Calculating Residence Time and Machine Shot Capacity
PVC Injection Molding Process
September 10, 2013
Injection molding is the most common way to manufacture plastic products in high volume. Injection molding machines operate horizontally or vertically to fasten and employ molds. Most machines today are horizontally-oriented, however vertical machines are primarily used for insert molding in order for the process to utilize gravity. A mold may be designed for either a cold runner system or a hot runner system. With a cold runner system, the plastic is injected through a carved channel and cools from the mold. The plastic from the carved channel forms a sprue, which is later removed. The more complex hot runner system operates using cartridge heaters to keep the plastic in the runners fluid so that it can be injected into the next part after the current part cools and is ejected.
With injection molding, parts are produced in their final shape upon ejection. Stabilizing this process can take a long time and defects (such as blistering, burn marks, color streaking, embedded foreign particles, flow lines, jetting, polymer degradation, sink marks, short shot, splash marks, stringing, voids, weld lines, and warping) can likely occur in the parts produced from irregularities in the mold or the molding process itself. In order to correct these issues, examination of the defective parts and process must specify the defects and address the design of the components and process. Trial runs are necessary prior to production to conduct the examination for defects.
One major type of polymer subjected to the injection molding process is PVC, or polyvinyl chloride. This polymer is characterized by its polarity of chlorine atoms in its amorphous molecular structure. Possession of chlorine atoms in PVC resins accounts for their chemical stability and attribute to their fire retarding properties, durability, and oil and/or chemical resistance. PVC's melt viscosity is relatively high compared to other thermoplastic materials, which makes it an unsuitable material for injection molding of large products in high shot capacity machines. It's heat stability is rather low, causing it to start decomposition at 140º C and melt at 160º C, therefore a heat stabilizer is necessary during the injection molding process for the PVC to keep its properties.
When running rigid PVC, or uPVC (unplasticized), through a hot runner system, it is important to keep heat stabilized and the molecular properties of the PVC intact. In order to prevent degradation of the PVC, monitoring the temperature of the mold and utilizing heat stabilizers to keep it under 140º C, disabling build up of excess water in the material's granules, restricting duration of the material's time spent in the barrel, and limiting the amount of regrind (post-industrial polymer waste from cut sprues, runners, chopped or ground flash, or non-contaminated rejected parts) added to the material. Vents should be added to the mold to prevent short shots and localized brining by allowing trapped gases and air to escape.
As previously stated, PVC is not ideal for high shot capacity injection machinery as they necessitate low compression screws and a short nozzle (the hollow metal hose screwed into the extrusion end of the heating cylinder which forms a seal under pressure between the cylinder and the mold).
Calculating residence time for molding includes machine shot size in grams, maximum injection screw stroke in diameters, shot size being molded converted to the same melt density as polystyrene in grams, and cycle time of molding in seconds.
- Multiply machine shot by two if maximum injection screw stroke is less than four.
- Multiply machine shot by 1.4 if maximum injection screw stroke is greater than or equal to four. For example, if machine shot is 200 grams and maximum injection screw stroke is four, the result is 280.
- Divide the result by the melt density for polystyrene. For example, 280 divided by 100 grams is 2.8.
- Multiply the result by the cycle time of molding. For example, 2.8 times 15 seconds yields 42. Under these conditions, the barrel residence time is 42 seconds.
To determine the recommended shot weight of PVC, optimum barrel usage should first be calculated and multiplied by the specific gravity of the PVC, 1.35 kg, versus the specific gravity for the material in which the machine is rated, which most machines are rated for general purpose polystyrene (GPPS) at 1.05 kg. For example, using a 60 oz (1.7 kg) barrel, the equation should look something like this:
60 oz x 1.35/1.05 = 77 oz (2.2 kg)