Plastic Injection Molding Machinery
From mold building to manufacturing, PM Plastics specializes in producing large quantities of custom plastic parts for companies throughout the U.S. The combination of our machinery and product design, mold and auxiliary equipment make PM Plastics your perfect partner for success! Our dedication to precision and quality ensures you can depend on parts that meet your exact specifications and cost requirements order after order, year after year.
We have a vast assortment of plastic injection machine sizes and related auxiliary equipment. With 3 factories, 53 presses ranging in size from 30 to 1,000 tons and warehouse 165,000 warehouse, they allow us to adapt production to the custom requirements of every application. We operate 24 hours a day, 5-7 days a week for maximum efficiency.
In order for molding successfully, the machine must perform. These are things our engineers consider:
- Accuracy and control of screw position
- In order to achieve shot-to-shot consistency, injection must start with the same amount of material in the barrel, and transfer to the holding phase must occur at the same position on each and every shot. Another important (and often overlooked) point is screw-bottom zeroing. In order for the injection unit to transfer pressure to the plastic during pack and hold, we must maintain a cushion. If the screw-bottom zeroing is off, the molder may not be maintaining a cushion, resulting in short shots and dimensional variation.
- Load sensitivity:
- This test qualifies the machine’s ability to maintain consistency in fill time under varying loads (material viscosity variation, thermal cycling, etc.). Think of it like cruise control in your car, which is intended to help your car maintain a consistent speed even when road conditions change. It is very important that the load-sensitivity test be performed at multiple injection-speed settings. Many machines will pass this test at one speed but fail at another. So test and confirm that the machine can accurately maintain injection velocity control at all speeds within the range that you intend to use.
- Non-return valve performance:
- Each time we begin to inject plastic into the mold, at least a small amount of material leaks past the valve. As the valve (usually a sliding check-ring style) and the barrel start to wear, this leakage increases, as does the variation in leakage. This results in process variation. It is critical that we perform a static or a dynamic test (or both) to monitor the condition of the non-return valve. Excessively worn components must be replaced.
- Injection velocity linearity and accuracy:
- In this test, the machine’s injection velocity control at various speed settings is evaluated. Note that there are some inaccuracies in how this test is traditionally performed, primarily due to the time that it takes to accelerate the movement of the screw. One easy trick for improving the test accuracy is to use more than 80% of the shot capacity of the machine. If the shot requirement for the mold is too small, we can “cheat” by adding extra decompression, forcing the machine to use more of the shot capacity. Please confirm that the mold can handle the extra air introduced.
- It’s also important to understand how the machine handles deceleration of injection (overshoot). Although a great deal of this overshoot is due to momentum, other factors come into play such as controller speed, scan rates, hydraulic valve response, and drive technology (hydraulic, electric servo, or hybrid). These factors influence how much uncontrolled overshoot occurs at transfer to hold. Generally speaking, all-electric machines have less overshoot than hydraulic machines, although some machine manufacturers offer advanced hydraulic drive technology that helps to minimize this condition.
- Switchover response:
- This is perhaps the most subtle and complicated, yet one of the most critical of the machine tests. Problems at switchover are much more common than you may think and can lead to problems with dimensional control, shorts, flash, and other molding defects. In extreme cases this can even lead to tool damage
- Backpressure control:
- Backpressure problems often go undetected; Problems with backpressure can lead to dimensional variation, short shots, flash, black specks, color problems, and several other molded defects. It can also lead to inconsistencies in, or extension of, the cycle time. The best method to evaluate backpressure control is to use some sort of graphic process-monitoring capability, either within the molding machine controller or with an external device (such as the RJG eDART system). Using the graphic screen, compare the actual backpressure to the set backpressure throughout screw recovery. The greater the discrepancy, the greater the likelihood of problems. This same method can be used to evaluate the accuracy of hold-pressure control
- Temperature control:
- Injection molding machines are notorious for temperature-control issues. Part of the problem is the disconnect between the steel temperature that we can control directly and the plastic temperature that we need to control, but this isn’t the only problem. Placement and style of thermocouples and heater bands greatly influence our ability to control temperatures throughout the injection system (barrel, nozzle, hot runner, etc.). In the nozzle area alone, you often find temperature variances of up to 100° F when measuring the temperature of everything from the end cap to the nozzle tip
- Clamp system:
- The importance of qualifying performance of the machine does not end with the injection unit. It is critical that the molder also evaluates the clamp system (platen parallelism and deflection, clamp-force control, etc.). The procedure for this evaluation is often included in the machine’s maintenance manual. It is also important to understand the differences in clamping systems (toggle, hydraulic, two-platen, etc.) and how clamp force is influenced. In the case of toggle systems, exact control of clamp force is often very difficult due to the influence of thermal expansion.
- Machine safety systems and equipment:
- Far too many molders fail to confirm the proper operation of key safety systems. Additionally, guards are removed or missing, limit switches are bypassed, and older machines are not updated to meet current safety standards. These unsafe practices should not be tolerated under any circumstances.
Since 1946 PM Plastics, has remained a family-owned and operated full-service plastic injection molding contract manufacturer as an American Manufacture. Because of our 70 years of experience with plastic tooling and injection molding our engineers can help with a part design for manufacturability as well as meeting the structural and design requirements for its intended use.