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John Deere Achieving Excellence

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* Achieving Excellence 5 Year Hall of Fame Partner 2006 Harvester Division

* Achieving Excellence John Deere Partner
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What Differentiates Us

The aggressive pursuit of new technology and current engineering advancements puts P & P Industries is a world class injection molder.

Engineer's Monthly Tip

Increasing wall thickness generally improves part strength, stiffness and impact resistance. However, overly thick walls can cause high internal stress and therefore reduce part strength. Impact resistance can suffer if the part is too stiff and unable to deflect and distribute the applied force.

Injection Molding Process

P & P Industries makes the complex process of injection plastic molding simple because of the years of experience, education, certifications our team offers.

For the injection molding cycle to begin, P & P engineers will help design your part with you or we will review your current molds. There are five criteria must be met during the injection molding cycle: clamping, injection, cooling, mold open and part removal. Typically the injection molding cycle is done as fast as possible while maintaining the dimensions and quality of the part.

Injection Molding Cycle: Animated display of the 4 processes of injection molding; 1. Clamping, 2. Injection, 3. Cooling, 4. Ejection.

The cycle begins with the mold closing fast to just before the leader pins where mold safety is set, the closing speed and pressure is set low to prevent accidental mold damage. When both halves of the mold touch high pressure is built to create clamp tonnage.

Plastic is melted by 80% of the screw and 20% by heater bands. The melted plastic is injected into the mold. The melted plastic travels to the mold via a sprue bushing, then a runner system and finally the gate. This process is called plastication.

When the part is about 95% full, called the 1st stage of injection is complete the part begins to take shape in the mold cavity. This begins the pack and hold phase of the process. The final 5% of the part is controlled by velocity and pressure. The pack and hold phase continues until the gate is sealed to stabilize the part dimensions.

Cooling time is dependent on wall thickness, plastic actual melt temperature, mold temperature and shrinkage. The better designed part more efficient the cooling the faster the cycle time lower the cost. Water is channeled through the mold to improve the cooling. During the cooling time plastication takes place.

Plastication is the process of melting plastic and preparing the next shot. The plastic is dried to a manufacturer specification as specified on a material data sheet. Once dried the plastic is conveyed from a hopper to JIT loader and enters the feed throat. The feed throat has water lines through it to prevent bridging of plastic at the feed throat.

The barrel contains a screw that primarily uses shear to melt the pellets and consists of three sections.

  • Feed Section – where the plastic first enters the barrel and screw, it conveys the plastic forward
  • Transition Section – the plastic starts to melt by shear heat, the flight depth of the screw continue to get decrease
  • Metering Section – the final section, with shallowest of the flight depth, it improves the melt and color quality
Attached to the front of the screw is a non-return valve. The purpose of the non-return valve is to act as a plunger and extruder. The non-return assembly allows material to flow in front of the screw creating the next shot. During injection, the non-return assembly prevents the shot from flowing back into the screw sections.

The mold will open when the next shot is created and the cooling time has timed out. The mold will be set to open slowly to release the vacuum that is caused by the injection molding process and prevent the part staying on the stationary side of the mold. Then pick up speed and then slow down at end of travel. The end of travel will be determined by the design of the part to minimize the cycle time. Once the mold is open, the part is ejected usually by the ejector pins. When the ejector pins are retracted and all criteria of the molding cycle are met the next cycle begins.

This animation shows how the plastic will flow if the gate is placed right in the center of the part.

This animation shows how the plastic will flow if the gate is placed right in the center of the part. The gate location was determined by the gate advisers of the Mold Flow software. According to this software this would be the best possible gate location for this part design. As you will notice the plastic has hesitation when it goes thru the three knockout holes due to that thin wall area. Three things may occur do to this plastic hesitation. The part may have a burn, knit/weld line, or even a non-fill.