Injection Molding - Process Characteristics

Process Characteristics

• Utilizes a ram or screw-type plunger to force molten plastic material into a mold cavity
• Produces a solid or open-ended shape that has conformed to the contour of the mold
• Used to process both thermoplastic and thermosetting polymers, with the former being considerably more prolific in terms of annual material volumes processed. The prevalence of thermoplastics is a result of many factors including:

• simultaneous economic, engineering, and manufacturing feasibility
• unique and diverse material properties, well suited for a larger proportion of consumer and engineering applications
• the ability to soften and flow upon heating

• facilitates recycling post consumer waste, reject parts, and the melt delivery system
• reduces manufacturing risks associated with the injection unit. Thermosets, which cannot flow after the chemical crosslinking occurs, are susceptible to crosslinking in the injection barrel and encapsulating the screw and check valves leading to flow obstructions, mechanical seizures, damage to expensive components, and timely/costly down-times during the mechanical removal of thermosetting polymer from the injection components.
• reduces manufacturing time per part produced in many scenarios because the thermal solidification of thermoplastics be achieved in a shorter time duration than the chemical solidification of thermosets

• The process consists of high pressure injection of the raw material into a mold which shapes the polymer into the desired shape. Molds can be of a single cavity or multiple cavities. In multiple cavity molds, each cavity can be identical and form the same parts or can be unique and form multiple different geometries during a single cycle. Molds are generally made from tool steels but stainless steels and aluminum molds are suitable for certain applications. Aluminum molds typically are ill-suited for high volume production or parts with narrow dimensional tolerances as they have inferior mechanical properties and are more prone to wear and damage and deformation during the injection and clamping cycles, but are cost effective in low volume applications as mold fabrication costs and time are considerably reduced. Many steel molds are designed to process well over a million parts during their lifetime and can cost hundreds of thousands of dollars to fabricate.
• For thermoplastics, typically pelletized raw material is fed through a hopper into a heated barrel with a reciprocating screw. Upon entrance to the barrel the thermal energy increases and the Van der Waals forces that resist relative flow of individual chains are weakened as a result of increased space between molecules at higher thermal energy states. This reduces its viscosity, which enables the ability to flow with the driving force of the injection unit. The screw delivers the raw material forward, mixes and homogenizes the thermal and viscous distributions of the polymer, and reduces the required heating time by mechanically shearing the material and adding a significant amount of frictional heating to the polymer. The materials feeds forward through a check valve and collects at the front of the screw into a volume known as a "shot". A "shot" refers to the volume of material which is used to fill the mold cavity, compensate for shrinkage, and provide a cushion (approximately 10% of the total shot volume which remains in the barrel and prevents the screw from bottoming out) to transfer pressure from the screw to the mold cavity. When enough material has gathered, the material is forced at high pressure and velocity into the part forming cavity. To prevent spikes in pressure the process normally utilizes a transfer position corresponding to a 95-98% full cavity where the screw shifts from a constant velocity to a constant pressure control. Often injection times are well under 1 second. Once the screw reaches the transfer position the packing pressure is applied which completes mold filling and compensates for thermal shrinkage, which is quite high for thermoplastics relative to many other materials. The packing pressure is applied until the gate (cavity entrance) solidifies. The gate is is normally the first place to solidify through its entire thickness due to its small size. Once the gate solidifies, no more material can enter the cavity; accordingly, the screw reciprocates and acquires material for the next cycle while the material within the mold cools so that it can be ejected and be dimensionally stable. This cooling duration is dramatically reduced by the use of cooling lines circulating water or oil from a thermolator. Once the required temperature has been achieved, the mold opens and an array of pins, sleeves, strippers, etc. are driven forward to demold the article. Then, the mold closes and the process is repeated.
• For thermosets, typically two different chemical components are injected into the barrel. These components immediately begin irreversible chemical reactions which eventually crosslinks the material into a single connected network of molecules. As the chemical reaction occurs the two fluid components permanently transform into a viscoelastic solid. The solidification in the injection barrel and screw can be problematic and have financial repercussions; therefore, minimizing the thermoset curing within the barrel is vital. This typically means that the residence time and temperature of the chemical precursors is minimized in the injection unit. The residence time can be reduced by minimizing the barrel's volume capacity and by maximizing the cycle times. These factors have led to the use of a thermally isolated, cold injection unit that injects the reacting chemicals into a thermally isolated hot mold, which increases the rate of chemical reactions and results in shorter time required to achieve a solidified thermoset component. After the part has solidified vales close, isolating the injection system and chemical precursors, and the mold opens ejecting the molded parts. Then, the mold closes and the process repeats.
• A parting line, sprue, gate marks, and ejector pin marks are usually present on the final part. None of these features are typically desired, but are unavoidable due to the nature of the process. Gate marks occurs at the gate which joins the melt-delivery channels (sprue and runner) to the part forming cavity. The only way to eliminate gate marks would be to eliminate the gate itself, rendering the injection impossible to achieve. Parting line and ejector pin marks result from minute misalignments, wear, gaseous vents, clearances for adjacent parts in relative motion, and/or dimensional differences of the mating surfaces contacting the injected polymer. Dimensional differences can be attributed to non-uniform, pressure-induced deformation during injection, machining tolerances, and non-uniform thermal expansion and contraction of mold components, which experience rapid cycling during the injection, packing, cooling, and ejection phases of the process. Mold components are often designed with materials of various coefficients of thermal expansion. These factors cannot be simultaneously accounted for without astronomical increases in the cost for design, fabrication, processing, and quality monitoring. The skillful mold and part designer will position these aesthetic detriments in hidden areas if feasible.