injection molding cost sheet

There are innumerable different styles of estimate sheets. Some include information regarding the estimated figures only, while others are used to record facts pertaining to production and actual cost comparisons. Figure 1 exemplifies a concise form of standard estimate sheet which is applicable to any item where it is required to prepare selling prices for the customer.

Figure 1

It will be noted that provision is made for overhead cost after each operation inasmuch as sometimes this quantity is a variable and, therefore, appears as a different percentage on the corresponding labor cost. After we arrive at the primeplastic molding companies cost we then apply the sales and administrative estimated values. Finally, the percentage of profit is added to the total to obtain the selling price. It should be remembered, however, that net profit is based on selling price and that the amount used to obtain the selling price is not actually the amount of net profit realized. In other words, if the full factory cost amounted to $80.00 per thousand and to this was added 25% for profit the selling price would then be $100.00 but the net profit in such a case is not 25% but 20%. The actual net profit is the percentage ratio of the difference between the full plastic molding companies cost and the selling price. If it is desired to find the correct percentage to apply to a full plastic molding companies cost in order to obtain a given net profit on the selling price this can be accomplished by merely substituting in the following formula:

P is the unknown percentage which should be applied to the full plastic molding companies cost in order to get the desired profit wanted in percentage. The percentage of profit wanted is designated by p. Thus if it is desired to get a 20% net profit on an item which has a full plastic moulding supply factory of any value whatsoever we can see that it must have 25% added to it.

This will apply to any value and is therefore helpful in assuring that the real net profit based on selling price will be obtained.
Now that we have studied the estimate form and thoroughly understand it let us work out a problem utilizing this knowledge.
Let us say, for instance, that a customer submits a blue print showing a piece rectangular in shape, about 4f/ X 2″ X 1〃 deep. It has two holes in the base that have to be punched out and he asks for a custom plastic piece price based on quantities of fifty thousand lots. He stipulates that he would like to have figures furnished on a ten cavity mold, the piece to be made of black phenolic material. At the outset we take off quantities from the print and find that the part would have an approximate weight of 154 pounds per thousand. This is, of course, net weight so we have to add a certain percentage in order to get the gross. Let us now look at Figure 2. Here is the estimate form made out on the basis of the information submitted. We can follow this through from the beginning and see how the ultimate selling price was obtained.

In the first place the material price is found to be twelve cents a pound and inasmuch as no provision is made for tabletting in the estimate the powder price must include this preparation cost. Some firms prefer to set up a separate operation for pilling, but inasmuch as it is labor performed on material prior to its function in the molding process it is commonly included in the material cost per pound. After finding the net material cost, 20% is then added for flash or overflow to give the gross material figure. We turn next to the molding operation. In deciding the pressing cost the cycle time is usually used for a basis, so in this instance three minutes was presumed to be sufficient. If it takes three minutes to complete one heat we know that twenty heats should be ejected in an hour, and if we multiply the number of heats by the mold capacity we then obtain the total hourly production. In this case we have a ten cavity die and therefore two hundred units should be produced in an hour. Converting this into the number of hours required to make a thousand and multiplying by the prevailing labor amounts to $2.50 and the overhead is then obtained by taking a percentage of the labor. In the example given, overhead has been assumed to be 150% in the press room and 100% in the finishing room, this accounts for the values given in the overhead column. The method of arriving at overhead percentages will be dealt with in detail later. For the present at least is suffice to note that the total labor and overhead is added to the material cost, after which is appended the rejection and carton price figures to obtain the prime factory cost. In looking back at the finishing operations we find that the piece was charged with first and final inspection, sanding, punching, and, of course, packing.

to be continued…

When you buy metal for injection mold making, it is either in ingot or semifinished form. In making the injection mold, the metal is either melted and cast, or mechanically worked or machined to form the finished shape. Consequently, there is little, if any, difference in the composition and properties of the metal as-purchased and the metal in the completed tool. The producer of the metal has predetermined its properties by alloying or processing it to its as-purchased form.

When you buy a tooling plastic, it is usually in the form of a liquid (or paste)resin (a partially polymerized monomer) —or, basically, a chemical compound. In making the tool, the resin is mixed with other chemicals and often solid materials such as fillers or reinforcements, and is cast or laminated to the finished shape. Chemical reactions within the material actually change the material, producing its final performance capabilities. From one standpoint, you are actually making the material, along with the tool.

