of the screw tip is often referred to as the shot or screw cushion. As the screw rotates, the screw cushion grows in size, forcing the screw to retreat up the barrel until the required cushion size is achieved. Once this is obtained, screw rotation is halted. At this stage, the injection unit is said to be primed and ready for the next operation, known as the injection phase.
To inject the molten resin through the nozzle into the mould, the screw is forced forward closing the ring valve in doing so. With the ring valve firmly seated shut against the screw shoulder, the screw cushion is thus pressurized. The cushion pressure builds until sufficient to force molten resin through the nozzle into the mould feed system. The applied pressure continues until the mould impression is full of molten resin. This sequence of events is referred to as the injection 'first stage' or high-pressure phase. If the injection first stage is allowed to continue beyond the mould fill point, overpacking of the moulding will result. Overpacked mouldings suffer from high moulded-in stress levels resulting in poor component dimensional stability and inconsistent mechanical properties.
On completion of the first stage of injection, the applied pressure is reduced to a level sufficient to avoid component overpacking. Conversely, over-reduction of pressure could result in component underpacking which leads to high component after-shrinkage and distortion. Known as holding, dwell, packing, or second-stage pressure, it is maintained until sufficient solidification of the feed system has occurred to allow the removal of the applied pressure altogether. When the second stage has been completed, the injection unit can be re-primed in preparation for the next shot.

The Clamp Unit
In order to inject the molten resin into the mould under considerable pressure, the mould must be sufficiently clamped together to resist the applied injection force. Under-clamping would result in the mould being forced apart creating a flash about the split line. Apart from providing sufficient clamping force for the mould, the clamping unit is also utilized for mould opening, closing, and sometimes component ejection when required. The hydraulic actuation cylinder controls the movement of the toggle assembly arms and linkages using collapsing the link assembly to open the moving platen and erection the assembly to close and lock the platens/mould. The clamping force is applied mechanically as a result of stretching the machine tie bars by the locking-out of the toggle link assembly contained within them. Due to the design of the linkage assembly, a mechanical advantage is generated on actuation which results in a force multiplication in the region of 20:1 being achieved over the applied piston loading. Setting of the clamping force is obtained by adjustment of the lock height which can be achieved by jacking the assembly forward or back on the lock jacking screw. Clamp tonnages can be calculated by measuring the resultant tie bar stretch generated by the subsequent locking down of the press after adjustment.

Maintenance Tips for Bottle Cap Injection Molding Machine

 

Perform Basic Maintenance Checks before and after Each Production Cycle

Before starting a new production cycle, it is important to thoroughly inspect the injection molds. Examine the surfaces of the injection molding die to make sure they are free of plastic or contaminants. Any foreign debris may cause interference when the tool clamps down and cause the plastic to seep into the parting area during mold injection. You should also be on the lookout for anomalous marks or damage on the mold’s surfaces. It is also good practice to inspect the final part from the previous cycle. This will be a good indicator of the mold’s performance. For example, excessive flash on the parting line may mean that the injection mold cavity and core are not clamping down correctly.

 

Clean the Mold Core and Cavity

Clean your mold after each production cycle to prevent the build-up of foreign materials. Cleaning can be done with the injection mold tool in the press or on the bench. Press mold cleaning is done after each production run, while on-bench cleaning takes place alongside major injection molding maintenance activities and is not done as frequently. Some resins will foul the molds quicker than others, so your cleaning cycle will depend heavily on the resin used. Wipe down the mold with a standard degreaser or mild solvent using a cotton cloth. Other techniques like dry ice blasting can help you clean the mold in place without leaving any residue. If the plastic molding tool has been removed from the machine for more extensive maintenance, it must be disassembled, and each part cleaned individually. One of the more efficient methods for doing so is to clean the parts in an ultrasonic bath. This allows you to clean every surface—even cooling channels and mold vents—that are hard to reach. The mold material must be carefully considered as some materials are not conducive to ultrasonic cleaning. In this case, traditional cleaning techniques must be employed.

 

Blow Out Dust, Dirt, and Water with Compressed Air

It is important to remove any dirt or dust that has settled on the mold surface during cleaning. The water used to clean the mold can also leave stubborn scale marks when it dries, especially on highly polished molds. The molds should be dried as soon as possible after cleaning. An injection molding technician can use compressed air to accomplish this. Cooling channels must also be blown dry as water can easily get trapped in these channels. Anti-rust coatings will make debris like dust or cotton fibers stick to the injection mold die surface. They may later end up on the injection molded parts, so care should be taken to carefully inspect the mold surfaces afterward.

 

Check the Mold's Runners, Sprues, and Other Places

It's important to inspect the quality of the mold runners, sprues, gates, and vents. All these features are critical to efficient production. Wear and material build-up are two of the most common failure modes and can cause parts to stick to the injection molding tools. Molds are most likely to wear at the plastic’s entry gate, so this area should be thoroughly inspected. Make sure there are no plastic buildup or surface blemishes on the runner or sprue, as these may impede the flow of plastic and cause sticking. Scratches from previous attempts to remove stuck plastic can further exacerbate the problem, causing future plastic to stick more easily. Sprues and runners must be cleaned and re-polished if flaws are found. Check that the hot runner nozzle tips are free of burrs.

 

Examine the Mold's Connectors and Hardware

Check all cooling water hoses, pipe connectors, and threads to ensure that there are no leaks. Remove pipes and fittings and clean off any potential build-up of calcium or other deposits. If the mold is not removed, ensure that all the bolts connecting the mold to the platen plates are properly torqued. If the mold is removed, inspect the platen plates for warping and rusting. Dents and burrs on the platen can damage the mold and reduce its effective lifespan. Any small burrs or other defects must be removed with a polishing stone or similar tool. Due to the softer metal of the platen, the mold fastening hole threads can get damaged over time, so these should be periodically checked. The use of a torque wrench is recommended to prevent over-tensioning damage to the threads. Check the platen for any deformation.

 

Before Storing the Mold, Make Sure It's Totally Dry

If the mold is to be stored for an extended period, it must be fully dried and protected beforehand. Moisture can cause corrosion even if the mold is sealed. Drying can be done with compressed air. An additional rust inhibitor is then needed—spray the entire mold to protect it from atmospheric moisture.