This is an important difference between metals and plastics, because the mixing, formulating and curing of the plastics determine to a great extent the properties of the finished material—just as the alloying of the metal in the producers plant has a determining effect on properties of the alloy. Consequently the engineer must know a great deal more about the basic chemical technology of plastics than he had to know about the alloying of metals. The subsequent chapters on materials and fabricating techniques attempt to clarify this area of technology. For more detailed information references at the ends of appropriate chapters should be consulted.

Behavior Differences

From the behavior standpoint, one of the important differences between plastics and metals is the sensitivity of plastics to the effects of time. Consequently they are much more sensitive than metals to rate of stressing, and aging under various environmental conditions. In spite of the newness of plastics, a substantial amount of design information is available. But long periods of time required for testing to determine effects of long-time use have limited the amount of data on creep, fatigue and long-time exposure to heat and chemicals.

Engineering properties as determined by standard test procedures are the only practical measure we have of predicting the behavior of a material in service. The engineering properties of plastics most pertinent to tooling use include

• mechanical properties (both short-term and longterm )，such as strength, hardness, and wear resistance, and their relationship to physical properties such as density and moisture absorption；
• thermal characteristics, such as heat resistance, thermal conductivity and thermal expansion characteristics； and
• chemical resistance.

One aid to development or restoration of curiosity is to train oneself to be observant. An injection mold design engineer, especially, should be observant of objects about him that have been created by man. He must ask how the object is made, of what materials it is constructed, why it was designed of a particular size and shape, why and how it was finished as it was, and how much it cost. These observations lead the creative thinker to see ways in which it can be improved, or to devise a better object to take its place. In the world of competitive industry, he may also be led to see a way of reducing its cost. Observation often leads to a revolutionary idea that but he also must have a comprehensive knowledge of a considerable variety of plastics manufacturing materials and of industrial processes. He must be familiar with the organization and functioning of industrial concerns and the human factors involved in a manufacturing enterprise. He must realize that progress is made through working with others, and that honesty, acceptance of responsibility, and the meeting of commitments cheerfully form a basis for good relationships with his fellow workers. He must know that high production rates are achieved when manufacturing operations can be carried on in safety and relative comfort by satisfied shop personnel. He must also be conscious that appearance is a factor in selling a product and either design his product with that in mind or get help in the problem of achieving good appearance without sacrificing utility. Most important of all, he must be cost conscious, as almost all actions are based eventually on cost.may satisfy a public need. Witness the experience of DeForrest, who was led to develop the thermionic electron tube by observing the effects of an electrical discharge on a nearby gas jet. This observation, seemingly irrelevant to what he was doing, caused him to wonder why the jet behaved in an unexplained manner. This was an incentive to develop the relationship of heat to electron flow and, having arrived at the explanation of that phenomenon, he progressed to the invention of the three-element electron tube, which is the core of radio broadcasting.

One significant creative idea usually opens up fields of activities that lead to many ideas. As a student, Dr. C. R. Hanna, Associate Director of the Westinghouse Research Laboratories, was interested in acoustics. This led him to research work on loudspeakers when radios were first produced, and resulted in the development of both large and small speakers with various types of power units. The use of mechanical equivalents for electric currents and electric circuit equivalents for mechanical devices led to many variations and combinations of electrical and mechanical devices. The microphones, pickup devices, and speakers that were developed used mechanical or electrical actuators. When the sound movie came into existence, it was troubled by excessive noise in the system used for recording, pickup, and broadcasting. When fundamental principles of electrical and mechanical filtering and damping were incorporated in the camera and projector, the present-day high-fidelity sound movies were obtained. These achievements in the field of sound attracted many similar problems to Dr. Hanna’s department, such as speed regulation, damping, and elimination of noise in other types of apparatus. As a result, improved speed regulators, quiet rotating equipment, and an automobile shock absorber were developed. The control of large amounts of power by very sensitive electric circuits, mechanical devices, and a hydraulic valve system was a natural step in the solution of damping problems and power control. The control of speed and stability of equipment led to the use of the gyroscope, which is used to keep guns on the target when mounted in tanks, on ships, and in airplanes. The master stabilizer equipment on battleships and the automatic pilot for airplanes and guided missiles are some of the latter applications of the foregoing systems. A less spectacular but equally significant application is the use of a gyroscope to regulate the speed of a relatively slow-moving part, such as that of a gearless elevator motor as it decelerates to a stop. The damping systems have been applied to railroad passenger cars in order to reduce the sway and road shock. The railroad cars also can be tilted to compensate for centrifugal forces in going around the curve. Thus it can be seen that possibilities of an idea are endless and most profitable to the person who recognizes its significance. The idea may be the basis of one’s life work.